Method and apparatus to eliminate discontinuities in adaptively filtered signals

1. A method of filtering an audio signal, the audio signal including successive signal frames, comprising:

(a) filtering a beginning portion of a current signal frame using a past set of filter coefficients, thereby producing a first filtered frame portion;

(b) filtering the beginning portion of the current signal frame using a current set of filter coefficients, thereby producing a second filtered frame portion; and

(c) modifying the second filtered frame portion with the first filtered frame portion so as to smooth a possible filtered signal discontinuity between the second filtered frame portion and a past filtered frame produced using the past filter coefficients.

2. The method of claim 1 , wherein step (c) comprises performing an overlap-add operation over the second filtered frame portion and the first filtered frame portion.

3. The method of claim 1 , wherein step (c) comprises:

(d)(i) weighting the first filtered frame portion with a first weighting function to produce a first weighted filtered frame portion;

(d)(ii) weighting the second filtered frame portion with a second weighting function to produce a second weighted filtered frame portion;

(d)(iii) combining the first and second weighted filtered frame portions.

4. The method of claim 3 , wherein step (d)(iii) comprises:

adding together the first and second weighted filtered frame portions.

5. The method of claim 3 , wherein each of the first and second weighting functions is one of a triangular function and a raised cosine function.

6. The method of claim 3 , further comprising:

deriving the current filter coefficients based on at least a part of the current signal frame; and

deriving the past filter coefficients based on at least a part of a past signal frame.

7. The method of claim 1 , further comprising:

prior to step (a), filtering the past signal frame using the past set of filter coefficients, thereby producing the past filtered frame,

wherein step (c) comprises modifying the second filtered frame portion with the first filtered frame portion so as to smooth a possible filtered signal discontinuity between the second filtered frame portion and the past filtered frame.

8. The method of claim 1 , wherein the signal is a decoded speech (DS) signal including successive DS frames, and the beginning portion of the current signal frame is a beginning portion of a current DS frame.

9. The method of claim 8 , wherein:

step (a) comprises at least one of short-term and long-term filtering the beginning portion of the current DS frame using at least one of past short-term filter coefficients and past long-term filter coefficients, respectively; and

step (b) comprises at least one of short-term and long-term filtering the beginning portion of the current frame using at least one of current short-term and current long-term filter coefficients, respectively.

10. The method of claim 9 , wherein:

step (a) further comprises gain scaling, with a past gain, a first intermediate filtered DS frame portion resulting from said at least of short-term and long-term filtering; and

step (b) further comprises gain scaling, with a current gain, a second intermediate filtered DS frame portion resulting from said at least one of short-term and long-term filtering.

11. The method of claim 9 , further comprising:

deriving the current short-term filter coefficients based on at least a part of the current DS frame; and

deriving the past short-term filter coefficients based on at least a part of the past DS frame.

12. A computer program product (CPP) comprising a computer usable medium having computer readable program code (CRPC) means embodied in the medium for causing an application program to execute on a computer processor to filter an audio signal, the audio signal including successive signal frames, comprising:

first CRPC means for causing the processor to filter a beginning portion of a current signal frame using a past set of filter coefficients, thereby producing a first filtered frame portion;

second CRPC means for causing the processor to filter the beginning portion of the current signal frame using a current set of filter coefficients, thereby producing a second filtered frame portion; and

third CRPC means for causing the processor to modify the second filtered frame portion with the first filtered frame portion so as to smooth a possible filtered signal discontinuity between the second filtered frame portion and a past filtered frame produced using the past filter coefficients.

13. The CPP of claim 12 , wherein the third CRPC means includes CRPC means for causing the processor to performing an overlap-add operation over the second filtered frame portion and the first filtered frame portion.

14. The CPP of claim 12 , wherein the third CRPC means includes:

first weighting CRPC means for causing the processor to weight the first filtered frame portion with a first weighting function to produce a fist weighted filtered frame portion;

second weighting CRPC means for causing the processor to weight the second filtered frame portion with a second weighting function to produce a second weighted filtered frame portion; and

combining CRPC means for causing the processor to combine the first and second weighted filtered frame portions.

15. The CPP of claim 14 , wherein the combining CRPC means includes CRPC means for causing the processor to add together the first and second weighted filtered frame portions.

16. The CPP of claim 14 , wherein each of the first and second weighting functions is one of a triangular function and a raised cosine function.

17. The CPP of claim 12 , wherein the signal is a decoded speech (DS) signal including successive DS frames, and the beginning portion of the current signal frame is a beginning portion of a current DS frame.

18. The CPP of claim 17 , wherein:

the first CRPC means includes at least one of

CRPC means for causing the processor to short-term filter the beginning portion of the current DS frame using past short-term filter coefficients, and

CRPC means for causing the processor to long-term filter the beginning portion of the current DS frame using past long-term filter coefficients; and

the second CRPC means includes at least one of

CRPC means for causing the processor to short-term filter the beginning portion of the current DS frame using current short-term filter coefficients, and

CRPC means for causing the processor to long-term filter the beginning portion of the current DS frame using current long-term filter coefficients.

19. The CPP of claim 18 , wherein:

the first CRPC means further includes CRPC means for causing the processor to gain scale, with a past gain, a first intermediate filtered DS frame portion resulting from said at least of short-term and long-term filtering; and

the second CRPC means further includes CRPC means for causing the processor to gain scale, with a current gain, a second intermediate filtered DS frame portion resulting from said at least one of short-term and long-term filtering.

20. An apparatus for filtering an audio signal, the audio signal including successive signal frames, comprising:

first means for filtering a beginning portion of a current signal frame using a past set of filter coefficients, thereby producing a first filtered frame portion;

second means for filtering the beginning portion of the current signal frame using a current set of filter coefficients, thereby producing a second filtered frame portion; and

third means for modifying the second filtered frame portion with the first filtered frame portion so as to smooth a possible filtered signal discontinuity between the second filtered frame portion and a past filtered frame produced using the past filter coefficients.

21. The apparatus of claim 20 , wherein the third means comprises means for performing an overlap-add operation over the second filtered frame portion and the first filtered frame portion.

22. The apparatus of claim 20 , wherein the third means comprises:

means for weighting the first filtered frame portion with a first weighting function to produce a fist weighted filtered frame portion;

means for weighting the second filtered frame portion with a second weighting function to produce a second weighted filtered frame portion; and

means for combining the overlapped first and second weighted filtered frame portions.

23. The apparatus of claim 22 , wherein the combining means comprises means for adding together the first and second weighted filtered frame portions.

24. The apparatus of claim 22 , wherein each of the first and second weighting functions is one of a triangular function and a raised cosine function.

25. The apparatus of claim 20 , wherein the signal is a decoded speech (DS) signal including successive DS frames, and the beginning portion of the current signal frame is a beginning portion of a current DS frame.