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Usually, the performance of a low-bit-rate speech coder degrades seriously in the presence of various interfering signals such as the background noise, acoustic echo, co-talkers' speech and other unwanted signals. This comes from the mismatch between the input signal and the assumed speech production model on which the design of the given speech coder is based. In this paper, we present an approach to modify the input signal such that it can be coded more effectively within the generalized analysis-by-synthesis framework. Signal modification in the presented approach is performed according to a criterion which makes a compromise between the modification and coder quantization errors. The coder-decoder (CODEC) characteristic is described in terms of a transfer matrix, and an on-line method using the recursive least square (RLS) technique is proposed to estimate it. Since each part of the speech signal is differently affected by the modification, we also devise an adaptive method based on the signal-to-quantization noise ratio (SQNR). In contrast to the conventional modification techniques, our approach can be implemented as a simple front-end for any analysis-by-synthesis type coders.