Research on a new high-grade plan of medium bit rate coding for speech

A new mixed speech coding method at medium bit rate is presented. It utilizes the characteristics of adaptive differential pulse code modulation (ADPCM) and vector quantization (VQ). The authors adopt the ADPCM algorithm which has a good coding effect, to deal with important baseband signals and a fast VQ algorithm is used to quantize the highband signal which is less important to synthetic speech. For the coding program of VQ, the authors propose a very efficient searching method that searches the codebook at a speed of 5-10 times faster than that of a full search scheme, while both have the same quantized noise. Simulations prove that recovered speech has good naturalness, clearness and articulation. In addition, baseband and highband signals can be operated in parallel, so it is easy to realize this new system in real time