Low delay speech coding

Good speech quality with low-delay coding at 8-16 kb/s has been obtained using backward adaptive analysis-by-synthesis codecs, such as low delay CELP (code excited linear prediction) (future 16 kb/s standard), low-delay vector excitation coding (LD-VXC), and backward adaptive tree/trellis codecs. The author presents design and performance tradeoffs for the low-delay analysis-by-synthesis codecs at rates of 8-16 kb/s. A number of approaches for improving the speech quality at 8 kb/s are discussed. Backward pitch prediction configuration is compared to a closed-loop forward configuration similar to that used in the conventional forward CELP for the adaptive codebook. Finally, the robustness to transmission errors is discussed, and a number of tradeoffs for reducing the sensitivity to transmission errors are presented