A 4 kbps adaptive fixed code-excited linear prediction speech coder

We propose an adaptive fixed code-excited linear prediction (AF-CELP) speech coder operating at 4 kbps. By exploiting the fact that a fixed codebook contribution to the speech signal is also periodic as the corresponding adaptive codebook contribution, the adaptive fixed codebook model efficiently represents excitation signals. In order to overcome the quality degradation caused by the coarse quantization of excitation, a paired pulse algebraic codebook structure is also applied to the excitation model. Additionally, a pitch prefiltering, a noise spreading, and a harmonic enhancement technique are adopted in the decoding process. The spectrogram reading and informal listening tests proved that the AF-CELP reproduces high quality speech