This paper describes a speech coder with low complexity and optimized codebook. A 13 kb/s speech coder is used for upbanded IS-95, a personal communications system (PCS) standard. This speech coder is called Qualcomm code excited linear prediction (QCELP). The QCELP coder uses the closed-loop search for all pitch delays in each pitch subframe. This pitch search requires a large computation time. 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 does not have the speech property. We used a two-stage pitch analysis for complexity reduction and an optimized codebook, which has a speech property and a circular structure, for better Korean speech quality. The improved coder shows a better performance than that of the original QCELP coder in terms of complexity and speech quality.
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