In CELP coding, stochastic (noise-like) excitation is used in exciting a cascade of long-term and short-term all-pole linear synthesis filters. This appr oach is based on the observation that the ideal excitation, obtained by inverse-filte ring the speech signal, can be modeled for simplicity as Gaussian white noise. Although such stochastic excitation resembles the ideal excitation in its global statisti cal properties, it contains a noisy component that is irrelevant to the synthesis pr ocess. This component introduces some roughness and noisiness in the synthesized speech . The present invention reduces this effect by adaptively controlling the level of the stochastic excitation. The proposed control mechanism links the stochastic excitation to the long-term predictor in such a way that the excitation level is inversely related to the efficiency of the predictor. As a result, during voiced sounds, the excitation level is considerably attenuated and the synthesis is mainly accomplished by exciting the short-term filter with the periodic output of the long-term filter. This reduces the noisiness, enhances the pitch structure of th e synthesized speech and its perceptual quality.
CONSTRAINED-STOCHASTIC-EXCITATION CODING