Improved Speech Quality and Efficient Vector Quantization in SELP

A SELP algorithm consisting of a two-stage vector quantization using an adaptive codebook and a stochastic codebook is described. The adaptive codebook is similar to a closed loop long-term prediction filter if the predictor delay is more than one frame length. The performance of the adaptive codebook is improved by extending it to include candidate vectors constructed from past synthetic excitation displaying a high level of periodicity.