Digital vocoder

Abstract

This invention relates to a digital vocoder where speech is translated to a digital version thereof. An adaptive filter in the form of an N stage Itakura type cascade derives from the speech a prediction residual and N spectral parameter in the form of weighting coefficients of the filter. The pitch period, derived from the prediction residual, the voiced/unvoiced decision derived from the first of the weighting coefficients and the peak amplitude of a pitch pulse, and a digital signal representing filter gain, derived from the last of the weighting coefficients are multiplexed with each other and the N spectral parameters for transmission to a receiver. The receiver employs an excitation generator responsive to the pitch period, the gain signal and the voiced/unvoiced decision to produce an excitation signal to excite an N/2 cascade connected stage receive filter. Each stage of the receive filter operates on a time shared basis on an adjacent pair of the N spectral parameters. The receive adaptive filter operation is the inverse of the transmit adaptive filter so that after digital to analog conversion the original speech is substantially reproduced.