The Research And Simulation Of Selectable Mode Vocoder Algorithm

The modern mobile communication requires low coding rate to expand system capacity and high synthesized speech quality to guarantee fine communication. Variable rate speech coding technology is the core one of the third generation mobile communication system, and it's able to achieve an ideal balance between both above.As one of the important variable rate speech coding standards, SMV (Selectable Mode Vocoder) algorithm was standardized by 3GPP2 for applying to the Wideband Spread Spectrum Communication Systems. It achieves low multi-rate speech coding and decoding. Meanwhile, it can provide good synthesized speech quality at low coding rate as well as the flexibility of allowing the system operator to make tradeoffs between speech quality and system capacity. SMV represents the developing direction of the present speech coding techonology. Therefore, making research on SMV is of great value in theory and practical significance.At first, this paper introduced the encoding and decoding principles of the SMV algorithm, and mainly studied the key technologies in SMV encoder, including rate decision algorithm, signal modification mechanism, gain determination mechanism, quantization mechanism, adaptive codebook search and fixed codebook search, which are used to improve the synthesized speech quality. The Frame Erasure Concealment algorithm and Post-processing algorithm, which are selectable, were also studied.Then the SMV algorithm was simulated under Linux environment on the basis of its theoretical research. This paper showed the contrastive results between the original speech signals and the synthesized signals of SMV under various modes and circumstances. It also tested the performance measures of SMV such as average coding rate, encoder and decoder complexities, PESQ score and Itakura Distance Measure. It took objective evaluations of SMV's decoded speech quality, showed the waveforms and analysis of the experimental results.These experimental results proved that SMV has the characters of flexibility in adjusting average coding rate, fine synthesized speech, accepctable hardware costs in fact and balancing between communication quality and channel availability, so it can completely satisfy the third generation mobile communication system's commands of high quality and high capacity ,and be an ideal option for it.