Research on MBE algorithm at bit rate 800 bps-2.4 kbps vocoder

This paper presents a variable-rate speech coding at 800 bps, 1.2 kbps and 2.4 kbps based on the algorithm of multi-band excitation (MBE). It takes the advantages of MBE algorithm in pitch estimation, UV decision and the estimation of spectral amplitude vector. The main difficulty of at the low bit rate of the MBE algorithm lies in an efficient quantization of the spectral amplitude vector. The technique of linear prediction (LP) is used, and the spectral amplitude vectors are approached by an all-pole model with formants to be enhanced. The techniques of vector quantization (VQ) is adopted to quantize the LP parameters. The split-VQ (SPVQ), and multi-stage VQ (MSVQ) are employed to reduce the storage capacity and computing complexity, and max-min method is used to accelerate the searching of the VQ codebook. The 800 bps, 1.2 kbps and 2.4 kbps vocoder has been implemented in real-time using a single fixed point DSP ADSP2181 chip. The synthesized speech of this vocoder is clear and intelligible even in the presence of background noise at 80 dB. The speech quality of this vocoder has reached the current international level.