Method and Apparatus for Encoding and Decoding

• US 20100280823A1
• Filed: 06/22/2010
• Published: 11/04/2010
• Est. Priority Date: 03/26/2008
• Status: Active Grant

• Current Assignee: Huawei Technologies Co., Ltd. (Huawei Investment & Holding Co., Ltd.)
• Original Assignee: Huawei Technologies Co., Ltd. (Huawei Investment & Holding Co., Ltd.)
• Inventors: Shlomot, Eyal, Zhang, Libin, Dai, Jinliang
• Granted Patent: US 8,370,135 B2
• Time in Patent Office: Days
• US Class Current: 704/201
• CPC Class Codes: G10L 19/012 Comfort noise or silence coding

Abstract

An encoding method includes extracting background noise characteristic parameters within a hangover period, for a first superframe after the hangover period, performing background noise encoding based on the extracted background noise characteristic parameters, for superframes after the first superframe, performing background noise characteristic parameter extraction and DTX decision for each frame in the superframes after the first superframe, and for the superframes after the first superframe, performing background noise encoding based on extracted background noise characteristic parameters of the current superframe, background noise characteristic parameters of a plurality of superframes previous to the current superframe, and a final DTX decision. Also, a decoding method and apparatus and an encoding apparatus are disclosed. Bandwidth occupancy may be reduced substantially while the signal quality may be guaranteed.

Claims

1. An encoding method comprising:

extracting background noise characteristic parameters within a hangover period;

for a first superframe after the hangover period, performing background noise encoding based on the extracted background noise characteristic parameters within the hangover period and background noise characteristic parameters of the first superframe;

for superframes after the first superframe, performing background noise characteristic parameter extraction and Discontinuous Transmission (DTX) decision for each frame in the superframes after the first superframe; and

for the superframes after the first superframe, performing background noise encoding based on extracted background noise characteristic parameters of a current superframe, background noise characteristic parameters of a plurality of superframes previous to the current superframe, and a final DTX decision.

2. The method according to claim 1, wherein the process of extracting the background noise characteristic parameters within the hangover period comprises:

for each frame of a superframe within the hangover period, obtaining an autocorrelation coefficient of the each frame of the superframe within the hangover period.

3. The method according to claim 1, wherein the process of, for the first superframe after the hangover period, performing background noise encoding based on the extracted background noise characteristic parameters within the hangover period and the background noise characteristic parameters of the first superframe comprises:

within a first frame and a second frame of the first superframe after the hangover period, storing an autocorrelation coefficient of the corresponding first frame and second frame of the first superframe after the hangover period; and

within the second frame of the first superframe after the hangover period, extracting an LPC filter coefficient and a residual energy E_t of the first superframe based on the autocorrelation coefficients of the first frame and second frame and the extracted background noise characteristic parameters within the hangover period, and performing background noise encoding.

4. The method according to claim 3, wherein:

the process of extracting the LPC filter coefficient and a residual energy E_t comprises

calculating the average of the autocorrelation coefficients of the first superframe and four superframes which are previous to the first superframe and within the hangover period, and

calculating the LPC filter coefficient and the residual energy from the average of the autocorrelation coefficients based on a Levinson-Durbin algorithm; and

the process of performing background noise encoding within the second frame further comprises transforming the LPC filter coefficient into the LSF domain for quantization encoding; and

performing linear quantization encoding on the residual energy in the logarithm domain.

5. The method according to claim 1, wherein the process of, for superframes after the first superframe, performing background noise characteristic parameter extraction for each frame in the superframes after the first superframe comprises:

calculating the stationary average autocorrelation coefficient of the current frame based on the values of the autocorrelation coefficients of four recent consecutive frames, the stationary average autocorrelation coefficients being the average of the autocorrelation coefficients of two frames having intermediate norm values of autocorrelation coefficients in the four recent consecutive frames; and

calculating the LPC filter coefficient and the residual energy from the stationary average autocorrelation coefficient based on the Levinson-durbin algorithm.

6. The method according to claim 5, wherein after the residual energy is calculated, the method further comprises:

performing a long-term smoothing on the residual energy to obtain the energy estimate of the current frame, the smoothing algorithm being

E_LT=α

E_LT+(1−

α

)E_t,k, with 0<

α

<

1, wherein the smoothed energy estimate of the current frame is assigned as the residual energy for quantization, as follows

E_t,k=E_LT,

where k=1,2, representing the first frame and the second frame respectively.

7. The method according to claim 1, wherein the process of, for superframes after the first superframe, performing DTX decision for each frame in the superframes after the first superframe further comprises:

if the LPC filter coefficient of the current frame and the LPC filter coefficient of the previous SID superframe exceed a preset threshold or the energy estimate of the current frame is substantially different from the energy estimate of the previous SID superframe, setting a parameter change flag of the current frame to 1; and

if the LPC filter coefficient of the current frame and the LPC filter coefficient of the previous SID superframe do not exceed the preset threshold or the energy estimate of the current frame is not substantially different from the energy estimate of the previous SID superframe, setting the parameter change flag of the current frame to 0.

8. The method according to claim 1, wherein the process of performing DTX decision for each frame in the superframes after the first superframe further comprises:

if a frame of the current superframe has a DTX decision of 1, the DTX decision for the Lower-band component of the current superframe represents 1.

9. The method according to claim 8, wherein if a final DTX decision of the current superframe represents 1, the process of for superframes after the first superframe, performing background noise encoding based on the extracted background noise characteristic parameters of a current superframe, background noise characteristic parameters of a plurality of superframes previous to the current superframe, and a final DTX decision comprises:

determining a smoothing factor for the current superframe, wherein if the DTX decision of the first frame of the current superframe represents zero and the DTX decision of the second frame represents 1, the smoothing factor is 0.1;

otherwise, the smoothing factor is 0.5;

performing parameter smoothing for the first frame and second frame of the current superframe, the smoothed parameters being the characteristic parameters of the current superframe for performing background noise encoding, wherein the parameter smoothing comprises;

calculating the smoothed average R^t(j) from the stationary average autocorrelation coefficient of the first frame and the stationary average autocorrelation coefficient of the second frame, as follows;

R^t(j)=smooth_rateR^t,1(j)+(1−

smooth_rate)R^t,2(j), where smooth_rate is the smoothing factor, R^t,1(j) is the stationary average autocorrelation coefficient of the first frame, and R^t,2(j) is the stationary average autocorrelation coefficient of the second frame;

calculating an LPC filter coefficient from the smoothed average R^t(j) based on the Levinson-durbin algorithm; and

calculating the smoothed average Ē

_t from the energy estimate of the first frame and the energy estimate of the second frame, as follows;

Ē

_t=smooth_rateĒ

_t,1+(1−

smooth_rate)Ē

_t,2, where Ē

_t,1 is the energy estimate of the first frame and Ē

_t,2 is the energy estimate of the second frame.

10. The method according to claim 1, wherein the process of performing background noise encoding based on the extracted background noise characteristic parameters of the current superframe, background noise characteristic parameters of a plurality of superframes previous to the current superframe, and a final DTX decision comprises:

calculating the average of the autocorrelation coefficients of a plurality of superframes previous to the current superframe;

calculating the average LPC filter coefficient of the plurality of superframes previous to the current superframe based on the average of the autocorrelation coefficients of a plurality of superframes previous to the current superframe;

if the difference between the average LPC filter coefficient and the LPC filter coefficient of the current superframe is less than or equal to a preset value, transforming the average LPC filter coefficient to the LSF domain for quantization encoding;

if the difference between the average LPC filter coefficient and the LPC filter coefficient of the current superframe is more than the preset value, transforming the LPC filter coefficient of the current superframe to the LSF domain for quantization encoding; and

performing linear quantization encoding on an energy parameter in the logarithm domain.

11. A decoding method comprising:

obtaining Comfort Noise Generator (CNG) parameters for a first frame of a first superframe from a speech encoding frame previous to the first frame of the first superframe; and

performing background noise decoding for the first frame of the first superframe based on the CNG parameters, wherein the CNG parameters includes a target excited gain, determined by a long-term smoothed fixed codebook gain which is smoothed from the fixed codebook gain of the speech encoding frames, and an LPC filter coefficient, defined by a long-term smoothed LPC filter coefficient which is smoothed from the LPC filter coefficient of the speech encoding frames.

12. The method according to claim 11, wherein after the process of performing background noise decoding for the first frame of the first superframe, the method further comprises:

for frames other than the first frame of the first superframe, after obtaining CNG parameters from the previous SID superframe, performing background noise decoding based on the obtained CNG parameters.

13. The method according to claim 11, wherein the target excited gain is determined as follows:

the target excited gain=γ

*fixed codebook gain, 0<

γ

<

1.

14. The method according to claim 11, wherein the LPC filter coefficient is defined as follows:

the LPC filter coefficient=a long-term smoothed LPC filter coefficient which is smoothed from the LPC filter coefficient of the speech encoding frames.

15. An encoding apparatus comprising:

a first extracting unit, configured to extract background noise characteristic parameters within a hangover period;

a second encoding unit configured to, for a first superframe after the hangover period, perform background noise encoding based on the extracted background noise characteristic parameters within the hangover period and background noise characteristic parameters of the first superframe;

a second extracting unit configured to, for superframes after the first superframe, perform background noise characteristic parameter extraction for each frame in the superframes after the first superframe;

a Discontinuous Transmission (DTX) decision unit configured to, for superframes after the first superframe, perform DTX decision for each frame in the superframes after the first superframe; and

a third encoding unit configured to, for the superframes after the first superframe, perform background noise encoding based on extracted background noise characteristic parameters of a current superframe, background noise characteristic parameters of a plurality of superframes previous to the current superframe, and a final DTX decision.

16. The apparatus according to claim 15, wherein the first extracting unit further comprises:

a buffer module configured to, for each frame of a superframe within the hangover period, obtain an autocorrelation coefficient of the each frame of the superframe within the hangover period.

17. The apparatus according to claim 15, wherein the second encoding unit comprises:

an extracting module configured to, within a first frame and a second frame of the first superframe after the hangover period, store an autocorrelation coefficient of the corresponding first frame and second frame of the first superframe after the hangover period ; and

an encoding module configured to, within the second frame of the first superframe after the hangover period, extract an LPC filter coefficient and a residual energy E_t of the first superframe based on the autocorrelation coefficients of the first frame and second frame and the extracted background noise characteristic parameters within the hangover period, and perform background noise encoding.

18. The apparatus according to claim 15, wherein the second extracting unit comprises:

a first calculating module configured to, calculate the stationary average autocorrelation coefficient of the current frame based on the values of the autocorrelation coefficients of four recent consecutive frames, the stationary average of the autocorrelation coefficients being the average of the autocorrelation coefficients of two frames having intermediate norm values of autocorrelation coefficients in the four recent consecutive frames; and

a second calculating module configured to calculate the LPC filter coefficient and the residual energy from the stationary average autocorrelation coefficient based on the Levinson-Durbin algorithm.

19. The apparatus according to claim 18, wherein the second extracting unit further comprises:

a second residual energy smoothing module configured to perform a long-term smoothing on the residual energy to obtain the energy estimate of the current frame, the smoothing algorithm being;

E_LT=α

E_LT+(1−

α

)E_t,k, with 0<

α

<

1, wherein the smoothed energy estimate of the current frame is assigned as the residual energy for quantization, as follows;

E_t,k=E_LT, where k=1, 2, representing the first frame and the second frame respectively.

20. The apparatus according to claim 15, wherein the DTX decision unit comprises:

a threshold comparing module configured to, if the LPC filter coefficient of the current frame and the LPC filter coefficient of the previous SID superframe exceed a preset threshold, generate a decision command;

an energy comparing module configured to calculate the average of the residual energies of the current frame and three recent previous frames as the energy estimate of the current frame;

quantize the average of the residual energies with a quantizer in the logarithmic domain, and if the difference between the decoded logarithmic energy and the decoded logarithmic energy of the previous SID superframe exceeds a preset value, generate a decision command; and

a first decision module configured to set a parameter change flag of the current frame to 1 according to the decision command.

21. The apparatus according to claim 20, wherein the DTX decision unit further comprises:

a second decision unit configured to, if the DTX decision for a frame of the current superframe represents 1, the DTX decision for the Lower-band component of the current superframe represents 1;

wherein the third encoding unit comprises

(a) a smoothing command module configured to, if a final DTX decision of the current superframe represents 1, generate a smoothing command;

(b) a smoothing factor determining module configured to, upon receipt of the smoothing command, determine a smoothing factor for the current superframe, wherein if the DTX decision of the first frame of the current superframe represents zero and the DTX decision of the second frame of the current superframe represents 1, the smoothing factor is 0.1, otherwise, the smoothing factor is 0.5; and

(c) a parameter smoothing module configured to perform parameter smoothing for the first frame and second frame of the current superframe, and the smoothed parameters being the characteristic parameters of the current superframe for performing background noise encoding, wherein the parameter smoothing comprises

(a) calculating the smoothed average R^t(j) from the stationary average autocorrelation coefficient of the first frame and the stationary average autocorrelation coefficient of the second frame, as follows;

R^t(j)=smooth_rateR^t,1(j)+(1−

smooth_rate)R^t,2(j), where smooth_rate is the smoothing factor, R^t,1(j) is the stationary average autocorrelation coefficients of the first frame, and R^t,2 (j) is the stationary average autocorrelation coefficients of the second frame;

(b) calculating an LPC filter coefficient from the smoothed average R^t(j) based on the Levinson-Durbin algorithm; and

(c) calculating the smoothed average Ē

_t from the energy estimate of the first frame and the energy estimate of the second frame, as follows;

Ē

_t=smooth_rateĒ

_t,1+(1−

smooth_rate)Ē

_t,2, where Ē

_t,1is the energy estimate of the first frame and Ē

_t,2 is the energy estimate of the second frame.

22. The apparatus according to claim 15, wherein the third encoding unit comprises:

a third calculating module configured to calculate the average LPC filter coefficient of the plurality of superframes previous to the current superframe, based on a calculated average of the autocorrelation coefficients of a plurality of superframes previous to the current superframe;

a first encoding module configured to, if the difference between the average LPC filter coefficient and the LPC filter coefficient of the current superframe is less than or equal to a preset value, transform the average LPC filter coefficient to the LSF domain for quantization encoding;

a second encoding module configured to, if the difference between the average LPC filter coefficient and the LPC filter coefficient of the current superframe is more than the preset value, transform the LPC filter coefficient of the current superframe to the LSF domain for quantization encoding; and

a third encoding module configured to perform linear quantization encoding on an energy parameter in the logarithm domain.

23. A decoding apparatus comprising:

a Comfort Noise Generator (CNG) parameter obtaining unit, configured to obtain CNG parameters for a first frame of a first superframe from a speech encoding frame previous to the first frame of the first superframe; and

a first decoding unit configured to perform background noise decoding for the first frame of the first superframe based on the CNG parameters, wherein the CNG parameters includes a target excited gain, determined by a long-term smoothed fixed codebook gain which is smoothed from the fixed codebook gain of the speech encoding frames, and an LPC filter coefficient, defined by a long-term smoothed LPC filter coefficient which is smoothed from the LPC filter coefficient of the speech encoding frames.

24. The apparatus according to claim 23, further comprising:

a second decoding unit configured to, for frames other than the first frame of the first superframe, after obtaining CNG parameters from the previous SID superframe, perform background noise decoding based on the obtained CNG parameters.