Frequency domain postfiltering for quality enhancement of coded speech
First Claim
1. A method of postfiltering a speech signal using linear predictive coefficients of the speech signal for enhancing human perceptual quality of the speech signal, the method comprising the steps of:
- generating a postfilter by performing a non-linear transformation the linear predictive coefficients spectrum in the frequency domain;
applying the generated postfilter to the synthesized speech signal in the frequency domain; and
transforming the filtered frequency domain synthesized speech signal into a speech signal in the time domain;
wherein the step of generating a postfilter further comprises the steps of;
representing the linear predictive coefficients spectrum by a time domain vector;
transforming the time domain vector into a frequency domain vector by a Fourier transformation;
inversing the frequency domain vector; and
calculating gains according to the magnitude of the all-pole model vector, wherein the gains include a magnitude and a phase response.
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Abstract
A method and system of performing postfiltering in the frequency domain to improve the quality of a speech signal, especially for synthesized speech resulting from codecs of low bit-rate, is provided. The method comprises LPC tilt computation and compensation methods and modules, a formant filter gain computation method and module, and an anti-aliasing method and module. The formant filter gain calculation employs an LPC representation, an all-pole modeling, a non-linear transformation and a phase computation. The LPC used for deriving the postfilter may be transmitted from an encoder or may be estimated from a synthesized or other speech signal in a decoder or receiver. The invention may be implemented in a linked decoder and encoder. A separate LPC evaluation unit that is responsible for processing and or deriving the LPC may be implemented within the invention.
34 Citations
18 Claims
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1. A method of postfiltering a speech signal using linear predictive coefficients of the speech signal for enhancing human perceptual quality of the speech signal, the method comprising the steps of:
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generating a postfilter by performing a non-linear transformation the linear predictive coefficients spectrum in the frequency domain;
applying the generated postfilter to the synthesized speech signal in the frequency domain; and
transforming the filtered frequency domain synthesized speech signal into a speech signal in the time domain;
wherein the step of generating a postfilter further comprises the steps of;
representing the linear predictive coefficients spectrum by a time domain vector;
transforming the time domain vector into a frequency domain vector by a Fourier transformation;
inversing the frequency domain vector; and
calculating gains according to the magnitude of the all-pole model vector, wherein the gains include a magnitude and a phase response. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A computer-readable medium having computer-readable instructions for performing steps to postfilter a synthesized speech signal using the linear predictive coefficients spectrum of the speech signal comprising the steps of:
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computing the tilt of the linear predictive coefficients spectrum;
compensating the linear predictive coefficients spectrum using the computed tilt;
generating a postfilter by executing a non-linear transformation of the compensated linear predictive coefficients spectrum in the frequency domain; and
applying the generated postfilter to the synthesized speech signal in the frequency domain;
wherein the step of generating a postfilter further comprises the steps of;
representing the linear predictive coefficients by a time domain vector;
transforming the time domain vector into a frequency domain vector by a Fourier transformation;
transferring the frequency domain vector into an all-pole model vector; and
calculating gains according to the magnitude of the all-pole model vector, wherein the gains include a magnitude and phase response. - View Dependent Claims (8, 9, 10, 11)
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12. A computer-readable medium having computer-readable instructions for performing steps to postfilter a synthesized speech signal using the linear predictive coefficients spectrum of the speech signal comprising the steps of:
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computing the tilt of the linear predictive coefficients spectrum;
compensating the linear predictive coefficients spectrum using the computed tilt;
generating a postfilter by executing a non-linear transformation of the compensated linear predictive coefficients spectrum in the frequency domain and executing an anti-aliasing procedure in the time domain; and
applying the generated postfilter to the synthesized speech signal in the frequency domain.
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13. An apparatus for postfiltering a speech signal using a plurality of linear predictive coefficients of the speech signal for enhancing human perceptual quality of the speech signal, the apparatus comprising:
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a Fourier transformation module operable for conducting a Fourier transformation;
an inverse Fourier transformation module operable for conducting inverse Fourier transformation; and
a formant filter comprising formant filter gains, wherein the gains are calculated in the frequency domain by performing a non-linear transformation of the linear predictive coefficients;
wherein the formant filter further comprises;
a linear predictive coefficients tilt computation module for computing the tilt of the linear predictive coefficients spectrum;
a linear predictive coefficients tilt compensation module for compensating the linear predictive coefficients according to the computed tilt of the linear predictive coefficients spectrum;
a formant gain calculation module for calculating formant filter gains in the frequency domain by performing a non-linear transformation of the linear predictive coefficients after tilt compensation, wherein the gains include a magnitude and phase response; and
a gain application module for applying the format filter gains to a speech signal by multiplying the gains and the speech signal in the frequency domain. - View Dependent Claims (14, 15, 16, 17)
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18. An apparatus for use with a postfilter for processing linear predictive coefficients of a signal and providing a frequency domain formant filter gains for a formant filter, the apparatus comprising:
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a linear predictive coefficients tilt computation module for computing the tilt of the linear predictive coefficients;
a linear predictive coefficients tilt compensation module for compensating the linear predictive coefficients spectrum according to the computed tilt of the linear predictive coefficients spectrum; and
a formant filter gain computation module for calculating the frequency domain formant filter gains according to the linear predictive coefficients, wherein the gains include a magnitude and a phase response.
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Specification