FREQUENCY DOMAIN PARAMETER SEQUENCE GENERATING METHOD, ENCODING METHOD, DECODING METHOD, FREQUENCY DOMAIN PARAMETER SEQUENCE GENERATING APPARATUS, ENCODING APPARATUS, DECODING APPARATUS, PROGRAM, AND RECORDING MEDIUM

Abstract

The present invention reduces encoding distortion in frequency domain encoding compared to conventional techniques, and obtains LSP parameters that correspond to quantized LSP parameters for the preceding frame and are to be used in time domain encoding from coefficients equivalent to linear prediction coefficients resulting from frequency domain encoding. When p is an integer equal to or greater than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], an LSP linear transformation unit (300) determines the value of each converted frequency domain parameter ˜ω[i] (i=1, 2, . . . , p) in a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, through linear transformation which is based on the relationship of values between ω[i] and one or more frequency domain parameters adjacent to ω[i].

Claims

1. A frequency domain parameter sequence generating method comprising: where p is an integer equal to or greater than 1, γ1 is a positive constant equal to or smaller than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], a parameter sequence conversion step of determining a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, wherein the parameter sequence conversion step determines a value of each converted frequency domain parameter ˜ω[i] (i−1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] through linear transformation which is based on a relationship of values between ω[i] and one or more frequency domain parameters adjacent to ω[i], and each ω[i] (i=1, 2, . . . , p) in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] is a frequency domain parameter equivalent to a.sub.γ1[1], a.sub.γ1[2], . . . , a.sub.γ1[p], or a quantized value of said frequency domain parameter, where a.sub.γ1[i]=a[i]×(γ1).sup.i.

2. The frequency domain parameter sequence generating method according to claim 1, wherein the linear transformation is linear transformation that makes intervals between parameter values more uniform or less uniform in the converted frequency domain parameter sequence than in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p].

3. A frequency domain parameter sequence generating method comprising: where p is an integer equal to or greater than 1, and a linear prediction coefficient sequence obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p]; ω[1], ω[2], ω[p] is one of an LSP parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], an ISP parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], an ISF parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], an ISF parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], and a frequency domain parameter sequence which is derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p] and in which all of ω[1], ω[2], . . . , ω[p−1] are present from 0 to π and, when all of linear prediction coefficients contained in the linear prediction coefficient sequence are 0, ω[1], ω[2], . . . , ω[p−1] are present from 0 to π at equal intervals; and each γ1 and γ2 is a positive constant equal to or smaller than 1, and K is a predetermined p×p band matrix in which diagonal elements and elements that neighbor the diagonal elements in row direction have non-zero values, a parameter sequence conversion step of generating a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] defined by a following formula. $(\begin{matrix} \tilde{ω} [1] \\ \tilde{ω} [2] \\ .Math. \\ \tilde{ω} [p] \end{matrix}) = K (\begin{matrix} ω [1] - \frac{π}{p - 1} \\ ω [2] - \frac{2 .Math. π}{p + 1} \\ .Math. \\ ω [p] - \frac{p .Math. .Math. π}{p - 1} \end{matrix}) .Math. (γ2 - γ1) + (\begin{matrix} ω [1] \\ ω [2] \\ .Math. \\ ω [p] \end{matrix})$

4. The frequency domain parameter sequence generating method according to claim 3, wherein the band matrix K has positive values in the diagonal elements and negative values in elements that neighbor the diagonal elements in row direction.

5. A frequency domain parameter sequence generating method comprising: where p is an integer equal to or greater than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence which is derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p] and in which all of ω[1], ω[2], . . . , ω[p−1] are present from 0 to π and, when all of linear prediction coefficients contained in the linear prediction coefficient sequence are 0, ω[1], ω[2], . . . , ω[p−1] are present from 0 to π at equal intervals, and where each γ1 and γ2 is a positive constant equal to or smaller than 1, and K is a predetermined p×p band matrix in which diagonal elements and elements that neighbor the diagonal elements in row direction have non-zero values, a parameter sequence conversion step of determining a converted frequency domain parameter sequence ˜ω[1], ˜θ[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], θ[2], . . . , ω[p] as input, wherein the parameter sequence conversion step determines each ˜ω[i] (i=1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] such that when ω[i] is closer to ω[i+1] relative to a midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that a value of ˜ω[i+1]−˜ω[i] will be smaller than ω[i+1]−ω[i], and when ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be smaller than ω[i]−ω[i−1], and a converted frequency domain parameter sequence ˜ω[1], ˜θ[2], . . . , ˜θ[p] defined by $(\begin{matrix} \tilde{ω} [1] \\ \tilde{ω} [2] \\ .Math. \\ \tilde{ω} [p] \end{matrix}) = K (\begin{matrix} ω [1] - \frac{π}{p - 1} \\ ω [2] - \frac{2 .Math. π}{p + 1} \\ .Math. \\ ω [p] - \frac{p .Math. .Math. π}{p - 1} \end{matrix}) .Math. (γ2 - γ1) + (\begin{matrix} ω [1] \\ ω [2] \\ .Math. \\ ω [p] \end{matrix})$ is generated.

6. The frequency domain parameter sequence generating method according to claim 3, wherein the band matrix K has values greater than or equal to 0 in diagonal elements and values smaller than or equal to 0 in elements that neighbor the diagonal elements in row direction.

7. A frequency domain parameter sequence generating method comprising: where p is an integer equal to or greater than 1, γ1 is a positive constant equal to or smaller than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], a parameter sequence conversion step of determining a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, wherein the parameter sequence conversion step determines each ˜ω[i] (i=1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] such that when ω[i] is closer to ω[i+1] relative to a midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that a value of ˜ω[i+1]−˜ω[i] will be smaller than ω[i+1]−ω[i], and when ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be smaller than ω[i]−ω[i−1], and each ω[i] (i=1, 2, . . . , p) in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] is a frequency domain parameter equivalent to a.sub.γ1[1], a.sub.γ2[2], . . . , a.sub.γ1[p], or a quantized value of said frequency domain parameter, where a.sub.γ1[i]=a[i]×(γ1).sup.i.

8. A frequency domain parameter sequence generating method comprising: where p is an integer equal to or greater than 1, γ1 is a positive constant equal to or smaller than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], a parameter sequence conversion step of determining a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, wherein the parameter sequence conversion step determines each ˜ω[i] (i=1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] such that when ω[i] is closer to ω[i+1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that a value of ˜ω[i+1]−˜ω[i] will be greater than ω[i+1]−ω[i], and when ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be greater than ω[i]−ω[i−1], and each ω[i] (i=1, 2, p) in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] is a frequency domain parameter equivalent to a.sub.γ1[1], a.sub.γ1[2], . . . , a.sub.γ1[p], or a quantized value of said frequency domain parameter, where a.sub.γ1[i]=a[i]×(γ1).sup.i.

9. The frequency domain parameter sequence generating method according to any one of claims 3 to 6, wherein γ1 is a positive constant equal to or smaller than 1, and each ω[i] (i=1, 2, . . . , p) in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] is a frequency domain parameter equivalent to a.sub.γ1[1], a.sub.γ1[2], . . . , a.sub.γ1[p], or a quantized value of said frequency domain parameter, where a.sub.γ1[i]=a[i]×(γ1).sup.i.

10. A encoding method including the steps of the frequency domain parameter sequence generating method according to any one of claims 1 to 8, the encoding method comprising: where γ is a adjustment factor which is a positive constant equal to or smaller than 1, a linear prediction coefficient adjustment step of generating a adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p] by adjusting the linear prediction coefficient sequence a[1], a[2], . . . , a[p] using the adjustment factor γ; a adjusted LSP generation step of generating a adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[p] using the adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p]; a adjusted LSP encoding step of encoding the adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[p] to generate adjusted LSP codes and a adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] corresponding to the adjusted LSP codes; an LSP linear transformation step of, with the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] being the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p], and γ1=γ and γ2=1, executing the parameter sequence conversion step to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ω[p] as an approximate quantized LSP parameter sequence ̂θ.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a quantized linear prediction coefficient sequence generation step of generating a adjusted quantized linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p] by converting the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] into linear prediction coefficients; a quantized smoothed power spectral envelope series calculation step of calculating a quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N] which is a series in frequency domain corresponding to the adjusted quantized linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p]; a frequency domain encoding step of generating frequency domain signal codes by encoding a frequency domain sample sequence X[1], X[2], . . . , X[N] corresponding to the audio signals using the quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP generation step of generating an LSP parameter sequence θ[1], θ[2], . . . , e[p] using the linear prediction coefficient sequence a[1], a[2], . . . , a[p]; an LSP encoding step of encoding the LSP parameter sequence θ[1], θ[2], . . . , θ[p] to generate LSP codes and a quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] corresponding to the LSP codes; and a time domain encoding step of encoding the audio signals to generate time domain signal codes using either a quantized LSP parameter sequence obtained in the LSP encoding step for a preceding time segment or an approximate quantized LSP parameter sequence obtained in the LSP linear transformation step for the preceding time segment, and the quantized LSP parameter sequence for the predetermined time segment.

11. A encoding method including the steps of the frequency domain parameter sequence generating method according to any one of claims 1 to 8, the encoding method comprising: where γ is a adjustment factor which is a positive constant equal to or smaller than 1, a linear prediction coefficient adjustment step of generating a adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p] by adjusting the linear prediction coefficient sequence a[1], a[2], . . . , a[p] using the adjustment factor γ; a adjusted LSP generation step of generating a adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[p] using the adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p]; a adjusted LSP encoding step of encoding the adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[p] to generate adjusted LSP codes and a adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] corresponding to the adjusted LSP codes; an LSP linear transformation step of, with the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] being the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p], and γ1=γ and γ2=1, executing the parameter sequence conversion step to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] as an approximate quantized LSP parameter sequence ̂θ.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a quantized smoothed power spectral envelope series calculation step of calculating a quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N] based on the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p]; a frequency domain encoding step of generating frequency domain signal codes by encoding a frequency domain sample sequence X[1], X[2], X[N] corresponding to the audio signals using the quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP generation step of generating an LSP parameter sequence θ[1], θ[2], . . . , θ[p] using the linear prediction coefficient sequence a[1], a[2], . . . , a[p]; an LSP encoding step of encoding the LSP parameter sequence θ[1], θ[2], . . . , θ[p] to generate LSP codes and a quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] corresponding to the LSP codes; and a time domain encoding step of encoding the audio signals to generate time domain signal codes using either a quantized LSP parameter sequence obtained in the LSP encoding step for a preceding time segment or an approximate quantized LSP parameter sequence obtained in the LSP linear transformation step for the preceding time segment, and the quantized LSP parameter sequence for the predetermined time segment.

12. The encoding method according to claim 10, further comprising: an output step of outputting either the frequency domain signal codes generated in the frequency domain encoding step or the time domain signal codes generated in the time domain encoding step, wherein at the time domain encoding step, when frequency domain signal codes have been output in the output step for the preceding time segment, encoding that uses the approximate quantized LSP parameter sequence obtained in the LSP linear transformation step for the preceding time segment is performed, and when time domain signal codes have been output in the output step for the preceding time segment, encoding that uses the quantized LSP parameter sequence obtained in the LSP generation step for the preceding time segment is performed.

13. A decoding method including the steps of the frequency domain parameter sequence generating method according to any one of claims 1 to 8, the decoding method comprising: a adjusted LSP code decoding step of decoding input adjusted LSP codes to obtain a decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], Aθ.sub.γ[2], . . . , ̂θ.sub.γ[p]; a decoded LSP linear transformation step of, with the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] being the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p], and γ1=γ and γ2=1, executing the parameter sequence conversion step to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] as a decoded approximate LSP parameter sequence ̂θ.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a decoded linear prediction coefficient sequence generation step of generating a decoded adjusted linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p] by converting the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] into linear prediction coefficients; a decoded smoothed power spectral envelope series calculation step of calculating a decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], ̂W.sub.γ[N] which is a series in frequency domain corresponding to the decoded adjusted linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p]; a frequency domain decoding step of generating decoded sound signals using a frequency domain signal sequence resulting from decoding of input frequency domain signal codes and the decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP code decoding step of decoding input LSP codes to obtain a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p]; and a time domain decoding step of decoding input time domain signal codes, and generating decoded sound signals by synthesizing the time domain signal codes using either the decoded LSP parameter sequence obtained in the LSP code decoding step for the preceding time segment or the decoded approximate LSP parameter sequence obtained in the LSP linear transformation step for the preceding time segment, and the decoded LSP parameter sequence for the predetermined time segment.

14. A decoding method including the steps of the frequency domain parameter sequence generating method according to any one of claims 1 to 8, the decoding method comprising: a adjusted LSP code decoding step of decoding input adjusted LSP codes to obtain a decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ [2], . . . , ̂θ.sub.γ[p]; a decoded LSP linear transformation step of, with the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] being the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂a.sub.γ[2], ̂θ.sub.γ[p], and γ1=γ and γ2=1, executing the parameter sequence conversion step to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] as a decoded approximate LSP parameter sequence ̂θ.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a decoded smoothed power spectral envelope series calculation step of calculating a decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N] based on the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p]; a frequency domain decoding step of generating decoded sound signals using the frequency domain signal sequence resulting from decoding of input frequency domain signal codes and the decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; a frequency domain decoding step of generating decoded sound signals using the frequency domain signal sequence resulting from decoding of the input frequency domain signal codes and the decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP code decoding step of decoding input LSP codes to obtain a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂[p]; and a time domain decoding step of decoding input time domain signal codes, and generating decoded sound signals by synthesizing the time domain signal codes using either the decoded LSP parameter sequence obtained in the LSP code decoding step for the preceding time segment or the decoded approximate LSP parameter sequence obtained in the LSP linear transformation step for the preceding time segment, and the decoded LSP parameter sequence for the predetermined time segment.

15. A frequency domain parameter sequence generating apparatus comprising: where p is an integer equal to or greater than 1, γ1 is a positive constant equal to or smaller than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], a parameter sequence converting unit that determines a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, wherein the parameter sequence converting unit determines a value of each converted frequency domain parameter ˜ω[i] (i=1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] through linear transformation which is based on a relationship of values between ω[i] and one or more frequency domain parameters adjacent to ω[i], and each ω[1] (i=1, 2, . . . , p) in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] is a frequency domain parameter equivalent to a.sub.γ1[1], a.sub.γ1[2], . . . , a.sub.γ1[p], or a quantized value of said frequency domain parameter, where a.sub.γ1[i]=a[i]×(γ1).sup.i.

16. A frequency domain parameter sequence generating apparatus comprising: where p is an integer equal to or greater than 1, and a linear prediction coefficient sequence obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p]; ω[1], ω[2], . . . , ω[p] is one of an LSP parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], an ISP parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], an LSF parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], an ISF parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], and a frequency domain parameter sequence which is derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p] and in which all of ω[1], ω[2], . . . , ω[p−1] are present from 0 to π and, when all of linear prediction coefficients contained in the linear prediction coefficient sequence are 0, ω[1], ω[2], . . . , ω[p−1] are present from 0 to π at equal intervals; and each γ1 and γ2 is a positive constant equal to or smaller than 1, and K is a predetermined p×p band matrix in which diagonal elements and elements that neighbor the diagonal elements in row direction have non-zero values, a parameter sequence converting unit that generates a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] defined by a following formula. $(\begin{matrix} \tilde{ω} [1] \\ \tilde{ω} [2] \\ .Math. \\ \tilde{ω} [p] \end{matrix}) = K (\begin{matrix} ω [1] - \frac{π}{p - 1} \\ ω [2] - \frac{2 .Math. π}{p + 1} \\ .Math. \\ ω [p] - \frac{p .Math. .Math. π}{p - 1} \end{matrix}) .Math. (γ2 - γ1) + (\begin{matrix} ω [1] \\ ω [2] \\ .Math. \\ ω [p] \end{matrix})$

17. The frequency domain parameter sequence generating apparatus according to claim 16, wherein the band matrix K has positive values in the diagonal elements and negative values in elements that neighbor the diagonal elements in row direction.

18. A frequency domain parameter sequence generating apparatus comprising: where p is an integer equal to or greater than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence which is derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p] and in which all of ω[1], ω[2], . . . , ω[p−1] are present from 0 to π and, when all of linear prediction coefficients contained in the linear prediction coefficient sequence are 0, ω[1], ω[2], . . . , ω[p−1] are present from 0 to π at equal intervals, and where each γ1 and γ2 is a positive constant equal to or smaller than 1, and K is a predetermined p×p band matrix in which diagonal elements and elements that neighbor the diagonal elements in row direction have non-zero values, a parameter sequence converting unit that determines a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, wherein the parameter sequence converting unit determines each ˜ω[i] (i=1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] such that when ω[i] is closer to ω[i+1] relative to a midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that a value of ˜ω[i+1]−˜ω[i] will be smaller than ω[i+1]−ω[i], and when ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be smaller than ω[i]−ω[i−1], and a converted frequency domain parameter sequence −ω[1], ˜ω[2], . . . , ˜ω[p] defined by $(\begin{matrix} \tilde{ω} [1] \\ \tilde{ω} [2] \\ .Math. \\ \tilde{ω} [p] \end{matrix}) = K (\begin{matrix} ω [1] - \frac{π}{p - 1} \\ ω [2] - \frac{2 .Math. π}{p + 1} \\ .Math. \\ ω [p] - \frac{p .Math. .Math. π}{p - 1} \end{matrix}) .Math. (γ2 - γ1) + (\begin{matrix} ω [1] \\ ω [2] \\ .Math. \\ ω [p] \end{matrix})$ is generated.

19. The frequency domain parameter sequence generating apparatus according to claim 16, wherein the band matrix K has values greater than or equal to 0 in diagonal elements and values smaller than or equal to 0 in elements that neighbor the diagonal elements in row direction.

20. A frequency domain parameter sequence generating apparatus comprising: where p is an integer equal to or greater than 1, γ1 is a positive constant equal to or smaller than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], θ[2], . . . , ω[p] are a frequency domain parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], a parameter sequence converting unit that determines converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, wherein the parameter sequence converting unit determines each ω[i] (i=1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] such that when ω[i] is closer to ω[i+l] relative to a midpoint between ω[i+l] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that a value of ˜ω[i+1]−˜ω[i] will be smaller than ω[i+1]−ω[i], and when ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be smaller than ω[i]−ω[i−1], and each ω[i] (i=1, 2, . . . , p) in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] is a frequency domain parameter equivalent to a.sub.γ1[1], a.sub.γ1[2], . . . , a.sub.γ1[p], or a quantized value of said frequency domain parameter, where a.sub.γ1[i]=a[i]×(γ1).sup.i.

21. A frequency domain parameter sequence generating apparatus comprising: where p is an integer equal to or greater than 1, γ1 is a positive constant equal to or smaller than 1, a linear prediction coefficient sequence which is obtained by linear prediction analysis of audio signals in a predetermined time segment is represented as a[1], a[2], . . . , a[p], and ω[1], ω[2], . . . , ω[p] are a frequency domain parameter sequence derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], a parameter sequence converting unit that determines a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] using the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] as input, wherein the parameter sequence converting unit determines each ˜ω[i] (i=1, 2, . . . , p) in the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] such that when ω[i] is closer to ω[i+1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ω[i−1] and that a value of ω[i+1]−˜ω[i] will be greater than ω[i+1]−˜ω[i], and when ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be greater than ω[i]−ω[i−1], and each ω[i] (i=1, 2, . . . , p) in the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] is a frequency domain parameter equivalent to a.sub.γ1[1], a.sub.γ1[2], . . . , a.sub.γ1[p], or a quantized value of said frequency domain parameter, where a.sub.γ1[i]=a[i]×(γ1).sup.i.

22. A encoding apparatus including the units of the frequency domain parameter sequence generating apparatus according to any one of claims 15 to 21, the encoding apparatus comprising: where γ is a adjustment factor which is a positive constant equal to or smaller than 1, a linear prediction coefficient adjusting unit that generates a adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p] by adjusting the linear prediction coefficient sequence a[1], a[2], . . . , a[p] using the adjustment factor γ; a adjusted LSP generating unit that generates a adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[p] using the adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p]; a adjusted LSP encoding unit that encodes the adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[p] to generate adjusted LSP codes and a adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] corresponding to the adjusted LSP codes; an LSP linear transformation unit that, with the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] being the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p], and γ1=γ and γ2=1, executes the parameter sequence converting unit to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] as an approximate quantized LSP parameter sequence ̂.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a quantized linear prediction coefficient sequence generating unit that generates a adjusted quantized linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p] by converting the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] into linear prediction coefficients; a quantized smoothed power spectral envelope series calculating unit that calculates a quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N] which is a series in frequency domain corresponding to the adjusted quantized linear prediction coefficient sequence ̂a.sub.γ[1], ̂.sub.γ[2], . . . , ̂a.sub.γ[p]; a frequency domain encoding unit that generates frequency domain signal codes by encoding a frequency domain sample sequence X[1], X[2], X[N] corresponding to the audio signals using the quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP generating unit that generates an LSP parameter sequence θ[1], θ[2], . . . , θ[p] using the linear prediction coefficient sequence a[1], a[2], . . . , a[p]; an LSP encoding unit that encodes the LSP parameter sequence θ[1], θ[2], . . . , θ[p] to generate LSP codes and a quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] corresponding to the LSP codes; and a time domain encoding unit that encodes the audio signals to generate time domain signal codes using either a quantized LSP parameter sequence obtained in the LSP encoding unit for a preceding time segment or an approximate quantized LSP parameter sequence obtained in the LSP linear transformation unit for the preceding time segment, and the quantized LSP parameter sequence for the predetermined time segment.

23. A encoding apparatus including the units of the frequency domain parameter sequence generating apparatus according to any one of claims 15 to 21, the encoding apparatus comprising: where γ is a adjustment factor which is a positive constant equal to or smaller than 1, a linear prediction coefficient adjusting unit that generates a adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], a.sub.γ[p] by adjusting the linear prediction coefficient sequence a[1], a[2], . . . , a[p] using the adjustment factor γ; a adjusted LSP generating unit that generates a adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], θ.sub.γ[p] using the adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p]; a adjusted LSP encoding unit that encodes the adjusted LSP parameter sequence θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[p] to generate adjusted LSP codes and a adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] which is determined by quantization of values in the adjusted LSP parameter sequence corresponding to the adjusted LSP codes; an LSP linear transformation unit that, with the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] being the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p], and γ1=γ and γ2=1, executes the parameter sequence converting unit to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] as an approximate quantized LSP parameter sequence ̂θ.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a quantized smoothed power spectral envelope series calculating unit that calculates a quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N] based on the adjusted quantized LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p]; a frequency domain encoding unit that generates frequency domain signal codes by encoding a frequency domain sample sequence X[1], X[2], X[N] corresponding to the audio signals using the quantized smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP generating unit that generates an LSP parameter sequence θ[1], θ[2], . . . , θ[p] using the linear prediction coefficient sequence a[1], a[2], . . . , a[p]; an LSP encoding unit that encodes the LSP parameter sequence θ[1], θ[2], . . . , θ[p] to generate LSP codes and a quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] corresponding to the LSP codes; and a time domain encoding unit that encodes the audio signals to generate time domain signal codes using either a quantized LSP parameter sequence obtained in the LSP encoding unit for a preceding time segment or an approximate quantized LSP parameter sequence obtained in the LSP linear transformation unit for the preceding time segment, and the quantized LSP parameter sequence for the predetermined time segment.

24. A decoding apparatus including the units of the frequency domain parameter sequence generating apparatus according to any one of claims 15 to 21, the decoding apparatus comprising: a adjusted LSP code decoding unit that decodes input adjusted LSP codes to obtain a decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p]; a decoded LSP linear transformation unit that, with the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p] being the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p], and γ1=γ and γ2==1, executes the parameter sequence converting unit to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] as a decoded approximate LSP parameter sequence ̂θ.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a decoded linear prediction coefficient sequence generating unit that generates a decoded adjusted linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p] by converting the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p] into linear prediction coefficients; a decoded smoothed power spectral envelope series calculating unit that calculates a decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N] which is a series in frequency domain corresponding to the decoded adjusted linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p]; a frequency domain decoding unit that generates decoded sound signals using a frequency domain signal sequence resulting from decoding of input frequency domain signal codes and the decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP code decoding unit that decodes input LSP codes to obtain a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p]; and a time domain decoding unit that decodes input time domain signal codes, and generates decoded sound signals by synthesizing the time domain signal codes using either the decoded LSP parameter sequence obtained by the LSP code decoding unit for the preceding time segment or the decoded approximate LSP parameter sequence obtained in the decoded LSP linear transformation unit for the preceding time segment, and the decoded LSP parameter sequence for the predetermined time segment.

25. A decoding apparatus including the units of the frequency domain parameter sequence generating apparatus according to any one of claims 15 to 21, the decoding apparatus comprising: a adjusted LSP code decoding unit that decodes input adjusted LSP codes to obtain a decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p]; a decoded LSP linear transformation unit that, with the frequency domain parameter sequence ω[1], θ[2], . . . , ω[p] being the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.γ[2], . . . , ̂θ.sub.γ[p], and γ1=γ and γ2=1, executes the parameter sequence converting unit to thereby generate the converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p] as a decoded approximate LSP parameter sequence ̂θ.sub.app[1], ̂θ.sub.app[2], . . . , ̂θ.sub.app[p]; a decoded smoothed power spectral envelope series calculating unit that calculates a decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . ̂W.sub.γ[N] based on the decoded adjusted LSP parameter sequence ̂θ.sub.γ[1], ̂θ.sub.1[2], . . . , ̂θ.sub.γ[p]; a frequency domain decoding unit that generates decoded sound signals using the frequency domain signal sequence resulting from decoding of input frequency domain signal codes and the decoded smoothed power spectral envelope series ̂W.sub.γ[1], ̂W.sub.γ[2], . . . , ̂W.sub.γ[N]; an LSP code decoding unit that decodes input LSP codes to obtain a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p]; and a time domain decoding unit that decodes input time domain signal codes, and generates decoded sound signals by synthesizing the time domain signal codes using either the decoded LSP parameter sequence obtained in the LSP code decoding unit for the preceding time segment or the decoded approximate LSP parameter sequence obtained in the decoded LSP linear transformation unit for the preceding time segment, and the decoded LSP parameter sequence for the predetermined time segment.

26. A program for causing a computer to carry out the steps of the frequency domain parameter sequence generating method according to any one of claims 1, 3, 5, 7, and 8.

27-28. (canceled)

29. A non-transitory computer-readable recording medium having a program recorded thereon for causing a computer to carry out the steps of the frequency domain parameter sequence generating method according to any one of claims 1, 3, 5, 7, and 8.

30-31. (canceled)

32. The encoding method according to claim 11, further comprising: an output step of outputting either the frequency domain signal codes generated in the frequency domain encoding step or the time domain signal codes generated in the time domain encoding step, wherein at the time domain encoding step, when frequency domain signal codes have been output in the output step for the preceding time segment, encoding that uses the approximate quantized LSP parameter sequence obtained in the LSP linear transformation step for the preceding time segment is performed, and when time domain signal codes have been output in the output step for the preceding time segment, encoding that uses the quantized LSP parameter sequence obtained in the LSP generation step for the preceding time segment is performed.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0060] FIG. 1 is a diagram illustrating the functional configuration of a conventional encoding apparatus.

[0061] FIG. 2 is a diagram illustrating the process flow of a conventional encoding method.

[0062] FIG. 3 is a diagram illustrating the relation between a encoding apparatus and a decoding apparatus.

[0063] FIG. 4 is a diagram illustrating the functional configuration of a encoding apparatus in a first embodiment.

[0064] FIG. 5 is a diagram illustrating the process flow of the encoding method in the first embodiment.

[0065] FIG. 6 is a diagram illustrating the functional configuration of a decoding apparatus in the first embodiment.

[0066] FIG. 7 is a diagram illustrating the process flow of the decoding method in the first embodiment.

[0067] FIG. 8 is a diagram illustrating the functional configuration of the encoding apparatus in a second embodiment.

[0068] FIG. 9 is a diagram for describing the nature of LSP parameters.

[0069] FIG. 10 is a diagram for describing the nature of LSP parameters.

[0070] FIG. 11 is a diagram for describing the nature of LSP parameters.

[0071] FIG. 12 is a diagram illustrating the process flow of the encoding method in the second embodiment.

[0072] FIG. 13 is a diagram illustrating the functional configuration of the decoding apparatus in the second embodiment.

[0073] FIG. 14 is a diagram illustrating the process flow of the decoding method in the second embodiment.

[0074] FIG. 15 is a diagram illustrating the functional configuration of a encoding apparatus in a modification of the second embodiment.

[0075] FIG. 16 is a diagram illustrating the process flow of the encoding method in the modification of the second embodiment.

[0076] FIG. 17 is a diagram illustrating the functional configuration of the encoding apparatus in a third embodiment.

[0077] FIG. 18 is a diagram illustrating the process flow of the encoding method in the third embodiment.

[0078] FIG. 19 is a diagram illustrating the functional configuration of the decoding apparatus in the third embodiment.

[0079] FIG. 20 is a diagram illustrating the process flow of the decoding method in the third embodiment.

[0080] FIG. 21 is a diagram illustrating the functional configuration of the encoding apparatus in a fourth embodiment.

[0081] FIG. 22 is a diagram illustrating the process flow of the encoding method in the fourth embodiment.

[0082] FIG. 23 is a diagram illustrating the functional configuration of a frequency domain parameter sequence generating apparatus in a fifth embodiment.

DETAILED DESCRIPTION OF THE EMBODIMENTS

[0083] Embodiments of the present invention will be described below. In the drawings used in the description below, components having the same function or steps that perform the same processing are denoted with the same reference characters and repeated descriptions are omitted.

First Embodiment

[0084] A encoding apparatus according to a first embodiment obtains, in a frame for which time domain encoding is performed, LSP codes by encoding LSP parameters that have been converted from linear prediction coefficients. In a frame for which frequency domain encoding is performed, the encoding apparatus obtains adjusted LSP codes by encoding adjusted LSP parameters that have been converted from adjusted linear prediction coefficients. When time domain encoding is to be performed in a frame following a frame for which frequency domain encoding was performed, linear prediction coefficients generated by inverse adjustment of linear prediction coefficients that correspond to LSP parameters corresponding to adjusted LSP codes are converted to LSPs, which are then used as LSP parameters in the time domain encoding for the following frame.

[0085] A decoding apparatus according to the first embodiment obtains, in a frame for which time domain decoding is performed, linear prediction coefficients that have been converted from LSP parameters resulting from decoding of LSP codes and uses them for time domain decoding. In a frame for which frequency domain decoding is performed, the decoding apparatus uses adjusted LSP parameters generated by decoding adjusted LSP codes for the frequency domain decoding. When time domain decoding is to be performed in a frame following a frame for which frequency domain decoding was performed, linear prediction coefficients generated by inverse adjustment of linear prediction coefficients that correspond to LSP parameters corresponding to the adjusted LSP codes are converted to LSPs, which are then used as LSP parameters in the time domain decoding for the following frame.

[0086] In the encoding and decoding apparatuses according the first embodiment, as illustrated in FIG. 3, input sound signals input to a encoding apparatus 1 are coded into a code sequence, which is then sent from the encoding apparatus 1 to the decoding apparatus 2, in which the code sequence is decoded into decoded sound signals and output.

[0087] <Encoding Apparatus>

[0088] As shown in FIG. 4, the encoding apparatus 1 includes, as with the conventional encoding apparatus 9, an input unit 100, a linear prediction analysis unit 105, an LSP generating unit 110, an LSP encoding unit 115, a feature amount extracting unit 120, a frequency domain encoding unit 150, a delay input unit 165, a time domain encoding unit 170, and an output unit 175, for example. The encoding apparatus 1 further includes a linear prediction coefficient adjusting unit 125, a adjusted LSP generating unit 130, a adjusted LSP encoding unit 135, a quantized linear prediction coefficient generating unit 140, a first quantized smoothed power spectral envelope series calculating unit 145, a quantized linear prediction coefficient inverse adjustment unit 155, and an inverse-adjusted LSP generating unit 160, for example.

[0089] The encoding apparatus 1 is a specialized device build by incorporating special programs into a known or dedicated computer having a central processing unit (CPU), main memory (random access memory or RAM), and the like, for example. The encoding apparatus 1 performs various kinds of processing under the control of the central processing unit, for example. Data input to the encoding apparatus 1 or data resulting from various kinds of processing are stored in the main memory, for example, and data stored in the main memory are retrieved for use in other processing as necessary. At least some of the processing components of the encoding apparatus 1 may be implemented by hardware such as an integrated circuit.

[0090] As shown in FIG. 4, the encoding apparatus 1 in the first embodiment differs from the conventional encoding apparatus 9 in that, when the feature amount extracted by the feature amount extracting unit 120 is smaller than a predetermined threshold (i.e., when the temporal variation in the input sound signal is small), the encoding apparatus 1 encodes a adjusted LSP parameter sequence θ.sub.γR[1], θ.sub.γR[2], . . . , θ.sub.γR[p], which is a series generated by converting a adjusted linear prediction coefficient sequence a.sub.γR[1], a.sub.γR[2], . . . , a.sub.γR[p] into LSP parameters, and outputs adjusted LSP code Cγ, instead of encoding an LSP parameter sequence θ[1], θ[2], . . . , θ[p] which is a series generated by converting linear prediction coefficient sequence a[1], a[2], . . . , a[p] into LSP parameters and outputting LSP code C1.

[0091] With the configuration of the first embodiment, when the feature amount extracted by the feature amount extracting unit 120 in the preceding frame was smaller than the predetermined threshold (i.e., when temporal variation in the input sound signal was small), the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂0 [p] is not generated and thus cannot be input to the delay input unit 165. The quantized linear prediction coefficient inverse adjustment unit 155 and the inverse-adjusted LSP generating unit 160 are processing components added for addressing this: when the feature amount extracted by the feature amount extracting unit 120 in the preceding frame was smaller than the predetermined threshold (i.e., when temporal variation in the input sound signal was small), they generate a series of approximations of the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] for the preceding frame to be used in the time domain encoding unit 170, from the adjusted quantized linear prediction coefficient sequence ̂a.sub.γR[1], ̂a.sub.γR[2], . . . , ̂a.sub.γR[p]. In this case, an inverse-adjusted LSP parameter sequence ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] is the series of approximations of the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p]

[0092] <Encoding Method>

[0093] Referring to FIG. 5, the encoding method according to the first embodiment will be described. The following description mainly focuses on differences from the conventional technique described above.

[0094] At step S125, the linear prediction coefficient adjusting unit 125 determines a series of coefficient, a.sub.γR[i]=a[i]×γR.sup.i, which is the product of each coefficient a[i] (i=1, . . . , p) in the linear prediction coefficient sequence a[1], a[2], . . . , a[p] output by the linear prediction analysis unit 105 and the ith power of adjustment factor γR, and outputs it. In the following description, the series a.sub.γR[1], a.sub.γR[2], a.sub.γR[p] determined will be called a adjusted linear prediction coefficient sequence.

[0095] The adjusted linear prediction coefficient sequence a.sub.γR[1], a.sub.γR[2], . . . , a.sub.γR[p] output by the linear prediction coefficient adjusting unit 125 is input to the adjusted LSP generating unit 130.

[0096] At step S130, the adjusted LSP generating unit 130 determines and outputs a adjusted LSP parameter sequence θ.sub.γR[1], θ.sub.γR[2], θ.sub.γR[p], which is a series of LSP parameters corresponding to the adjusted linear prediction coefficient sequence a.sub.γR[1], a.sub.γR[2], . . . , a.sub.γR[p] output by the linear prediction coefficient adjusting unit 125. The adjusted LSP parameter sequence θ.sub.γR[1], θ.sub.γR[2], . . . , θ.sub.γR[p] is a series in which values are arranged in ascending order. That is, it satisfies

0<θ.sub.γR[1]<θ.sub.γR[2]< . . . <θ.sub.γR[p]<π.

[0097] The adjusted LSP parameter sequence θ.sub.γR[1], θ.sub.γR[2], . . . , θ.sub.γR[p] output by the adjusted LSP generating unit 130 is input to the adjusted LSP encoding unit 135.

[0098] At step S135, the adjusted LSP encoding unit 135 encodes the adjusted LSP parameter sequence θ.sub.γR[1], θ.sub.γR[2], θ.sub.γR[p] output by the adjusted LSP generating unit 130, and generates adjusted LSP code Cγ and a series of quantized adjusted LSP parameters, ̂θ.sub.γR[1], ̂a.sub.γR[2], . . . , ̂θ.sub.γR[p], corresponding to the adjusted LSP code Cγ, and outputs them. In the following description, the series ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] will be called a adjusted quantized LSP parameter sequence.

[0099] The adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] output by the adjusted LSP encoding unit 135 is input to the quantized linear prediction coefficient generating unit 140. The adjusted LSP code Cγ output by the adjusted LSP encoding unit 135 is input to the output unit 175.

[0100] At step S140, the quantized linear prediction coefficient generating unit 140 generates and outputs a series of linear prediction coefficients, ̂a.sub.γR[.sup.1], ̂a.sub.γR[2], . . . , ̂a.sub.γR[p], from the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] output by the adjusted LSP encoding unit 135. In the following description, the series ̂a.sub.γR[1], ̂a.sub.γR[2], . . . , ̂a.sub.γR[p] will be called a adjusted quantized linear prediction coefficient sequence.

[0101] The adjusted quantized linear prediction coefficient sequence ̂a.sub.γ[1], ̂a.sub.γ[2], . . . , ̂a.sub.γ[p] output by the quantized linear prediction coefficient generating unit 140 is input to the first quantized smoothed power spectral envelope series calculating unit 145 and the quantized linear prediction coefficient inverse adjustment unit 155.

[0102] At step S145, the first quantized smoothed power spectral envelope series calculating unit 145 generates and outputs a quantized smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] according to Formula (8) using each coefficient ̂a.sub.γR[i] in the adjusted quantized linear prediction coefficient sequence ̂a.sub.γR[1], ̂a.sub.γR[2], . . . , ̂a.sub.γR[p] output by the quantized linear prediction coefficient generating unit 140.

[00006] $\begin{matrix} {\tilde{W}}_{γ .Math. .Math. R} [n] = \frac{σ^{2}}{2 .Math. π | 1 + {.Math.}_{i = 1}^{p} .Math. .Math. {\hat{a}}_{γ .Math. .Math. R} [i] .Math. \exp (- ijn) .Math. |^{2}} & (8) \end{matrix}$

[0103] The quantized smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] output by the first quantized smoothed power spectral envelope series calculating unit 145 is input to the frequency domain encoding unit 150.

[0104] Processing in the frequency domain encoding unit 150 is the same as that performed by the frequency domain encoding unit 150 of the conventional encoding apparatus 9 except that it uses the quantized smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] in place of the approximate smoothed power spectral envelope series ˜W.sub.γR[1], ˜W.sub.γR[2], . . . , ˜W.sub.γR[N].

[0105] At step S155, the quantized linear prediction coefficient inverse adjustment unit 155 determines a series ̂a.sub.γ[1]/(γR), ̂a.sub.γ[2]/(γR).sup.2, . . . , ̂a.sub.γ[p]/(γR).sup.p of value a.sub.γ[1]/(γR).sup.i determined by dividing each value ̂a.sub.γR[i] in the adjusted quantized linear prediction coefficient sequence ̂a.sub.R[1], ̂a.sub.γR[2], . . . , ̂a.sub.γRt[p] output by the quantized linear prediction coefficient generating unit 140 by the ith power of the adjustment factor γR, and outputs it. In the following description, the series ̂a.sub.γ[1]/(γR), ̂a.sub.γ[2]/(γR).sup.2, . . . , ̂a.sub.γ[p]/(γR).sup.p will be called an inverse-adjusted linear prediction coefficient sequence. The adjustment factor γR is set to the same value as the adjustment factor γR used in the linear prediction coefficient adjusting unit 125.

[0106] The inverse-adjusted linear prediction coefficient sequence ̂a.sub.γ[1]/(γR), ̂a.sub.γ[2]/(γR).sup.2, . . . , ̂a.sub.γ[p]/(γR).sup.p output by the quantized linear prediction coefficient inverse adjustment unit 155 is input to the inverse-adjusted LSP generating unit 160.

[0107] At step S160, the inverse-adjusted LSP generating unit 160 determines and outputs a series of LSP parameters, ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p], from the inverse-adjusted linear prediction coefficient sequence ̂a.sub.γ[1]/(γR), ̂a.sub.γ[2]/(γR).sup.2, . . . , ̂a.sub.γ[p]/(γR).sup.p output by the quantized linear prediction coefficient inverse adjustment unit 155. In the following description, the LSP parameter series ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] will be called an inverse-adjusted LSP parameter sequence. The inverse-adjusted LSP parameter sequence ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] is a series in which values are arranged in ascending order. That is, it is a series that satisfies

0<̂θ′[1]<̂θ′[2]< . . . <̂θ′[p]<π.

[0108] The inverse-adjusted LSP parameters ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] output by the inverse-adjusted LSP generating unit 160 are input to the delay input unit 165 as a quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p]. That is, the inverse-adjusted LSP parameters ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] are used in place of the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p].

[0109] At step S175, the encoding apparatus 1 sends, by way of the output unit 175, the LSP code C1 output by the LSP encoding unit 115, the identification code Cg output by the feature amount extracting unit 120, the adjusted LSP code Cγ output by the adjusted LSP encoding unit 135, and either the frequency domain signal codes output by the frequency domain encoding unit 150 or the time domain signal codes output by the time domain encoding unit 170, to the decoding apparatus 2.

[0110] <Decoding Apparatus>

[0111] As illustrated in FIG. 6, the decoding apparatus 2 includes an input unit 200, an identification code decoding unit 205, an LSP code decoding unit 210, a adjusted LSP code decoding unit 215, a decoded linear prediction coefficient generating unit 220, a first decoded smoothed power spectral envelope series calculating unit 225, a frequency domain decoding unit 230, a decoded linear prediction coefficient inverse adjustment unit 235, a decoded inverse-adjusted LSP generating unit 240, a delay input unit 245, a time domain decoding unit 250, and an output unit 255, for example.

[0112] The decoding apparatus 2 is a specialized device build by incorporating special programs into a known or dedicated computer having a central processing unit (CPU), main memory (random access memory or RAM), and the like, for example. The decoding apparatus 2 performs various kinds of processing under the control of the central processing unit, for example. Data input to the decoding apparatus 2 or data resulting from various kinds of processing are stored in the main memory, for example, and data stored in the main memory are retrieved for use in other processing as necessary. At least some of the processing components of the decoding apparatus 2 may be implemented by hardware such as an integrated circuit.

[0113] <Decoding Method>

[0114] Referring to FIG. 7, the decoding method in the first embodiment will be described.

[0115] At step S200, a code sequence generated in the encoding apparatus 1 is input to the decoding apparatus 2. The code sequence contains the LSP code C1, identification code Cg, adjusted LSP code Cγ, and either frequency domain signal codes or time domain signal codes.

[0116] At step S205, the identification code decoding unit 205 implements control so that the adjusted LSP code decoding unit 215 will execute the subsequent processing if the identification code Cg contained in the input code sequence corresponds to information indicating the frequency domain encoding method, and so that the LSP code decoding unit 210 will execute the subsequent processing if the identification code Cg corresponds to information indicating the time domain encoding method.

[0117] The adjusted LSP code decoding unit 215, the decoded linear prediction coefficient generating unit 220, the first decoded smoothed power spectral envelope series calculating unit 225, the frequency domain decoding unit 230, the decoded linear prediction coefficient inverse adjustment unit 235, and the decoded inverse-adjusted LSP generating unit 240 are executed when the identification code Cg contained in the input code sequence corresponds to information indicating the frequency domain encoding method (step S206).

[0118] At step S215, the adjusted LSP code decoding unit 215 obtains a decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] by decoding the adjusted LSP code Cγ contained in the input code sequence, and outputs it. That is, it obtains and outputs a decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] which is a sequence of LSP parameters corresponding to the adjusted LSP code Cγ. The same symbols are used because the decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] obtained here is identical to the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] generated by the encoding apparatus 1 if the adjusted LSP code Cγ output by the encoding apparatus 1 is accurately input to the decoding apparatus 2 without being affected by code errors or the like.

[0119] The decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] output by the adjusted LSP code decoding unit 215 is input to the decoded linear prediction coefficient generating unit 220.

[0120] At step S220, the decoded linear prediction coefficient generating unit 220 generates and outputs a series of linear prediction coefficients, ̂a.sub.γR[1], ̂a.sub.γR[2], . . . , ̂a.sub.γR[p], from the decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] output by the adjusted LSP code decoding unit 215. In the following description, the series ̂a.sub.γR[1], ̂a.sub.γR[2], ̂a.sub.γR[p] will be called a decoded adjusted linear prediction coefficient sequence.

[0121] The decoded linear prediction coefficient sequence ̂a.sub.γR[1], ̂a.sub.γR[2], . . . , ̂a.sub.γR[p] output by the decoded linear prediction coefficient generating unit 220 is input to the first decoded smoothed power spectral envelope series calculating unit 225 and the decoded linear prediction coefficient inverse adjustment unit 235.

[0122] At step S225, the first decoded smoothed power spectral envelope series calculating unit 225 generates and outputs a decoded smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] according to Formula (8) using each coefficient ̂a.sub.γR[1] in the decoded adjusted linear prediction coefficient sequence ̂a.sub.γR[1], ̂a.sub.γR[2], ̂a.sub.γR[p] output by the decoded linear prediction coefficient generating unit 220.

[0123] The decoded smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] output by the first decoded smoothed power spectral envelope series calculating unit 225 is input to the frequency domain decoding unit 230.

[0124] At step S230, the frequency domain decoding unit 230 decodes the frequency domain signal codes contained in the input code sequence to determine a decoded normalized frequency domain signal sequence X.sub.N[1], X.sub.N[2], . . . , X.sub.N[N]. Next, the frequency domain decoding unit 230 obtains a decoded frequency domain signal sequence X[1], X[2], . . . , X[N] by multiplying each value X.sub.N[n] (n=1, . . . , N) in the decoded normalized frequency domain signal sequence X.sub.N[1], X.sub.N[2], . . . , X.sub.N[N] by the square root of each value ̂W.sub.γR[n] in the decoded smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N], and outputs it. That is, it calculates X[n]=X.sub.N[n]×sqrt(̂W.sub.γR[n]). It then converts the decoded frequency domain signal sequence X[1], X[2], . . . , X[N] into the time domain to obtain and output decoded sound signals.

[0125] At step S235, the decoded linear prediction coefficient inverse adjustment unit 235 determines and outputs a series, ̂a.sub.γR[1]/(γR), ̂a.sub.γR[2]/(γR).sup.2, . . . , ̂a.sub.γR[p]/(γR).sup.p, of value ̂a.sub.γ[i]/(γR).sup.i by dividing each value ̂a.sub.γR[i] in the decoded adjusted linear prediction coefficient sequence ̂a.sub.γR[1], ̂a.sub.γR[2], . . . , ̂a.sub.γR[p] output by the decoded linear prediction coefficient generating unit 220 by the ith power of the adjustment factor γR. In the following description, the series ̂a.sub.γR[1]/(γR), ̂a.sub.γR[2]/(γR).sup.2, . . . , ̂a.sub.γR[p]/(γR).sup.p will be called a decoded inverse-adjusted linear prediction coefficient sequence. The adjustment factor γR is set to the same value as the adjustment factor γR used in the linear prediction coefficient adjusting unit 125 of the encoding apparatus 1.

[0126] The decoded inverse-adjusted linear prediction coefficient sequence ̂a.sub.γR[1]/(γR), ̂a.sub.γR[2]/(γR).sup.2, . . . , ̂a.sub.γR[p]/(γR).sup.p output by the decoded linear prediction coefficient inverse adjustment unit 235 is input to the decoded inverse-adjusted LSP generating unit 240.

[0127] At step S240, the decoded inverse-adjusted LSP generating unit 240 determines an LSP parameter series ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] from the decoded inverse-adjusted linear prediction coefficient sequence ̂a.sub.γR[1]/(γR), ̂a.sub.γR[2]/(γR).sup.2, . . . , ̂a.sub.γR[p]/(γR).sup.p, and outputs it. In the following description, the LSP parameter series ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] will be called a decoded inverse-adjusted LSP parameter sequence.

[0128] The decoded inverse-adjusted LSP parameters ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] output by the decoded inverse-adjusted LSP generating unit 240 are input to the delay input unit 245 as a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p].

[0129] The LSP code decoding unit 210, the delay input unit 245, and the time domain decoding unit 250 are executed when the identification code Cg contained in the input code sequence corresponds to information indicating the time domain encoding method (step S206).

[0130] At step S210, the LSP code decoding unit 210 decodes the LSP code C1 contained in the input code sequence to obtain a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p], and outputs it. That is, it obtains and outputs a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p], which is a sequence of LSP parameters corresponding to the LSP code C1.

[0131] The decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] output by the LSP code decoding unit 210 is input to the delay input unit 245 and the time domain decoding unit 250.

[0132] At step S245, the delay input unit 245 holds the input decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] and outputs it to the time domain decoding unit 250 with a delay equivalent to the duration of one frame. For instance, if the current frame is the fth frame, the decoded LSP parameter sequence for the f−1th frame, ̂θ.sup.[f-1][1], ̂θ.sup.[f-1][2], . . . , ̂θ.sup.[f-1][p], is output to the time domain decoding unit 250.

[0133] When the identification code Cg contained in the input code corresponds to information indicating the frequency domain encoding method, the decoded inverse-adjusted LSP parameter sequence ̂θ′[1], ̂θ′[2], . . . , ̂θ′[p] output by the decoded inverse-adjusted LSP generating unit 240 is input to the delay input unit 245 as the decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p].

[0134] At step S250, the time domain decoding unit 250 identifies the waveforms contained in the adaptive codebook and waveforms in the fixed codebook from the time domain signal codes contained in the input code sequence. By applying the synthesis filter to a signal generated by synthesis of the waveforms in the adaptive codebook and the waveforms in the fixed codebook that have been identified, a synthesized signal from which the effect of the spectral envelope has been removed is determined, and the synthesized signal determined is output as a decoded sound signal.

[0135] The filter coefficients for the synthesis filter are generated using the decoded LSP parameter sequence for the fth frame, ̂θ[1], ̂θ[2], . . . , ̂θ[p], and the decoded LSP parameter sequence for the f−1th frame, ̂θ.sup.[f-1][1], ̂θ.sup.[f-1][2], . . . , ̂θ.sup.[f-1][p].

[0136] Specifically, a frame is first divided into two subframes, and the filter coefficients for the synthesis filter are determined as follows.

[0137] In the latter-half subframe, a series of values

̂a[1]×(γR),̂a[2]×(γR).sup.2, . . . ,̂a[p]×(γR).sup.p

is used as filter coefficients for the synthesis filter. This is obtained by multiplying each coefficient ̂a[i] of the decoded linear prediction coefficients ̂a[1], ̂a[2], . . . ̂a[p], which is a coefficient sequence generated by converting the decoded LSP parameter sequence for the fth frame, ̂θ[1], ̂θ[2], . . . , ̂θ[p], into linear prediction coefficients, by the ith power of the adjustment factor γR.

[0138] In the first-half subframe, a series of values

˜a[1]×(γR),˜a[2]×(γR).sup.2, . . . ,˜a[p]×(γR).sup.p

which is obtained by multiplying each coefficient ˜a[i] of decoded interpolated linear prediction coefficients ˜a[1], ˜a[2], . . . , ˜a[p] by the ith power of the adjustment factor γR, is used as filter coefficients for the synthesis filter. The decoded interpolated linear prediction coefficients ˜a[1], ˜a[2], . . . , ˜a[p] is a coefficient sequence generated by converting, into linear prediction coefficients, the decoded interpolated LSP parameter sequence ˜θ[1], ˜θ[2], . . . , ˜θ[p], which is a series of intermediate values between each value ̂θ[i] in the decoded LSP parameter sequence for the fth frame, ̂θ[1], ̂θ[2], . . . , ̂θ[p], and each value ̂θ.sup.[f-1][i] in the decoded LSP parameter sequence for the f−1th frame, θ.sup.[f-1][1], θ.sup.[f-1][2], . . . , θ.sup.[f-1][p]. That is,

˜θ[i]=0.5×̂θ.sup.[f-1][i]+0.5×̂θ[i](i=1, . . . ,p).

[0139] <Effects of the First Embodiment>

[0140] The adjusted LSP encoding unit 135 of the encoding apparatus 1 determines such a adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] that minimizes the quantizing distortion between the adjusted LSP parameter sequence θ.sub.γR[1], θ.sub.γR[2], . . . , θ.sub.γR[p] and the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] This can determine the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] so that a power spectral envelope series that takes into account the sense of hearing (i.e., that has been smoothed with adjustment factor γR) is approximated with high accuracy. The quantized smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N], which is a power spectral envelope series obtained by expanding the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] into the frequency domain, can approximate the smoothed power spectral envelope series W.sub.γR[1], W.sub.γR[2], . . . , W.sub.γR[N] with high accuracy. When the code amount of the LSP code C1 is the same as that of the adjusted LSP code Cγ, the first embodiment yields smaller encoding distortion in frequency domain encoding than the conventional technique. In addition, assuming an equal encoding distortion to that in the conventional encoding method, the adjusted LSP code Cγ achieves a further smaller code amount compared to the conventional method than the LSP code C1 does. Thus, with a encoding distortion equal to that in the conventional method, the code amount can be reduced compared to the conventional method, whereas with the same code amount as the conventional method, encoding distortion can be reduced compared to the conventional method.

Second Embodiment

[0141] The encoding apparatus 1 and decoding apparatus 2 of the first embodiment are expensive in terms of calculation in the inverse-adjusted LSP generating unit 160 and the decoded inverse-adjusted LSP generating unit 240 in particular. To address this, a encoding apparatus 3 in a second embodiment directly generates an approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, ̂θ[p].sub.app, which is a series of approximations of the values in the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p], from the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] without the intermediation of linear prediction coefficients. Similarly, a decoding apparatus 4 in the second embodiment directly generates a decoded approximate LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app, which is a series of approximations of the values in the decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p], from the decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] without the intermediation of linear prediction coefficients.

[0142] <Encoding Apparatus>

[0143] FIG. 8 shows the functional configuration of the encoding apparatus 3 in the second embodiment.

[0144] The encoding apparatus 3 differs from the encoding apparatus 1 of the first embodiment in that it does not include the quantized linear prediction coefficient inverse adjustment unit 155 and the inverse-adjusted LSP generating unit 160 but includes an LSP linear transformation unit 300 instead.

[0145] Utilizing the nature of LSP parameters, the LSP linear transformation unit 300 applies approximate linear transformation to a adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] to generate an approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app.

[0146] First, the nature of LSP parameters will be described.

[0147] Although the LSP linear transformation unit 300 applies approximate transformation to a series of quantized LSP parameters, the nature of an unquantized LSP parameter sequence will be discussed first because the nature of a quantized LSP parameter series is basically the same as the nature of an unquantized LSP parameter sequence.

[0148] An LSP parameter sequence θ[1], θ[2], . . . , θ[p] is a parameter sequence in the frequency domain that is correlated with the power spectral envelope of the input sound signal. Each value in the LSP parameter sequence is correlated with the frequency position of the extreme of the power spectral envelope of the input sound signal. The extreme of the power spectral envelope is present at a frequency position between θ[i] and θ[i+1]; and with a steeper slope of a tangent around the extreme, the interval between θ[i] and θ[i+1] (i.e., the value of θ[i+1]−θ[i]) becomes smaller. In other words, as the height difference in the waves of the amplitude of the power spectral envelope is larger, the interval between θ[i] and θ[i+1] becomes less even for each i (i=1, 2, . . . , p−1). Conversely, when there is almost no height difference in the waves of the power spectral envelope, the interval between θ[i] and θ[i+1] is close to an equal interval for each value of i.

[0149] As the value of the adjustment factor γ becomes smaller, the height difference in the waves of the amplitude of smoothed power spectral envelope series W.sub.γ[1], W.sub.γ[2], . . . , W.sub.γ[N], defined by Formula (7), becomes smaller than the height difference in the waves of the amplitude of the power spectral envelope series W[1], W[2], . . . , W[N] defined by Formula (6). It can be accordingly said that a smaller value of the adjustment factor γ makes the interval between θ[i] and θ[i+1] closer to an equal interval. When γ has no influence (i.e., γ=0), this corresponds to the case of a flat power spectral envelope.

[0150] When the adjustment factor γ=0, adjusted LSP parameters θ.sub.γ=0[1], θ.sub.γ=0[2], . . . , θ.sub.γ=0[p] are

[00007] $θ_{γ = 0} (i) = \frac{i .Math. .Math. π}{p + 1},$

in which case the interval between θ[i] and θ[i+l] is equal for all i=1, . . . , p−1. When γ=1, the adjusted LSP parameter sequence θ.sub.γ=1[1], θ.sub.γ=1[2], . . . , θ.sub.γ=1[p] and the LSP parameter sequence θ[1], θ[2], θ[p] are equivalent. The adjusted LSP parameters satisfy the property:

0<θ.sub.γ[1]<θ.sub.γ[2] . . . <θ.sub.γ[p]<π.

[0151] FIG. 9 is an example of the relation between the adjustment factor γ and adjusted LSP parameter θ.sub.γ[1] (i=1, 2, . . . , p). The horizontal axis represents the value of adjustment factor γ and the vertical axis represents the adjusted LSP parameter value. The plot illustrates the values of θ.sub.γ[1], θ.sub.γ[2], . . . , θ.sub.γ[16] in order from the bottom assuming the order of prediction p=16. The value of each θ.sub.γ[i] is derived by determining a adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p] for each value of γ through processing similar to the linear prediction coefficient adjusting unit 125 by use of a linear prediction coefficient sequence a[1], a[2], . . . , a[p] which has been obtained by linear prediction analysis on a certain speech sound signal, and then converting the adjusted linear prediction coefficient sequence a.sub.γ[1], a.sub.γ[2], . . . , a.sub.γ[p] into LSP parameters through similar processing to the adjusted LSP generating unit 130. When γ=1, θ.sub.γ=1[i] is equivalent to θ[i].

[0152] As shown in FIG. 9, given 0<γ<1, the LSP parameter θ.sub.γ[i] is an internal division point between θ.sub.γ=0[i] and θ.sub.γ=1[i]. On a two-dimensional plane where the horizontal axis represents the value of adjustment factor γ and the vertical axis represents the LSP parameter value, each LSP parameter θ.sub.γ[i], when seen locally, is in a linear relationship with increase or decrease of γ. Given two different adjustment factors γ1 and γ2 (0l<γ1<γ2≦1), the magnitude of the slope of a straight line connecting a point (γ1, θ.sub.γ1[i]) and a point (γ2, θ.sub.γ2[i]) on the two-dimensional plane is correlated with the relative interval between the LSP parameters that precede and follow θ.sub.γ1[i] in the LSP parameter sequence, θ.sub.γ1 [1], θ.sub.γ1[2], . . . , θ.sub.γ1[p] (i.e., θ.sub.γ1[i−1] and θ.sub.γ1[i+1]), and θ.sub.γ1[i]. Specifically,

[0153] when

|θ.sub.γ1[i]−θ.sub.γ1[i−1]|>|θ.sub.γ1[i+1]−θ.sub.γ1[i]| (9)

then the following properties hold:

|θ.sub.γ2[i]−θ.sub.γ2[i−1]|>|θ.sub.γ2[i+1]−θ.sub.γ2[i]|, and

|θ.sub.γ2[i]−θ.sub.γ2[i−1]|>|θ.sub.γ1[i]−θ.sub.γ1[i−1]| (10).

[0154] When

|θ.sub.γ1[i]−θ.sub.γ1[i−1]|<|θ.sub.γ1[i+1]−θ.sub.γ1[i]| (11)

then the following properties hold:

|θ.sub.γ2[i+1]−θ.sub.γ2[i]|>|θ.sub.γ1[i+1]−θ.sub.γ1|, and

|θ.sub.γ2[i]−θ.sub.γ2[i−1]|>|θ.sub.γ1[i]−θ.sub.γ1[i−1]| (12).

[0155] Formulas (9) and (10) indicate that when θ.sub.γ1[i] is closer to θ.sub.γ1[i+1] with respect to the midpoint between θ.sub.γ1[i+1] and θ.sub.γ1[i−1], θ.sub.γ2[i] will assume a value that is further closer to θ.sub.γ2[i+1] (see FIG. 10). This means that on a two-dimensional plane with the horizontal axis being the γ value and the vertical axis being the LSP parameter value, the slope of straight line L2 connecting the point (γ1, θ.sub.γ1[i]) and the point (γ2, θ.sub.γ2[i]) is larger than the slope of straight line L1 connecting a point (0, θ.sub.γ=0[i]) and a point (γ1, θ.sub.γ1[i]) (see FIG. 11).

[0156] Formulas (11) and (12) indicate that when θ.sub.γ1[i] is closer to θ.sub.γ1[i−1] with respect to the midpoint between θ.sub.γ1[i+1] and θ.sub.γ1[i−1], θ.sub.γ2[1] will assume a value that is further closer to θ.sub.γ2[i−1]. This means that on a two-dimensional plane with the horizontal axis being the γ value and the vertical axis being the LSP parameter value, the slope of straight line connecting the point (γ1, θ.sub.γ1[i]) and the point (γ2, θ.sub.γ2[i]) is smaller than the slope of a straight line connecting the point (0, θ.sub.γ=0[i]) and the point (γ1, θ.sub.γ1[i]).

[0157] Based on the properties above, the relationship between θ.sub.γ1[1], θ.sub.γ1[2], . . . , θ.sub.γ1[p] and θ.sub.γ2[1], θ.sub.γ2[2], . . . , θ.sub.γ2[p] can be modeled with Formula (13), where Θ.sub.γ1=(θ.sub.γ1[1], θ.sub.γ1[2], . . . , θ.sub.γ1 [p]).sup.T and Θ.sub.γ2(θ.sub.γ2[1], θ.sub.γ2[2], . . . , θ.sub.γ2[p]).sup.T:

Θ.sub.γ2≈K(Θ.sub.γ1−Θ.sub.γ=0)(γ.sub.2−γ.sub.1)+Θ.sub.γ1 (13)

where K is a p×p matrix defined by Formula (14).

[00008] $\begin{matrix} K = (\begin{matrix} x_{1} & y_{1} & 0 .Math. \\ z_{2} & x_{2} & y_{2} \\ z_{3} & x_{3} & y_{3} \\ ⋱ & ⋱ & ⋱ \\ ⋱ & ⋱ \\ 0 & z_{p} & x_{p} \end{matrix}) & (14) \end{matrix}$

[0158] In this case, 0<γ1, γ2≦1, and γ1≠γ2 hold. Although Formulas (9) to (12) describe the relationships on the assumption of γ1<γ2, the model of Formula (13) has no limitation on the relation of magnitude between γ1 and γ2; they may be either γ1<γ2 or γ1>γ2.

[0159] The matrix K is a band matrix that has non-zero values only in the diagonal components and elements adjacent to them and is a matrix representing the correlations described above that hold between LSP parameters corresponding to the diagonal components and the neighboring LSP parameters. Note that although Formula (14) illustrates a band matrix with a band width of three, the band width is not limited to three.

[0160] Assuming that

{tilde over (Θ)}.sub.γ2=K(Θ.sub.γ1−Θ.sub.γ=0)(γ.sub.2−γ.sub.1)+Θ.sub.γ1 (13a),

then

˜Θ.sub.γ2=(˜θ.sub.γ2[1],˜θ.sub.γ2[2], . . . ,˜θ.sub.γ2[p]).sup.T

is an approximation of Θ.sub.γ2.

[0161] Expanding Formula (13a) gives Formula (15) below:

{tilde over (θ)}.sub.γ2[i]=z.sub.i(θ.sub.γ1[i−1]−θ.sub.γ=0[−1])+γ.sub.i(θ.sub.γ1[i+1]−θ.sub.γ=0[i+1])+x.sub.i(θ.sub.γ1[i]−θ.sub.γ=0[i])+θ.sub.γ1[i] (15)

where i=2, . . . , p−1.

[0162] On a two-dimensional plane with the horizontal axis representing the γ value and the vertical axis representing the LSP parameter value, let .sup.−θ.sub.γ2[i] denote the value on the vertical axis corresponding to γ2 on an extension of straight line L1 that connects between the point (γ1, θ.sub.γ1[i]) and the point (0, θ.sub.γ=0[i]), namely the value on the vertical axis corresponding to γ2 as approximated by straight line approximation from the slope of straight line L1 connecting θ.sub.γ1[i] and θ.sub.γ=0[i] (see FIG. 11). Then,

[00009] ${\overline{θ}}_{γ2} [i] = \frac{θ_{γ1} [i] - θ_{γ = 0} [i]}{γ_{1}} .Math. (γ_{2} - γ_{1}) + θ_{γ .Math. .Math. 1} [i]$

holds. When γ1>γ2, it means straight line interpolation, while when γ1<γ2, it means straight line extrapolation.

[0163] In Formula (14), given that

[00010] $x_{i} = \frac{1}{γ_{1}}, y_{i} = 0, z_{i} = 0,$

then ˜θ.sub.γ2[i]=.sup.−θ.sub.γ2[i], and ˜θ.sub.γ2[i] obtained with the model of Formula (13a) matches the estimation .sup.−θ.sub.γ2[i] of the LSP parameter value corresponding to γ2 as approximated by straight line approximation with a straight line that connects the point (γ1, θ.sub.γ1[i]) and the point (0, θ.sub.γ=0[i]) on the two-dimensional plane.

[0164] Given that u.sub.i and v.sub.i are positive values equal to or smaller than 1, assuming

[00011] $\begin{matrix} x_{i} = u_{i} + v_{i} + \frac{γ_{2} - γ_{1}}{γ_{1}}, y_{i} = - v_{i}, z_{i} = - u_{i} & (16) \end{matrix}$

in the Formula (14) above, Formula (15) can be rewritten as:

[00012] $\begin{matrix} \begin{matrix} {\tilde{θ}}_{γ2} [i] .Math. = & .Math. u_{i} (θ_{γ1} [i] - θ_{γ - 0} [i] - (θ_{γ1} [i - 1] - θ_{γ - 0} [i - 1])) + \\ .Math. v_{i} (θ_{γ1} [i] - θ_{γ = 0} [i] - (θ_{γ1} [i + 1] - θ_{γ = 0} [i + 1])) + \\ .Math. \frac{γ_{2} - γ_{1}}{γ_{1}} .Math. (θ_{γ1} [i] - θ_{γ = 0} [i]) + θ_{γ1} [i] \\ = & .Math. u_{i} (θ_{γ1} [i] - θ_{γ1} [i - 1] - (θ_{γ = 0} [i] - θ_{γ = 0} [i - 1])) + \\ .Math. v_{i} (θ_{γ1} [i] - θ_{γ1} [i + 1] - (θ_{γ = 0} [i] - θ_{γ = 0} [i + 1])) + {\overline{θ}}_{γ2} [i] \\ = & .Math. u_{i} (θ_{γ1} [i] - θ_{γ1} [i - 1] - \frac{π}{p + 1}) - \\ .Math. v_{i} (θ_{γ1} [i + 1] - θ_{γ1} [i] - \frac{π}{p + 1}) + {\overline{θ}}_{γ2} [i] \end{matrix} & (17) \end{matrix}$

[0165] Formula (17) means adjusting the value of .sup.−θ.sub.γ2[i] by weighting the differences between the ith LSP parameter θ.sub.γ1[i] in the LSP parameter sequence, θ.sub.γ1[1], θ.sub.γ1[2], . . . , θ.sub.γ1[p], and its preceding and following LSP parameter values (i.e., θ.sub.γ1 [i]−θ.sub.γ1 [i−1] and θ.sub.γ1[i+1]−θ.sub.γ1[i]) to obtain ˜θ.sub.γ2[i]. That is to say, correlations such as shown in Formulas (9) through (12) above are reflected in the elements in the band portion (non-zero elements) of the matrix K in Formula (13a).

[0166] The values ˜θ.sub.γ2[1], ˜θ.sub.γ2[2], . . . , ˜θ.sub.γ2[p] given by Formula (13a) are approximate values (estimated values) of LSP parameter values θ.sub.γ2[1], θ.sub.γ2[2], . . . , θ.sub.γ2[p] when the linear prediction coefficient sequence a[1]×(γ2), . . . , a[p]×(γ2).sup.p is converted to LSP parameters.

[0167] Especially when γ2>γ1, the matrix K in Formula (14) tends to have positive values in the diagonal components and negative values in elements in the vicinity of them, as indicated by Formulas (16) and (17).

[0168] The matrix K is a preset matrix, which is pre-learned using learning data, for example. How to learn the matrix K will be discussed later.

[0169] Similar properties also apply to quantized LSP parameters. That is, vectors Θ.sub.γ1 and Θ.sub.γ2 in the LSP parameter sequence in Formula (13) can be replaced with the vectors ̂Θ.sub.γ1 and ̂Θ.sub.γ2 in the quantized LSP parameter sequence, respectively. Specifically, ̂Θ.sub.γ1=(̂θ.sub.γ1 [1], ̂θ.sub.γ1[2], . . . , ̂θ.sub.γ1 [p]).sup.T and ̂Θ.sub.γ2=(̂θ.sub.γ2[1], ̂θ.sub.γ2[2], . . . , ̂θ.sub.γ2[p]).sup.T, then the following formula holds:

{circumflex over (Θ)}.sub.γ2≈K({circumflex over (Θ)}.sub.γ1−{circumflex over (Θ)}.sub.γ=0)(γ.sub.2−γ.sub.1)+{circumflex over (Θ)}.sub.γ1 (13b).

[0170] Since matrix K is a band matrix, calculation cost required for calculating Formulas (13), (13a), and (13b) is very small.

[0171] The LSP linear transformation unit 300 included in the encoding apparatus 3 of the second embodiment generates an approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app from the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] based on Formula (13b). Note that the adjustment factor γR used in generation of the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] is the same as the adjustment factor γR used in the linear prediction coefficient adjusting unit 125.

[0172] <Encoding Method>

[0173] Referring to FIG. 12, the encoding method in the second embodiment will be described. The following description mainly focuses on differences from the foregoing embodiment.

[0174] Processing performed in the adjusted LSP encoding unit 135 is the same as the first embodiment. However, the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] output by the adjusted LSP encoding unit 135 is also input to the LSP linear transformation unit 300 in addition to the quantized linear prediction coefficient generating unit 140.

[0175] The LSP linear transformation unit 300, given ̂Θ.sub.γ1=(̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p]).sup.T, determines and outputs an approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app according to

[00013] $\begin{matrix} (\begin{matrix} {\hat{θ} [1]}_{app} \\ .Math. \\ {\hat{θ} [p]}_{app} \end{matrix}) = K ({\hat{Θ}}_{γ1} - {\hat{Θ}}_{γ .Math. .Math. R = 0}) .Math. (γ_{2} - γ_{1}) + {\hat{Θ}}_{γ1} . & (18) \end{matrix}$

That is, using Formula (13b), the LSP linear transformation unit 300 determines a series of approximations, ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app, of the quantized LSP parameter sequence. As γ1 and γ2 are constants, matrix K′ which is generated by multiplying the individual elements of matrix K by (γ2−γ1) may be used instead of the matrix K of Formula (18), and the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app may also be determined by

[00014] $\begin{matrix} (\begin{matrix} {\hat{θ} [1]}_{app} \\ .Math. \\ {\hat{θ} [p]}_{app} \end{matrix}) = K^{'} ({\hat{Θ}}_{γ1} - {\hat{Θ}}_{γ .Math. .Math. R = 0}) + {\hat{Θ}}_{γ1} . & (18 .Math. a) \end{matrix}$

[0176] The approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, ̂θ[p].sub.app output by the LSP linear transformation unit 300 is input to the delay input unit 165 as the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p]. That is to say, in the time domain encoding unit 170, when the feature amount extracted by the feature amount extracting unit 120 for the preceding frame is smaller than the predetermined threshold (i.e., when temporal variation in the input sound signal was small, that is, when encoding in the frequency domain was performed), the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂[p].sub.app for the preceding frame is used in place of the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] for the preceding frame.

[0177] <Decoding Apparatus>

[0178] FIG. 13 shows the functional configuration of the decoding apparatus 4 in the second embodiment.

[0179] The decoding apparatus 4 differs from the decoding apparatus 2 in the first embodiment in that it does not include the decoded linear prediction coefficient inverse adjustment unit 235 and the decoded inverse-adjusted LSP generating unit 240 but includes a decoded LSP linear transformation unit 400 instead.

[0180] <Decoding Method>

[0181] Referring to FIG. 14, the decoding method in the second embodiment will be described. The following description mainly focuses on differences from the foregoing embodiment.

[0182] Processing in the adjusted LSP code decoding unit 215 is the same as the first embodiment. However, the decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] output by the adjusted LSP code decoding unit 215 is also input to the decoded LSP linear transformation unit 400 in addition to the decoded linear prediction coefficient generating unit 220.

[0183] The decoded LSP linear transformation unit 400 determines a decoded approximate LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app according to Formula (18) with ̂Θ.sub.γ1=(̂θ.sub.γR[1], ̂.sub.γR[2], . . . , ̂θ.sub.γR[p]).sup.T, and outputs it. That is, Formula (13b) is used to determine a series of approximations, ̂θ[1].sub.app, ̂θ[2].sub.app, ̂θ[p].sub.app, of the decoded LSP parameter sequence. As with the LSP linear transformation unit 300, the decoded approximate LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app may be determined by use of Formula (18a).

[0184] The decoded approximate LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app output by the decoded LSP linear transformation unit 400 is input to the delay input unit 245 as a decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p]. It means that in the time domain decoding unit 250, when the identification code Cg for the preceding frame corresponds to information indicating the frequency domain encoding method, the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, ̂θ[p].sub.app for the preceding frame is used in place of the decoded LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] for the preceding frame.

[0185] <Learning Process for Transformation Matrix K>

[0186] The transformation matrix K used in the LSP linear transformation unit 300 and the decoded LSP linear transformation unit 400 is determined in advance through the following process and prestored in storages (not shown) of the encoding apparatus 3 and the decoding apparatus 4.

[0187] (Step 1) For prepared sample data for speech sound signals corresponding to M frames, each sample data is subjected to linear prediction analysis to obtain linear prediction coefficients. A linear prediction coefficient sequence produced by linear prediction analysis of the mth (1≦m≦M) sample data is represented as a.sup.(m)[1], a.sup.(m)[2], . . . , a.sup.(m)[p], and referred to as a linear prediction coefficient sequence a.sup.(m)[1], a.sup.(m)[2], . . . , a.sup.(m)[p] corresponding to the mth sample data.

[0188] (Step 2) For each m, LSP parameters θ.sub.γ=1.sup.(m)[1], θ.sub.γ=1.sup.(m)[2], . . . , θ.sub.γ=1.sup.(m)[p] are determined from the linear prediction coefficient sequence a.sup.(m)[1], a.sup.(m)[2], . . . , a.sup.(m)[p]. The LSP parameters θ.sub.γ=1.sup.(m)[1], θ.sub.γ=1.sup.(m)[2], . . . , θ.sub.γ=1.sup.(m)[p] are coded in a similar manner to the LSP encoding unit 115, thereby generating a quantized LSP parameter sequence ̂θ.sub.γ=1.sup.(m)[1], ̂θ.sub.γ=1.sup.(m)[2], . . . , ̂θ.sub.γ=1.sup.(m)[p] Here,

̂Θ.sup.(m).sub.γ1=(̂θ.sub.γ=1.sup.(m)[1], . . . ,̂θ.sub.γ=1.sup.(m)[p]).sup.T.

[0189] (Step 3) For each m, setting γL as a predetermined positive constant smaller than 1 (for example, γL=0.92), a adjusted linear prediction coefficient,

a.sub.γ.sup.(m)[i]=a.sup.(m)[i]×(γL).sup.i

is calculated.

[0190] (Step 4) For each m, a adjusted LSP parameter sequence θ.sub.γL.sup.(m)[1], θ.sub.γL.sup.(m)[p] is determined from the adjusted linear prediction coefficient sequence a.sub.γL.sup.(m)[1], . . . , a.sub.γL.sup.(m)[p]. The adjusted LSP parameter sequence θ.sub.L.sup.(m)[1], . . . , θ.sub.γL.sup.(m)[p] is coded in a similar manner to the adjusted LSP encoding unit 135, thereby generating a quantized LSP parameter sequence ̂θ.sub.γL.sup.(m)[1], . . . , ̂θ.sub.γL.sup.(m)[p]. Here,

̂Θ.sup.(m).sub.γ2=(̂θ.sub.γL.sup.(m)[1], . . . ,̂.sub.γL.sup.(m)[p]).sup.T.

[0191] Through Steps 1 to 4, M pairs of quantized LSP parameter sequences (̂Θ.sup.(m).sub.γ1, ̂Θ.sup.(m).sub.γ2) are obtained. This set is used as learning data set Q, where Q={(̂Θ.sup.(m).sub.γ1, ̂Θ.sup.(m).sub.γ2)|m=1, . . . , M}. Note that all of the values of adjustment factor γL used in generation of the learning data set Q are common fixed values.

[0192] (Step 5) Each pair of LSP parameter sequences (̂Θ.sup.(m).sub.γ1, ̂Θ.sup.(m).sub.γ2) contained in the learning data Q is substituted into the model of Formula (13b), where γ1=γL, γ2=1, ̂Θ.sub.γ1=̂Θ.sup.(m).sub.γ1, and ̂Θ.sub.γ2=̂Θ.sup.(m).sub.γ2, and the coefficients for matrix K are learned with the square error criterion. That is, a vector in which the components in the band portion of the matrix K are arranged in order from the top is defined as:

[00015] $B = (\begin{matrix} x_{1} \\ y_{1} \\ z_{2} \\ x_{2} \\ y_{2} \\ z_{3} \\ .Math. \\ x_{p} \end{matrix})$

and B is obtained by

[00016] $\begin{matrix} B = \frac{1}{(γ2 - γ1)} .Math. {({.Math.}_{m = 1}^{M} .Math. .Math. J_{m}^{T} .Math. J_{m})}^{- 1} .Math. {.Math.}_{m = 1}^{M} .Math. .Math. J_{m}^{T} ({\hat{Θ}}_{γ1}^{(m)} - {\hat{Θ}}_{γ2}^{(m)}) \\ = \frac{1}{(1 - γ .Math. .Math. L)} .Math. {({.Math.}_{m = 1}^{M} .Math. .Math. J_{m}^{T} .Math. J_{m})}^{- 1} .Math. {.Math.}_{m = 1}^{M} .Math. .Math. J_{m}^{T} ({\hat{Θ}}_{γ1}^{(m)} - {\hat{Θ}}_{γ2}^{(m)}) \end{matrix} .$

Here,

[0193] [00017] $J_{m} = (\begin{matrix} d_{1} & d_{2} \\ d_{1} & d_{2} & d_{3} \\ ⋱ & ⋱ \\ d_{p - 2} & d_{p - 1} & d_{p} \\ d_{p - 1} & d_{p} \end{matrix}), .Math. \begin{matrix} d_{i} = {\hat{θ}}_{γ2}^{(m)} [i] - {\hat{θ}}_{γ .Math. .Math. L = 0}^{(m)} [i] \\ = {\hat{θ}}_{γ2}^{(m)} [i] - \frac{i .Math. .Math. π}{p + 1} \end{matrix}$

[0194] Learning of the matrix K is performed with the value of γL fixed. However, the matrix K used in the LSP linear transformation unit 300 does not have to be one that has been learned using the same value as the adjustment factor γR used in the encoding apparatus 3.

[0195] By way of example, values obtained by multiplying (γ2−γ1) and the elements in the band portion of the matrix K generated by the above-described method given that p=15 and γL=0.92, namely the values of the elements in the band portion of matrix K′, are shown below. That is, the products of the values x.sub.1, x.sub.2, . . . , x.sub.15, y.sub.1, y.sub.2, . . . , y.sub.14, z.sub.2, z.sub.3, . . . , z.sub.15 in Formula (14) and γ2−γ1 are xx.sub.1, xx.sub.2, . . . , xx.sub.15, yy.sub.1, yy.sub.2, . . . , yy.sub.14, zz.sub.2, zz.sub.3, . . . , zz.sub.15 below:

xx1=1.11499, yy1=−0.54272,
zz2=−0.83414f, xx2=1.59810f, yy2=−0.70966,
zz3=−0.49432, xx3=1.38370, yy3=−0.78076,
zz4=−0.39319, xx4=1.23032, yy4=−0.67921,
zz5=−0.39166, xx5=1.18521, yy5=−0.69088,
zz6=−0.34784, xx6=1.04839, yy6=−0.60619,
zz7=−0.41279, xx7=1.13305, yy7=−0.63247,
zz8=−0.36450, xx8=0.95694, yy8=−0.53039,
zz9=−0.43984, xx9=1.01910, yy9=−0.51707,
zz10=−0.40120, xx10=0.90395, yy10=−0.44594,
zz11=−0.49262, xx11=1.07345, yy11=−0.51892,
zz12=−0.41695, xx12=0.96596, yy12=−0.49247,
zz13=−0.45002, xx13=1.00336, yy13=−0.48790,
zz14=−0.46854, xx14=0.93258, yy14=−0.41927,
zz15=−0.45020, xx15=0.88783.

[0196] When γ2>γ1 as in the above example, in which γ1=γL=0.92 and γ2=1, the diagonal components of matrix K′ assume values close to 1 as in the above example, while components neighboring the diagonal component assume negative values.

[0197] Conversely, when γ1>γ2, the diagonal components of matrix K′ assume negative values as in the example shown below, while components neighboring the diagonal component assume positive values. Values obtained by multiplying (γ2−γ1) and the elements in the band portion of the matrix K with p=15, γ1=1, and γ2=γL=0.92, namely the values of the elements in the band portion of matrix K′ can be as below, for example:

xx1=−0.557012055, yy1=0.213853042,
zz2=0.110112745, xx2=−0.534830085, yy2=0.2440903,
zz3=0.149879603, xx3=−0.522734808, yy3=0.23494022,
zz4=0.144479327, xx4=−0.533013231, yy4=0.259021145,
zz5=0.136523255, xx5=−0.502606738, yy5=0.248139539,
zz6=0.138005088, xx6=−0.478327709, yy6=0.244219107,
zz7=0.133771751, xx7=−0.467186849, yy7=0.243988642,
zz8=0.13667916, xx8=−0.408737408, yy8=0.192803054,
zz9=0.160602461, xx9=−0.427436157, yy9=0.190554547,
zz10=0.147621742, xx10=−0.383087812, yy10=0.165954888,
zz11=0.18358465, xx11=−0.434034351, yy11=0.183004742,
zz12=0.166249458, xx12=−0.409482196, yy12=0.170107295,
zz13=0.162343147, xx13=−0.409804718, yy13=0.165221097,
zz14=0.178158258, xx14=−0.400869431, yy14=0.123020055,
zz15=0.171958144, xx15=−0.447472325.

[0198] When γ1>γ2, this corresponds to a case where ̂Θ.sup.(m).sub.γ1 is set as

̂Θ.sup.(m).sub.γ1=(̂θ.sub.γL.sup.(m)[1], . . . ,̂θ.sub.γL.sup.(m)[p]).sup.T

in Step 2 of <Learning Process for Transformation Matrix K>, ̂θ.sup.(m).sub.γ2 is set as

̂Θ.sup.(m).sub.γ2=(̂θ.sub.γ=1.sup.(m)[1], . . . ,̂θ.sub.γ=1.sup.(m)[p]).sup.T

in Step 4, and each pair of LSP parameter sequences (̂Θ.sup.(m).sub.γ1, ̂Θ.sup.(m).sub.γ2) contained in learning data Q is substituted into the model of Formula (13b) with γ1=1, γ2=γL, ̂Θ.sub.γ1=̂Θ.sup.(m).sub.γ1, and ̂Θ.sub.γ2=̂Θ.sup.(m).sub.γ2 in Step 5 and the coefficients for matrix K are learned with the square error criterion.

[0199] <Effects of the Second Embodiment>

[0200] The encoding apparatus 3 according to the second embodiment provides similar effects to the encoding apparatus 1 in the first embodiment because, as with the first embodiment, it has a configuration in which the quantized linear prediction coefficient generating unit 900, the quantized linear prediction coefficient adjusting unit 905, and the approximate smoothed power spectral envelope series calculating unit 910 of the conventional encoding apparatus 9 are replaced with the linear prediction coefficient adjusting unit 125, adjusted LSP generating unit 130, adjusted LSP encoding unit 135, quantized linear prediction coefficient generating unit 140, and the first quantized smoothed power spectral envelope series calculating unit 145. That is, when the encoding distortion is equal to that in a conventional method, the code amount can be reduced compared to the conventional method, whereas when the code amount is the same as in the conventional method, encoding distortion can be reduced compared to the conventional method.

[0201] In addition, the calculation cost of the encoding apparatus 3 in the second embodiment is low because K is a band matrix in calculation of Formula (18). By replacing the quantized linear prediction coefficient inverse adjustment unit 155 and the inverse-adjusted LSP generating unit 160 in the first embodiment with the LSP linear transformation unit 300, a series of approximations of the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] can be generated with a smaller amount of calculation than the first embodiment.

[0202] [Modification of Second Embodiment]

[0203] The encoding apparatus 3 in the second embodiment decides whether to code in the time domain or in the frequency domain based on the magnitude of temporal variation in the input sound signal for each frame. However, even for a frame in which the temporal variation in the input sound signal was large and frequency domain encoding was selected, it is possible that actually a sound signal reproduced by encoding in the time domain leads to smaller distortion relative to the input sound signal than a signal reproduced by encoding in the frequency domain. Likewise, even for a frame in which the temporal variation in the input sound signal was small and encoding in the time domain was selected, it is possible that actually a sound signal reproduced by encoding in the frequency domain leads to smaller distortion relative to the input sound signal than a sound signal reproduced by encoding in the time domain. That is to say, the encoding apparatus 3 in the second embodiment cannot always select one of the time domain and frequency domain encoding methods that provides smaller distortion relative to the input sound signal. To address this, a encoding apparatus 8 in a modification of the second embodiment performs both time domain and frequency domain encoding on each frame and selects either of them that yields smaller distortion relative to the input sound signal.

[0204] <Encoding Apparatus>

[0205] FIG. 15 shows the functional configuration of the encoding apparatus 8 in a modification of the second embodiment.

[0206] The encoding apparatus 8 differs from the encoding apparatus 3 in the second embodiment in that it does not include the feature amount extracting unit 120 and includes a code selection and output unit 375 in place of the output unit 175.

[0207] <Encoding Method>

[0208] Referring to FIG. 16, the encoding method in the modification of the second embodiment will be described. The following description mainly focuses on differences from the second embodiment.

[0209] In the encoding method according to the modification of the second embodiment, the LSP generating unit 110, LSP encoding unit 115, linear prediction coefficient adjusting unit 125, adjusted LSP generating unit 130, adjusted LSP encoding unit 135, quantized linear prediction coefficient generating unit 140, first quantized smoothed power spectral envelope series calculating unit 145, delay input unit 165, and LSP linear transformation unit 300 are also executed in addition to the input unit 100 and the linear prediction analysis unit 105 for all frames regardless of whether the temporal variation in the input sound signal is large or small. The operations of these components are the same as the second embodiment. However, the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app generated by the LSP linear transformation unit 300 is input to the delay input unit 165.

[0210] The delay input unit 165 holds the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] input from the LSP encoding unit 115 and the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app input from the LSP linear transformation unit 300 at least for the duration of one frame. When the frequency domain encoding method was selected by the code selection and output unit 375 for the preceding frame (i.e., when the identification code Cg output by the code selection and output unit 375 for the preceding frame is information indicating the frequency domain encoding method), the delay input unit 165 outputs the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app for the preceding frame input from the LSP linear transformation unit 300 to the time domain encoding unit 170 as the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] for the preceding frame. When the time domain encoding method was selected by the code selection and output unit 375 for the preceding frame (i.e., when the identification code Cg output by the code selection and output unit 375 for the preceding frame is information indicating the time domain encoding method), the delay input unit 165 outputs the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] for the preceding frame input from the LSP encoding unit 115 to the time domain encoding unit 170 (step S165).

[0211] As with the frequency domain encoding unit 150 in the second embodiment, the frequency domain encoding unit 150 generates and outputs frequency domain signal codes, and also determines and outputs the distortion or an estimated value of the distortion of the sound signal corresponding to the frequency domain signal codes relative to the input sound signal. The distortion or an estimation thereof may be determined either in the time domain or in the frequency domain. This means that the frequency domain encoding unit 150 may determine the distortion or an estimated value of the distortion of a frequency-domain sound signal series corresponding to frequency domain signal codes relative to the frequency-domain sound signal series that is obtained by converting the input sound signal into the frequency domain.

[0212] The time domain encoding unit 170, as with the time domain encoding unit 170 in the second embodiment, generates and outputs time domain signal codes, and also determines the distortion or an estimated value of the distortion of the sound signal corresponding to the time domain signal codes relative to the input sound signal.

[0213] Input to the code selection and output unit 375 are the frequency domain signal codes generated by the frequency domain encoding unit 150, the distortion or an estimated value of distortion determined by the frequency domain encoding unit 150, the time domain signal codes generated by the time domain encoding unit 170, and the distortion or an estimated value of distortion determined by the time domain encoding unit 170.

[0214] When the distortion or estimated value of distortion input from the frequency domain encoding unit 150 is smaller than the distortion or an estimated value of distortion input from the time domain encoding unit 170, the code selection and output unit 375 outputs the frequency domain signal codes and identification code Cg which is information indicating the frequency domain encoding method. When the distortion or estimated value of distortion input from the frequency domain encoding unit 150 is greater than the distortion or an estimated value of distortion input from the time domain encoding unit 170, the code selection and output unit 375 outputs the time domain signal codes and identification code Cg which is information indicating the time domain encoding method. When the distortion or an estimated value of distortion input from the frequency domain encoding unit 150 is equal to the distortion or an estimated value of distortion input from the time domain encoding unit 170, the code selection and output unit 375 outputs either the time domain signal codes or the frequency domain signal codes according to predetermined rules, as well as identification code Cg which is information indicating the encoding method corresponding to the codes being output. That is to say, of the frequency domain signal codes input from the frequency domain encoding unit 150 and the time domain signal codes input from the time domain encoding unit 170, the code selection and output unit 375 outputs either one that leads to a smaller distortion of the sound signal reproduced from the codes relative to the input sound signal, and also outputs information indicative of the encoding method that yields smaller distortion as identification code Cg (step S375).

[0215] The code selection and output unit 375 may also be configured to select either one of the sound signals reproduced from the respective codes that has smaller distortion relative to the input sound signal. In such a configuration, the frequency domain encoding unit 150 and the time domain encoding unit 170 reproduce sound signals from the codes and output them instead of distortion or an estimated value of distortion. The code selection and output unit 375 outputs either the sound signal reproduced by the frequency domain encoding unit 150 or the sound signal reproduced by the time domain encoding unit 170 respectively from frequency domain signal codes and time domain signal codes that has smaller distortion relative to the input sound signal, and also outputs information indicating the encoding method that yields smaller distortion as identification code Cg.

[0216] Alternatively, the code selection and output unit 375 may be configured to select either one that has a smaller code amount. In such a configuration, the frequency domain encoding unit 150 outputs frequency domain signal codes as in the second embodiment. The time domain encoding unit 170 outputs time domain signal codes as in the second embodiment. The code selection and output unit 375 outputs either the frequency domain signal codes or the time domain signal codes that have a smaller code amount, and also outputs information indicating the encoding method that yields a smaller code amount as identification code Cg.

[0217] <Decoding Apparatus>

[0218] A code sequence output by the encoding apparatus 8 in the modification of the second embodiment can be decoded by the decoding apparatus 4 of the second embodiment as with a code sequence output by the encoding apparatus 3 of the second embodiment.

[0219] <Effects of Modification of the Second Embodiment>

[0220] The encoding apparatus 8 in the modification of the second embodiment provides similar effects to the encoding apparatus 3 of the second embodiment and further has the effect of reducing the code amount to be output compared to the encoding apparatus 3 of the second embodiment.

Third Embodiment

[0221] The encoding apparatus 1 of the first embodiment and the encoding apparatus 3 of the second embodiment once convert the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] into linear prediction coefficients and then calculate the quantized smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N]. A encoding apparatus 5 in the third embodiment directly calculates the quantized smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] from the adjusted quantized LSP parameter sequence ̂a.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] without converting the adjusted quantized LSP parameter sequence to linear prediction coefficients. Similarly, a decoding apparatus 6 in the third embodiment directly calculates the decoded smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] from the decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] without converting the decoded adjusted LSP parameter sequence to linear prediction coefficients.

[0222] <Encoding Apparatus>

[0223] FIG. 17 shows the functional configuration of the encoding apparatus 5 according to the third embodiment.

[0224] The encoding apparatus 5 differs from the encoding apparatus 3 in the second embodiment in that it does not include the quantized linear prediction coefficient generating unit 140 and the first quantized smoothed power spectral envelope series calculating unit 145 but includes a second quantized smoothed power spectral envelope series calculating unit 146 instead.

[0225] <Encoding Method>

[0226] Referring to FIG. 18, the encoding method in the third embodiment will be described. The following description mainly focuses on differences from the foregoing embodiments.

[0227] At step S146, the second quantized smoothed power spectral envelope series calculating unit 146 uses the adjusted quantized LSP parameters ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] output by the adjusted LSP encoding unit 135 to determine a quantized smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] according to Formula (19) and outputs it.

[00018] $\begin{matrix} {\hat{W}}_{γ .Math. .Math. R} [k] = \sqrt{\frac{δ^{2}}{2 .Math. π} .Math. \frac{1}{| A (\exp (j .Math. .Math. ω_{k})) .Math. |^{2}}}, .Math. | A (\exp (j .Math. .Math. ω_{k})) .Math. |^{2} = {\begin{matrix} 2^{p - 1} [(1 - \cos .Math. .Math. ω_{k}) .Math. {.Math.}_{n = 1}^{p .Math. / .Math. 2} .Math. .Math. {(\cos .Math. .Math. {\hat{θ}}_{γ .Math. .Math. R} [2 .Math. n] - \cos .Math. .Math. ω_{k})}^{2} + (1 + \cos .Math. .Math. ω_{k}) .Math. {.Math.}_{n = 1}^{p .Math. / .Math. 2} .Math. .Math. {(\cos .Math. .Math. {\hat{θ}}_{γ .Math. .Math. R} [2 .Math. n - 1] - \cos .Math. .Math. ω_{k})}^{2}] .Math. & (p .Math. : .Math. .Math. odd) .Math. \\ 2^{p - 1} [(1 - \cos .Math. .Math. ω_{k}) .Math. (1 + \cos .Math. .Math. ω_{k}) .Math. {.Math.}_{n = 1}^{(p - 1) .Math. / .Math. 2} .Math. .Math. {(\cos .Math. .Math. {\hat{θ}}_{γ .Math. .Math. R} [2 .Math. n] - \cos .Math. .Math. ω_{k})}^{2} + {.Math.}_{n = 1}^{(p + 1) .Math. / .Math. 2} .Math. .Math. {(\cos .Math. .Math. {\hat{θ}}_{γ .Math. .Math. R} [2 .Math. n - 1] - \cos .Math. .Math. ω_{k})}^{2}] & (p .Math. : .Math. .Math. even) \end{matrix} .Math. .Math. ω_{k} = - \frac{2 .Math. π .Math. .Math. k}{N} & (19) \end{matrix}$

[0228] <Decoding Apparatus>

[0229] FIG. 19 shows the functional configuration of the decoding apparatus 6 in the third embodiment.

[0230] The decoding apparatus 6 differs from the decoding apparatus 4 in the second embodiment in that it does not include the decoded linear prediction coefficient generating unit 220 and the first decoded smoothed power spectral envelope series calculating unit 225 but includes a second decoded smoothed power spectral envelope series calculating unit 226 instead.

[0231] <Decoding Method>

[0232] Referring to FIG. 20, the decoding method in the third embodiment will be described. The following description mainly focuses on differences from the foregoing embodiments.

[0233] At step S226, as with the second quantized smoothed power spectral envelope series calculating unit 146, the second decoded smoothed power spectral envelope series calculating unit 226 uses the decoded adjusted LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] to determine a decoded smoothed power spectral envelope series ̂W.sub.γR[1], ̂W.sub.γR[2], . . . , ̂W.sub.γR[N] according to the Formula (19) above and outputs it.

Fourth Embodiment

[0234] The quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p] is a series that satisfies

0<̂θ[1]< . . . <̂θ[p]<π.

That is, it is a series in which parameters are arranged in ascending order. Meanwhile, the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app generated by the LSP linear transformation unit 300 is produced through approximate transformation, so it could not be in ascending order. To address this, the fourth embodiment adds processing for rearranging the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app output by the LSP linear transformation unit 300 into ascending order.

[0235] <Encoding Apparatus>

[0236] FIG. 21 shows the functional configuration of a encoding apparatus 7 in the fourth embodiment.

[0237] The encoding apparatus 7 differs from the encoding apparatus 5 in the second embodiment in that it further includes an approximate LSP series modifying unit 700.

[0238] <Encoding Method>

[0239] Referring to FIG. 22, the encoding method in the fourth embodiment will be described. The following description mainly focuses on differences from the foregoing embodiments.

[0240] The approximate LSP series modifying unit 700 outputs a series in which the values ̂θ[i].sub.app in the approximate quantized LSP parameter sequence ̂θ[1].sub.app, ̂θ[2].sub.app, . . . , ̂θ[p].sub.app output by the LSP linear transformation unit 300 have been rearranged in ascending order as a modified approximate quantized LSP parameter sequence ̂θ′[1].sub.app, ̂θ′[2].sub.app, . . . , ̂θ′[p].sub.app. The modified first approximate quantized LSP parameter sequence ̂θ′[1].sub.app, ̂θ′[2].sub.app, . . . , ̂θ′[p].sub.app output by the approximate LSP series modifying unit 700 is input to the delay input unit 165 as the quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p].

[0241] In addition to merely rearranging the values in the approximate quantized LSP parameter sequence, each value ̂θ[i].sub.app may be adjusted as ̂θ′[i].sub.app such that |̂θ[i+1].sub.app−̂θ[i].sub.app| is equal to or greater than a predetermined threshold for each value of i=1, . . . , p−1.

[0242] [Modification]

[0243] While the foregoing embodiments were described assuming use of LSP parameters, an ISP parameter sequence may be employed instead of an LSP parameter sequence. An ISP parameter sequence ISP[1], . . . , ISP[p] is equivalent to a series consisting of an LSP parameter sequence of the p−1th order and PARCOR coefficient k.sub.p of the pth order (the highest order). That is to say,

[0244] ISP[i]=θ[i] for i=1, . . . , p−1, and

[0245] ISP[p]=k.sub.p.

[0246] Specific processing will be illustrated for a case where input to the LSP linear transformation unit 300 is an ISP parameter sequence in the second embodiment.

[0247] Assume that input to the LSP linear transformation unit 300 is a adjusted quantized ISP parameter sequence ̂ISP.sub.γR[1], ̂ISP.sub.γR[2], . . . , ̂ISP.sub.γR[p]. Here,

[0248] ̂ISP.sub.γR[1]=̂θ.sub.γR[i], and

[0249] ̂ISP.sub.γR[p]̂k.sub.p.

The value ̂k.sub.p is the quantized value of k.sub.p.

[0250] The LSP linear transformation unit 300 determines an approximate quantized ISP parameter sequence ̂ISP[1].sub.app, . . . , ̂ISP[p].sub.app through the following process and outputs it.

[0251] (Step 1) Given ̂Θ.sub.γ1=(̂ISP.sub.γR[1], . . . , ̂ISP.sub.γR[p−1]).sup.T, p is replaced with p−1, and ̂θ[1].sub.app, . . . , ̂θ[p−1].sub.app are determined by calculating Formula (18). Here,

[0252] ̂ISP[i].sub.app=̂θ[i].sub.app (i=1, . . . , p−1).

[0253] (Step 2) ̂ISP[p].sub.app defined by the formula below is determined.

[0254] ̂ISP[p].sub.app=̂ISP.sub.γR[p].Math.(1/γR).sup.p.

Fifth Embodiment

[0255] The LSP linear transformation unit 300 included in the encoding apparatuses 3, 5, 7, 8 and the decoded LSP linear transformation unit 400 included in the decoding apparatuses 4, 6 may also be implemented as a separate frequency domain parameter sequence generating apparatus.

[0256] The following description illustrates a case where the LSP linear transformation unit 300 included in the encoding apparatuses 3, 5, 7, 8 and the decoded LSP linear transformation unit 400 included in the decoding apparatuses 4, 6 are implemented as a separate frequency domain parameter sequence generating apparatus.

[0257] <Frequency Domain Parameter Sequence Generating Apparatus>

[0258] A frequency domain parameter sequence generating apparatus 10 according to the fifth embodiment includes a parameter sequence converting unit 20 for example, as shown in FIG. 23, and receives frequency domain parameters ω[1], ω[2], . . . , ω[p] as input and outputs converted frequency domain parameters ˜ω[1], ˜θ[2], . . . , ˜ω[p].

[0259] The frequency domain parameters ω[1], ω[2], . . . , ω[p] to be input are a frequency domain parameter sequence derived from linear prediction coefficients, a[1], a[2], . . . , a[p], which are obtained by linear prediction analysis of sound signals in a predetermined time segment. The frequency domain parameters ω[1], ω[2], . . . , ω[p] may be an LSP parameter sequence θ[1], θ[2], . . . , θ[p] used in conventional encoding methods, or a quantized LSP parameter sequence ̂θ[1], ̂θ[2], . . . , ̂θ[p], for example. Alternatively, they may be the adjusted LSP parameter sequence θ.sub.γR[1], θ.sub.γR[2], . . . , θ.sub.γR[p] or the adjusted quantized LSP parameter sequence ̂θ.sub.γR[1], ̂θ.sub.γR[2], . . . , ̂θ.sub.γR[p] used in the aforementioned embodiments, for example. Further, they may be frequency domain parameters equivalent to LSP parameters, such as the ISP parameter sequence described in the modification above, for example. A frequency domain parameter sequence derived from linear prediction coefficients a[1], a[2], . . . , a[p] are a series in the frequency domain derived from a linear prediction coefficient sequence and represented by the same number of elements as the order of prediction, typified by an LSP parameter sequence, an ISP parameter sequence, an LSF parameter sequence, or an ISF parameter sequence each derived from the linear prediction coefficient sequence a[1], a[2], . . . , a[p], or a frequency domain parameter sequence in which all of the frequency domain parameters ω[1], ω[2], . . . , ω[p−1] are present from 0 to π and, when all of the linear prediction coefficients contained in the linear prediction coefficient sequence are 0, the frequency domain parameters ω[1], ω[2], . . . , ω[p−1] are present from 0 to π at equal intervals.

[0260] The parameter sequence converting unit 20, similarly to the LSP linear transformation unit 300 and the decoded LSP linear transformation unit 400, applies approximate linear transformation to the frequency domain parameter sequence ω[1], ω[2], . . . , ω[p−1] making use of the nature of LSP parameters to generate a converted frequency domain parameter sequence ˜ω[1], ˜ω[2], . . . , ˜ω[p]. The parameter sequence converting unit 20 determines the value of the converted frequency domain parameter ˜ω[i] according to one of the methods shown below for each i=1, 2, . . . , p, for example.

[0261] 1. The value of the converted frequency domain parameter ˜ω[i] is determined by linear transformation which is based on the relationship of values between ω[i] and one or more frequency domain parameters adjacent to ω[i]. For instance, linear transformation is performed so that the intervals between parameter values becomes more uniform or less uniform in the converted frequency domain parameter sequence ˜ω[i] than in the frequency domain parameter sequence ω[i]. Linear transformation that makes the parameter interval more uniform corresponds to processing that flats the waves of the amplitude of the power spectral envelope in the frequency domain (processing for smoothing the power spectral envelope). Linear transformation that makes the parameter interval less uniform corresponds to processing that emphasizes the height difference in the waves of the amplitude of the power spectral envelope in the frequency domain (processing for unsmoothing the power spectral envelope).

[0262] 2. When ω[i] is closer to ω[i+1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i+1]−˜ω[i] will be smaller than ω[i+1]−ω[i]. When ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be smaller than ω[i]−ω[i−1]. This corresponds to processing that emphasizes the height difference in the waves of the amplitude of the power spectral envelope in the frequency domain (processing for unsmoothing the power spectral envelope).

[0263] 3. When ω[i] is closer to ω[i+1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i+1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i+1]−˜ω[i] will be greater than ω[i+1]−ω[i]. When ω[i] is closer to ω[i−1] relative to the midpoint between ω[i+1] and ω[i−1], then ˜ω[i] is determined so that ˜ω[i] will be closer to ˜ω[i−1] relative to the midpoint between ˜ω[i+1] and ˜ω[i−1] and that the value of ˜ω[i]−˜ω[i−1] will be greater than ω[i]−ω[i−1]. This corresponds to processing that flats the waves of the amplitude of the power spectral envelope in the frequency domain (processing for smoothing the power spectral envelope).

[0264] For example, the parameter sequence converting unit 20 determines the converted frequency domain parameters ˜ω[1], ˜ω[2], . . . , ˜ω[p] according to Formula (20) below and outputs it.

[00019] $\begin{matrix} (\begin{matrix} \tilde{ω} [1] \\ \tilde{ω} [2] \\ .Math. \\ \tilde{ω} [p] \end{matrix}) = K (\begin{matrix} ω [1] - \frac{π}{p - 1} \\ ω [2] - \frac{2 .Math. π}{p + 1} \\ .Math. \\ ω [p] - \frac{p .Math. .Math. π}{p - 1} \end{matrix}) .Math. (γ2 - γ1) + (\begin{matrix} ω [1] \\ ω [2] \\ .Math. \\ ω [p] \end{matrix}) & (20) \end{matrix}$

[0265] Here, γ1 and γ2 are positive coefficients equal to or smaller than 1. Formula (20) can be derived by setting θ.sub.γ1=(ω[1], ω[2], . . . , ω[p]).sup.T and θ.sub.γ2=(˜ω[1], ˜ω[2], . . . , ˜ω[p]).sup.T in Formula (13), which models LSP parameters, and defining

[00020] $Θ_{γ = 0} = (\frac{π}{p + 1}, \frac{2 .Math. π}{p + 1}, .Math., \frac{p .Math. .Math. π}{p + 1}) .$

In this case, frequency domain parameters ω[1], ω[2], . . . , ω[p] are a frequency-domain parameter sequence or the quantized values thereof equivalent to

a[1]×(γ1),a[2]×(γ1).sup.2, . . . ,a[p]×(γ1).sup.p,

which is a coefficient sequence that has been adjusted by multiplying each coefficient a[i] of the linear prediction coefficients a[1], a[2], . . . , a[p] by the ith power of the factor γ1. The converted frequency domain parameters ˜ω[1], ˜ω[2], . . . , ˜ω[p] are a series that approximates a frequency-domain parameter sequence equivalent to

a[1]×(γ2),a[2]×(γ2).sup.2, . . . ,a[p]×(γ2).sup.p,

which is a coefficient sequence that has been adjusted by multiplying each coefficient a[i] of the linear prediction coefficients a[1], a[2], . . . , a[p] by the ith power of factor γ2.

[0266] <Effects of the Fifth Embodiment>

[0267] As with the encoding apparatuses 3, 5, 7, 8 or the decoding apparatuses 4, 6, the frequency domain parameter sequence generating apparatus in the fifth embodiment is able to determine converted frequency domain parameters from frequency domain parameters with a smaller amount of calculation than when converted frequency domain parameters are determined from frequency domain parameters by way of linear prediction coefficients as in the encoding apparatus 1 and the decoding apparatus 2.

[0268] The present invention is not limited to the above-described embodiments and it goes without saying that modifications may be made as necessary without departing from the scope of the invention. The various kinds of processing illustrated in the embodiments above could also be performed in parallel or separately in accordance with the processing capability of the device executing them or certain necessity in addition to being carried out chronologically in the orders described herein.

[0269] [Program and Recording Media]

[0270] When the various processing functions of the apparatuses described in the embodiments are implemented by a computer, the processing details of the functions supposed to be provided in the apparatuses are described by a program. The program is then executed by the computer so as to implement various processing functions of the individual apparatuses on the computer.

[0271] A program describing the processing details can be recorded in a computer-readable recording medium. The computer-readable recording medium may be any kind of media, such as a magnetic recording device, optical disk, magneto-optical recording medium, and semiconductor memory, for example.

[0272] Such a program may be distributed by selling, granting, or lending a portable recording medium, such as a DVD or CD-ROM for example, having the program recorded thereon. Alternatively, the program may be stored in a storage device at a server computer and transferred to other computers from the server computer over a network so as to distribute the program.

[0273] When a computer is to execute such a program, the computer first stores the program recorded on a portable recording medium or the program transferred from the server computer once in its own storage device, for example. Then, when it carries out processing, the computer reads the program stored in its recording medium and performs processing in accordance with the program that has been read. As an alternative form of execution of the program, the computer may directly read the program from a portable recording medium and perform processing in accordance with the program, or the computer may perform processing sequentially in accordance with a program it has received every time a program is transferred from the server computer to the computer. The above-described processing may also be implemented as a so-called application service provider (ASP) service, which implements processing functions only through requests for execution and acquisition of results without transfer of programs from a server computer to a computer. Programs in the embodiments described herein are intended to contain information that is used in processing by an electronic computer and subordinate to programs (such as data that is not a direct instruction on a computer but has properties governing the processing of the computer).

[0274] Additionally, while the apparatuses of the present invention have been described as being implemented through execution of predetermined programs on computer in such embodiments, at least part of these processing details may also be implemented by hardware.

FREQUENCY DOMAIN PARAMETER SEQUENCE GENERATING METHOD, ENCODING METHOD, DECODING METHOD, FREQUENCY DOMAIN PARAMETER SEQUENCE GENERATING APPARATUS, ENCODING APPARATUS, DECODING APPARATUS, PROGRAM, AND RECORDING MEDIUM

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Cpc classification

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G10L19/12

PHYSICS

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G10L19/02

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G10L25/06

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G10L25/12

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G10L19/07

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International classification

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G10L19/07

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G10L25/06

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G10L19/12

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Abstract

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