In this paper, we propose a simplified transform-based and variable-rate vocoder, which is evolved from the code-excited linear prediction (CELP) coding structure. With pre-emphasis and de-emphasis filters, the transformed-based CELP vocoder incorporates a long-term predictor, a discrete cosine transform (DCT), and pre-filters and postfilters for achieving perceptually weighted quantization. The proposed transform-based vocoder requires less computational complexity with slightly worse quality than the CELP coders. Furthermore, the proposed DCT-based coding structure easily figured with additional DCT coefficients could simultaneously offer low, middle, and high bit rates to adapt the variation of bandwidth for modern Internet or wireless communications.

A fast pitch search algorithm using the skipping technique is proposed to reduce the computation time in CELP vocoder. Based on the characteristics of the correlation function of speech signal, the proposed algorithm skips over certain ranges in the full pitch search range in a simple way. Though the search range is reduced, high speech quality can be maintained since those lags having high correlation values are not skipped over and are used for search by closed-loop analysis. To improve the efficiency of the proposed method, we develop three variants of the skipping technique. The experimental results show that the proposed and the modified algorithm can reduce the computation time in the pitch search considerably, over 60% reduction compared with the traditional full search method.