RELP-vocoding with uniform and non-uniform down-sampling

Residual-excited base-band vocoders (RELP-vocoders) are known to be one of the better alternatives for speech vocoding in the range 4.8 to 9.6 kbit/s. This paper proposes non-uniform decimation as one candidate for improving the quality of RELP vocoders. Both fixed and adaptive schemes for the time-varying decimation are discussed. It is shown that an adaptive decimation allows for coding the full band residual if desired. In fact the decimation can be used to adaptively allocate bits in accordance with the instantaneous spectrum. The work is concentrated on developing the possibilities of non-uniform down-sampling and a fully optimized implementation requires further work. Preliminary listening tests indicate that the methods proposed do improve the subjective quality significantly.