Frequency band table design for high frequency reconstruction algorithms

Abstract

The present document relates to audio encoding and decoding. In particular, the present document relates to audio coding schemes which make use of high frequency reconstruction (HFR) methods. A system configured to determine a master scale factor band table of a highband signal (105) of an audio signal is described. The highband signal (105) is to be generated from a lowband signal (101) of the audio signal using a high frequency reconstruction (HFR) scheme. The master scale factor band table is indicative of a frequency resolution of a spectral envelope of the highband signal (105).

Claims

1. A system configured to determine a master scale factor band table of a highband signal of an audio signal, wherein the master scale factor band table is indicative of a frequency resolution of a spectral envelope of the highband signal; wherein the system is configured to: receive a set of parameters transmitted from an audio encoder along with an audio bitstream being indicative of a lowband signal of the audio signal, the set of parameters including a selection parameter and one or more index parameters; store a plurality of pre-determined scale factor band tables in a memory of the system independently from the audio encoder; wherein at least one scale factor band of the pre-determined scale factor band tables comprises a plurality of frequency bands; determine the master scale factor band table by selecting a particular one of the pre-determined scale factor band tables based on the selection parameter of the received set of parameters and by selecting some or all of the scale factor bands of the selected pre-determined scale factor band table using the one or more index parameters of the received set of parameters, the one or more index parameters representing indexes into the selected pre-determined scale factor band table; and reconstruct the highband signal from the lowband signal using the master scale factor band table.

2. The system of claim 1, wherein the master scale factor band table is determined by truncating the selected pre-determined scale factor band table using the set of parameters.

3. The system of claim 1, wherein the master scale factor band table comprises only scale factor bands from the selected pre-determined scale factor band table.

4. The system of claim 1, wherein the one or more index parameters of the set of parameters comprise a start frequency parameter indicative of a scale factor band of the master scale factor band table having the lowest frequency of the scale factor bands of the master scale factor band table; and the system is configured to remove zero, one or more scale factor bands at a lower frequency end of the selected pre-determined scale factor band table for determining the master scale factor band table.

5. The system of claim 4, wherein the start frequency parameter comprises a 3 bit value taking on values between 0 and 7.

6. The system of claim 4, wherein the system is configured to remove an even number of scale factor bands at the lower frequency end of the selected pre-determined scale factor band table; and the even number is twice the start frequency parameter.

7. The system of claim 1, wherein the one or more index parameters of the set of parameters comprise a stop frequency parameter indicative of the scale factor band of the master scale factor band table having the highest frequency of the scale factor bands of the master scale factor band table; and the system is configured to remove zero, one or more scale factor bands at an upper frequency end of the selected pre-determined scale factor band table for determining the master scale factor band table.

8. The system of claim 7, wherein the stop frequency parameter comprises a 2 bit value taking on values between 0 and 3.

9. The system of claim 7, wherein the system is configured to remove an even number of scale factor bands at the upper frequency end of the selected pre-determined scale factor band table; and the even number is twice the stop frequency parameter.

10. The system of claim 1, wherein the selection parameter is a master scale parameter indicative of one of the plurality of pre-determined scale factor band tables, which is to be used to determine the master scale factor band table.

11. The system of claim 10, wherein the plurality of pre-determined scale factor band tables comprises a low bit rate scale factor band table and a high bit rate scale factor band table; and the low bit rate scale factor band table comprises one or more scale factor bands at lower frequencies than frequencies of any of the scale factor bands of the high bit rate scale factor band table; and/or the high bit rate scale factor band table comprises one or more scale factor bands at higher frequencies than frequencies of any of the scale factor bands of the low bit rate scale factor band table.

12. The system of claim 11, wherein the master scale parameter comprises a 1 bit value taking on values between 0 and 1, to distinguish between the low bit rate scale factor band table and the high bit rate scale factor band table.

13. The system of claim 11, wherein the low bit rate scale factor band table comprises one or more scale factor bands ranging from a first low frequency band to a first high frequency band; and the high bit rate scale factor band table comprises one or more scale factor bands ranging from a second low frequency band to a second high frequency band; and the first low frequency band is at a lower frequency than the second low frequency band; and/or the second high frequency band is at a higher frequency than the first high frequency band.

14. The system of claim 13, wherein a number of scale factor bands comprised within the high bit rate scale factor band table is higher than a number of scale factor bands comprised within the low bit rate scale factor band table.

15. The system of claim 13, wherein the frequency bands correspond to frequency bands generated by a 64 channel filter bank; and wherein the frequency bands range from band index 0 to band index 63.

16. The system of claim 15, wherein the low bit rate scale factor band table comprises some or all of the following scale factor bands from frequency band 10 up to frequency band 20, each scale factor band comprising a single frequency band; scale factor bands from frequency band 20 up to frequency band 32, each scale factor band comprising two frequency bands; scale factor bands from frequency band 32 up to frequency band 38, each scale factor band comprising three frequency bands; and/or scale factor bands from frequency band 38 up to frequency band 46, each scale factor band comprising four frequency bands.

17. The system of claim 16, wherein the high bit rate scale factor band table comprises some or all of the following scale factor bands from frequency band 18 up to frequency band 24, each scale factor band comprising a single frequency band; scale factor bands from frequency band 24 up to frequency band 44, each scale factor band comprising two frequency bands; and/or scale factor bands from frequency band 44 up to frequency band 62, each scale factor band comprising three frequency bands.

18. The system of claim 1, wherein a number of frequency bands comprised within the scale factor bands of the selected pre-determined scale factor band table increases with increasing frequency.

19. The system of claim 1, wherein a number of scale factor bands comprised within the selected pre-determined scale factor band table and/or a number of scale factor bands comprised within the master scale factor band table is an even number.

20. The system of claim 1, further configured to determine a high resolution frequency band table and a low resolution frequency band table based on the master scale factor band table.

21. The system of claim 20, wherein the set of parameters comprises a cross over band parameter indicative of zero, one or more scale factor bands at a lower frequency end of the master scale factor band table, which are to be excluded from high frequency reconstruction; and the system is configured to determine the high resolution frequency band table and the low resolution frequency band table from the master scale factor band table by excluding the zero, one or more scale factor bands at the lower frequency end of the master scale factor band table in accordance to the cross over band parameter.

22. The system of claim 21, wherein the cross over band parameter comprises a 2 or 3 bit value taking on values between 0 and 3 or 7, to indicate the 0 up to 3 or 7 scale factor bands at the lower frequency end of the master scale factor band table, which are to be excluded.

23. The system of claim 21, wherein the high resolution frequency band table corresponds to the master scale factor band table without the zero, one or more scale factor bands at the lower frequency end of the master scale factor band table.

24. The system of claim 21, further configured to determine the low resolution frequency band table by decimating the high resolution frequency band table.

25. The system of claim 1, wherein the frequency bands correspond to frequency bands generated by a quadrature mirror filter bank.

26. The system of claim 1, wherein the lowband signal and the highband signal are segmented into a sequence of frames comprising a pre-determined number of samples of the audio signal; the system is configured to receive an updated set of parameters for a set of frames from the sequence of frames; the system is configured to maintain the master scale factor band table unchanged, if the parameters of the updated set of parameters, which affect the master scale factor band table, remain unchanged; and the system is configured to determine an updated master scale factor band table, if the parameters of the updated set of parameters, which affect the master scale factor band table, change.

27. The system of claim 26, wherein the system is configured to receive an updated set of parameters for each frame of the sequence of frames.

28. The system of claim 26, further configured to determine a noise band table and/or a limiter band table and/or a patching scheme for transposition from the master scale factor band table and/or from the high and low resolution frequency band tables.

29. A method for determining a master scale factor band table for a highband signal of an audio signal, wherein the master scale factor band table is indicative of a frequency resolution of a spectral envelope of the highband signal; wherein the method comprises: receiving a set of parameters transmitted from an audio encoder along with an audio bitstream being indicative of the lowband signal of the audio signal, the set of parameters including a selection parameter and one or more index parameters; storing a plurality of pre-determined scale factor band tables in a memory independently from the audio encoder; wherein at least one scale factor band of the pre-determined scale factor band tables comprises a plurality of frequency bands; determining the master scale factor band table by selecting a particular one of the pre-determined scale factor band tables based on the selection parameter of the received set of parameters and by selecting some or all of the scale factor bands of the selected pre-determined scale factor band table using the one or more index parameters of the set of parameters, the one or more index parameters representing indexes into the selected pre-determined scale factor band table; and reconstructing the highband signal from the lowband signal using the master scale factor band table.

Description

SHORT DESCRIPTION OF THE FIGURES

(1) The invention is explained below in an exemplary manner with reference to the accompanying drawings, wherein

(2) FIG. 1 shows example lowband and highband signals;

(3) FIG. 2 shows example scale factor band tables;

(4) FIGS. 3a and 3b show comparisons of example master scale factor band tables; and

(5) FIG. 4 shows an example method for generating a highband signal using a pre-determined scale factor band table.

DETAILED DESCRIPTION

(6) Audio decoders which make use of HFR (High Frequency Reconstruction) techniques typically comprise an HFR unit for generating a high frequency audio signal (referred to as a highband signal) from a low frequency audio signal (referred to as a lowband signal) and a subsequent spectral envelope adjustment unit for adjusting the spectral envelope of the high frequency audio signal.

(7) In FIG. 1 a stylistically drawn spectrum 100, 110 of the output of an HFR unit is displayed, prior to going into the envelope adjuster. In the top-panel, a copy-up method (with two patches) is used to generate the highband signal 105 from the lowband signal 101, e.g. the copy-up method used in MPEG-4 SBR (Spectral Band Replication) which is outlined in “ISO/IEC 14496-3 Information Technology—Coding of audio-visual objects—Part 3: Audio” and which is incorporated by reference. The copy-up method translates parts of the lower frequencies 101 to higher frequencies 105. In the lower panel, a harmonic transposition method (with two non-overlapping transposition orders) is used to generate the highband signal 115 from the lowband signal 111, e.g. the harmonic transposition method of MPEG-D USAC which is described in “MPEG-D USAC: ISO/IEC 23003-3—Unified Speech and Audio Coding” and which is incorporated by reference. In the subsequent envelope adjustment stage, a target spectral envelope is applied onto the high frequency components 105, 115.

(8) In addition to the spectrum 100, 110, FIG. 1 illustrates example frequency bands 130 of the spectral envelope data representing the target spectral envelope. These frequency bands 130 are referred to as scale factor bands or target intervals. Typically, a target energy value, i.e. a scale factor energy (or scale factor), is specified for each target interval, i.e. for each scale factor band. In other words, the scale factor bands define the effective frequency resolution of the target spectral envelope, as there is typically only a single target energy value per target interval. Using the scale factors or target energies specified for the scale factor bands, a subsequent envelope adjuster strives to adjust the highband signal so that the energy of the highband signal within the scale factor bands equals the energy of the received spectral envelope data, i.e. the target energy, for the respective scale factor bands.

(9) The present document is directed at an efficient scheme for determining the frequency band tables (which are indicative of the scale factor bands 130 to be used within the HFR or SBR process) at an audio decoder. Furthermore, the present document is directed at reducing the signalling overhead for communicating the frequency band tables (referred to as scale factor band tables) from an audio encoder to the corresponding audio decoder. In addition, the present document is directed at simplifying the tuning of the audio encoder.

(10) A possible approach to determining the frequency band tables (in particular the master scale factor band table) at an audio decoder is based on pre-defined algorithms that make use of parameters which have been transmitted to the audio decoder. During run-time the pre-determined algorithms are executed to calculate the frequency band tables based on the transmitted parameters. The pre-determined algorithms provide a so called “master table” (also referred to as the master scale factor band table). The calculated “master table” may then be used to derive a set of tables needed to correctly decode and apply the parametric data corresponding to the High Frequency Reconstruction algorithm (e.g. the high resolution frequency band table, the low resolution frequency band table, the noise band table and/or a limiter band table).

(11) The above mentioned scheme for determining frequency band tables is disadvantageous, as it requires the transmission of parameters which are used by the audio decoder to calculate the “master tables”. Furthermore, the execution of the pre-determined algorithms for calculating the “master tables” requires computing resources at the audio decoder and therefore increases the cost of the audio decoder.

(12) In the present document, it is proposed to make use of one or more pre-determined, static, scale factor band tables. In particular, it is proposed to define two static scale factor band tables, a first table for low bit rates and a second table for high bit rates. The other tables, including the master table, which may be needed by the audio decoder to reconstruct the highband signal 105 may then be derived from the statically pre-defined tables. The derivation of the other tables (in particular the master scale factor band table) may be done in an efficient manner by indexing the pre-defined scale factor band tables with parameters transmitted from the audio encoder to the audio decoder within the data stream (also referred to as bitstream).

(13) The first and second static scale factor band tables may be defined in Matlab notation as a first table: sfbTableLow=[(10:20)′;(22:2:32)′;(35:3:38)′;(42:4:46)′]; and a second table: sfbTableHigh=[(18:24)′;(26:2:44)′;(47:3:62)′];
providing the scale factor band divisions 210 and 200, respectively, as shown in FIG. 2 (solid lines). In the above mentioned Matlab notation, the numbers indicate individual frequency bands 220 (e.g. quadrature mirror filter bank, QMF, bands or complex-valued QMF, CQMF, bands). The first table (i.e. the low bit rate scale factor band table) starts at frequency band 10 (reference numeral 201) and goes up to frequency band 46 (reference numeral 202). The second table (i.e. the high bit rate scale factor band table) starts at frequency band 18 (reference numeral 211) and goes up to frequency band 62 (reference numeral 212). As such, the first table (for relatively low bit rates, e.g. lower than a pre-determined bit rate threshold) comprises scale factor bands 130 from frequency band 10 to 20, which comprise a single frequency band 220 each, scale factor bands 130 from frequency band 20 to 32, which comprise two frequency bands 220 each, scale factor bands from frequency band 32 to 38, which comprise three frequency bands 220 each, and scale factor bands 130 from frequency band 38 to 46, which comprise four frequency bands 220 each.

(14) In a similar manner, the second table (for relatively high bit rates, e.g. higher than the pre-determined bit rate threshold) comprises scale factor bands 130 from frequency band 18 to 24, which comprise a single frequency band 220 each, scale factor bands 130 from frequency band 24 to 44, which comprise two frequency bands 220 each, and scale factor bands 130 from frequency band 44 to 62, which comprise three frequency bands 220 each.

(15) As can be seen from FIG. 2, the low bit rate scale factor band table 200 starts at CQMF band 10 and goes to band 46, having up to 20 scale factor bands 130. The high bit rate scale factor band table 210 supports up to 22 scale factor bands 130 ranging from band 18 to band 62.

(16) In order to derive the master table which is to be used for the decoding of a current frame from the static scale factor band tables 200, 210, three parameters may be used. These parameters may be transmitted from the audio encoder to the audio decoder, in order to enable the audio decoder to derive the master table for the current frame (i.e. in order to derive the current master table). These parameters are: 1. Start frequency (startFreq) parameter: The start frequency parameter may have a length of 3 bits and may take on values between 0 and 7. The start frequency parameter may be an index into the pre-determined scale factor band tables 200, 210 starting from the lowest frequency bands 201, 211 of the respective scale factor band tables 200, 210 (i.e. frequency band 10 or 18) moving upwards in steps of two scale factor bands 130. The parameter value startFreq=1 will hence point to frequency band 20 for the high bit rate scale factor band table 210. 2. Stop frequency (stopFreq) parameter: The stop frequency parameter may have a length of 2 bits and may take on values between 0 and 4. The stop frequency parameter may be an index into the scale factor band tables 200, 210 starting from the highest frequency band (46 or 62) going downwards in steps of two scale factor bands 130. The parameter value stopFreq=2 will hence point to band 50 in the high bit rate scale factor band table 210. 3. Master scale (masterScale) parameter. The master scale parameter may have a length of 1 bit and may take on value between 0 and 1. The master scale parameter may indicate which of the two pre-determined scale factor band tables 200, 210 is currently being used. By way of example, the parameter value masterScale=0 may indicate the low bit rate scale factor band table 200 and the parameter value masterScale=1 may indicate the high bit rate scale factor band table 210.

(17) The following tables 1 and 2 list the possible start and stop frequencies bands for the low bit rate scale factor band table 200 and for the high bit rate scale factor band table 210, respectively, using a sampling frequency of 48000 Hz.

(18) TABLE-US-00001 Frequency CQMF Frequency startFreq CQMF band [Hz] stopFreq band [Hz] 0 10 3750 0 46 17250 1 12 4500 1 38 14250 2 14 5250 2 32 12000 3 16 6000 3 28 10500 4 18 6750 5 20 7500 6 24 9000 7 28 10500

Table 1

Showing Start and Stop Frequencies for the Low Bitrate Scale Factor Band Table

(19) TABLE-US-00002 Frequency CQMF Frequency startFreq CQMF band [Hz] stopFreq band [Hz] 0 18 6750 0 62 23250 1 20 7500 1 56 21000 2 22 8250 2 50 18750 3 24 9000 3 44 16500 4 28 10500 5 32 12000 6 36 13500 7 40 15000

Table 2

Showing Start and Stop Frequencies for the High Bitrate Scale Factor Band Table

(20) Using the master scale parameter, the encoder may indicate to the decoder, which one of the pre-determined scale factor band tables 200, 210 is to be used to derive the master scale factor band table. Using the start frequency parameter and the stop frequency parameter, as outlined in the Tables 1 and 2, the actual master scale factor band table may be determined. By way of example, for masterScale=0, startFreq=1 and stopFreq=2, the master scale factor band table comprises the scale factor bands from the low bit rate scale factor band table 200 ranging from frequency band 12 up to frequency band 32.

(21) The master scale factor band table may correspond to a high resolution frequency band table which is used to perform HFR for continuous segments of an audio signal. A low resolution frequency band table may be derived from the master scale factor band table by decimating the high resolution frequency band table, e.g. by a factor of 2. The low resolution frequency band table may be used for transient segments of the audio signal (in order to allow for an increased temporal resolution, at the expense of a reduced frequency resolution). It can be seen from Tables 1 and 2 that the number of scale factor bands 130 for the high resolution frequency band tables 210, 210 may be an even number. Hence, a low resolution frequency band table may be a perfect decimation of the high resolution table by a factor 2. Moreover, as seen from Tables 1 and 2, the frequency band tables always start and end on an even numbered CQMF band 220.

(22) A fourth parameter that affects the currently used frequency band tables may be the cross over band (xOverBand) parameter. The cross over band parameter may have a length of 2 or 3 bits and may take on values between 0 and 3 (7). The xOverBand parameter may be an index into the high resolution frequency band table (or into the master scale factor band table) starting at the first bin, moving upward with a step of one scale factor band 130. Hence, usage of the xOverBand parameter will effectively truncate the beginning of the high resolution frequency band table and/or the master scale factor band table. The xOverBand parameter may be used to extend the frequency range of the lowband signal 101 and/or to reduce the frequency range of the highband signal 105. Since the xOverBand parameter changes the HFR bandwidth by truncating the existing tables, and in particular without changing the transposer patching scheme, the xOverBand parameter may be used to alter the bandwidth on runtime without audible artifacts, or to allow for different HFR bandwidths in a multi-channel setup, while all channels still use the same patching scheme. For some choices of the xOverBand parameter, the first scale factor band of the high and low resolution frequency band table will be identical (as can be seen e.g. in FIG. 3b).

(23) FIGS. 3a and 3b show a comparison of master scale factor band tables which have been derived based on the pre-determined scale factor band tables 200, 210 and master scale factor band tables which have been derived using an algorithmic approach. FIG. 3a shows a situation of a relatively low bit rate of 22 kbps (mono/parametric stereo). The upper half 300 of the diagram shows the master scale factor band table which has been derived using the static low bit rate scale factor band table 200 and the lower half 310 of the diagram shows the master scale factor band table which has been derived using an algorithmic approach. The lines 301, 311 represent the borders of the scale factor bands of the respective master scale factor band tables. The lower diamonds 302, 312 represent the borders of the high resolution scale factor bands and the higher diamonds 303, 313 represent the borders of the low resolution scale factor bands. It can be seen that the master scale factor band tables which are derived using the static, pre-determined scale factor band tables 200, 210 are substantially the same as the master scale factor band tables which are derived using the algorithmic approach.

(24) FIG. 3b shows a relatively high bit rate stereo case with a bit rate of 76 kb/s. In this case, the high bit rate scale factor band table 210 has been used to determine the master scale factor band table. Again, the upper diagram 320 shows the master scale factor band table which has been derived using the static scale factor band table 210, whereas the lower diagram 330 shows the master scale factor band table which has been derived using the algorithmic approach. The lines 321, 331 represent the borders of the scale factor bands of the respective master scale factor band tables. The lower diamonds 322, 332 represent the borders of the high resolution scale factor bands and the higher diamonds 323, 333 represent the borders of the low resolution scale factor bands. Again, it can be seen that the master scale factor band tables which are derived using the static, pre-determined scale factor band tables 200, 210 are substantially the same as the master scale factor band tables which are derived using the algorithmic approach.

(25) In the example of FIG. 3b, the xOverBand parameter has been set to a value unequal to zero. In particular, the xOverBand parameter has been set to 2 for the algorithmic approach, while the xOverBand parameter has been set to 1 for the approach which has been described in the present document. As a result of using the xOverBand parameter, a number of frequency bands 324, 334, which is equal to the xOverBand parameter is excluded from the high resolution tables and the low resolution tables.

(26) The current master scale factor band table (also referred to as the current master table) may be derived by the audio decoder using the pseudo code listed in Table 3.

(27) TABLE-US-00003 TABLE 3 if( masterReset == 1 ) { If( masterScale == 1 ) { nMfb = 22 − 2 * startFreq − 2 * stopFreq; For k = 0 to nMfb masterBandTable(k) = sfbTableHigh(2 * startFreq + k); } Else { nMfb = 20 − 2 * startFreq − 2 * stopFreq; For k = 0 to nMfb masterBandTable(k) = sfbTableLow(2 * startFreq + k); } }

(28) In the pseudo code of Table 3, the parameter masterReset is set to 1 if any of the following parameters has changed from the previous frame: the masterScale parameter, the startFreq parameter and/or the stopFreq parameter. As such, the reception of a changed masterScale parameter, startFreq parameter and/or stopFreq parameter triggers the determination of a new master table at the audio decoder. A current master table is used as long as a new (updated) master table is determined (subject to a changed master scale, start frequency and/or stop frequency parameter).

(29) In the pseudo code of Table 3, masterBandTable is the derived master scale factor band table and nMfb is the number of scale factor bands in the derived master scale factor band table. From the derived master scale factor band table all other tables which are used in the HFR process, e.g. the high and low resolution frequency band tables, the noise band table and the limiter band table, may be derived according to legacy SBR methods which are specified e.g. in “ISO/IEC 14496-3 Information Technology—Coding of audio-visual objects—Part 3: Audio”, which is incorporated by reference.

(30) FIG. 4 shows a flow chart of an example method 400 for determining a master scale factor band table for a highband signal 105, 115 of an audio signal. In other words, the method 400 is directed at determining a master scale factor band table (also referred to as the master table) which is used in the context of an HFR scheme to generate the highband signal 105, 115 from a lowband signal 101, 111 of the audio signal. The master scale factor band table is indicative of a frequency resolution of a spectral envelope of the highband signal 105, 115. The method 400 comprises the step of receiving 401 a set of parameters, e.g. the start frequency parameter, the stop frequency parameter and/or the master scale parameter. Furthermore, the method 400 comprises the step of providing 402 a pre-determined scale factor band table 200, 210. In addition, the method 400 comprises the step of determining 403 the master scale factor band table by selecting some or all of the scale factor bands 130 of the pre-determined scale factor band table 200, 210, using the set of parameters.

(31) In the present document, an efficient scheme for deriving the scale factor bands used for HFR is described. The scheme employs one or more pre-determined scale factor band tables from which the master scale factor band tables for HFR (e.g. for SBR) are derived. For this purpose, a set of parameters is inserted into the bitstream which is transmitted from the audio encoder to the audio decoder, thereby enabling the audio decoder to determine the master scale factor band table. The determination of the master scale factor band table only consists in table look-up operations, thereby providing a computationally efficient scheme for determining the master scale factor band table. In addition, the set of parameters which is inserted into the bitstream can be encoded in a bit rate efficient manner.

(32) The methods and systems described in the present document may be implemented as software, firmware and/or hardware. Certain components may e.g. be implemented as software running on a digital signal processor or microprocessor. Other components may e.g. be implemented as hardware and or as application specific integrated circuits. The signals encountered in the described methods and systems may be stored on media such as random access memory or optical storage media. They may be transferred via networks, such as radio networks, satellite networks, wireless networks or wired networks, e.g. the Internet. Typical devices making use of the methods and systems described in the present document are portable electronic devices or other consumer equipment which are used to store and/or render audio signals.