Low bit rate cepstral vocoder using the log magnitude approximation filter

By using the log magnitude approximation filter as the speech synthesizer, a low bit rate cepstral vocoder was obtained. The synthesis filter is of minimum phase and of pole-zero type, and this vocoder can provide a better spectral fit than that provided by all-pole model. The filter can be easily obtained without transforming the cepstrum to an impulse response and the computation for the filter coefficients is non-iterative. The spectral envelope information is transmitted in a form of cepstral values. Since the coefficient sensitivities of the synthesis filter is sufficiently small, a fairly coarse quantization of the cepstrum is allowed. A reasonable speech quality vocoder was realized at the overall bit rate of 1770 bits/s.