Cepstral residual vocoder for improved quality speech transmission at 4.8 kbps

The cepstral representation of the LPC residual signal was found in an earlier study to provide a convenient framework for efficiently representing the main spectral components of the residual signal. The analysis-synthesis system developed uses in general poles, zeros and cepstral residual terms to better represent the input speech signal spectrum and achieves improved synthesized speech quality. In this work we consider the quantization of the system parameters and its effect on the system performance in terms of spectral mismatch. The results are used to develop a cepstral residual vocoder system for 4.8 kbps transmission of speech. The system is based on an 8-pole LPC model and the cepstral representation of the residual signal. It applies optimal uniform quantization to each term of the cepstral residual and obtains most of the attainable spectral matching at this useful rate. The study is based on computer simulations with telephone bandwidth speech.