LD-CELP: a high quality 16 kb/s speech coder with low delay

The coder is basically a backward-adaptive version of the code-excited linear prediction (CELP) coder, and is named low-delay CELP, or LD-CELP. The low coding delay (less than 2 ms) is achieved by using backward-adaptive predictor and gain, and by using a small excitation vector size. This coder has been implemented in real-time using the AT&T DSP32C floating-point DSP chips. Currently the encoder uses about 90% of the processor time while the decoder takes about 40%. Fixed-point implementation of the coder is also possible with some modifications of the algorithm. The speech quality of 16 kb/s LD-CELP is approximately equivalent to that of the CCITT G.721 32 kb/s ADPCM (adaptive differential PCM) standard. The coder can pass 300, 1200, and 2400 b/s modem signals as well as DTMF (dual-tone multifrequency) tones. From the preliminary results of CCITT laboratory tests, it appears that this coder can meet all CCITT requirements for the 16 kb/s speech coding standard