Implementation of a channel vocoder synthesizer using a fast, time-multiplexed digital filter

This report describes the implementation of a 19-channel vocoder synthesizer, with a computer driven digital filter. The digital filter is a special purpose digital processor, which produces the transfer function of a second order section, with two poles and two zeros. Since up to 128 indipendent filter calculations can be completed during one sampling interval of the resulting speech wave, it can be time multiplexed into 128 indipendent filter sections; these can be interconnected in several ways, under computer control. The sections are used to implement the filter bank of a channel vocoder; coefficients determining both the structure of the system, and frequency response are also supplied by a computer, in this case a PDP 11/20. The basic scheme of the realized vocoder and its driving program will be discussed in detail. Possible applications include automated voice response systems in which the synthesizer is either controlled by data derived from naturally spoken sentences, or by parameters computed according to dyadic synthesis techniques.