Methods and apparatus for noise conditioning in digital speech compression systems using linear predictive coding
First Claim
1. A method for processing a speech signal comprising a plurality of successive signal intervals, each signal interval comprising a plurality of successive signal samples, the method comprising:
- classifying each signal interval containing no speech sounds as a noise interval;
classifying each signal interval containing speech sounds as a speech interval;
calculating long window LPC coefficients for each noise interval based on the samples of that noise interval and a plurality of preceding signal intervals;
calculating excitation parameters for each noise interval based on the samples of that noise interval and the long window LPC coefficients calculated for that noise interval;
calculating short window LFC coefficients for each speech interval based only on the samples of that speech interval;
calculating excitation parameters for each speech interval based on the samples of that speech interval and the short window LPC coefficients calculated for that speech interval; and
combining the LPC coefficients and the excitation parameters calculated for each signal interval to encode that signal interval.
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Abstract
In methods and apparatus for processing a speech signals comprising a plurality of successive signal intervals, each signal interval containing no speech sounds is classified as a noise interval, and LPC coefficients are calculated for each noise interval based on the samples of that noise interval and on the samples of a plurality of preceding signal intervals. When noise intervals encoded using LPC coefficients calculated as described above are reconstructed, the subjectively annoying "swishing" or "waterfall" effects encountered in conventional LPC speech processing systems are reduced or eliminated.
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Citations
20 Claims
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1. A method for processing a speech signal comprising a plurality of successive signal intervals, each signal interval comprising a plurality of successive signal samples, the method comprising:
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classifying each signal interval containing no speech sounds as a noise interval; classifying each signal interval containing speech sounds as a speech interval; calculating long window LPC coefficients for each noise interval based on the samples of that noise interval and a plurality of preceding signal intervals; calculating excitation parameters for each noise interval based on the samples of that noise interval and the long window LPC coefficients calculated for that noise interval; calculating short window LFC coefficients for each speech interval based only on the samples of that speech interval; calculating excitation parameters for each speech interval based on the samples of that speech interval and the short window LPC coefficients calculated for that speech interval; and combining the LPC coefficients and the excitation parameters calculated for each signal interval to encode that signal interval. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9, 10)
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11. Apparatus for processing a speech signal comprising a plurality of successive signal intervals, each signal interval comprising a plurality of successive samples, the apparatus comprising processing means and storage means for storing instructions for operation of the processing means, said instructions implementing functional blocks comprising:
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a speech detector for distinguishing signal intervals containing speech sounds from signal intervals containing no speech sounds; a short window LPC analyzer for calculating LPC coefficients for each signal interval containing speech sounds based only on the samples of that speech interval; a long window LPC analyzer for calculating LPC coefficients for each signal interval containing no speech sounds based on the samples of that signal interval and on the samples of a plurality of preceding signal intervals; an excitation analyzer for calculating excitation parameters for each signal interval based on the samples of that signal interval and the LPC coefficients selected for that signal interval; and an encoder for combining the LPC coefficients and the excitation parameters calculated for each signal interval to encode each signal interval. - View Dependent Claims (12, 13)
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14. Apparatus for processing an LPC encoded speech signal, the LPC encoded speech signal comprising a plurality of successive encoded signal intervals, each signal interval comprising a respective set of LPC coefficients and a respective set of excitation parameters representing the speech signal over a respective time interval, the apparatus comprising processing means and storage means for storing instructions for operation of the processing means, said instructions comprising:
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a decoder for extracting LPC coefficients and excitation parameters for each successive encoded signal interval from the LPC encoded speech signal; a synthesis filter for reconstructing speech signal intervals from the extracted LPC coefficients and excitation parameters, each reconstructed speech signal interval comprising a plurality of successive signal samples; a speech detector for distinguishing reconstructed speech signal intervals containing speech sounds from reconstructed speech signal intervals containing no speech sounds; and a low pass filter for attenuating spectral components of at least the reconstructed speech signal intervals containing no speech sounds at frequencies greater than 3500 Hz relative to spectral components of the reconstructed speech signal intervals at frequencies less than 3500 Hz, the low pass filter being switchable into an output signal path in response to detection by the speech detector of a reconstructed speech signal interval containing no speech sounds to provide an output speech signal interval processed by the low pass filter and being switchable out of the output signal path in response to detection by the speech detector of a reconstructed speech signal interval containing speech sounds to provide an output speech signal interval not processed by the low pass filter.
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15. Apparatus for processing an LPC encoded speech signal, the LPC encoded speech signal comprising a plurality of successive encoded signal intervals, each signal interval comprising a respective set of LPC coefficients and a respective set of excitation parameters representing the speech signal over a respective time interval, the apparatus comprising processing means and storage means for storing instructions for operation of the processing means, said instructions implementing functional blocks comprising:
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a decoder for extracting LPC coefficients and excitation parameters for each successive encoded signal interval from the LPC encoded speech signal; a first synthesis filter element operable to reconstruct speech signal intervals from the extracted LPC coefficients and excitation parameters, each reconstructed speech signal interval comprising a plurality of successive signal samples; a speech detector for distinguishing reconstructed speech signal intervals containing speech sounds from reconstructed speech signal intervals containing no speech sounds; a long window LPC analyzer operable to calculate long window LPC coefficients for at least the reconstructed speech signal intervals containing no speech sounds, the long window LPC coefficients for each reconstructed speech signal interval being based on samples of said reconstructed speech signal intervals and a plurality of reconstructed speech signal intervals preceding said reconstructed speech signal interval; and a second synthesis filter element operable to reconstruct speech signal intervals from the long window LPC coefficients and the extracted excitation parameters, each reconstructed speech signal interval comprising a plurality of successive signal samples; the long window LPC analyzer and the second synthesis filter being switchable into an output signal path in response to detection by the speech detector of a reconstructed speech signal interval containing no speech sounds to provide an output speech signal interval processed by the long window LPC analyzer and the second synthesis filter, and being switchable out of the output signal path in response to detection by the speech detector of a reconstructed speech signal interval containing speech sounds to provide an output speech signal interval not processed by the long window LPC analyzer and the second synthesis filter. - View Dependent Claims (16)
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17. Apparatus for processing an LPC encoded speech signal, the LPC encoded speech signal comprising a plurality of successive encoded signal intervals, each signal interval comprising a respective set of LPC coefficients and a respective set of excitation parameters representing the speech signal over a respective time interval, the apparatus comprising processing means and storage means for storing instructions for operation of the processing means, said instructions implementing functional blocks comprising:
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a decoder for extracting LPC coefficients and excitation parameters for each successive encoded signal interval from the LPC encoded speech signal; a long window LPC analyzer operable to compute long window LPC coefficients for at least the speech signal intervals containing no speech sounds from the extracted LPC coefficients for that signal interval and the extracted LPC coefficients for each of a plurality of preceding signal intervals; a synthesis filter operable to reconstruct speech signal intervals from LPC coefficients and excitation parameters, each reconstructed speech signal interval comprising a plurality of successive signal samples; a selector for selecting between the extracted LPC coefficients and the long window LPC coefficients for application to the synthesis filter; and a speech detector for distinguishing reconstructed speech signal intervals containing speech sounds from reconstructed speech signal intervals containing no speech sounds; the selector being responsive to the speech detector to apply the extracted LPC coefficients to the synthesis filter upon detecting reconstructed speech signal intervals containing speech sounds and to apply the long window LPC coefficients to the synthesis filter upon detecting reconstructed speech signal intervals containing no speech sounds. - View Dependent Claims (18, 19)
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20. A method for processing a speech signal comprising a plurality of successive signal intervals, each signal interval comprising a plurality of successive signal samples, the method comprising:
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classifying each signal interval containing no speech sounds as a noise interval; classifying each signal interval containing speech sounds as a speech interval; calculating LPC coefficients for each speech interval based on a respective first plurality of samples comprising the samples of that speech interval; calculating excitation parameters for each speech interval based on the samples of that speech interval and the LPC coefficients calculated for that speech interval; calculating LPC coefficients for each noise interval based on a respective second plurality of samples comprising the samples of that noise interval and a plurality of preceding signal intervals; and calculating excitation parameters for each noise interval based on the samples of that noise interval and the LPC coefficients calculated for that noise interval; wherein the each respective second plurality of samples contains at least ten times as many samples as each respective first plurality of samples.
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Specification