We have created a novel 8 kb/s Low-Delay CELP coder which achieves good speech quality with a one-way delay of 7 ms to 10 ms. In this coder, we used backward adaptation to update a 10-th order LPC predictor and the excitation gain. In addition, we designed a 3-tap pitch predictor where the pitch period was differentially coded into 4 bits and the 3 tap-weights were vector quantized into 5 to 6 bits, with the codebook search jointly optimizing the pitch period and the 3 taps in a closed-loop manner. The excitation codebook was closed-loop trained and pseudo Gray coded. A postfilter was used in the decoder to enhance speech quality. The resulting 8 kb/s LD-CELP coder now runs full-duplex in real-time on a single 80 ns AT&T DSP32C chip, and it achieves roughly the same speech quality (in terms of Mean Opinion Score) as the established 13 kb/s GSM RPE-LTP and 8 kb/s VSELP cellular radio coding standards but with only 1/5 of the coding delay.

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