Apparatus and method for coding residual signals of audio signals into a frequency domain and apparatus and method for decoding the same

• US 7,599,833 B2
• Filed: 05/26/2006
• Issued: 10/06/2009
• Est. Priority Date: 05/30/2005
• Status: Active Grant

• Current Assignee: Electronics and Telecommunications Research Institute
• Original Assignee: Electronics and Telecommunications Research Institute
• Inventors: Kim, Hyun-Woo, Sung, Jong-Mo, Kim, Do-Young, Lee, Mi-Suk
• Primary Examiner(s): Opsasnick; Michael N
• Application Number:
  US11/441,955
• Publication Number:
  US 20060277040A1
• Time in Patent Office: 1,229 Days
• Field of Search: 704/219, 704221-223
• US Class Current: 704/219
• CPC Class Codes: G10L 19/0212 using orthogonal transformation
• CPC Class Codes: G10L 19/035 Scalar quantisation
• CPC Class Codes: G10L 19/06 Determination or coding of the spectral characteristics, e.g. of the short-term prediction coefficients
• CPC Class Codes: G10L 19/24 Variable rate codecs, e.g. for generating different qualities using a scalable representation such as hierarchical encoding or layered encoding
• CPC Class Codes: G10L 21/038 using band spreading techniques

Abstract

Provided is a residual signal coding/decoding apparatus and method. The residual signal coding apparatus includes a transformer, an LPC coefficient extractor, an LPC coefficient quantizer, an LP analysis filter, a band splitter, a pulse searcher, and a pulse quantizer. The transformer transforms time-domain residual signals into a frequency domain to output transform coefficients. The LPC coefficient extractor extracts LPC coefficients from the transform coefficients. The LPC coefficient quantizer quantizes the LPC coefficients to output quantized LPC coefficients and corresponding indices. The LP analysis filter performs an LP analysis on the transform coefficients to output LP residual transform coefficients. The band splitter splits the LP residual transform coefficients into bands to output the LP residual transform coefficients. The pulse searcher searches the LP residual transform coefficients for the respective bands to select optimal pulses and output parameters of the optimal pulses. The pulse quantizer quantizes the parameters of the optimal pulses.

Claims

1. A residual signal coding apparatus, comprising:

a receiver for inputting audio signals and outputting time-domain residual signals of the inputted audio signals;

a transformer for transforming the time-domain residual signals into a frequency domain to output transform coefficients;

a linear predictive coding (LPC) coefficient extractor for extracting LPC coefficients from the transform coefficients;

an LPC coefficient quantizer for quantizing the LPC coefficients to output quantized LPC coefficients and corresponding indices;

a linear prediction (LP) analysis filter including a filter made of the quantized LPC coefficients and performing an LP analysis on the transform coefficients to output LP residual transform coefficients;

a band splitter for splitting the LP residual transform coefficients into a predetermined number of bands to output the LP residual transform coefficients on a per-band basis;

a pulse searcher for searching the LP residual transform coefficients for the respective bands to select an optimal pulse and output parameters of the optimal pulse; and

a pulse quantizer for quantizing the parameters of the optimal pulse,

wherein the residual signal coding apparatus outputs quantized LPC coefficients and corresponding indices, and quantized pulse parameters of the inputted audio signals.

2. The residual signal coding apparatus as recited in claim 1, wherein the transformer outputs the transform coefficients by performing Modified Discrete Cosine Transform (MDCT) on the time-domain residual signals.

3. The residual signal coding apparatus as recited in claim 1, wherein the transformer outputs MDCT coefficients by performing the MDCT on the time-domain residual signals based on an equation expressed as:

$X<br><br>\left(k\right)=\underset{n=1}{\stackrel{N-1}{\sum <br><br>}}<br><br>x<br><br>\left(n\right)<br><br>h<br><br>\left(n\right)<br><br>\cos <br><br>\left\{\frac{2<br><br>\mathrm{\pi <br><br>}}{N}<br><br>\left(k+\frac{1}{2}\right)<br><br>\left(n+\frac{N}{4}+\frac{1}{2}\right)\right\}$ $k=0,1,\mathrm{\dots <br><br>}<br><br>,\frac{N}{2}-1,n=0,1,\mathrm{\dots <br><br>}<br><br>,N-1$

where X(k) represents the MDCT coefficients;

x(n) represents the time-domain residual signals;

h(n) represents a window function;

n represents time-domain sample indices; and

N represents the size of an MDCT block.

4. The residual signal coding apparatus as recited in claim 1, wherein the LPC coefficient quantizer calculates the quantized LPC coefficients and the corresponding indices based on a vector quantization (VQ) scheme or a predictive split vector quantization (PSVQ) scheme.

5. The residual signal coding apparatus as recited in claim 1, wherein the LP analysis filter outputs the LP residual transform coefficients based on an equation expressed as:

$R<br><br>\left(k\right)=X<br><br>\left(k\right)-\underset{i=1}{\stackrel{p}{\sum <br><br>}}<br><br>a_1^{\mathrm{\prime <br><br>}}<br><br>X<br><br>\left(k-i\right)$

where R(k) represents the LP residual transform coefficients; and

a′

_i represents the quantized LPC coefficients.

6. The residual signal coding apparatus as recited in claim 1, wherein the pulse quantizer comprises:

a magnitude quantizer for quantizing pulse magnitude information out of the parameters of the optimal pulse with a predetermined number of bits using a predetermined codebook;

a sign quantizer for quantizing pulse sign information out of the parameters of the optimal pulse with a predetermined number of bits using a track structure of the pulse searcher; and

a position quantizer for quantizing pulse position information out of the parameters of the optimal pulse with a predetermined number of bits using the track structure of the pulse searcher.

7. The residual signal coding apparatus as recited in claim 1, wherein the LPC coefficient extractor extracts and outputs the LPC coefficients minimizing a function value of an equation expressed as:

$E=\underset{k=0}{\stackrel{N-1}{\sum <br><br>}}<br><br>{\left\{X<br><br>\left(k\right)-\underset{i=1}{\stackrel{p}{\sum <br><br>}}<br><br>a_i<br><br>X<br><br>\left(k-i\right)\right\}}^2$

where E is a function representing a squared prediction error between a current transform coefficient and predicted coefficient from the past p number of transform coefficients;

a_i represents the LPC coefficients; and

p represents an LP order.

8. The residual signal coding apparatus as recited in claim 7, wherein the LPC coefficient extractor calculates the LPC coefficients based on a Levinson-Durbin algorithm.

9. The residual signal coding apparatus as recited in claim 1, wherein the pulse searcher divides the LP residual transform coefficients for the respective bands into a predetermined number of tracks and searches the LP residual transform coefficients on a per-track basis to select a predetermined number of optimal pulses.

10. The residual signal coding apparatus as recited in claim 9, wherein the pulse searcher performs:

a first step of initializing a predetermined minimum error value;

a second step of selecting one of per-track pulse combinations depending on the number of pulses to be searched in each track;

a third step of generating per-band pulse combinations by setting a pulse value to a given value only at the selected per-band pulse combination but to 0 at the remaining positions;

a fourth step of outputting per-band transform coefficients that is LP-combined based on the per-band pulse combinations;

a fifth step of calculating an error value that is a difference between the per-band transform coefficients outputted in the fourth step and the original transform coefficients outputted from the transformer;

a sixth step of selecting the pulse in the per-track pulse combinations constituting the per-band pulse combination as the optimal pulse, when the calculated error value is smaller than the minimum error value stored in the first step; and

a seventh step of repeating the second to sixth steps with respect to the remaining per-track pulse combinations.

11. The residual signal coding apparatus as recited in claim 9, wherein the pulse searcher performs:

a first step of selecting one from a predetermined number of the tracks;

a second step of obtaining magnitude information on all pulses of the selected track;

a third step of selecting the optimal pulses in a descending order of the magnitudes of the obtained magnitude information according to the number of pulses to be searched from the selected track; and

a fourth step of repeating the first to third steps with respect to the remaining tracks.

12. The residual signal coding apparatus as recited in claim 11, wherein the number of pulses to be searched from each track is 1.

13. A residual signal coding method, comprising the steps of:

a) receiving an audio signal and transforming time-domain residual signals of the received audio signal into a frequency domain to output transform coefficients;

b) extracting linear predictive coding (LPC) coefficients from the transform coefficients;

c) quantizing the LPC coefficients to output quantized LPC coefficients and corresponding indices;

d) performing, using a filter made of the quantized LPC coefficients, a linear prediction (LP) analysis on the transform coefficients to output LP residual transform coefficients;

e) splitting the LP residual transform coefficients into a predetermined number of bands to output the LP residual transform coefficients on a per-band basis;

f) searching the LP residual transform coefficients for the respective bands to select an optimal pulse and output parameters of the optimal pulse; and

g) quantizing the parameters of the optimal pulse.

14. The residual signal coding method as recited in claim 13, wherein the quantized LPC coefficients and the corresponding indices are calculated in the step c) based on a vector quantization (VQ) scheme or a predictive split vector quantization (PSVQ) scheme.

15. The residual signal coding method as recited in claim 13, wherein the LP residual transform coefficients are outputted in the step d) based on an equation expressed as:

$R<br><br>\left(k\right)=X<br><br>\left(k\right)-\underset{i=1}{\stackrel{p}{\sum <br><br>}}<br><br>a_1^{\mathrm{\prime <br><br>}}<br><br>X<br><br>\left(k-i\right)$

where R(k) represents the LP residual transform coefficients, and a, represents the quantized LPC coefficients.

16. The residual signal coding method as recited in claim 13, wherein the transform coefficients are outputted in the step a) by performing Modified Discrete Cosine Transform (MDCT) on the time-domain residual signals.

17. The residual signal coding method as recited in claim 16, wherein MDCT coefficients are outputted in the step a) by performing the MDCT on the time-domain residual signals according to the following equation

$X$

( k ) = ∑

N - 1 n = 0 ⁢

x ⁡

( n ) ⁢

h ⁡

( n ) ⁢

cos ⁢

{ 2 ⁢

π

N ⁢

( k + 1 2 ) ⁢

( n + N 4 + 1 2 ) } $k=0,1,\mathrm{\dots <br><br>}<br><br>,\frac{N}{2}-1,n=0,1,\mathrm{\dots <br><br>}<br><br>,N-1$

where X(k) represents the MDCT coefficients;

x(n) represents the time-domain residual signals;

h(n) represents a window function;

n represents time-domain sample indices; and

N represents the size of an MDCT block.

18. The residual signal coding method as recited in claim 13, wherein the LPC coefficients minimizing a function value of an equation expressed as:

$E=\underset{k=0}{\stackrel{N-1}{\sum <br><br>}}<br><br>{\left\{X<br><br>\left(k\right)-\underset{i=1}{\stackrel{p}{\sum <br><br>}}<br><br>a_i<br><br>X<br><br>\left(k-i\right)\right\}}^2$

is outputted in the step b),

where E is a function representing a squared prediction error between a current transform coefficient and predicted coefficient from the past p number of previous transform coefficients, a_i represents the LPC coefficients, and p represents an LP degree.

19. The residual signal coding method as recited in claim 18, wherein the LPC coefficients are calculated in the Step b) base don a Levinson-Durbin algorithm.

20. The residual signal coding method as recited in claim 13, wherein the LP residual transform coefficients for The respective bands are split into a predetermined number of tracks and the LP residual transform coefficients of each track are searched to select a predetermined number of optimal pulses in the step f).

21. The residual signal coding method as recited in claim 20, wherein the step f) includes the steps of:

f5) initializing a predetermined minimum error value;

f6) selecting one of per-track pulse combinations depending on the number of pulses to be searched in each track;

f7) generating per-band pulse combinations by setting a pulse value to a given value only at the selected per-band pulse combination but to 0 at the remaining positions;

f8) outputting per-band transform coefficients that are LP-combined based on the per-band pulse combinations;

f9) calculating an error value that is a difference between the per-band transform coefficients outputted in the fourth step and the original transform coefficients outputted from the transformer;

f10) selecting the pulse in the per-track pulse combinations constituting the per-band pulse combination as the optimal pulse, when the calculated error value is smaller than the minimum error value stored in the first step; and

f11) repeating the second to sixth steps with respect to the remaining per-track pulse combinations.

22. The residual signal coding method as recited in claim 20, wherein the step f) includes the steps of:

f1) selecting one from a predetermined number of the tracks;

f2) obtaining magnitude information on all pulses of the selected track;

f3) selecting The optimal pulses in descending order of the magnitudes of the obtained magnitude information according to the number of pulses to be searched from the selected track; and

f4) repeating the first to third steps with respect to the remaining tracks.

23. The residual signal coding method as recited in claim 22, wherein the number of pulses to be searched from each track is 1.

24. A residual signal decoding apparatus comprising:

a linear predictive coding (LPC) de-quantizer receiving quantized LPC coefficients of an audio signal and de-quantizing indices of the received quantized LPC coefficients to output restored LPC coefficients;

a pulse de-quantizer receiving quantized pulse parameters of the audio signal and do-quantizing the received quantized Pulse parameters to output restored pulse parameters;

a pulse generator for generating pulses from the restored pulse parameters to output restored linear prediction (LP) residual transform coefficients for respective bands;

a band combiner for concatenating the restored LP residual transform coefficients for the respective bands with respect to all the bands to output restored LPC residual transform coefficients;

an LP synthesis filter including a filter made of the restored LPC coefficients and performing an LP synthesis on the restored LP residual transform coefficients to output restored transform coefficients; and

an inverse-transformer for inversely transforming the restored frequency-domain transform coefficients into a time domain to decode residual signals,

wherein the decoded residual signals are inputted to an audio signal decoder to output decoded audio signals.

25. The residual signal decoding apparatus as recited in claim 24, wherein the pulse de-quantizer includes:

a magnitude de-quantizer for de-quantizing magnitude information with a predetermined number of bits among quantized pulse parameters to restore a pulse magnitude;

a sign de-quantizer for de-quantizing sign information with a predetermined number of bits among the quantized pulse parameters to restore a pulse sign; and

a position de-quantizer for de-quantizing position information with a predetermined number of bits among the quantized pulse parameters to restore a pulse position.

26. A residual signal decoding method, comprising the steps of:

a) receiving quantized linear predictive coding (LPC) coefficients of an audio signal and de-quantizing the indices of the quantized linear predictive coding (LPC) coefficients to output restored LPC coefficients;

b) receiving quantized pulse parameters of the audio signal and de-quantizing the quantized pulse parameters to output restored pulse parameters;

c) generating pulses from the restored pulse parameters to output restored linear prediction (LP) residual transform coefficients for respective bands;

d) adding the restored LP residual transform coefficients for the respective bands with respect to all the bands to output restored LPC residual transform coefficients;

e) performing, using a filter made of the restored LPC coefficients, an LP synthesis on the restored LP residual transform coefficients to output restored transform coefficients; and

f) inversely transforming the restored frequency-domain transform coefficients into a time domain to decode residual signals,

g) providing the decoded residual signals to an audio signal decoder and outputting a decoded audio signal.