NOISE FILLING WITHOUT SIDE INFORMATION FOR CELP-LIKE CODERS

Abstract

An audio decoder provides a decoded audio information on the basis of an encoded audio information including linear prediction coefficients (LPC) and includes a tilt adjuster to adjust a tilt of a noise using linear prediction coefficients of a current frame to acquire a tilt information and a noise inserter configured to add the noise to the current frame in dependence on the tilt information. Another audio decoder includes a noise level estimator to estimate a noise level for a current frame using a linear prediction coefficient of at least one previous frame to acquire a noise level information; and a noise inserter to add a noise to the current frame in dependence on the noise level information provided by the noise level estimator. Thus, side information about a background noise in the bit-stream may be omitted. Methods and computer programs serve a similar purpose.

Claims

1. An audio decoder for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the audio decoder comprising: a tilt adjuster configured to adjust a tilt of a background noise, wherein the tilt adjuster is configured to use linear prediction coefficients of a current frame to acquire a tilt information; and a decoder core configured to decode an audio information of the current frame using the linear prediction coefficients of the current frame to acquire a decoded core coder output signal; and a noise inserter configured to add the adjusted background noise to the current frame, to perform a noise filling; wherein the tilt adjuster is configured to obtain the tilt information using a calculation of a gain g of the linear prediction coefficients of the current frame,
wherein g=(ak.Math.ak+1)/(ak.Math.ak), wherein a.sub.k is a linear prediction coefficient of the current frame, located at LPC index k.

2. The audio decoder according to claim 1, wherein the audio decoder comprises a frame type determinator for determining a frame type of the current frame, the frame type determinator being configured to activate the tilt adjuster to adjust the tilt of the background noise when the frame type of the current frame is detected to be of a speech type.

3. The audio decoder according to claim 1, wherein the tilt adjuster is configured to use a result of a first-order analysis of the linear prediction coefficients of the current frame to acquire the tilt information.

4. The audio decoder according to claim 3, wherein the tilt adjuster is configured to acquire the tilt information using a calculation of a gain g of the linear prediction coefficients of the current frame as the first-order analysis.

5. The audio decoder according to claim 1, wherein the audio decoder furthermore comprises: a noise level estimator configured to estimate a noise level for a current frame using a plurality of linear prediction coefficient of at least one previous frame to acquire a noise level information; wherein the noise inserter configured to add the background noise to the current frame in dependence on the noise level information provided by the noise level estimator; wherein the audio decoder is adapted to decode an excitation signal of the current frame and to compute its root mean square e.sub.rms; wherein the audio decoder is adapted to compute a peak level p of a transfer function of an LPC filter of the current frame; wherein the audio decoder is adapted to compute a spectral minimum m.sub.f of the current audio frame by computing the quotient of the root mean square e.sub.rms and the peak level p to acquire the noise level information; wherein the noise level estimator is adapted to estimate the noise level on the basis of two or more quotients of different audio frames.

6. An audio decoder for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the audio decoder comprising: a noise inserter configured to add a noise to the current frame in dependence on a noise level information; wherein the audio decoder is adapted to decode an excitation signal of the current frame and to compute its root mean square e.sub.rms; wherein the audio decoder is adapted to compute a peak level p of a transfer function of an LPC filter of the current frame; wherein the audio decoder is adapted to compute a spectral minimum m.sub.f of the current audio frame by computing the quotient of the root mean square e.sub.rms and the peak level p to acquire the noise level information; wherein the noise level estimator is adapted to estimate the noise level on the basis of two or more quotients of different audio frames; wherein the audio decoder comprises a decoder core configured to decode an audio information of the current frame using linear prediction coefficients of the current frame to acquire a decoded core coder output signal and wherein the noise inserter adds the noise depending on linear prediction coefficients used in decoding the audio information of the current frame and used in decoding the audio information of one or more previous frames.

7. The audio decoder according to claim 6, wherein the audio decoder comprises a frame type determinator for determining a frame type of the current frame, the frame type determinator being configured to identify whether the frame type of the current frame is speech or general audio, so that the noise level estimation can be performed depending on the frame type of the current frame.

8. The audio decoder according to claim 6, wherein the audio decoder is adapted to compute the root mean square e.sub.rms of the current frame from the time domain representation of the current frame to acquire the noise level information under the condition that the current frame is of a speech type.

9. The audio decoder according to claim 6, wherein the audio decoder is adapted to decode an unshaped MDCT-excitation of the current frame and to compute its root mean square e.sub.rms from the spectral domain representation of the current frame to acquire the noise level information if the current frame is of a general audio type.

10. The audio decoder according to claim 6, wherein the audio decoder is adapted to enqueue the quotient acquired from the current audio frame in the noise level estimator regardless of the frame type, the noise level estimator comprising a noise level storage for two or more quotients acquired from different audio frames.

11. The audio decoder according to claim 6, wherein the noise level estimator is adapted to estimate the noise level on the basis of statistical analysis of two or more quotients of different audio frames.

12. The audio decoder according to claim 1, wherein the audio decoder comprises a de-emphasis filter to de-emphasize the current frame, the audio decoder being adapted to applying the de-emphasis filter on the current frame after the noise inserter added the noise to the current frame.

13. The audio decoder according to claim 1, wherein the audio decoder comprises a noise generator, the noise generator being adapted to generate the noise to be added to the current frame by the noise inserter.

14. The audio decoder according to claim 1, wherein the audio decoder comprises a noise generator configured to generate random white noise.

15. The audio decoder according to claim 1, wherein the audio decoder is configured to use a decoder based on one or more of the decoders AMR-WB, G.718 or LD-USAC (EVS) in order to decode the encoded audio information.

16. A method for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the method comprising: adjusting a tilt of a background noise, wherein linear prediction coefficients of a current frame are used to acquire a tilt information; and decoding an audio information of the current frame using the linear prediction coefficients of the current frame to acquire a decoded core coder output signal; and adding the adjusted background noise to the current frame, to perform a noise filling; wherein the tilt information is obtained using a calculation of a gain g of the linear prediction coefficients of the current frame,
wherein g=(ak.Math.ak+1)/(ak.Math.ak), wherein ak is a linear prediction coefficient of the current frame, located at LPC index k.

17. A method for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the method comprising: adding a noise to the current frame in dependence on a noise level information; wherein an excitation signal of the current frame is decoded and wherein its root mean square e.sub.rms is computed; wherein a peak level p of a transfer function of an LPC filter of the current frame is computed; wherein a spectral minimum m.sub.f of the current audio frame is computed by computing the quotient of the root mean square e.sub.rms and the peak level p to acquire the noise level information; wherein the noise level is estimated on the basis of two or more quotients of different audio frames; wherein the method comprises decoding an audio information of the current frame using linear prediction coefficients of the current frame to acquire a decoded core coder output signal and wherein the method comprises adding the noise depending on linear prediction coefficients used in decoding the audio information of the current frame and used in decoding the audio information of one or more previous frames.

18. An audio decoder for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the audio decoder comprising: a tilt adjuster configured to adjust a tilt of a background noise, wherein the tilt adjuster is configured to use linear prediction coefficients of a current frame to acquire a tilt information; a decoder core configured to decode an audio information of the current frame using the linear prediction coefficients of the current frame to acquire a decoded core coder output signal; and a noise inserter configured to add the adjusted background noise to the current frame, to perform a noise filling, wherein the noise filling is used to fill spectral gaps or valleys; wherein the tilt adjuster is configured to obtain the tilt information using a calculation of a gain g of the linear prediction coefficients of the current frame,
wherein g=(ak.Math.ak+1)/(ak.Math.ak), wherein ak is a linear prediction coefficient of the current frame, located at LPC index k.

19. An audio decoder for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the audio decoder comprising: a tilt adjuster configured to adjust a tilt of a background noise, wherein the tilt adjuster is configured to use linear prediction coefficients of a current frame to acquire a tilt information; a decoder core configured to decode an audio information of the current frame using the linear prediction coefficients of the current frame to acquire a decoded core coder output signal; and a noise inserter configured to add the adjusted background noise to the current frame, to perform a noise filling, wherein noise is added in a frequency region of the decoded core coder output signal provided by the decoder core; wherein the tilt adjuster is configured to obtain the tilt information using a calculation of a gain g of the linear prediction coefficients of the current frame,
wherein g=(ak.Math.ak+1)/(ak.Math.ak), wherein ak is a linear prediction coefficient of the current frame, located at LPC index k.

20. The audio encoder of claim 19, wherein the audio decoder comprises a noise filling configured to fill spectral gaps or valleys in a decoded spectrum, wherein the audio decoder comprises a tilt determination configured to determine a tilt of a noise filling noise for the noise filling, and wherein the tilt determination is configured to use linear prediction coefficients of a current frame to acquire a tilt information.

21. A method for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the method comprising: adjusting a tilt of a background noise, wherein linear prediction coefficients of a current frame are used to acquire a tilt information; and adding the adjusted background noise to the current frame, to perform a noise filling; wherein the tilt information is obtained using a calculation of a gain g of the linear prediction coefficients of the current frame,
wherein g=(ak.Math.ak+1)/(ak.Math.ak), wherein ak is a linear prediction coefficient of the current frame, located at LPC index k.

22. An audio decoder for providing a decoded audio information on the basis of an encoded audio information comprising linear prediction coefficients (LPC), the audio decoder comprising: a tilt adjuster configured to adjust a tilt of a background noise, wherein the tilt adjuster is configured to use linear prediction coefficients of a current frame to acquire a tilt information; a noise inserter configured to add the adjusted background noise to the current frame, to perform a noise filling, wherein the tilt information is obtained using a calculation of a gain g of the linear prediction coefficients of the current frame,
wherein g=(ak.Math.ak+1)/(ak.Math.ak), wherein ak is a linear prediction coefficient of the current frame, located at LPC index k.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] Embodiments of the present invention will be detailed subsequently referring to the appended drawings, in which:

[0047] FIG. 1 shows a first embodiment of an audio decoder according to the present invention;

[0048] FIG. 2 shows a first method for performing audio decoding according to the present invention which can be performed by an audio decoder according to FIG. 1;

[0049] FIG. 3 shows a second embodiment of an audio decoder according to the present invention;

[0050] FIG. 4 shows a second method for performing audio decoding according to the present invention which can be performed by an audio decoder according to FIG. 3;

[0051] FIG. 5 shows a third embodiment of an audio decoder according to the present invention;

[0052] FIG. 6 shows a third method for performing audio decoding according to the present invention which can be performed by an audio decoder according to FIG. 5;

[0053] FIGS. 7a-7c shows an illustration of a method for calculating spectral minima m.sub.f for noise level estimations;

[0054] FIG. 8 shows a diagram illustrating a tilt derived from LPC coefficients; and

[0055] FIG. 9 shows a diagram illustrating how LPC filter equivalents are determined from a MDCT power-spectrum.

DETAILED DESCRIPTION OF THE INVENTION

[0056] The invention is described in detail with regards to the FIGS. 1 to 9. The invention is in no way meant to be limited to the shown and described embodiments.

[0057] FIG. 1 shows a first embodiment of an audio decoder according to the present invention. The audio decoder is adapted to provide a decoded audio information on the basis of an encoded audio information. The audio decoder is configured to use a coder which may be based on AMR-WB, G.718 and LD-USAC (EVS) in order to decode the encoded audio information. The encoded audio information comprises linear prediction coefficients (LPC), which may be individually designated as coefficients a.sub.k The audio decoder comprises a tilt adjuster configured to adjust a tilt of a noise using linear prediction coefficients of a current frame to obtain a tilt information and a noise inserter configured to add the noise to the current frame in dependence on the tilt information obtained by the tilt calculator. The noise inserter is configured to add the noise to the current frame under the condition that the bitrate of the encoded audio information is smaller than 1 bit per sample. Furthermore, the noise inserter may be configured to add the noise to the current frame under the condition that the current frame is a speech frame. Thus, noise may be added to the current frame in order to improve the overall sound quality of the decoded audio information which may be impaired due to coding artifacts, especially with regards to background noise of speech information. When the tilt of the noise is adjusted in view of the tilt of the current audio frame, the overall sound quality may be improved without depending on side information in the bitstream. Thus, the amount of data to be transferred with the bit-stream may be reduced.

[0058] FIG. 2 shows a first method for performing audio decoding according to the present invention which can be performed by an audio decoder according to FIG. 1. Technical details of the audio decoder depicted in FIG. 1 are described along with the method features. The audio decoder is adapted to read the bitstream of the encoded audio information. The audio decoder comprises a frame type determinator for determining a frame type of the current frame, the frame type determinator being configured to activate the tilt adjuster to adjust the tilt of the noise when the frame type of the current frame is detected to be of a speech type. Thus, the audio decoder determines the frame type of the current audio frame by applying the frame type determinator. If the current frame is an ACELP frame, the frame type determinator activates the tilt adjuster. The tilt adjuster is configured to use a result of a first-order analysis of the linear prediction coefficients of the current frame to obtain the tilt information. More specifically, the tilt adjuster calculates a gain g using the formula g=[a.sub.k.Math.a.sub.k+1]/[a.sub.k.Math.a.sub.k] as a first-order analysis, wherein a.sub.k are LPC coefficients of the current frame. FIG. 8 shows a diagram illustrating a tilt derived from LPC coefficients. FIG. 8 shows two frames of the word see. For the letter s, which has a high amount of high frequencies, the tilt goes up. For the letters ee, which have a high amount of low frequencies, the tilt goes down. The spectral tilt shown in FIG. 8 is the transfer function of the direct form filter x(n)g.Math.x(n1), g being defined as given above. Thus, the tilt adjuster makes use of the LPC coefficients provided in the bitstream and used to decode the encoded audio information. Side information may be omitted accordingly which may reduce the amount of data to be transferred with the bitstream. Furthermore, the tilt adjuster is configured to obtain the tilt information using a calculation of a transfer function of the direct form filter x(n)g.Math.x(n1). Accordingly, the tilt adjuster calculates the tilt of the audio information in the current frame by calculating the transfer function of the direct form filter x(n)g.Math.x(n1) using the previously calculated gain g. After the tilt information is obtained, the tilt adjuster adjusts the tilt of the noise to be added to the current frame in dependence on the tilt information of the current frame. After that, the adjusted noise is added to the current frame. Furthermore, which is not shown in FIG. 2, the audio decoder comprises a de-emphasis filter to de-emphasize the current frame, the audio decoder being adapted to apply the de-emphasis filter on the current frame after the noise inserter added the noise to the current frame. After de-emphasizing the frame, which also serves as a low-complexity, steep IIR high-pass filtering of the added noise, the audio decoder provides the decoded audio information. Thus, the method according to FIG. 2 allows to enhance the sound quality of an audio information by adjusting the tilt of a noise to be added to a current frame in order to improve the quality of a background noise.

[0059] FIG. 3 shows a second embodiment of an audio decoder according to the present invention. The audio decoder is again adapted to provide a decoded audio information on the basis of an encoded audio information. The audio decoder again is configured to use a coder which may be based on AMR-WB, G.718 and LD-USAC (EVS) in order to decode the encoded audio information. The encoded audio information again comprises linear prediction coefficients (LPC), which may be individually designated as coefficients a.sub.k. The audio decoder according to the second embodiment comprises a noise level estimator configured to estimate a noise level for a current frame using a linear prediction coefficient of at least one previous frame to obtain a noise level information and a noise inserter configured to add a noise to the current frame in dependence on the noise level information provided by the noise level estimator. The noise inserter is configured to add the noise to the current frame under the condition that the bitrate of the encoded audio information is smaller than 0.5 bit per sample. Furthermore, the noise inserter is configured to add the noise to the current frame under the condition that the current frame is a speech frame. Thus, again, noise may be added to the current frame in order to improve the overall sound quality of the decoded audio information which may be impaired due to coding artifacts, especially with regards to background noise of speech information. When the noise level of the noise is adjusted in view of the noise level of at least one previous audio frame, the overall sound quality may be improved without depending on side information in the bitstream. Thus, the amount of data to be transferred with the bit-stream may be reduced.

[0060] FIG. 4 shows a second method for performing audio decoding according to the present invention which can be performed by an audio decoder according to FIG. 3. Technical details of the audio decoder depicted in FIG. 3 are described along with the method features. According to FIG. 4, the audio decoder is configured to read the bitstream in order to determine the frame type of the current frame. Furthermore, the audio decoder comprises a frame type determinator for determining a frame type of the current frame, the frame type determinator being configured to identify whether the frame type of the current frame is speech or general audio, so that the noise level estimation can be performed depending on the frame type of the current frame. In general, the audio decoder is adapted to compute a first information representing a spectrally unshaped excitation of the current frame and to compute a second information regarding spectral scaling of the current frame to compute a quotient of the first information and the second information to obtain the noise level information. For example, if the frame type is ACELP, which is a speech frame type, the audio decoder decodes an excitation signal of the current frame and computes its root mean square e.sub.rms for the current frame f from the time domain representation of the excitation signal. This means, that the audio decoder is adapted to decode an excitation signal of the current frame and to compute its root mean square e.sub.rms from the time domain representation of the current frame as the first information to obtain the noise level information under the condition that the current frame is of a speech type. In another case, if the frame type is MDCT or DTX, which is a general audio frame type, the audio decoder decodes an excitation signal of the current frame and computes its root mean square e.sub.rms for the current frame f from the time domain representation equivalent of the excitation signal. This means, that the audio decoder is adapted to decode an unshaped MDCT-excitation of the current frame and to compute its root mean square e.sub.rms from the spectral domain representation of the current frame as the first information to obtain the noise level information under the condition that the current frame is of a general audio type. How this is done in detail is described in WO 2012/110476 A1. Furthermore, FIG. 9 shows a diagram illustrating how an LPC filter equivalent is determinated from a MDCT power-spectrum. While the depicted scale is a Bark scale, the LPC coefficient equivalents may also be obtained from a linear scale. Especially when they are obtained from a linear scale, the calculated LPC coefficient equivalents are very similar to those calculated from the time domain representation of the same frame, for example when coded in ACELP.

[0061] In addition, the audio decoder according to FIG. 3, as illustrated by the method chart of FIG. 4, is adapted to compute a peak level p of a transfer function of an LPC filter of the current frame as a second information, thus using a linear prediction coefficient to obtain the noise level information under the condition that the current frame is of a speech type. That means, the audio decoder calculates the peak level p of the transfer function of the LPC analysis filter of the current frame f according to the formula p=|a.sub.k, wherein a.sub.k is a linear prediction coefficient with k=0 . . . 15. If the frame is a general audio frame, the LPC coefficient equivalents are obtained from the spectral domain representation of the current frame, as shown in FIG. 9 and described in WO 2012/110476 A1 and above. As seen in FIG. 4, after calculating the peak level p, a spectral minimum m.sub.f of the current frame f is calculated by dividing e.sub.rms by p. Thus, The audio decoder is adapted to compute a first information representing a spectrally unshaped excitation of the current frame, in this embodiment e.sub.rms, and a second information regarding spectral scaling of the current frame, in this embodiment peak level p, to compute a quotient of the first information and the second information to obtain the noise level information. The spectral minimum of the current frame is then enqueued in the noise level estimator, the audio decoder being adapted to enqueue the quotient obtained from the current audio frame in the noise level estimator regardless of the frame type and the noise level estimator comprising a noise level storage for two or more quotients, in this case spectral minima m.sub.f, obtained from different audio frames. More specifically, the noise level storage can store quotients from 50 frames in order to estimate the noise level. Furthermore, the noise level estimator is adapted to estimate the noise level on the basis of statistical analysis of two or more quotients of different audio frames, thus a collection of spectral minima m.sub.f. The steps for computing the quotient m.sub.f are depicted in detail in FIG. 7, illustrating the calculation steps that may be used. In the second embodiment, the noise level estimator operates based on minimum statistics as known from [3]. The noise is scaled according to the estimated noise level of the current frame based on minimum statistics and after that added to the current frame if the current frame is a speech frame. Finally, the current frame is de-emphasized (not shown in FIG. 4). Thus, this second embodiment also allows to omit side information for noise filling, allowing to reduce the amount of data to be transferred with the bitstream. Accordingly, the sound quality of the audio information may be improved by enhancing the background noise during the decoding stage without increasing the data rate. Note that since no time/frequency transforms are necessary and since the noise level estimator is only run once per frame (not on multiple sub-bands), the described noise filling exhibits very low complexity while being able to improve low-bit-rate coding of noisy speech.

[0062] FIG. 5 shows a third embodiment of an audio decoder according to the present invention. The audio decoder is adapted to provide a decoded audio information on the basis of an encoded audio information. The audio decoder is configured to use a coder based on LD-USAC in order to decode the encoded audio information. The encoded audio information comprises linear prediction coefficients (LPC), which may be individually designated as coefficients a.sub.k. The audio decoder comprises a tilt adjuster configured to adjust a tilt of a noise using linear prediction coefficients of a current frame to obtain a tilt information and a noise level estimator configured to estimate a noise level for a current frame using a linear prediction coefficient of at least one previous frame to obtain a noise level information. Furthermore, the audio decoder comprises a noise inserter configured to add the noise to the current frame in dependence on the tilt information obtained by the tilt calculator and in dependence on the noise level information provided by the noise level estimator. Thus, noise may be added to the current frame in order to improve the overall sound quality of the decoded audio information which may be impaired due to coding artifacts, especially with regards to background noise of speech information, in dependence on the tilt information obtained by the tilt calculator and in dependence on the noise level information provided by the noise level estimator. In this embodiment, a random noise generator (not shown) which is comprised by the audio decoder generates a spectrally white noise, which is then both scaled according to the noise level information and shaped using the g-derived tilt, as described earlier. FIG. 6 shows a third method for performing audio decoding according to the present invention which can be performed by an audio decoder according to FIG. 5. The bitstream is read and a frame type determinator, called frame type detector, determines whether the current frame is a speech frame (ACELP) or general audio frame (TCX/MDCT). Regardless of the frame type, the frame header is decoded and the spectrally flattened, unshaped excitation signal in perceptual domain is decoded. In case of speech frame, this excitation signal is a time-domain excitation, as described earlier. If the frame is a general audio frame, the MDCT-domain residual is decoded (spectral domain). Time domain representation and spectral domain representation are respectively used to estimate the noise level as illustrated in FIG. 7 and described earlier, using LPC coefficients also used to decode the bitstream instead of using any side information or additional LPC coefficients. The noise information of both types of frames is enqueued to adjust the tilt and noise level of the noise to be added to the current frame under the condition that the current frame is a speech frame. After adding the noise to the ACELP speech frame (Apply ACELP noise filling) the ACELP speech frame is de-emphasized by a IIR and the speech frames and the general audio frames are combined in a time signal, representing the decoded audio information. The steep high-pass effect of the de-emphasis on the spectrum of the added noise is depicted by the small inserted Figures I, II, and III in FIG. 6.

[0063] In other words, according to FIG. 6, the ACELP noise filling system described above was implemented in the LD-USAC (EVS) decoder, a low delay variant of xHE-AAC [6] which can switch between ACELP (speech) and MDCT (music/noise) coding on a per-frame basis. The insertion process according to FIG. 6 is summarized as follows: [0064] 1. The bitstream is read, and it is determined whether the current frame is an ACELP or

[0065] MDCT or DTX frame. Regardless of the frame type, the spectrally flattened excitation signal (in perceptual domain) is decoded and used to update the noise level estimate as described below in detail. Then the signal is fully reconstructed up to the de-emphasis, which is the last step.

[0066] 2. If the frame is ACELP-coded, the tilt (overall spectral shape) for the noise insertion is computed by first-order LPC analysis of the LPC filter coefficients. The tilt is derived from the gain g of the 16 LPC coefficients a.sub.k, which is given by g=[a.sub.k.Math.a.sub.k+1]/[a.sub.k.Math.a.sub.k]. [0067] 3. If the frame is ACELP-coded, the noise shaping level and tilt are employed to perform the noise addition onto the decoded frame: a random noise generator generates the spectrally white noise signal, which is then scaled and shaped using the g-derived tilt. [0068] 4. The shaped and leveled noise signal for the ACELP frame is added onto the decoded signal just before the final de-emphasis filtering step. Since the de-emphasis is a first order IIR boosting low frequencies, this allows for low-complexity, steep IIR high-pass filtering of the added noise, as in FIG. 6, avoiding audible noise artifacts at low frequencies.

[0069] The noise level estimation in step 1 is performed by computing the root mean square e.sub.rms of the excitation signal for the current frame (or in case of an MDCT-domain excitation the time domain equivalent, meaning the e.sub.rms which would be computed for that frame if it were an ACELP frame) and by then dividing it by the peak level p of the transfer function of the LPC analysis filter. This yields the level m.sub.f of the spectral minimum of frame f as in FIG. 7. m.sub.f is finally enqueued in the noise level estimator operating based on e.g. minimum statistics [3]. Note that since no time/frequency transforms are necessary and since the level estimator is only run once per frame (not on multiple sub-bands), the described CELP noise filling system exhibits very low complexity while being able to improve low-bit-rate coding of noisy speech.

[0070] Although some aspects have been described in the context of an audio decoder, it is clear that these aspects also represent a description of the corresponding method, where a block or device corresponds to a method step or a feature of a method step. Analogously, aspects described in the context of a method step also represent a description of a corresponding block or item or feature of a corresponding audio decoder. Some or all of the method steps may be executed by (or using) a hardware apparatus, like for example, a microprocessor, a programmable computer or an electronic circuit. In some embodiments, some one or more of the most important method steps may be executed by such an apparatus.

[0071] The inventive encoded audio signal can be stored on a digital storage medium or can be transmitted on a transmission medium such as a wireless transmission medium or a wired transmission medium such as the Internet.

[0072] Depending on certain implementation requirements, embodiments of the invention can be implemented in hardware or in software. The implementation can be performed using a digital storage medium, for example a floppy disk, a DVD, a Blu-Ray, a CD, a ROM, a PROM, an EPROM, an EEPROM or a FLASH memory, having electronically readable control signals stored thereon, which cooperate (or are capable of cooperating) with a programmable computer system such that the respective method is performed. Therefore, the digital storage medium may be computer readable.

[0073] Some embodiments according to the invention comprise a data carrier having electronically readable control signals, which are capable of cooperating with a programmable computer system, such that one of the methods described herein is performed.

[0074] Generally, embodiments of the present invention can be implemented as a computer program product with a program code, the program code being operative for performing one of the methods when the computer program product runs on a computer. The program code may for example be stored on a machine readable carrier.

[0075] Other embodiments comprise the computer program for performing one of the methods described herein, stored on a machine readable carrier.

[0076] In other words, an embodiment of the inventive method is, therefore, a computer program having a program code for performing one of the methods described herein, when the computer program runs on a computer.

[0077] A further embodiment of the inventive methods is, therefore, a data carrier (or a digital storage medium, or a computer-readable medium) comprising, recorded thereon, the computer program for performing one of the methods described herein. The data carrier, the digital storage medium or the recorded medium are typically tangible and/or non-transitionary.

[0078] A further embodiment of the inventive method is, therefore, a data stream or a sequence of signals representing the computer program for performing one of the methods described herein. The data stream or the sequence of signals may for example be configured to be transferred via a data communication connection, for example via the Internet.

[0079] A further embodiment comprises a processing means, for example a computer, or a programmable logic device, configured to or adapted to perform one of the methods described herein.

[0080] A further embodiment comprises a computer having installed thereon the computer program for performing one of the methods described herein.

[0081] A further embodiment according to the invention comprises an apparatus or a system configured to transfer (for example, electronically or optically) a computer program for performing one of the methods described herein to a receiver. The receiver may, for example, be a computer, a mobile device, a memory device or the like. The apparatus or system may, for example, comprise a file server for transferring the computer program to the receiver.

[0082] In some embodiments, a programmable logic device (for example a field programmable gate array) may be used to perform some or all of the functionalities of the methods described herein. In some embodiments, a field programmable gate array may cooperate with a microprocessor in order to perform one of the methods described herein. Generally, the methods are advantageously performed by any hardware apparatus.

[0083] The apparatus described herein may be implemented using a hardware apparatus, or using a computer, or using a combination of a hardware apparatus and a computer.

[0084] The methods described herein may be performed using a hardware apparatus, or using a computer, or using a combination of a hardware apparatus and a computer.

[0085] While this invention has been described in terms of several embodiments, there are alterations, permutations, and equivalents which fall within the scope of this invention. It should also be noted that there are many alternative ways of implementing the methods and compositions of the present invention. It is therefore intended that the following appended claims be interpreted as including all such alterations, permutations and equivalents as fall within the true spirit and scope of the present invention.

LIST OF CITED NON-PATENT LITERATURE

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