Acoustic signal transform coding method and decoding method having a high efficiency envelope flattening method therein

• US 5,684,920 A
• Filed: 03/13/1995
• Issued: 11/04/1997
• Est. Priority Date: 03/17/1994
• Status: Expired due to Term

• Current Assignee: Nippon Telegraph and Telephone Corporation
• Original Assignee: Nippon Telegraph and Telephone Corporation
• Inventors: Moriya, Takehiro, Miki, Satoshi, Iwakami, Naoki
• Primary Examiner(s): MacDonald, Allen R.
• Assistant Examiner(s): COLLINS, ALPHONSO
• Application Number:
  US08/402,660
• Time in Patent Office: 967 Days
• Field of Search: 395/2, 395/2.1, 395/2.12, 395/2.13, 395/2.25, 395/2.26, 395/2.28, 395/2.29, 395/2.33, 395/2.24, 395/2.39, 395/2.67, 395/2.71, 395/2.72, 395/2.78, 395/2.17
• US Class Current: 704/203
• CPC Class Codes: G10L 19/0204 using subband decomposition
• CPC Class Codes: G10L 19/0212 using orthogonal transformation
• CPC Class Codes: G10L 25/12 the extracted parameters being prediction coefficients
• CPC Class Codes: G10L 25/27 characterised by the analysis technique

Abstract

An input acoustic signal is subjected to modified discrete cosine transform processing to obtain its spectrum characteristics. Linear prediction coefficients are derived from the input acoustic signal in a linear prediction coding analysis part, and the prediction coefficients are subjected to Fourier transform in a spectrum envelope calculation part to obtain the envelope of the spectrum characteristics of the input acoustic signal. In a normalization part the spectrum characteristics are normalized by the envelope thereof to obtain residual coefficients. Another normalization part normalizes the residual coefficients by a residual-coefficients envelope predicted in a residual-coefficients envelope calculation part, thereby obtaining fine structure coefficients, which are vector-quantized in a quantization part. A de-normalization part de-normalizes the quantized fine structure coefficients. The residual-coefficients envelope calculation part uses the reproduced residual coefficients to predict the envelope of residual coefficients of the subsequent frame.

Claims

1. An acoustic signal transform coding method which transforms an input acoustic signal to frequency-domain coefficients and encodes them to produce coded output, said method comprising the steps of:

(a) obtaining residual coefficients having a flattened envelope of the frequency characteristics of said input acoustic signal on a frame-by-frame basis;

(b) predicting the envelope of said residual coefficients of the current frame on the basis of said residual coefficients of the current or previous frame to produce a predicted residual-coefficients envelope;

(c) normalizing said residual coefficients of the current frame by said predicted residual-coefficients envelope to produce fine structure coefficients; and

(d) quantizing said fine structure coefficients and outputting index information representative of said quantized fine structure coefficients as part of said coded output.

2. The coding method of claim 1, wherein said step (b) includes the steps of:

(e) de-normalizing said quantized fine structure coefficients by said predicted residual-coefficients envelope of the current frame to generate reproduced residual coefficients;

(f) processing said reproduced residual coefficients to produce their spectrum envelope; and

(g) synthesizing said predicted residual-coefficients envelope for residual coefficients of the next frame on the basis of said spectrum envelope.

3. The coding method of claim 2, wherein said step (g) includes synthesizing said predicted residual-coefficients envelope by linear combination of the spectrum envelopes of said reproduced residual coefficients of a predetermined one or more contiguous frames preceding the current frame.

4. The coding method of claim 3, wherein said step (b) includes a step (h) of controlling said linear combination of said spectrum envelopes of said previous frames so that said predicted residual-coefficients envelope, which is synthesized on the basis of the spectrum envelopes of said reproduced residual coefficients of said previous frames, approaches the envelope of said residual coefficients of the current frame as a target.

5. The coding method of claim 4, wherein optimum control of said linear combination is determined aiming at the spectrum envelope of said reproduced residual coefficients of the current frame as said target and the thus determined optimum control is applied to said linear combination in the next frame.

6. The coding method of claim 4, wherein optimum control of said linear combination is determined aiming at the spectrum envelope of said residual coefficients of the current frame as said target and the thus determined optimum control is applied to the linear combination of said predicted residual-coefficients envelope in the current control.

7. The coding method of claim 5 or 6, wherein said linear combination in said step (g) is a process of multiplying the spectrum envelopes of said reproduced residual coefficients of said previous frames by prediction coefficients, respectively, and adding the multiplied results to obtain said predicted residual-coefficients envelope, and said step (h) includes a process of determining said prediction coefficients so that said added result approaches said target.

8. The coding method of claim 7, wherein said step (h) includes a step (i) of outputting, as another part of said coded output, index information representing quantization of said prediction coefficients when said target for determining said prediction coefficients is the spectrum envelope of said residual coefficients of the current frame.

9. The coding method of claim 7, wherein said linear combination in said step (g) includes generating a first sample group and a second sample group displaced at least one sample on the frequency axis from a sample group of each of said previous frames in the positive and the negative direction, respectively, multiplying said first and second sample groups by prediction coefficients and adding all the multiplied results together with the prediction coefficients-multiplied results for said previous frames to obtain said predicted residual-coefficients envelope.

10. The coding method of claim 3, wherein said step (f) includes:

a step (j) of calculating, over the current frame and a plurality of previous frames, average values of corresponding samples of said spectrum envelopes obtained from said reproduced residual coefficients, or calculating an average value of the samples in the current frame; and

a step (k) of subtracting said average values or said average value from said spectrum envelope of the current frame and providing the subtracted results as said spectrum envelope to said step (g), and wherein said step (g) includes a step (l) of adding said average values or said average value to the result of said linear combination and calculating said predicted residual-coefficients envelope from said added result.

11. The coding method of claim 10, wherein said step (f) includes:

a step (m) of calculating the intraframe average amplitude of said subtracted result obtained in said step (k); and

a step (n) of dividing said subtracted result in said step (k) by the average amplitude of said subtracted result in said step (m) and providing the divided result as said spectrum envelope to said step (g), and wherein said step (g) includes a step (o) of multiplying the result of said linear combination by the average amplitude of said subtracted result in said step (m) and providing the multiplied result as the result of said linear combination to said step (l).

12. The coding method of claim 3, wherein said step (f) includes convoluting a window function into said spectrum envelope of said reproduced residual coefficients and said step (g) includes performing linear combination by using the convoluted result as said spectrum envelope.

13. The coding method of claim 3, wherein said step (g) includes adding a predetermined constant to the result of said linear combination to obtain said predicted residual-coefficients envelope.

14. The coding method of claim 4, wherein control of said linear combination in said step (h) includes segmenting the target frequency-domain coefficients and the spectrum envelope of said reproduced residual coefficients into pluralities of subbands, respectively, and processing them for each subband.

15. The coding method of claim 1, wherein said step (b) includes quantizing said spectrum envelope of said residual coefficients of the current frame so that said predicted residual-coefficients envelope comes as close to said spectrum envelope as possible, and outputting index information representative of the quantization as another part of said coded output.

16. The coding method of claim 15, wherein said step (b) includes linearly combining said quantized spectrum envelope of the current frame and a quantized spectrum envelope of a past frame through use of predetermined prediction coefficients, determining said quantized spectrums so that the linearly combined envelope comes as close as possible to said spectrum envelope, and obtaining said linear combined envelope at that time as said predicted residual-coefficients envelope.

17. The coding method of claim 15, wherein said step (b) includes linearly combining a quantized spectrum envelope of the current frame and said predicted residual-coefficients envelope of a past frame, determining said quantized spectrum envelope so that the linearly combined envelope comes as close to said spectrum envelope as possible, and obtaining said linearly combined value at that time as said predicted residual-coefficients envelope.

18. The coding method of claim 1, wherein said step (a) includes transforming said input acoustic signal to frequency-domain coefficients, subjecting said input acoustic signal to a linear prediction coding analysis for each frame to obtain linear prediction coefficients, transforming said linear prediction coefficients to frequency-domain coefficients to obtain the spectrum envelope of said input acoustic signal and normalizing said frequency-domain coefficients of said input acoustic signal by said spectrum envelope to obtain said residual coefficients.

19. The coding method of claim 1, wherein said step (a) includes transforming said input acoustic signal to frequency-domain coefficients, inversely transforming the spectrum envelope of said frequency-domain coefficients into a time-domain signal, subjecting said time-domain signal to a linear prediction coding analysis to obtain linear prediction coefficients, transforming said linear prediction coefficients to frequency-domain coefficients to obtain the spectrum envelope of said input acoustic signal and normalizing the frequency-domain coefficients of said input acoustic signal by said spectrum envelope to obtain said residual coefficients.

20. The coding method of claim 18 or 19, wherein a process of transforming said linear prediction coefficients to the frequency-domain coefficients includes quantizing said linear prediction coefficients to obtain quantized linear prediction coefficients, transforming said quantized linear prediction coefficients as said linear prediction coefficients to said frequency-domain coefficients and outputting index information representative of said quantized linear prediction coefficients as another part of said coded output.

21. The coding method of claim 1, wherein said step (a) includes transforming said input acoustic signal to frequency-domain coefficients, dividing said frequency-domain coefficients into a plurality of subbands, calculating scaling factors of said subbands and normalizing the frequency-domain coefficients of said input acoustic signal by said scaling factors to obtain said residual coefficients.

22. The coding method of claim 1, wherein said step (a) includes subjecting said input acoustic signal to a linear prediction coding analysis to obtain linear prediction coefficients, applying said input acoustic signal to an inverse filter controlled by said linear prediction coefficients to obtain a residual signal and transforming said residual signal to frequency-domain coefficients to obtain said residual coefficients.

23. The coding method of claim 22, wherein a process of obtaining said residual signal includes controlling said inverse filter by providing thereto, as said linear prediction coefficients, quantized linear prediction coefficients obtained by quantizing said linear prediction coefficients and outputting indexes representative of said quantized linear prediction coefficients as another part of said coded output.

24. The coding method of claim 18 or 19, wherein a process of transforming said input acoustic signal to the frequency-domain coefficients includes subjecting said input acoustic signal to lapped orthogonal transform processing on a frame-by-frame basis.

25. An acoustic signal decoding method for decoding an acoustic signal coded after being transformed to frequency-domain coefficients of a predetermined plurality of samples for each frame, said method comprising:

(a) a step wherein fine structure coefficients decoded from input first quantization index information are de-normalized by the envelope of residual coefficients predicted from information about a past frame, whereby reproduced residual coefficients in the current frame are obtained; and

(b) a step wherein an acoustic signal added with the envelope of the frequency characteristics of said coded acoustic signal is regenerated from said reproduced residual coefficients obtained in said step (a).

26. The decoding method of claim 25, wherein said step (a) includes a step (c) of synthesizing the envelope of said residual coefficients for a next frame on the basis of said reproduced residual coefficients.

27. The decoding method of claim 26, wherein said step (c) includes:

a step (d) of calculating the spectrum envelope of said reproduced residual coefficients; and

a step (e) wherein said spectrum envelope of predetermined one or more contiguous past frames preceding the current frame is multiplied by prediction coefficients to obtain the envelope of said residual coefficients of the current frame by linear combination.

28. The decoding method of claim 27, wherein said step (e) includes a step (f) of adaptively controlling said linear combination so that said residual-coefficient envelope obtained by said linear combination comes as close to the envelope of said reproduced residual coefficients in the current frame as possible.

29. The decoding method of claim 28, wherein control of said linear combination in said step (f) is effected for each of a plurality of subbands into which the spectrum envelope of said residual coefficients is divided.

30. The decoding method of claim 27, wherein said step (d) includes:

a step (g) of calculating, over the current and past plural frames, average values of corresponding samples of said spectrum envelope obtained from said reproduced residual coefficients, or calculating an average value of the samples in the current frame; and

a step (h) of subtracting said average values or average value from said spectrum envelope of the current frame and providing the subtracted result as said spectrum envelope to said step (e), and wherein said step (e) includes a step (i) of adding said average values or average value to the result of said linear combination to obtain said predicted residual coefficients.

31. The decoding method of claim 30, wherein said step (c) includes:

a step (j) of calculating an intra-frame average amplitude of said subtracted result obtained in said step (h);

a step (k) of dividing the subtracted result in said step (h) by said average amplitude and providing the divided result as said spectrum envelope to said step (e), and wherein said step (e) includes a step (l) of multiplying the result of said linear combination by the average amplitude of said subtracted result and providing the multiplied result as the result of said linear combination to said step (i).

32. The decoding method of any one of claim 27, 28, 30 or 31, wherein said step (d) includes convoluting a window function into the spectrum envelope of said reproduced residual coefficients, and said step (e) includes performing said linear combination by using the convoluted result as said spectrum envelope.

33. The decoding method of any one of claim 27, 28, 30 or 31, wherein said linear combination in said step (e) includes producing a first sample group and a second sample group displaced at least one sample on the frequency axis from a sample group of each of said past frames in the positive and the negative direction, respectively, multiplying said first and second sample groups by prediction coefficients and adding all the multiplied results together with the prediction coefficient-multiplied results for said past frames to obtain said predicted residual-coefficients envelope.

34. The decoding method of any one of claim 27, 28, 30 or 31, wherein said step (e) includes adding a predetermined constant to the result of said linear combination to obtain said residual-coefficients envelope.

35. The decoding method of claim 26, wherein said step (c) includes:

a step (e) of calculating the spectrum envelope of said reproduced residual coefficients; and

a step (e) of multiplying said spectrum envelopes of predetermined one or more past contiguous frames preceding the current frame by said prediction coefficients specified by inputted third quantization index information and adding the multiplied results to obtain the envelope of said reproduced residual coefficients of the current frame.

36. The decoding method of claim 25, wherein said reproduced residual-coefficients envelope in said step (a) is obtained by linearly combining quantized spectrum envelopes of current and past frames obtained by inverse quantization of index information sent from the coding side.

37. The decoding method of claim 25, wherein said reproduced residual-coefficients envelope in said step (a) is obtained by linearly combining a synthesized residual-coefficients envelope in a past frame and a quantized spectrum envelope of the current frame obtained by inverse quantization of index information sent from the coding side.

38. The decoding method of any one of claim 25, 26, 35, or 36, wherein said step (b) includes:

inversely quantizing inputted second quantization index information to decode envelope information of the frequency characteristics of said acoustic signal; and

reproducing said acoustic signal provided with the envelope of said frequency characteristics on the basis of the envelope information of said frequency characteristics.

39. The decoding method of claim 38, wherein said step (b) includes:

decoding linear prediction coefficients of said acoustic signal as envelope information of said frequency characteristics from said second index, obtaining the envelope of the frequency characteristics of said acoustic signal from said reproduced linear prediction coefficients, de-normalizing said reproduced residual coefficients in said step (a) by the envelope of the frequency characteristics of said acoustic signal to obtain said frequency-domain coefficients, and transforming said frequency-domain coefficients to a time-domain signal to obtain said acoustic signal.

40. The decoding method of claim 39, wherein a process of obtaining the envelope of said frequency characteristics includes subjecting said linear prediction coefficients to Fourier transform processing and obtaining the resulting spectrum amplitude as the envelope of said frequency characteristics.

41. The decoding method of claim 38, wherein said step (b) includes:

transforming said reproduced residual coefficients in said step (a) to a time-domain residual signal;

decoding linear prediction coefficients of said acoustic signal as envelope information of said frequency characteristics from inputted second quantization index information; and

reproducing said acoustic signal by subjecting said residual signal to inverse filter processing through use of said linear prediction coefficients as filter coefficients.

42. The decoding method of claim 38, wherein said step (b) includes dividing said reproduced residual coefficients in said step (a) into a plurality of subbands, decoding from an inputted quantization scaling factor indexes scaling factors corresponding to said subbands as envelope information of said frequency characteristics, de-normalizing said reproduced residual coefficients of the respective subbands by said scaling factors corresponding thereto to obtain frequency-domain coefficients added with the envelope of said frequency characteristics, and transforming said frequency-domain coefficients to a time-domain signal to reproduce said acoustic signal.

43. The decoding method of claim 39, wherein the transformation of said frequency-domain coefficients to said time-domain signal is performed by inverse lapped orthogonal transform.

44. The decoding method of claim 38, wherein said step (b) includes providing said reproduced residual coefficients with an envelope of said frequency characteristics based on the envelope information to produce frequency domain coefficients, and transforming said frequency domain coefficients into the time domain signal to be obtained as the reproduced acoustic signal.

45. The decoding method of claim 44, wherein the transformation of said frequency domain coefficients to said time domain signal is performed by inverse lapped orthogonal transform.