An improved 8~32kbps embedded speech codec based on ITU-T EV-VBR candidate codec

A candidate codec for ITU-T Embedded Variable Bit Rates (EV-VBR) standardization designed by Speech and Audio Signal Processing Laboratory (SASPL) of Beijing University of Technology is improved in this paper. In the improved codec, the second sub-frame's spectral parameters are quantized additionally to improve the overall quantization precision. Depth-first tree search algorithm replaces the full search and focus search in searching algebraic codebook. Transformation coding (TCX) process is redesigned and applied into the higher three layers which reduces the algorithm complexity dramatically. The objective mean opinion score (MOS) test results show that the improved codec achieves comparable speech quality as G.718 recommendation codec in most test items and has a lower algorithm delay.