Selectable Mode Vocoder Algorithm

With the booming of the third generation mobile telecommunication, the variable rate speech coding technology, which is the core of it, has been widely studied during recent years. The SMV (Selectable Mode Vocoder) algorithm is the latest speech coding standard submitted by 3GPP2. It adapts to the CDMA system and achieves multi-rate speech coding and decoding. Source and mode control are combines in SMV for rate selection, so it improves the flexibility of CDMA system, it will allow CDMA subscribers to enjoy superior quality while allowing service providers to increase capacity as needed. SMV is regarded as a breakthrough technology that provides significant capacity and quality gains on CDMA systems, so The researching of SMV is of great practical value.The paper introduces the 'algorithm of SMV from the view of encoding and decoding, studies on the technologies such as signal modification scheme, gain determination scheme,the scheme of quantization and fixedcodebook structure that SMV employed to improve the synthesized speech quality. The paper also contrasts it to other existed speech coding standard on the frame length, rate, MOS and complexity.The author realize the selectable mode vocoder under Visual C++6.0 environment. This paper shows the contrastive results between the original speech signal and the synthesized signal of SMV under various modes and environment, it also shows the results of the test of SMV's performance. At last the direction of the SMV's studying is introduced on the basis of analysis of speech signal.