Method and apparatus to eliminate discontinuities in adaptively filtered signals

• US 20030088408A1
• Filed: 06/28/2002
• Published: 05/08/2003
• Est. Priority Date: 10/03/2001
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Lee, Chris C., Chen, Juin-Hwey, Thyssen, Jes
• Granted Patent: US 7,353,168 B2
• Time in Patent Office: Days
• US Class Current: 704/228
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering

Abstract

A method to eliminate discontinuities in an adaptively filtered signal includes filtering a beginning portion of a current signal frame using a past set of filter coefficients, thereby producing a first filtered frame portion. The method also includes filtering the beginning portion of the current signal frame using a current set of filter coefficients, thereby producing a second filtered frame portion. The method also includes 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.

Claims

1. A method of filtering a signal, the 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 fist 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 method of filtering a signal with a time varying filter, the filter switching from a first filter to a second filter at a filter update time, comprising:

(a) filtering a signal segment beginning at the filter update time with the first filter to produce a first filtered segment;

(b) filtering the signal segment with the second filter to produce a second filtered segment; and

(c) modifying the second filtered segment with the first filtered segment so as to smooth a possible filtered signal discontinuity at the filter update time.

13. The method of claim 12, further comprising:

prior to step (a), filtering a second signal segment preceding the first signal segment with the first filter, thereby producing a third filtered segment preceding and adjacent to the first filtered segment,

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

14. The method of claim 12, wherein the first filter has a first filter transfer function and the second filter has a second filter transfer function.

15. The method of claim 12, further comprising:

deriving the first filter based on the second signal segment; and

deriving the second filter based on the signal segment.

16. The method of claim 12, wherein each signal frame includes a series of signal sample.

17. 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 a signal, the 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.

18. The CPP of claim 17, 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.

19. The CPP of claim 17, 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.

20. The CPP of claim 19, wherein the combining CRPC means includes CRPC means for causing the processor to add together the first and second weighted filtered frame portions.

21. The CPP of claim 19, wherein each of the first and second weighting functions is one of a triangular function and a raised cosine function.

22. The CPP of claim 17, 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.

23. The CPP of claim 22, 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.

24. The CPP of claim 23, 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.

25. An apparatus for filtering a signal, the 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.

26. The apparatus of claim 25, wherein the third means comprises means for performing an overlap-add operation over the second filtered frame portion and the first filtered frame portion.

27. The apparatus of claim 25, 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.

28. The apparatus of claim 27, wherein the combining means comprises means for adding together the first and second weighted filtered frame portions.

29. The apparatus of claim 27, wherein each of the first and second weighting functions is one of a triangular function and a raised cosine function.

30. The apparatus of claim 25, 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.