Variable rate CELP based on subband flatness

With the standardization of Qualcomm´s QCELP and the deployment of various digital multiple-access networks, the implementation of variable-rate speech coding schemes has become an area of significant interest. Code-excited linear prediction (CELP) is the predominant coding methodology for communications quality speech coding below 8 kbps, and several variable-rate CELP schemes have been discussed in the literature. We propose a variable-rate CELP architecture wherein cues for rate variation are derived from subband measures of spectral flatness using the entropy functional. We also discuss a variable-rate coding scheme for multiple-tap pitch filters. Using reasonable assumptions about voice activity and instantaneous speech bandwidths, our coder can achieve an average rate of below 2000 bits/sec while maintaining communications quality in the encoded speech