Synthesis part of a "Log area ratio" vocoder in analog hardware

A speech output terminal for use with a digital computer, corresponding to the synthesis part of a predictive vocoder, is described. It is implemented mainly by analog components, employing as the all-pole synthesis filter the author's previously published analog discrete-time filter. The inverse-hyperbolic-tangent values of the PARCOR (or reflection) coefficients are the control parameters, which are thus proportional to the "log area ratios", well known for their good quantization properties. The filter is excited by a noise and/or a pulse generator through an integrator for high-frequency deemphasis. Pulse frequency and excitation power can be controlled logarithmically. The control parameters are outputted from the computer through a single 6-bit D/A-converter with an analog demultiplexer; the frame rate is variable and software-defined. The parameters are smoothed by RC-circuits in order to avoid the roughness of the synthesized speech due to abrupt parameter changes.