Pitch-based pre-filtering and post-filtering for compression of audio signals

1. A system for enhancing the quality of an audio signal produced by an audio codec, comprising:

a pitch-based pre-filter that adaptively filters an input audio signal to produce a filtered audio signal, wherein adaptively filtering the input audio signal comprises filtering each of a plurality of segments of the input audio signal in a manner that is dependent upon an estimated pitch period associated therewith;

an audio encoder that encodes the filtered audio signal to generate a compressed audio bit stream;

an audio decoder that decodes the compressed audio bit stream to generate a decoded audio signal; and

a pitch-based post-filter that adaptively filters the decoded audio signal to produce an output audio signal, wherein adaptively filtering the decoded audio signal comprises filtering each of a plurality of segments of the decoded audio signal in a manner that is dependent upon an estimated pitch period associated therewith, and wherein the pitch-based post-filter operates to undo at least part of a signal-shaping effect of the pitch-based pre-filter.

2. The system of claim 1 , wherein the pitch-based pre-filter performs adaptive comb filtering of the input audio signal to suppress pitch harmonic peaks in the frequency domain when the input audio signal exhibits pitch periodicity; and

wherein the pitch-based post-filter performs adaptive comb filtering of the decoded audio signal to boost pitch harmonic peaks in the frequency domain when the decoded audio signal exhibits pitch periodicity.

3. The system of claim 1 , wherein the pitch-based pre-filter performs adaptive comb filtering of the input audio signal to boost spectral valleys between pitch harmonics in the frequency domain when the input audio signal exhibits pitch periodicity; and

wherein the pitch-based post-filter performs adaptive comb filtering of the decoded audio signal to attenuate spectral valleys between pitch harmonics in the frequency domain when the decoded audio signal exhibits pitch periodicity.

4. The system of claim 1 , wherein the pitch-based post-filter is an inverse filter of the pitch-based pre-filter.

5. The system of claim 1 , further comprising:

a pitch parameter estimator that processes the input audio signal to determine pitch parameters that are used to configure the pitch-based pre-filter for each segment of the input audio signal, wherein the pitch parameters include the estimated pitch period associated with each segment of the input audio signal and one or more filter coefficients associated with each segment of the input audio signal;

a pitch parameter quantizer that quantizes and encodes the pitch parameters to generate a compressed pitch parameters bit stream; and

a pitch parameter decoder that decodes the compressed pitch parameters bit stream to obtain decoded pitch parameters that are used to configure the pitch-based post-filter for each segment of the decoded audio signal.

6. The system of claim 1 , further comprising:

a second audio decoder that decodes the compressed audio bit stream to generate a second decoded audio signal;

a first pitch parameter estimator that processes the second decoded audio signal to determine first pitch parameters that are used to configure the pitch-based pre-filter for each segment of the input audio signal, wherein the first pitch parameters include the estimated pitch period associated with each segment of the input audio signal and one or more filter coefficients associated with each segment of the input audio signal; and

a second pitch parameter estimator that processes the decoded audio signal to determine second pitch parameters that are used to configure the pitch-based post-filter for each segment of the decoded audio signal, wherein the second pitch parameters include the estimated pitch period associated with each segment of the decoded audio signal and one or more filter coefficients associated with each segment of the decoded audio signal.

7. The system of claim 1 , wherein each of the pitch-based pre-filter and the pitch-based post-filter includes at least one filter tap that is defined to be proportional to a parameter that measures a correlation between adjacent pitch cycle waveforms.

8. The system of claim 1 , wherein each of the pitch-based pre-filter and the pitch-based post-filter is a single tap filter.

9. The system of claim 1 , wherein each of the pitch-based pre-filter and the pitch-based post-filter is a multi-tap filter.

10. The system of claim 1 , wherein the pitch-based pre-filter adaptively filters the input audio signal by adaptively filtering a predetermined sub-band of the input audio signal; and

wherein the pitch-based post-filter adaptively filters the decoded audio signal by adaptively filtering a predetermined sub-band of the decoded audio signal.

11. The system of claim 1 , wherein the pitch-based pre-filter comprises an all-zero Finite Impulse Response (FIR) filter H pre (z)=1−b z −p and the pitch-based post-filter comprises an all-pole filter

H

post

⁡

(

z

)

=

1

1

-

bz

-

p

.

12. The system of claim 1 , wherein the pitch-based pre-filter performs an overlap-add operation of a first filtered signal produced by the filter H pre (z)=1−b z −p when configured with pitch parameters corresponding to a current segment of the input audio signal and a second filtered signal produced by the filter H 0,pre (z)=1−b 0 z −p 0 when configured with pitch parameters corresponding to a previously-processed segment of the input audio signal to reduce discontinuities at segment boundaries of the filtered audio signal; and

wherein the pitch-based post-filter performs an overlap-add operation of a third filtered signal produced by an all-zero FIR filter b z −p in a feedback branch of the all-pole filter H post (z) when configured with pitch parameters corresponding to a current segment of the input signal and a fourth filtered signal produced by the all-zero FIR filter b 0 z −p 0 when configured with pitch parameters corresponding to a previously-processed segment of the input audio signal to reduce discontinuities at segment boundaries of the output audio signal.

13. A method for enhancing the quality of an audio signal produced by an audio codec, comprising:

filtering each of a plurality of segments of an input audio signal by a pitch-based pre-filter in a manner that is dependent upon an estimated pitch period associated therewith to produce a filtered audio signal;

encoding the filtered audio signal in an audio encoder to generate a compressed audio bit stream; and

providing the compressed audio bit stream to a system that includes an audio decoder that decodes the compressed audio bit stream to generate a decoded audio signal and a pitch-based post-filter that filters each of a plurality of segments of the decoded audio signal in a manner that is dependent upon an estimated pitch period associated therewith to undo at least part of a signal-shaping effect of the pitch-based pre-filter.

14. The method of claim 13 , wherein filtering each of the plurality of segments of the input audio signal by the pitch-based pre-filter comprises performing adaptive comb filtering to suppress pitch harmonic peaks in the frequency domain when a segment of the input audio signal exhibits pitch periodicity; and

wherein the pitch-based post-filter comprises a pitch-based post-filter that filters each of the plurality of segments of the decoded audio signal by performing adaptive comb filtering to boost pitch harmonic peaks in the frequency domain when a segment of the decoded audio signal exhibits pitch periodicity.

15. The method of claim 13 , wherein filtering each of the plurality of segments of the input audio signal by the pitch-based pre-filter comprises performing adaptive comb filtering to boost spectral valleys between pitch harmonics in the frequency domain when a segment of the input audio signal exhibits pitch periodicity; and

wherein the pitch-based post-filter comprises a pitch-based post-filter that filters each of the plurality of segments of the decoded audio signal by performing adaptive comb filtering to attenuate spectral valleys between pitch harmonics in the frequency domain when a segment of the decoded audio signal exhibits pitch periodicity.

16. A method for enhancing the quality of an audio signal produced by an audio codec, comprising:

receiving a compressed audio bit stream generated by a system that includes a pitch-based pre-filter that filters each of a plurality of segments of an input audio signal in a manner that is dependent upon an estimated pitch period associated therewith to produce a filtered audio signal and an audio encoder that encodes the filtered audio signal to generate the compressed audio bit stream;

decoding the compressed audio bit stream in an audio decoder to generate a decoded audio signal; and

filtering each of a plurality of segments of the decoded audio signal by a pitch-based post-filter in a manner that is dependent upon an estimated pitch period associated therewith to produce an output audio signal, wherein the filtering operates to undo at least part of a signal-shaping effect of the pitch-based pre-filter.

17. The method of claim 16 , wherein the pitch-based pre-filter filters comprises a pitch-based pre-filter that filters each of the plurality of segments of the input audio signal by performing adaptive comb filtering to suppress pitch harmonic peaks in the frequency domain when a segment of the input audio signal exhibits pitch periodicity; and

wherein filtering each of the plurality of segments of the decoded audio signal by a pitch-based post-filter comprises performing adaptive comb filtering to boost pitch harmonic peaks in the frequency domain when a segment of the decoded audio signal exhibits pitch periodicity.

18. The method of claim 16 , wherein the pitch-based pre-filter comprises a pitch-based pre-filter that filters each of the plurality of segments of the input audio signal by performing adaptive comb filtering to boost spectral valleys between pitch harmonics in the frequency domain when a segment of the input audio signal exhibits pitch periodicity; and

wherein filtering each of the plurality of segments of the decoded audio signal by a pitch-based post-filter comprises performing adaptive comb filtering to attenuate spectral valleys between pitch harmonics in the frequency domain when a segment of the decoded audio signal exhibits pitch periodicity.

19. A method for avoiding frame boundary discontinuities when performing pitch-based pre-filtering and pitch-based post-filtering of an audio signal, comprising:

(a) obtaining a first set of filter parameters associated with a previously-received frame of the audio signal, wherein at least one parameter in the first set of filter parameters is determined based on an estimated pitch period associated with the previously-received frame;

(b) obtaining a second set of filter parameters associated with a current frame of the audio signal, wherein at least one parameter in the second set of filter parameters is determined based on an estimated pitch period associated with the current frame; and

(c) for each of a predetermined number of samples at a beginning of the current frame, consecutively performing an operation that effectively calculates and overlap adds a first filtered audio signal sample that corresponds to the sample of the current frame and is obtained using the first set of filter parameters and a second filtered audio signal sample that corresponds to the sample of the current frame and is obtained using the second set of filter parameters, thereby obtaining a corresponding sample of a filter output signal.

20. The method of claim 19 , wherein step (c) comprises performing, for consecutive values of an index n from 1 to K:

{tilde over (s)} ( n )= d ( n )+ w o ( n ) b 0 {tilde over (s)} ( n−p 0 )+ w i ( n ) b{tilde over (s)} ( n−p );

wherein K represents the predetermined number of samples at the beginning of the current frame, {tilde over (s)}(n) represents an n-th sample of the filter output signal, {tilde over (d)}(n) represents an n-th sample of a filter input signal, b 0 represents a filter tap associated with the previously-received frame, p 0 represents the estimated pitch period associated with the previously-received frame, b represents a filter tap associated with the current frame, p represents the estimated pitch period associated with the current frame, w 0 represents an n-th coefficient of a fade-out window, and w i represents an n-th coefficient of a fade-in window.

21. A system, comprising:

an audio encoder that includes:

a band splitter that splits an input audio signal into at least a first sub-band audio signal and a second sub-band audio signal,

a pitch-based pre-filter that filters the first sub-band audio signal to produce a pre-filtered first sub-band audio signal,

a first sub-band encoder that encodes the pre-filtered first sub-band audio signal to produce an encoded first sub-band audio signal,

a second sub-band encoder that encodes the second sub-band audio signal to produce an encoded second sub-band audio signal, and

a bit multiplexer that combines at least the encoded first sub-band audio signal and the encoded second sub-band audio signal to generate a compressed audio bit stream; and

an audio decoder that includes:

a bit demultiplexer that obtains at least the encoded first sub-band audio signal and the encoded second sub-band audio signal from the compressed audio bit stream,

a first sub-band decoder that decodes the encoded first sub-band audio signal to produce a decoded first sub-band audio signal,

a second sub-band decoder that decodes the encoded second sub-band audio signal to produce a decoded second sub-band audio signal,

a pitch-based post-filter that filters the decoded first sub-band audio signal to produce a post-filtered decoded first sub-band audio signal, and

a band combiner that combines at least the post-filtered decoded first sub-band audio signal and the decoded second sub-band audio signal to produce an output audio signal.