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Adaptive encoding of fixed codebook in CELP coders

We propose an adaptive encoding method of fixed codebook in CELP coders and implement an adaptive fixed code excited linear prediction (AF-CELP) speech coder. The AF-CELP exploits the fact that the fixed codebook contribution to the speech signal is also periodic as the adaptive codebook (or pitch filter) contribution. By modeling the fixed codebook with the pitch lag and the gain from the adaptive codebook, the AF-CELP can be implemented at low bit rates as well as low complexity. Listening tests show that a 6.4 kbit/s AF-CELP has a comparable quality to the 8 kbit/s CS-ACELP.