Variable rate techniques for CELP speech coding

The development of variable-rate speech coding techniques has recently become an area of significant research interest, and several variable-rate schemes have been discussed in the literature. Here, we discuss general techniques for efficient variable-rate coding and the analysis cues from which these coding modes are derived. These techniques include fixed-update, variable-allocation and variable-update, fixed-allocation approaches to formant and pitch filter encoding. Since code-excited linear prediction (CELP) is popular for communications quality speech coding below 8 kbps, we provide simulation results for the several techniques in the context of variable-rate CELP coders with average encoded rate below 2 kbps.