Improved 13 Kb/s speech coder for PCS

This paper describes efforts to improve of a 13 Kb/s speech coder. The coder is used for upbanded IS-95, a personal communications system (PCS) standard. This speech coder is called Qualcomm code excited linear prediction (QCELP). QCELP coder uses the closed-loop search for all pitch delays in each pitch subframe. This pitch search requires much computation time. And the coder has two codebooks, each of which consists of 128 circular entries which are generated from a Gaussian source. This kind of circular Gaussian codebook reduces the storage complexity, but doesn't have the speech property. The improved speech coder uses the two-stage pitch analysis for complexity reduction and the optimized codebooks which have the speech property and the circular structure. The improved coder shows better performance than that of the original QCELP coder in speech quality.