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

• US 20040117177A1
• Filed: 08/29/2003
• Published: 06/17/2004
• Est. Priority Date: 09/18/2002
• Status: Active Grant

• Current Assignee: Dolby International AB (Dolby Laboratories Incorporated)
• Original Assignee: Coding Technologies Sweden AB
• Inventors: Villemoes, Lars, Kjorling, Kristofer
• Granted Patent: US 7,577,570 B2
• Time in Patent Office: Days
• US Class Current: 704/225
• 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 spectral envelope adjustment of a signal, comprising:

means for providing a plurality of subband signals, a subband signal having associated therewith a channel number k indicating a frequency range covered by the subband signal, the subband signal originating from a channel filter having the channel number k in an analysis filterbank having a plurality of channel filters, wherein the channel filter having the channel number k has a channel response which is overlapped with a channel response of an adjacent channel filter having a channel number k−

1 in an overlapping range;

means for examining the subband signal having associated therewith the channel number k and for examining an adjacent subband signal having associated therewith the channel number k−

1 to determine, whether the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range;

means for calculating a first gain adjustment value and a second gain adjustment value for the subband signal and the adjacent subband signal in response to a positive result of the means for examining, wherein the means for calculating is operative to determine the first gain adjustment value and the second gain adjustment value dependent on each other; and

means for gain adjusting the subband signal and the adjacent subband signal using the first and the second gain adjusting values or for outputting the first and the second gain adjustment values for transmission or storing.

2. Apparatus in accordance with claim 1, in which the means for examining is operative to calculate signs of subband signals based on coefficients of prediction polynomials for the subband signal and the adjacent subband signal, and to indicate a positive result, when the signs have a predetermined relationship to each other.

3. Apparatus in accordance with claim 2, in which the means for examining is operative to apply an auto-correlation method or a co-variance method.

4. Apparatus in accordance with claim 2, 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 means for examining is operative to use the first order coefficient for calculating the signs of the subband signals.

5. Apparatus in accordance with claim 2, in which the means for examining is operative to calculate the sign for a subband signal based on the following equation:

6. Apparatus in accordance with claim 2, in which the predetermined relationship is defined such that the subband signal having associated therewith the channel number k has a first sign and the adjacent subband signal having associated therewith the channel number k−

1 has a second sign, which is opposite to the first sign.

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

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

9. Apparatus in accordance with claim 8, in which the means for examining is operative to determine, whether the tonal component is in the overlapping range of channel k and channel k−

1.

10. Apparatus in accordance with claim 1, further comprising means for providing a first reference spectral envelope value for the subband signal and a second reference spectral envelope value for the adjacent subband signal,

in which the means for calculating is operative to determine a first energy measure indicating a signal energy of the subband signal and a second energy measure indicating a signal energy of the adjacent subband signal, and

in which the means for examining is further operative to calculate the first and the second gain adjustment values based on a linear combination of the first reference spectral envelope value and the second reference spectral envelope value or a linear combination of the first energy measure or the second energy measure.

11. Apparatus in accordance with claim 1, in which the means for calculating is operative to calculate the first and the second gain adjustment values such that they differ by less than a predetermined threshold or are equal to each other.

12. Apparatus in accordance with claim 11, in which the predetermined threshold is lower than or equal to 6 dB.

13. Apparatus in accordance with claim 1, further comprising means for providing an unmodified first gain adjustment value for the subband signal and an unmodified second gain adjustment value for the adjacent subband signal, and

in which the means for calculating is operative to calculate the first and the second gain adjustment values so that both are larger than or equal to a lower value of the first and the second unmodified gain adjustment values and smaller or equal to a higher value of the first and the second unmodified gain adjustment values.

14. Apparatus in accordance with claim 13, in which the unmodified first gain adjustment value and the unmodified second gain adjustment value are indicative for a spectral envelope of an original signal in a frequency band, wherein the frequency band is to be reconstructed by a spectral band replication.

15. Apparatus in accordance with claim 1, further comprising a synthesis filterbank for filtering the gain adjusted subband signals to obtain a synthesized output signal.

16. Apparatus in accordance with claim 1, in which the analysis filterbank is a real valued filterbank, and

in which the synthesis filterbank is a real valued filterbank.

17. Apparatus in accordance with claim 1, in which the analysis filterbank is a complex-valued filterbank, and in which the synthesis filterbank is a real-valued filterbank.

18. Apparatus in accordance with claim 1, in which the means for calculating is operative to calculate the first gain adjustment value and the second gain adjustment value based on an average energy of the subband signal and the adjacent subband signal.

19. A method of spectral envelope adjustment of a signal, comprising:

providing a plurality of subband signals, a subband signal having associated therewith a channel number k indicating the frequency range covered by the subband signal, the subband signal originating from a channel filter having the channel number k in an analysis filterbank having a plurality of channel filters, wherein the channel filter having the channel number k has a channel response which is overlapped with a channel response of an adjacent channel filter having a channel number k−

1 in an overlapping range;

examining the subband signal having associated therewith the channel number k and for examining an adjacent subband signal having associated therewith the channel number k−

1 to determine, whether the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range;

calculating a first gain adjustment value and a second gain adjustment value for the subband signal and the adjacent subband signal in response to a positive result of the means for examining, wherein the means for calculating is operative to determine the first gain adjustment value and the second gain adjustment value dependent on each other; and

gain adjusting the subband signal and the adjacent subband signal using the first and the second gain adjusting values or outputting the first and the second gain adjustment values for transmission or storing.

20. Computer program having a program code for performing a method, when the computer program runs on a computer, the method having the following steps:

providing a plurality of subband signals, a subband signal having associated therewith a channel number k indicating the frequency range covered by the subband signal, the subband signal originating from a channel filter having the channel number k in an analysis filterbank having a plurality of channel filters, wherein the channel filter having the channel number k has a channel response which is overlapped with a channel response of an adjacent channel filter having a channel number k−

1 in an overlapping range;

examining the subband signal having associated therewith the channel number k and for examining an adjacent subband signal having associated therewith the channel number k−

1 to determine, whether the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range;

calculating a first gain adjustment value and a second gain adjustment value for the subband signal and the adjacent subband signal in response to a positive result of the means for examining, wherein the means for calculating is operative to determine the first gain adjustment value and the second gain adjustment value dependent on each other; and

gain adjusting the subband signal and the adjacent subband signal using the first and the second gain adjusting values or outputting the first and the second gain adjustment values for transmission or storing.

21. A method for spectral envelope adjustment of a signal, using a filterbank where the filterbank comprises a real valued analysis part and a real valued synthesis part or where said filterbank comprises a complex analysis part and a real valued synthesis part, where a lower, in frequency, channel and an adjacent higher, in frequency, channel are modified using the same gain value, if the lower channel has a positive sign and the higher channel has a negative sign, so that a relation between subband samples of the lower channel and subband samples of the higher channel is maintained.