With the standardization of Qualcomm's QCELP [1] 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 [2-4]. Here, 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.
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