Modified interpolation of LSFs based on optimization of distortion measures

The new telecommunications services have been pushing toward the development of improvements in speech coding, because of the need to improve encoded speech quality, using the lowest transmission rate possible. This study analyzes and proposes a method to adjust LSF parameters in order to improve their accuracy, minimizing the losses in the encoded LSFs interpolation process. With this scheme, the synthesized speech perceptual quality at the decoder end is increased, without relying on an increase of the transmission rate. We present a mathematical optimization method that minimizes different distortion measures, namely the Euclidean distortion measure and an approximation of the spectral distance in a detailed way. To evaluate the performance of the proposed improvements, the method is implemented in a speech coder with average rates below 2 kb/s. The results confirm that it is possible to obtain significant reduction in distortion measures using the proposed adjustment method of LSFs.