An 8-bit/s speech coder based on conjugate structure CELP

A high-quality 8-bit/s speech coder based on CS:CELP (conjugate structure code excited linear prediction) with 10 ms frame length is presented. To provide high quality in both error-free and error conditions, it uses four schemes: LSP (line spectrum pair) quantization using interframe correlation, preselection of codebook search, a conjugate structure, and backward adaptation of the VQ (vector quantization) gain. LSP parameters are quantized by multistage VQ with MA prediction. The preselection of the codebook reduces computational complexity and improves robustness. The CS improves the ability to handle random bit errors and reduces memory requirements. The backward adaptation of the VQ gain provides high quality and robustness without having to transmit input speech power information. Subjective testing indicates that the quality of the proposed coder is equivalent to that of the 32 kbit/s ADPCM (adaptive differential pulse code modulation) under error-free conditions. It is also found that the proposed coder is robust against random bit errors.<>