Real-time implementation of a variable rate CELP speech codec
Robert Zopf · Summit (Simon Fraser University) · 1995
In a typical voice codec application, we wish to maximize system capacity while at the same time maintain an acceptable level of speech quality. Conventional speech coding algorithms operate at fixed rates regardless of the input speech. In applications where the system capacity is determined by the average rate, better performance can be achieved by using a variable-rate codec. Examples of such applications are CDMA based digital cellular and digital voice storage. . In order to achieve a high quality, low average bit-rate Code Excited Linear Prediction (CELP) system, it is necessary to adjust the output bit-rate according to an analysis of the immediate input speech statistics. This thesis describes a lowcomplexity variable-rate CELP speech coder for implementation on the TMS320C51 Digital Signal Processor. The system implementation is user-switchable between a fixed-rate 8 kbit/s configuration and a variable-rate configuration with a peak rate of 8 kbit/s and an average rate of 4-5 kbit/s based on a one-way conversation with 30% silence. In variable-rate mode, each speech frame is analyzed by a frame classifier in order to determine the desired coding rate. A number of techniques are considered for reducing the complexity of the CELP algorithm for implementation while minimizing speech quality degradation. In a fixed-point implementation, the limited dynamic range of the processor leads to a loss in precision and hence a loss in performance compared with a floating-point system. As a result, scaling is necessary to maintain signal precision and minimize speech quality degradation. A scaling strategy is described which offers no degradation in speech quality between the fixed-point and floating-point systems. We present results which show that the variable-rate system obtains near equivalent quality compared with an 8 kbit/s fixed-rate system and significantly better quality than a fixedrate system with the same average rate. To my parents and my fiance, with love.