Support of a multichannel audio extension

• US 20070165869A1
• Filed: 03/21/2003
• Published: 07/19/2007
• Est. Priority Date: 03/04/2003
• Status: Active Grant

• Current Assignee: Conversant Wireless Licensing S.à.r.l. (f/k/a Core Wireless Licensing S.a.r.l.) (Conversant Intellectual Property Management, Inc. (f/k/a MOSAID Technologies))
• Original Assignee: Nokia Corporation
• Inventors: Ojanpera, Juha
• Granted Patent: US 7,787,632 B2
• Time in Patent Office: Days
• US Class Current: 381/23
• CPC Class Codes: G10L 19/008 Multichannel audio signal coding or decoding using interchannel correlation to reduce redundancy, e.g. joint-stereo, intensity-coding or matrixing
• CPC Class Codes: H04S 1/007 in which the audio signals are in digital form data reduction aspects thereof based on psychoacoustics G10L19/02
• CPC Class Codes: H04S 3/00 Systems employing more than two channels, e.g. quadraphonic H04S5/00, H04S7/00 take precedence

Abstract

The invention relates to methods and units supporting a multichannel audio extension. In order to allow an efficient extension requiring a low computational complexity, it is proposed that at an encoding end, at least state information is provided as side information for a provided mono audio signal (M) generated out of a multichannel audio signal. The state information indicates for each of a plurality of frequency bands how a predetermined or equally provided gain value is to be applied in the frequency domain to the mono audio signal (M) for obtaining first and a second channel signals (L,R) of a reconstructed multichannel audio signal.

Claims

1. A method for supporting a multichannel audio extension at an encoding end of a multichannel audio coding system, said method comprising:

transforming a first channel signal (L) of a multichannel audio signal into the frequency domain, resulting in a spectral first channel signal (L_MDCT);

transforming a second channel signal (R) of said multichannel audio signal into the frequency domain, resulting in a spectral second channel signal (R_MDCT); and

determining for each of a plurality of adjacent frequency bands whether said spectral first channel signal (L_MDCT), said spectral second channel signal (R_MDCT) or none of said spectral channel signals (L_MDCT,R_MDCT) is dominant in the respective frequency band and providing a corresponding state information for each of said frequency bands.

2. The method according to claim 1, comprising in addition

combining said first channel signal (L) and said second channel signal (R) to a mono audio signal (M) and encoding said mono signal (M) to a mono signal bitstream;

and

multiplexing at least said mono signal bitstream and said provided state information into a single bitstream.

3. The method according to claim 1, wherein said first channel signal (L) and said second channel signal (R) are arranged in a sequence of frames, and wherein said state information is provided for each frame of said first channel signal (L) and said second channel signal (R).

4. The method according to claim 1, further comprising in case it was determined that one of said spectral first channel signal (L_MDCT) and said spectral second channel signal (R_MDCT) is dominant in at least one of said frequency bands calculating and providing at least one gain value representative of the degree of said dominance.

5. The method according to claim 4, comprising

combining said first channel signal (L) and said second channel signal (R) to a mono audio signal (M) and encoding said mono signal (M) to a mono signal bitstream;

and

multiplexing said mono signal bitstream, said provided state information and said provided at least one gain value into a single bitstream.

6. The method according to claim 4, wherein said first channel signal (L) and said second channel signal (R) are arranged in a sequence of frames, and wherein said at least one gain is provided for each frame of said first channel signal (L) and said second channel signal (R).

7. The method according to claim 4, wherein said at least one gain value comprises a dedicated gain value for each of said frequency bands, each dedicated gain value being representative of the degree of the determined dominance of the respective dominant one of said spectral first channel signal (L_MDCT) and said spectral second channel signal (R_MDCT) in the respective frequency band.

8. The method according to claim 7, wherein channel weights are calculated for said spectral first channel signal (L_MDCT) and for said spectral second channel signal (R_MDCT) separately for each of said frequency bands based on the levels of spectral samples in said spectral channel signals (L_MDCT,R_MDCT), and wherein said dedicated gain value for a particular frequency band is determined to correspond to the ratio between the higher weight calculated for one of said spectral channel signals (L_MDCT,R_MDCT) for said particular frequency band and the lower weight calculated for the respective other one of said spectral channel signals (R_MDCT,L_MDCT) for said particular frequency band.

9. The method according to claim 4, wherein said at least one gain value comprises a common gain value representing an average degree of a dominance of said spectral first channel signal (L_MDCT) and said spectral second channel signal (R_MDCT) in all of said frequency bands.

10. The method according to claim 9, wherein channel weights are calculated for said spectral first channel signal (L_MDCT) and for said spectral second channel signal (R_MDCT) separately for each of said frequency bands based on the levels of spectral samples in said spectral channel signals (L_MDCT,R_MDCT), wherein a preliminary dedicated gain value for each frequency band is determined to correspond to the ratio between the higher weight calculated for one of said spectral channel signals (L_MDCT,R_MDCT) for a respective frequency band and the lower weight calculated for the respective other one of said spectral channel signals (R_MDCT,L_MDCT) for said respective frequency band, and wherein said common gain value is determined to be the average of said preliminary dedicated gain values.

11. The method according to claim 4, wherein the dynamic range of said at least one gain value is limited to a predetermined value at least for the lower ones of said frequency bands.

12. The method according to claim 1, wherein said state information is coded according to one of several coding schemes, the coding scheme being selected at least partly depending on which one of said spectral first channel signal (L_MDCT) and said spectral second channel signal (R_MDCT) is more frequently dominant in all of said frequency bands.

13. The method according to claim 1, wherein channel weights are calculated for said spectral first channel signal (L_MDCT) and for said spectral second channel signal (R_MDCT) separately for each of said frequency bands based on the levels of spectral samples in said spectral channel signals (L_MDCT,R_MDCT), and wherein the presence of a dominance in a particular one of said frequency bands is assumed in case the ratio between the higher channel weight resulting for said frequency band and the lower channel weight resulting for said frequency band reaches or exceeds a predetermined threshold value.

14. The method according to claim 1, further comprising

generating a reconstructed spectral first channel signal ({tilde over (L)}_f) and a reconstructed spectral second channel signal ({tilde over (R)}_f) based on said state information and on a mono channel version of said first channel signal (L) and said second channel signal (R);

and

generating and providing for those frequency bands, for which said state information indicates that one of said channel signals (L,R) is dominant, an enhancement information which reflects on a sample basis the difference between said reconstructed spectral first and second channel signals ({tilde over (L)}_f, {tilde over (R)}_f) on the one hand and said original spectral first and second channel signals on the other hand.

15. The method according to claim 14, wherein generating said enhancement information comprises quantizing said difference on a frequency band basis sample-by-sample to a predetermined range by adjusting a quantization gain for the respective frequency band, said quantizing resulting in quantized spectral enhancement samples, wherein said quantization gain employed for a respective frequency band are provided as part of said enhancement information.

16. The method according to claim 15, wherein said quantized spectral enhancement samples are provided for said enhancement information only for those frequency bands for which quantized spectral enhancement samples having non-zero values are available and which frequency bands require a quantization gain exceeding a specific threshold, an identification of those frequency bands for which said quantized spectral enhancement samples are provided for said enhancement information being provided as part of said enhancement information.

17. The method according to claim 15, wherein generating said enhancement information further comprises assigning said quantized spectral enhancement samples in groups of a predetermined number of samples to a respective codebook index, said codebook indices being provided as part of said enhancement information.

18. The method according to claim 17, wherein a respective codebook index is assigned only to those groups of quantized spectral enhancement samples, which comprise at least one quantized spectral enhancement sample having a value unequal to zero.

19. The method according to claim 14, further comprising providing an information on a bitrate employed for providing at least said state information and said enhancement information, said information on said bitrate being provided as part of said enhancement information.

20. The method according to claim 1, wherein said first channel signal (L) is a left channel signal of a stereo audio signal and wherein said second channel signal (R) is a right channel signal of said stereo audio signal.

37. A multichannel audio encoder (20) comprising means (22-26;

30-38) for realizing the steps of the method of claim 1.

38. A multichannel extension encoder (26) for a multichannel audio encoder (20), said multichannel extension encoder (26) comprising means (30-38) for realizing the steps of the method of claim 1.

41. A multichannel audio coding system comprising an encoder (20) with means (22-26;

30-38) for realizing the steps of the method of claim 1, and a decoder (21) with means (27-29;

40-46) for realizing the steps of the method of claim 21.

21. A method for supporting a multichannel audio extension at a decoding end of a multichannel audio coding system, said method comprising:

transforming a received mono audio signal (M) into the frequency domain, resulting in a spectral mono audio signal; and

generating a spectral first channel signal (L_MDCT, {tilde over (L)}_f) and a spectral second channel signal (R_MDCT, {tilde over (R)}_f) out of said spectral mono audio signal by weighting said spectral mono audio signal separately in each of a plurality of adjacent frequency bands for each of said spectral first channel signal (L_MDCT, {tilde over (L)}_f) and said spectral second channel signal (R_MDCT, {tilde over (R)}_f) based on at least one gain value and in accordance with a received state information, said state information indicating for each of said frequency bands whether said spectral first channel signal (L_MDCT, {tilde over (L)}_f), said spectral second channel signal (R_MDCT, {tilde over (R)}_f) or none of said spectral channel signals (L_MDCT, {tilde over (L)}_f,R_MDCT, {tilde over (R)}_f) is to be dominant within the respective frequency band.

22. The method according to claim 21, comprising

generating said spectral first channel signal (L_MDCT) within each of said frequency bands by multiplying one of said at least one gain values valid for a respective frequency band with samples of said spectral mono audio signal within said respective frequency band in case said state information indicates for said respective frequency band a dominance of said first channel signal (L_MDCT), by multiplying the reciprocal value of said gain value with samples of said spectral mono audio signal within said respective frequency band in case said state information indicates for said respective frequency band a dominance of said second channel signal (R_MDCT), and by taking over said spectral mono audio signal within said respective frequency band otherwise;

and

generating said spectral second channel signal (R_MDCT) within each of said frequency bands by multiplying one of said at least one gain values valid for a respective frequency band with samples of said spectral mono audio signal within said respective frequency band in case said state information indicates for said respective frequency band a dominance of said second channel signal (R_MDCT), by multiplying the weighted or not-weighted reciprocal value of said gain value with samples of said spectral mono audio signal within said respective frequency band in case said state information indicates for said respective frequency band a dominance of said first channel signal (L_MDCT), and by taking over said spectral mono audio signal within said respective frequency band otherwise.

23. The method according to claim 21, comprising as a preceding step demultiplexing a received bitstream at least into a mono signal bitstream and a state information bitstream, decoding said mono signal bitstream into said mono audio signal (M) and decoding said state information bitstream into said state information.

24. The method according to claim 23, wherein said received bitstream is demultiplexed into a mono signal bitstream, a state information bitstream and a gain bitstream, said method further comprising decoding said gain bitstream into said at least one gain value.

25. The method according to claim 21, wherein said mono audio signal (M) is delayed before being transformed into the time domain, in case said mono audio signal (M) is not time-aligned with an original multichannel audio signal which is to be reconstructed.

26. The method according to claim 21, wherein said at least one gain value comprises a dedicated gain value for each of said plurality of frequency bands.

27. The method according to claim 26, wherein said mono audio signal (M) is arranged in frames, wherein said gain values are smoothed at the start of each frame by averaging the gain value valid for the respective frequency band and the gain value valid for the respective next lower frequency band, and wherein said gain values are smoothed at the end of each frame by averaging the gain value valid for the respective frequency band and the gain value valid for the respective next higher frequency band.

28. The method according to claim 21, wherein for obtaining said state information, a received state information bitstream is decoded, which state information bitstream comprises at least partly in addition to said state information a coding scheme information, said coding scheme information indicating a coding scheme which has been employed for encoding said state information, said state information being decoded based on said coding scheme information.

29. Method The method according to claim 21, further comprising transforming said spectral first and second channel signals (L_MDCT,R_MDCT) into the time domain, resulting in a first channel signal (L) and a second channel signal (R) of a reconstructed multichannel audio signal.

30. The method according to claim 21, further comprising

receiving enhancement information which reflects at least for some spectral sample of those frequency bands, for which said state information indicates that one of said channel signals (L,R) is dominant, on a sample basis the difference between said generated spectral first and second channel signals ({tilde over (L)}_f, {tilde over (R)}_f) on the one hand and original spectral first and second channel signals on the other hand;

generating enhanced spectral first and second channel signals by taking into account on a sample-by-sample basis said difference reflected by said enhancement information; and

transforming said enhanced spectral first and second channel signals into the time domain, resulting in a first channel signal ({tilde over (L)}_new)and a second channel signal ({tilde over (R)}_new)of a reconstructed multichannel audio signal.

31. The method according to claim 30, wherein said difference is obtained by dequantizing quantized spectral enhancement samples obtained from said received enhancement information, said dequantizing employing a dedicated quantization gain for each frequency band for which quantized spectral enhancement samples are available, wherein said quantization gains are indicated in said enhancement information.

32. The method according to claim 31, wherein said received enhancement information identifies in addition those frequency bands among all frequency bands for which said state information indicates that one of said channel signals (L,R) is dominant, for which frequency bands quantized spectral enhancement samples are available, and wherein said identification of frequency bands is taken into account in generating said enhanced spectral first and second channel signals.

33. The method according to claim 31, wherein said quantized spectral enhancement samples are obtained from said received enhancement information by an inverse codebook mapping of codebook indices comprised in said received enhancement information to values of a respective group of a predetermined number of quantized spectral enhancement samples.

34. The method according to claim 33, wherein said received enhancement information comprises only codebook indices for selected groups of samples, wherein said enhancement information further comprises an identification of said groups for which codebook indices are comprised, and wherein said identification of groups is taken into account in generating said enhanced spectral first and second channel signals.

35. The method according to claim 30, wherein said enhancement information further comprises an indication of a bitrate with which at least said state information and said enhancement information are provided, which bitrate indication is employed for determining the amount of received enhancement information.

36. The method according to claim 21, wherein said first channel signal (L) is a left channel signal of a stereo audio signal and wherein said second channel signal (R) is a right channel signal of said stereo audio signal.

39. A multichannel audio decoder (21) comprising means (27-29;

40-46) for realizing the steps of the method of claim 21.

40. A multichannel extension decoder (29) for a multichannel audio decoder (20), said multichannel extension decoder (29) comprising means (40-46) for realizing the steps of the method of claim 21.