Audio codec post-filter

• US 20060271354A1
• Filed: 05/31/2005
• Published: 11/30/2006
• Est. Priority Date: 05/31/2005
• Status: Active Grant

• Current Assignee: Microsoft Technology Licensing LLC (Microsoft Corporation)
• Original Assignee: Microsoft Corporation
• Inventors: Sun, Xiaoqin, Khalil, Hosam A., Chen, Wei-Ge, Koishida, Kazuhito, Wang, Tian
• Granted Patent: US 7,707,034 B2
• Time in Patent Office: Days
• US Class Current: 704/205
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering

Abstract

Techniques and tools are described for processing reconstructed audio signals. For example, a reconstructed audio signal is filtered in the time domain using filter coefficients that are calculated, at least in part, in the frequency domain. As another example, producing a set of filter coefficients for filtering a reconstructed audio signal includes clipping one or more peaks of a set of coefficient values. As yet another example, for a sub-band codec, in a frequency region near an intersection between two sub-bands, a reconstructed composite signal is enhanced.

Claims

1. A computer-implemented method comprising:

calculating a set of filter coefficients for application to a reconstructed audio signal, wherein the calculating the set of filter coefficients comprises performing one or more frequency domain calculations; and

producing a filtered audio signal by filtering at least a portion of the reconstructed audio signal in a time domain using the set of filter coefficients.

2. The method of claim 1, wherein the filtered audio signal represents a frequency sub-band of the reconstructed audio signal.

3. The method of claim 1, wherein calculating the set of filter coefficients comprises:

performing a transform of a set of initial time domain values from a time domain into a frequency domain, thereby producing a set of initial frequency domain values;

performing the one or more frequency domain calculations using the frequency domain values to produce a set of processed frequency domain values;

performing a transform of the processed frequency domain values from the frequency domain into the time domain, thereby producing a set of processed time domain values; and

truncating the set of time domain values in the time domain.

4. The method of claim 1, wherein calculating the set of filter coefficients comprises processing a set of linear prediction coefficients.

5. The method of claim 4, wherein processing the set of linear prediction coefficients comprises capping a spectrum derived from the set of linear prediction coefficients.

6. The method of claim 4, wherein processing the set of linear prediction coefficients comprises reducing a range of a spectrum derived from the set of linear prediction coefficients.

7. The method of claim 1, wherein the one or more frequency domain calculations comprises one or more calculations in a logarithmic domain.

8. A method comprising:

producing a set of filter coefficients for application to a reconstructed audio signal, including processing a set of coefficient values representing one or more peaks and one or more valleys, wherein the processing the set of coefficient values comprises clipping one or more of the peaks or valleys; and

filtering at least a portion of the reconstructed audio signal using the filter coefficients.

9. The method of claim 8, wherein the clipping comprises capping the set of coefficient values at a clip value.

10. The method of claim 9, wherein producing a set of filter coefficients further comprises calculating the clip value as a function of an average of the set of coefficient values.

11. The method of claim 8, wherein the set of coefficient values is based at least in part on a set of linear prediction coefficient values.

12. The method of claim 8, wherein the clipping is performed in a frequency domain.

13. The method of claim 8, wherein the filtering is performed in a time domain.

14. The method of claim 8, further comprising reducing a range of the set of coefficient values before the clipping.

15. A computer-implemented method comprising:

receiving a reconstructed composite signal synthesized from plural reconstructed frequency sub-band signals, the plural reconstructed frequency sub-band signals including a reconstructed first frequency sub-band signal for a first frequency band and a reconstructed second frequency sub-band signal for a second frequency band; and

selectively enhancing the reconstructed composite signal at a frequency region around an intersection between the first frequency band and the second frequency band.

16. The method of claim 15 further comprising:

decoding coded information to produce the plural reconstructed frequency sub-band signals; and

synthesizing the plural reconstructed frequency sub-band signals to produce the reconstructed composite signal.

17. The method of claim 15, wherein enhancing the reconstructed composite signal comprises passing the reconstructed composite signal through a band pass filter, wherein a pass band of the band pass filter corresponds to the frequency region around the intersection between the first frequency band and the second frequency band.

18. The method of claim 17, wherein the band pass filter comprises a low pass filter in series with a high pass filter.

19. The method of claim 17, wherein the band pass filter has unity gain at one or more stop bands and greater than unity gain at the pass band.

20. The method of claim 15, wherein the enhancing comprises increasing signal energy in the frequency region.