An Enhanced Adaptive Codebook for a CELP Coder

An adaptive codebook for a onetap pitch filter has been used for determining the pitch filter using an analysis-by-synthesis procedure in CELP coders. In this paper, we first present the formulations for designing an enhanced adaptive codebook for a pseudo-three-tap pitch synthesis filter, which gives a better performance than a conventional one-tap pitch filter. Then, we focus on the stability analysis of the pseud~three-tap pitch filters. W e propose a sufficient test condition with a relaxed stability, which gives a better performmce than a strict stability check. W e have employed the enhanced adaptive codebook based on a pseud*threetap pitch filter with fractional pitch lags for a 4.8 kb/s CELP speech coder. Both objective and subjective quality have been improved with the enhanced adaptive codebook.