Excitation modeling based on speech residual information

Speech codecs based on code excited linear prediction (CELP) traditionally use an adaptive short-term filter, an adaptive codebook (long-term filter), and a fixed (stochastic) excitation codebook. The authors examined the possibility of replacing the fixed stochastic codebook by an adaptive codebook with adaptation based on the characteristics of the unquantized residual. In a typical 4-kb/s CELP codec, the authors use the spectral magnitude and phase of the unquantized residual to experimentally estimate an upper bound on the performance improvement which could be obtained by excitation codebook adaptation. The results suggest that adaptation methods based only on the spectral magnitude (including fractal-based codebooks) are unlikely to result in significant improvement. Adaptation based on the spectral phase information, on the other, shows a significant potential for improving CELP speech quality. The authors also present results of a preliminary test designed to investigate the effect of quantization noise on phased-based adaptation of excitation codebooks