Joint Optimization Of Linear Predictors In Speech Coders

Low bit rate speech coders often employ both formant and pitch predictors to remove near-sample and distant-sample redundancies in the speech signal. The coefficients of these predictors are usually determined for one prediction filter and then for the other (a sequential solution). This paper deals with formant and pitch predictors which are jointly optimized. The first configuration considered is a combination prediction error filter (in either a transversal or a lattice form) that performs the functions of both a formant and a pitch filter. Although a transversal combination filter outperforms the conventional F-P (formant followed by pitch) sequential solution, the combination filter exhibits a high incidence of nonminimum phase filters. For an F-P cascade connection, combined solutions and iterated sequential solutions are found. They yield higher prediction gains than the conventional F-P sequential solution. Furthermore, a practical implementation of the iterated sequential solution is developed such that both the formant and pitch filters are minimum phase. This implementation leads to decoded speech of higher perceptual quality than the conventional sequential solution.