Improvement On G.728 Recommendation And Research On Low Rate LD-CELP Algorithe

The LD-CELP algorithm with low rate and low complexity had very important meaning in the field of communication. G.728 was the only 16kbit/s ITU Recommendation in the low delay speech coding algorithms at present. The research aimed to reduce its rate and its complexity to improve G.728 algorithm. In this research, the complexity of G.728 could be greatly reduced and the average segmented SNR could be increased about 2dB by using normal shape codebook and the exact denotation of exciting gain.It was observed that how different recursive windows affect upon the three filters of G.728. To the fixed quantization of gain, this paper proposed a scheme about gain adaptive predict and gain adaptive quantization, which brought about satisfying results. Based on these results, we proposed a method of codebook search, which decreased the compute amount of G.728 codebook search by 85%. The more experiment of reducing the order of synthesis filter showed that, on the premise of not changing the average segmented SNR, the complexity of G.728 could be decreased by 40%.Adopting normal shape codebook had another good use, which afforded the possibility of directly constructing a new shape codebook without codebook training which needed mass of calculation amount. We tentatively constructed a normal shape codebook in which every codevector included ten samples. An 8kbit/s low delay CELP algorithm was proposed, and based on it, an 8kbit/s algorithm with pitch predictor was proposed too. The experiment programs of these two algorithms afforded an experiment environment for thefuture research of 8kbit/s low delay CELP algorithm.