Excitation problem in speech synthesis Free

Linear predictive coding methods require efficient representation of both the LPC filter and its input excitation to synthesize high‐quality speech at low bit rates. Considerable progress has been made so far in encoding the filter parameters and it is possible to quantize these parameters with only 1600 bits/s without introducing distortion in the synthetic speech signal. However, it is still not possible to encode the LPC filter excitation at low bit rates and maintain high voice quality in the synthetic speech signal. In this paper, the problems associated with low bit representation of the excitation are discussed. To achieve low bit rates, a parametric representation is needed that can provide a compact yet accurate representation of the excitation. Such a compact representation is obtained by expressing the excitation waveform as a linear combination of the eigenvectors of the autocorrelation matrix of the LPC filter's impulse response. This representation allows the study of the effect of changes in the filter excitation on the speech output in a systematic manner. The signal‐to‐noise ratios necessary to represent various eigenvector components in the excitation without producing perceptible distortion in the output speech signal have been determined. Thus the minimum number of bits necessary to reproduce a speech signal is estimated. These results will be discussed in the paper.