Method for reduction of aliasing introduced by spectral envelope adjustment in real-valued filterbanks

• US 7,548,864 B2
• Filed: 09/26/2007
• Issued: 06/16/2009
• Est. Priority Date: 09/18/2002
• Status: Expired due to Term

• Current Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Original Assignee: Coding Technologies Sweden AB
• Inventors: Kjorling, Kristofer, Villemoes, Lars
• Primary Examiner(s): Hudspeth; David R.
• Assistant Examiner(s): Sked; Matthew J
• Application Number:
  US11/862,143
• Publication Number:
  US 20080015868A1
• Time in Patent Office: 629 Days
• Field of Search: 704/500
• US Class Current: 704/500
• CPC Class Codes: G10L 19/00 Speech or audio signals analysis-synthesis techniques for redundancy reduction, e.g. in vocoders; Coding or decoding of speech or audio signals, using source filter models or psychoacoustic analysis in musical instruments G10H
• CPC Class Codes: G10L 19/0204 using subband decomposition
• CPC Class Codes: G10L 19/025 Detection of transients or attacks for time/frequency resolution switching
• CPC Class Codes: G10L 19/083 the excitation function being an excitation gain G10L25/90 takes precedence
• CPC Class Codes: G10L 19/12 the excitation function being a code excitation, e.g. in code excited linear prediction [CELP] vocoders
• CPC Class Codes: G10L 19/26 Pre-filtering or post-filtering
• CPC Class Codes: G10L 25/12 the extracted parameters being prediction coefficients
• CPC Class Codes: G10L 25/18 the extracted parameters being spectral information of each sub-band
• CPC Class Codes: G10L 25/21 the extracted parameters being power information
• CPC Class Codes: G10L 25/48 specially adapted for particular use
• CPC Class Codes: H03H 17/0266 Filter banks

Abstract

The present invention proposes a new method for improving the performance of a real-valued filterbank based spectral envelope adjuster. By adaptively locking the gain values for adjacent channels dependent on the sign of the channels, as defined in the application, reduced aliasing is achieved. Furthermore, the grouping of the channels during gain-calculation, gives an improved energy estimate of the real valued subband signals in the filterbank.

Claims

1. Apparatus for assessing which channels among a plurality of channels are channels for which coupled gain factors are required, the gain factors being for the purpose of gain adjusting the channels comprising:

an analysis filterbank for filtering a signal to generate a plurality of channels,the analysis filter bank having subband filters, adjacent filters of the filterbank having transition bands overlapping in an overlapping range; and

an examiner for examining the adjacent channels, whether the adjacent channels have aliasing generating signal components in the overlapping range,

wherein the examiner is operative to calculate signs of the adjacent channels based on coefficients of prediction polynomials for the adjacent channels, and

wherein the examiner is operative to indicate adjacent channels as channels for which coupled gain factors are required, when the signs of the adjacent channels have a predetermined relationship to each other.

2. Apparatus in accordance with claim 1, in which the examiner is operative to apply an autocorrelation method or a covariance method.

3. Apparatus in accordance with claim 1, in which the prediction polynomial is a low order polynomial having a first order coefficient, wherein the order of the low order polynomial is smaller than 4, and in which the examiner is operative to use the first order coefficients for calculating the signs of the channels.

4. Apparatus in accordance with claim 1, in which the examiner is operative to calculate the sign for a subband signal based on the following equation:

$\mathrm{sign}\left(k\right)=\{\begin{array}{ccc}{\left(-1\right)}^k& \mathrm{if}& {\mathrm{\alpha <br><br>}}_1<<br><br>0\\ {\left(-1\right)}^{k+1}& \mathrm{if}& {\mathrm{\alpha <br><br>}}_1\ge <br><br>0\end{array}<br><br><br><br>0<<br><br>k<<br><br>M$

wherein k is the channel number, and α

₁ is the first-order coefficient.

5. Apparatus in accordance with claim 1, in which the examiner is operative to apply, as the predetermined relationship, such that the first channel has a first sign and the adjacent second channel has a second sign, wherein the second sign is configured opposite to the first sign.

6. Apparatus in accordance with claim 5, in which the first sign is negative, and the second sign is positive.

7. Apparatus in accordance with claim 5, in which the first sign is negative, and the second sign is positive.

8. Apparatus in accordance with claim 1, in which the examiner is operative to perform a tonal analysis for the channel and the adjacent channel for determining a tonal component having a tonality measure above a tonality threshold.

9. Apparatus in accordance with claim 1, in which the examiner is operative to apply, as the predetermined relationship, such that the first channel has a first sign and the adjacent second channel has a second sign, wherein the second sign is configured opposite to the first sign.

10. Apparatus in accordance with claim 1, in which the examiner is operative to perform a tonal analysis for the channel and the adjacent channel for determining a tonal component having a tonality measure above a tonality threshold.

11. Method of assessing which channels among a plurality of channels generated by filtering a signal using an analysis filterbank are channels, for which coupled gain factors are required, the gain factors being for the purpose of gain adjusting the channels, the filterbank having subband filter, adjacent filter of the filterbank having transition bands overlapping is an overlapping range, comprising:

examining the adjacent channels, whether the adjacent channels have, aliasing generating signal components in the overlapping range,

wherein the examining the adjacent channels comprises calculating signs of the adjacent channels based on coefficients of prediction polynomials for the adjacent channels, and

wherein the examining the adjacent channels comprises indicating the adjacent channels as channels for which couples gain factors required, when the sign of the adjacent channels have a predetermined relationship to each other.

12. A digital storage medium having stored thereon a program having a program code for performing, when being executed on a computer, a method of assessing which channels among a plurality of channels generated by filtering a signal using an analysis filterbank are channels, for which coupled gain factors are required, the gain factors being for the purpose of gain adjusting the channels, the filterbank having subband filter, adjacent filters of the filterbank having transition bands overlapping in an overlapping range, the method comprising:

examining the adjacent channels whether the adjacent channels have aliasing generating signal components in the overlapping range,

wherein the examining the adjacent channels comprises calculating signs of the adjacent channels based on coefficients of predicted polynomials for the adjacent channels, and

wherein the examining the adjacent channels comprises indicating the adjacent channels as channels for which coupled gain factors are required, when the signs of the adjacent channels have a predetermined relationship to each other.

13. Apparatus for assessing which channels among a plurality of channels generated by filtering a signal using an analysis filterbank are channels, for which coupled gain factors are required, the gain factors being for purpose of gain adjusting the channels, the filterbank having subband filters, adjacent filters of the filterbank having transition bands overlapping in an overlapping range, comprising:

an examiner for examining the adjacent channels, whether the adjacent channels have aliasing generating signal components in the overlapping range,

wherein the examiner is operative to calculate signs of the adjacent channels bases on the following equation;

$\mathrm{sign}<br><br>\left(k\right)=\{\begin{array}{c}{\left(-1\right)}^k<br><br><br><br>\mathrm{if}<br><br><br><br>{\mathrm{\alpha <br><br>}}_1<<br><br>0\\ {\left(-1\right)}^{k+1}<br><br><br><br>\mathrm{if}<br><br><br><br>{\mathrm{\alpha <br><br>}}_1\ge <br><br>0\end{array}<br><br>0<<br><br>k<<br><br>M$

wherein k is the channel number, and α

₁ is the first-order coefficient; and

wherein the examiner is operative to indicate the adjacent channels as channels for which coupled gain factors are required, when the signs of the adjacent channels have a predetermined relationship to each other.

14. Apparatus in accordance with claim 13, in which the examiner is operative to apply an autocorrelation method or a covariance method.

15. Apparatus in accordance with claim 13, in which the prediction polynomial is a low order polynomial having a first order coefficient, wherein the order of the low order polynomial is smaller than 4, and in which the examiner is operative to use the first order coefficients for calculating the sign of the channels.

16. Method of assessing which channels among a plurality of channels generated by filtering a signal using an analysis filterbank are channels, for which coupled gain factors are required, the gain factors being for the purpose of gain adjusting the channels, the filterbank having subband filters, adjacent filters of the filterbank having transition bands overlapping in an overlapping range, range, comprising:

examining the adjacent channels with an examiner, whether the adjacent channels have aliasing generating signal components in the overlapping range, wherein signs of the adjacent channels are calculated based on the following equation;

$\mathrm{sign}<br><br>\left(k\right)=\{\begin{array}{c}{\left(-1\right)}^k<br><br><br><br>\mathrm{if}<br><br><br><br>{\mathrm{\alpha <br><br>}}_1<<br><br>0\\ {\left(-1\right)}^{k+1}<br><br><br><br>\mathrm{if}<br><br><br><br>{\mathrm{\alpha <br><br>}}_1\ge <br><br>0\end{array}<br><br>0<<br><br>k<<br><br>M$

wherein k the channel number, and α

₁ is the first-order coefficient, and

wherein the adjacent channels are indicated as channels for which coupled gain factors are required, when the signs of the adjacent channels have a predetermined relationship to each other.