| This thesis examines further enhancement to ITU-T G.711.1 speech coder. The original G.711 coder is effectively a low band mu-law quantizer. The G.711.1 extension adds noise feed-back and lower band enhancement layer apart from the higher-band. To further improve the core lower-band coding performance the use of both vector quantization and delayed decision multi-path tree encoder in the above coder at the low band portion is studied. The delayed decision multi-path tree encoding is implemented by the (M, L) -- algorithm. The new quantizer takes into account past history, and hence, the error propagation due to noise feed-back, and codes multiple samples under mu-law. The final bitstream is compatible with the G.711.1 decoder and, hence, with the original G.711 decoder. An evaluation method, ITU-T P.862 perceptual evaluation of speech quality (PESQ), is used to evaluate the performance. Both the vector quantizer and tree encoder have better performance than the original core layer encoder in terms of perceptual quality, though they are limited by the increased computational complexity. Future studies are suggested. |
