Method for reduction of aliasing introduced by spectral envelope adjustment in realvalued filterbanks

0Associated
Cases 
0Associated
Defendants 
0Accused
Products 
18Forward
Citations 
0
Petitions 
2
Assignments
First Claim
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,wherein 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, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, when the signs have a predetermined relationship to each other, wherein the means for examining is operative to calculate the sign for a subband signal based on the following equation;
$\mathrm{sign}\left(k\right)=\{\begin{array}{cc}{\left(1\right)}^{k}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{0}{\left(1\right)}^{k+1}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{\ge 0},0kM,\end{array}$ wherein k is the channel number, and α
_{1 }is the firstorder coefficient of a prediction polynomial;
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 the 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 Assignments
0 Petitions
Accused Products
Abstract
The present invention proposes a new method for improving the performance of a realvalued 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 gaincalculation, gives an improved energy estimate of the real valued subband signals in the filterbank.
29 Citations
View as Search Results
Method and system for preventing speech comprehension by interactive voice response systems  
Patent #
US 7,979,274 B2
Filed 05/20/2009

Current Assignee
Nuance Communications Inc.

Sponsoring Entity
ATT Inc.

Method and System for Preventing Speech Comprehension by Interactive Voice Response Systems  
Patent #
US 20090228271A1
Filed 05/20/2009

Current Assignee
Nuance Communications Inc.

Sponsoring Entity
ATT Inc.

HIERARCHICAL ENCODING/DECODING DEVICE  
Patent #
US 20090326931A1
Filed 07/07/2006

Current Assignee
Orange S.A.

Sponsoring Entity
Orange S.A.

Hierarchical encoding/decoding device  
Patent #
US 8,374,853 B2
Filed 07/07/2006

Current Assignee
Orange S.A.

Sponsoring Entity
Orange S.A.

PostQuantization Gain Correction in Audio Coding  
Patent #
US 20130339038A1
Filed 07/04/2011

Current Assignee
Telefonaktiebolaget LM Ericsson

Sponsoring Entity
Telefonaktiebolaget LM Ericsson

Efficient combined harmonic transposition  
Patent #
US 8,983,852 B2
Filed 05/25/2010

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Advanced stereo coding based on a combination of adaptively selectable left/right or mid/side stereo coding and of parametric stereo coding  
Patent #
US 9,082,395 B2
Filed 03/05/2010

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Metadata time marking information for indicating a section of an audio object  
Patent #
US 9,105,300 B2
Filed 10/14/2010

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Method, apparatus, and medium for detecting frequency extension coding in the coding history of an audio signal  
Patent #
US 9,117,440 B2
Filed 04/30/2012

Current Assignee
Dolby International AB, Dolby Laboratories Incorporated

Sponsoring Entity
Dolby International AB, Dolby Laboratories Incorporated

Efficient combined harmonic transposition  
Patent #
US 9,190,067 B2
Filed 02/04/2015

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Realtime separation of signal components in spectrum analyzer  
Patent #
US 9,577,798 B1
Filed 04/30/2014

Current Assignee
Keysight Technologies Inc.

Sponsoring Entity
Keysight Technologies Inc.

Efficient combined harmonic transposition  
Patent #
US 9,881,597 B2
Filed 10/14/2015

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Advanced stereo coding based on a combination of adaptively selectable left/right or mid/side stereo coding and of parametric stereo coding  
Patent #
US 9,905,230 B2
Filed 06/09/2015

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Postquantization gain correction in audio coding  
Patent #
US 10,121,481 B2
Filed 07/04/2011

Current Assignee
Telefonaktiebolaget LM Ericsson

Sponsoring Entity
Telefonaktiebolaget LM Ericsson

Advanced stereo coding based on a combination of adaptively selectable left/right or mid/side stereo coding and of parametric stereo coding  
Patent #
US 10,297,259 B2
Filed 01/17/2018

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Efficient combined harmonic transposition  
Patent #
US 10,304,431 B2
Filed 12/21/2017

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Postquantization gain correction in audio coding  
Patent #
US 10,460,739 B2
Filed 08/04/2017

Current Assignee
Telefonaktiebolaget LM Ericsson

Sponsoring Entity
Telefonaktiebolaget LM Ericsson

Efficient combined harmonic transposition  
Patent #
US 10,657,937 B2
Filed 04/05/2019

Current Assignee
Dolby International AB

Sponsoring Entity
Dolby International AB

Digital signal processing apparatus and digital signal processing method  
Patent #
US 7,197,093 B2
Filed 07/21/2004

Current Assignee
Sony Corporation

Sponsoring Entity
Sony Corporation

Audio signal coding with a signal adaptive filterbank  
Patent #
US 5,732,189 A
Filed 12/22/1995

Current Assignee
Lucent Technologies Inc.

Sponsoring Entity
Lucent Technologies Inc.

Audio decoding apparatus and method for band expansion with aliasing adjustment  
Patent #
US 7,069,212 B2
Filed 09/11/2003

Current Assignee
Matsushita Electric Industrial Company Limited, NEC Corporation

Sponsoring Entity
Matsushita Electric Industrial Company Limited, NEC Corporation

Partial band reconstruction of frequency channelized filters  
Patent #
US 20040042557A1
Filed 11/25/2002

Current Assignee
Callahan Cellular LLC

Sponsoring Entity
BAE Systems Information and Electronic Systems Integration Incorporated

Aliasing reduction using complexexponential modulated filterbanks  
Patent #
US 20030016772A1
Filed 04/02/2002

Current Assignee
Dolby International AB

Sponsoring Entity
Coding Technologies AB

Subband adaptive signal processing in an oversampled filterbank  
Patent #
US 20030108214A1
Filed 08/07/2002

Current Assignee
Semiconductor Components Industries LLC

Sponsoring Entity
ON Semiconductor Corporation

Method and apparatus for filtering and compressing sound signals  
Patent #
US 20020015503A1
Filed 08/07/2001

Current Assignee
Ototronix LLC

Sponsoring Entity
APHERMA CORPORATION

Quantization in perceptual audio coders with compensation for synthesis filter noise spreading  
Patent #
US 6,363,338 B1
Filed 04/12/1999

Current Assignee
Dolby Laboratories Incorporated

Sponsoring Entity
Dolby Laboratories Incorporated

Filterbank structure and method for filtering and separating an information signal into different bands, particularly for audio signal in hearing aids  
Patent #
US 6,236,731 B1
Filed 04/16/1998

Current Assignee
KS HIMPP

Sponsoring Entity
Bank of Novia Scotia

Framebased audio coding with video/audio data synchronization by dynamic audio frame alignment  
Patent #
US 6,124,895 A
Filed 10/17/1997

Current Assignee
Dolby Laboratories Incorporated

Sponsoring Entity
Dolby Laboratories Incorporated

Method and apparatus for encoding and decoding multiple audio channels at low bit rates using adaptive selection of encoding method  
Patent #
US 5,890,125 A
Filed 07/16/1997

Current Assignee
Dolby Laboratories Incorporated

Sponsoring Entity
Dolby Laboratories Incorporated

25 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,wherein 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, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, when the signs have a predetermined relationship to each other, wherein the means for examining is operative to calculate the sign for a subband signal based on the following equation; $\mathrm{sign}\left(k\right)=\{\begin{array}{cc}{\left(1\right)}^{k}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{0}{\left(1\right)}^{k+1}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{\ge 0},0kM,\end{array}$ wherein k is the channel number, and α
_{1 }is the firstorder coefficient of a prediction polynomial;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 the 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.  View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16)
 17. 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,wherein, in the step of examining, signs of subband signals are calculated based on coefficients of prediction polynomials for the subband signal and the adjacent subband signal, and a positive result is indicated, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, when the signs have a predetermined relationship to each other, wherein the sign for a subband signal is calculated based on the following equation; $\mathrm{sign}\left(k\right)=\{\begin{array}{cc}{\left(1\right)}^{k}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{0}{\left(1\right)}^{k+1}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{\ge 0},0kM,\end{array}$ wherein k is the channel number, and α
_{1 }is the firstorder coefficient of a prediction polynomial;calculating a first gain adjustment value and a second gain adjustment value for the subband signal and the adjacent subband signal in response to the positive result of the step of examining, wherein the first gain adjustment value and the second gain adjustment value are determined 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.
 18. Digital storage medium having stored thereon a computer program having a program code for performing a method of spectral envelope adjustment of a signal, when the computer program runs on a computer, the method 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;wherein, in the step of examining, signs of subband signals are calculated based on coefficients of prediction polynomials for the subband signal and the adjacent subband signal, and a positive result is indicated, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, when the signs have a predetermined relationship to each other, wherein the sign for a subband signal is calculated based on the following equation; $\mathrm{sign}\left(k\right)=\{\begin{array}{cc}{\left(1\right)}^{k}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{0}{\left(1\right)}^{k+1}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{\ge 0},0kM,\end{array}$ wherein k is the channel number, and α
_{1 }is the firstorder coefficient of a prediction polynomial;calculating a first gain adjustment value and a second gain adjustment value for the subband signal and the adjacent subband signal in response to the positive result of the step of examining, wherein the first gain adjustment value and the second gain adjustment value are determined 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.
 19. A method for spectral envelope adjustment of a signal, comprising:
filtering the 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, modifying a lower, in frequency, channel and an adjacent higher, in frequency, channel 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, wherein the sign for a subband signal is calculated based on the following equation; $\mathrm{sign}\left(k\right)=\{\begin{array}{cc}{\left(1\right)}^{k}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{0}{\left(1\right)}^{k+1}& \mathrm{if}\phantom{\rule{0.8em}{0.8ex}}{\mathrm{\alpha 1}}_{\ge 0},0kM,\end{array}$ wherein k is the channel number, and a_{1 }is firstorder coefficient of a prediction polynomial.
 20. 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 providing an unmodified first gain adjustment value for the subband signal and an unmodified second gain adjustment value for the adjacent subband signal; 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, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, wherein the means for calculating is operative to determine the first gain adjustment value and the second gain adjustment value dependent on each other, wherein 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; 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.
 21. 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, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, wherein the means for calculating is operative to determine the first gain adjustment value and the second gain adjustment value dependent on each other, wherein 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; 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.
 22. 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 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;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;providing an unmodified first gain adjustment value for the subband signal and an unmodified second gain adjustment value for the adjacent subband signal; 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 step of examining, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, wherein the first gain adjustment value and the second gain adjustment value are determined dependent on each other, wherein, in the step of calculating, the first and the second gain adjustment values are calculated 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; 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.
 23. 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 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;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 step of examining, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, wherein the first gain adjustment value and the second gain adjustment value are calculated dependent on each other, wherein, in the step of calculating, the first gain adjustment value and the second gain adjustment value are calculated based on an average energy of the subband signal and the adjacent subband signal; 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.
 24. Digital storage medium having stored thereon a computer program having a program code for performing a method of spectral envelope adjustment of a signal, when the computer program runs on a computer, the method comprising:
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;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;providing an unmodified first gain adjustment value for the subband signal and an unmodified second gain adjustment value for the adjacent subband signal; 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 step of examining, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, wherein the first gain adjustment value and the second gain adjustment value are determined dependent on each other, wherein, in the step of calculating, the first and the second gain adjustment values are calculated 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; 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.
 25. Digital storage medium having stored thereon a computer program having a program code for performing a method of spectral envelope adjustment of a signal, when the computer program runs on a computer, the method comprising:
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;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 step of examining, the positive result being that the subband signal and the adjacent subband signal have aliasing generating signal components in the overlapping range, wherein the first gain adjustment value and the second gain adjustment value are calculated dependent on each other, wherein, in the step of calculating, the first gain adjustment value and the second gain adjustment value are calculated based on an average energy of the subband signal and the adjacent subband signal; 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.
1 Specification
The present invention relates to systems comprising spectral envelope adjustment of audio signals using a realvalued subband filterbank. It reduces the aliasing introduced when using a realvalued subband filterbank for spectral envelope adjustment. It also enables an accurate energy calculation for sinusoidal components in a realvalued subband filterbank.
It has been shown in PCT/SE02/00626 “Aliasing reduction using complex exponential modulated filterbanks”, that a complexexponential modulated filterbank is an excellent tool for spectral envelope adjustment audio signals. In such a procedure the spectral envelope of the signal is represented by energyvalues corresponding to certain filterbank channels. By estimating the current energy in those channels, the corresponding subband samples can be modified to have the desired energy, and hence the spectral envelope is adjusted. If restraints on computational complexity prevents the usage of a complex exponential modulated filterbank, and only allows for a cosine modulated (realvalued) implementation, severe aliasing is obtained when the filterbank is used for spectral envelope adjustment. This is particularly obvious for audio signals with a strong tonal structure, where the aliasing components will cause intermodulation with the original spectral components. The present invention offers a solution to this by putting restraints on the gainvalues as a function of frequency in a signal dependent manner.
It is the object of the present invention to provide an improved technique for spectral envelope adjustment.
In accordance with a first aspect of the invention, this object is achieved by an 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.
In accordance with a second aspect of the invention, this object is achieved by 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.
In accordance with a third aspect of the invention, this object is achieved by a computer program having a program code for performing the above method, when the computer program runs on a computer.
In accordance with a fourth aspect of the invention, this object is achieved by 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.
The present invention relates to the problem of intermodulation introduced by aliasing in a realvalued filterbank used for spectral envelope adjustment. The present invention analyses the input signal and uses the obtained information to restrain the envelope adjustment capabilities of the filterbank by grouping gainvalues of adjacent channel in an order determined by the spectral characteristic of the signal at a given time. For a realvalued filterbank e.g. a pseudoQMF where transition bands overlap with closest neighbour only, it can be shown that due to aliasing cancellation properties the aliasing is kept below the stopband level of the prototype filter. If the prototype filter is designed with a sufficient aliasing suppression the filterbank is of perfect reconstruction type from a perceptual point of view, although this is not the case in a strict mathematical sense. However, if the channel gain of adjacent channels are altered between analysis and synthesis, the aliasing cancellation properties are violated, and aliasing components will appear audible in the output signal. By performing a loworder linear prediction on the subband samples of the filterbank channels, it is possible to assess, by observing the properties of the LPC polynomial, where in a filterbank channel a strong tonal component is present. Hence it is possible to assess which adjacent channels that must not have independent gainvalues in order to avoid a strong aliasing component from the tonal component present in the channel.
The present invention comprises the following features:
 Analysing means of the subband channels to assess where in a subband channel a strong tonal component is present;
 Analysing by means of a loworder linear predictor in every subband channel;
 Gain grouping decision based on the location of the zeros of the LPC polynomial;
 Accurate energy calculation for a realvalued implementation.
The present invention will now be described by way of illustrative examples, not limiting the scope or spirit of the invention, with reference to the accompanying drawings, in which:
The belowdescribed embodiments are merely illustrative for the principles of the present invention for improvement of a spectral envelope adjuster based on a realvalued filterbank. It is understood that modifications and variations of the arrangements and the details described herein will be apparent to others skilled in the art. It is the intent, therefore, to be limited only by the scope of the impending patent claims and not by the specific details presented by way of description and explanation of the embodiments herein.
In the following description a realvalued pseudoQMF is used comprising a realvalued analysis as well as a real valued synthesis.
It should be understood however, that the aliasing problem addressed by the present invention also appears for systems with a complex analysis and a realvalued synthesis, as well as any other cosinemodulated filterbank apart from 25 the pseudoQMF (QMF=Quadrature Mirror Filter) used in this description. The present invention is applicable for such systems as well. In a pseudoQMF every channel essentially only overlaps its adjacent neighbour in frequency. The frequencyresponse of the channels is shown in the subsequent figures by the dashed lines. This is only for illustrative purposes to indicate the overlapping of the channels, and should not be interpreted as the actual channel response given by the prototype filter. In
In order to assess which channels should have coupled gainfactors, the present invention teaches the usage of inband linear prediction. If a low order linear prediction is used, e.g. a second order LPC, this frequency analysis tool is able to resolve one sinusoidal component in every channel. By observing the sign of the first predictor polynomial coefficient it is easy to determine if the sinusoidal component is situated in the upper or lower half of the frequency range of the subband channel.
A second order prediction polynomial
A(z)=1−α_{1}z^{−1}−α_{2}z^{2} (1)
is obtained by linear prediction using the autocorrelation method or the covariance method for every channel in the QMF filterbank that will be affected by the spectral envelope adjustment. The sign of the QMFbank channel is defined according to:
where k is the channel number, M is the number of channels, and where the frequency inversion of every other QMF channel is taken into account. Hence, it is possible for every channel to assess where a strong tonal component is situated, and thus grouping the channels together that share a strong sinusoidal component. In
where E_{k}^{ref}(m) is the reference energy, and E_{k}(m) is the estimated energy, at the point m in time. This ensures that the grouped channels get the same gain value. Such grouping of the gain factors preserves the aliasing cancellation properties of the filterbank and gives the output according to
In a realvalued filterbank, the energy estimation is not straightforward as in a complex representation. If the energy is calculated by summing the squared subband samples of a single channel, there is a risk of tracking the time envelope of the signal rather than the actual energy. This is due to the fact that a sinusoidal component can have an arbitrary frequency from 0 to the filterbank channel width. If a sinusoidal component is present in a filterbank channel it can have a very low relative frequency, albeit being a high frequency sinusoidal in the original signal. Assessing the energy of this signal becomes difficult in a realvalued system since, if the averaging time is badly chosen with respect to the frequency of the sinusoidal, a tremolo (amplitudevariation) can be introduced, when in fact the signal energy actually is constant. The present invention teaches however, that the filterbank channels should be grouped twobytwo given the location of the sinusoidal components. This significantly reduces the tremoloproblem, as will be outlined below.
In a cosinemodulated filterbank the analysis filters h_{k}(n) are cosinemodulated versions of a symmetric lowpass prototype filter p_{0}(n) as
where M is the number of channels, k=0, 1, . . . , M−1, N is the prototype filter order and n=0, 1, . . . , N. The symmetry of the prototype filter is assumed here to be with respect to n=N/2. The derivations below are similar in case of half sample symmetry.
Given a sinusoidal input signal x(n)=A cos(ωn+θ) with frequency 0≦Ω≦π, the subband signal of channel k≧1 can be computed to be approximately
where P(ω) is the real valued discrete time Fourier transform of the shifted prototype filter p_{0}(n+N/2). The approximation is good when P(Ω+π(k+½)/M) is small, and this holds in particular if P(ω) is negligible for ω≧π/M, a hypothesis underlying the discussion which follows. For spectral envelope adjustment, the averaged energy within a subband k might be calculated as
where w(n) is a window of length L. Inserting equation (5) in equation (6) leads to
where Ψ(Ω) is a phase term which is independent of k and W(ω) is the discrete time Fourier transform of the window. This energy can be highly fluctuating if Ω is close to an integer multiple of π/M, although the input signal is a stationary sinusoid. Artifacts of tremolo type will appear in a system based on such single real analysis bank channel energy estimates.
On the other hand, assuming that π(k−½)/M≦Ω≦π(k+½)/M and that P(ω) is negligible for ω≧π/M, only the subband channels k and k−1 have nonzero outputs, and these channels will be grouped together as proposed by the present invention. The energy estimate based on these two channels is
where
For most useful designs of prototype filters, it holds that S(Ω) is approximately constant in the frequency range given above. Furthermore, if the window w(n) has a lowpass filter character, then ε(Ω) is much smaller than W(0), so the fluctuation of the energy estimate of equation (8) is significantly reduced compared to that of equation (7).
The subband signals output by the means 80 from
The means 82 for examining is operative to identify two adjacent subband signals, which have an aliasing generating signal component in the overlapping range. The means 82 is coupled to a means 84 for calculating gain adjustment values for adjacent subband signals. In particular, the means 84 is operative to calculate the first gain adjustment value and a second gain adjustment value for the subband signal on the one hand and the adjacent subband signal on the other hand. The calculation is performed in response to a positive result of the means for examining. In particular, the means for calculating is operative to determine the first gain adjustment value and the second gain adjustment value not independent on each other but dependent on each other.
The means 84 outputs a first gain adjustment value and a second gain adjustment value. It is to be noted at this point that, preferably, the first gain adjustment value and the second gain adjustment value are equal to each other in a preferred embodiment. In the case of modifying gain adjustment values, which have been calculated for example in a spectral band replication encoder, the modified gain adjustment values corresponding to the original SBR gain adjustment values are both smaller than the higher value of the original values and higher than the lower value of the original values as will be outlined later on.
The means 84 for calculating gain adjustment values therefore calculates two gain adjustment values for the adjacent subband signals. These gain adjustment values and the subband signals themselves are supplied to a means 86 for gain adjusting the adjacent subband signals using the calculated gain adjustment values. Preferably, the gain adjustment performed by the means 86 is performed by a multiplication of subband samples by the gain adjustment values so that the gain adjustment values are gain adjustment factors. In other words, the gain adjustment of a subband signal having several subband samples is performed by multiplying each subband sample from a subband by the gain adjustment factor, which has been calculated for the respective subband. Therefore, the fine structure of the subband signal is not touched by the gain adjustment. In other words, the relative amplitude values of the subband samples are maintained, while the absolute amplitude values of the subband samples are changed by multiplying these samples by the gain adjustment value associated with the respective subband signal.
At the output of means 86, gainadjusted subband signals are obtained. When these gainadjusted subband signals are input into a synthesis filterbank, which is preferably a realvalued synthesis filterbank, the output of the synthesis filterbank, i.e., the synthesized output signal does not show significant aliasing components as has been described above with respect to
It is to be noted here that a complete cancellation of aliasing components can be obtained, when the gain values of the adjacent subband signals are made equal to each other. Nevertheless, at least a reduction of aliasing components can be obtained when the gain adjustment values for the adjacent subband signals are calculated dependent on each other. This means that an improvement of the aliasing situation is already obtained, when the gain adjustment values are not totally equal to each other but are closer to each other compared to the case, in which no inventive steps have been taken.
Normally, the present invention is used in connection with spectral band replication (SBR) or high frequency reconstruction (HFR), which is described in detail in WO 98/57436 A2.
As it is known in the art, spectral envelope replication or high frequency reconstruction includes certain steps at the encoderside as well as certain steps at the decoderside.
In the encoder, an original signal having a full bandwidth is encoded by a source encoder. The sourceencoder produces an output signal, i.e., an encoded version of the original signal, in which one or more frequency bands that were included in the original signal are not included any more in the encoded version of the original signal. Normally, the encoded version of the original signal only includes a low band of the original bandwidth. The high band of the original bandwidth of the original signal is not included in the encoded version of the original signal. At the encoderside, there is, in addition, a spectral envelope analyser for analysing the spectral envelope of the original signal in the bands, which are missing in the encoded version of the original signal. This missing band(s) is, for example, the high band. The spectral envelope analyser is operative to produce a coarse envelope representation of the band, which is missing in the encoded version of the original signal. This coarse spectral envelope representation can be generated in several ways. One way is to pass the respective frequency portion of the original signal through an analysis filterbank so that respective subband signals for respective channels in the corresponding frequency range are obtained and to calculate the energy of each subband so that these energy values are the coarse spectral envelope representation.
Another possibility is to conduct a Fourier analysis of the missing band and to calculate the energy of the missing frequency band by calculating an average energy of the spectral coefficients in a group such as a critical band, when audio signals are considered, using a grouping in accordance with the wellknown Bark scale.
In this case, the coarse spectral envelope representation consists of certain reference energy values, wherein one reference energy value is associated with a certain frequency band. The SBR encoder now multiplexes this coarse spectral envelope representation with the encoded version of the original signal to form an output signal, which is transmitted to a receiver or an SBRready decoder.
The SBRready decoder is, as it is known in the art, operative to regenerate the missing frequency band by using a certain or all frequency bands obtained by decoding the encoded version of the original signal to obtain a decoded version of the original signal. Naturally, the decoded version of the original signal also does not include the missing band. This missing band is now reconstructed using the bands included in the original signal by spectral band replication. In particular, one or several bands in the decoded version of the original signal are selected and copied up to bands, which have to be reconstructed. Then, the fine structure of the copied up subband signals or frequency/spectral coefficients are adjusted using gain adjustment values, which are calculated using the actual energy of the subband signal, which has been copied up on the one hand, and using the reference energy which is extracted from the coarse spectral envelope representation, which has been transmitted from the encoder to the decoder. Normally, the gain adjustment factor is calculated by determining the quotient between the reference energy and the actual energy and by taking the square root of this value.
This is the situation, which has been described before with respect to
The inventive device illustrated in
In the other case, in which the inventive device is used for enhancing the operability of an existing SBRready decoder, the means for calculating gain adjustment values for adjacent subband signals can be implemented such that it retrieves the gain adjustment values of two adjacent subband signals, which have an aliasing problem. Since a typical SBRready encoder does not pay any attention to aliasing problems, these gain adjustment values for these two adjacent subband signals are independent on each other. The inventive means for calculating the gain adjustment values is operative to derive calculated gain adjustment values for the adjacent subband signals based on the two retrieved “original” gain adjustment values. This can be done in several ways. The first way is to make the second gain adjustment value equal to the first gain adjustment value. The other possibility is to make the first gain adjustment value equal to the second gain adjustment value. The third possibility is to calculate the average of both original gain adjustment values and to use this average as the first calculated gain adjustment value and the second calculated envelope adjustment value. Another opportunity would be to select different or equal first and second calculated gain adjustment values, which are both lower than the higher original gain adjustment value and which are both higher than the lower gain adjustment value of the two original gain adjustment values. When
In accordance with another embodiment of the present invention, in which the SBRready encoder already performs the features of providing subband signals (block 80 of
In this case, the decoder will receive an already “aliasingreduced” coarse spectral envelope representation together with preferably an indication that the aliasingreducing grouping of adjacent subband signals has already been conducted. Then, no modifications to a normal SBRdecoder are necessary, since the gain adjustment values are already in good shape so that the synthesized signal will show no aliasing distortion.
In the following, certain implementations of the means 84 for providing subband signals are described. In case the present invention is implemented in a novel encoder, the means for providing a plurality of subband signals is the analyser for analysing the missing frequency band, i.e., the frequency band that is not included in the encoded version of the original signal.
In case the present invention is implemented in a novel decoder, the means for providing a plurality of subband signals can be an analysis filterbank for analysing the decoded version of the original signal combined with an SBR device for transposing the low band subband signals to high band subband channels. In case, however, the encoded version of the original signal includes quantized and potentially entropyencoded subband signals themselves, the means for providing does not include an analysis filterbank. In this case, the means for providing is operative to extract entropydecoded and requantized subband signals from the transmitted signal input to the decoder. The means for providing is further operative to transpose such low band extracted subband signals in accordance with any of the known transposition rules to the high band as it is known in the art of spectral band replication or high frequency reconstruction.
As has been outlined earlier, the inventive calculation of gain adjustment values in dependence from each other allows to combine a complex analysis filterbank and a realvalued synthesis filterbank or to combine a realvalued analysis filterbank and a realvalued synthesis filterbank in particular for low cost decoder applications.
In particular, in accordance with the preferred embodiment of the present invention, the means 104 is operative to determine subband signals as subband signals having aliasinggenerating signal components, in case the subband signal having the lower channel number has a positive sign and the subband signal having the higher channel number has a negative sign. When
It is to be noted here that, alternatively, also another prediction polynomial, i.e., a prediction polynomial of third, forth or fifth order can be used, and that also another polynomial coefficient can be used for determining the sign such as the second, third or forth order prediction polynomial coefficient. The procedure shown with respect to equations 1 and 2 is, however, preferred since it involves a low calculation overhead.
Depending on the circumstances, the inventive method of spectral envelope adjustment can be implemented in hardware or in software. The implementation can take place on a digital storage medium such as a disk or a CD having electronically readable control signals, which can cooperate with a programmable computer system so that the inventive method is carried out. Generally, the present invention, therefore, is a computer program product having a program code stored on a machinereadable carrier, for performing the inventive method, when the computerprogram product runs on a computer. In other words, the invention is, therefore, also a computer program having a program code for performing the inventive method, when the computer program runs on a computer.