A preprocessor for low-bit-rate speech coding

In this letter, we propose a preprocessor that modifies the input signal such that it can be coded more effectively in a low-bit-rate speech coder. Since most of the low-bit-rate speech coders are designed based on the human speech production mechanism, the perceived quality of the speech reconstructed in the decoder degrades seriously if the original input signal deviates from the pure speech. In order to alleviate this problem, we introduce a criterion that compromises the quantization error with the distortion incurred due to the modification. The coder-decoder characteristic is described in terms of a transfer matrix, and it is estimated according to the least squares criterion. The proposed approach is found to be effective in reducing audible distortions through a number of listening tests.