A Military Semi-Vocoder for Analog Telephone Transmission

Abstract

A semi-vocoder for telephone speech is described. It was developed for an Air Force envisioned system of multiple analog vocoders fed from any telephone line to operate over a single voice frequency channel. Such a semi-vocoding plan is shown to offer advantages in the maintenance of voice quality, an important factor in user acceptance. It comprises 500 cps of unvocoded baseband and 250 cps, or 13 spectrum channels, of vocoded information. The direct baseband provides spectrum and excitation information; it facilitates the derivation of pitch when the fundamental frequencies of talkers are missing. Also, the processing of the direct baseband to provide automatic voice excitation obviates the need for an arbitrary voiced-unvoiced decision and excitation by buzz-hiss switching. Errors associated with the latter method are accentuated by the limited frequency range of telephone inputs. The vocoded information is processed by the direct time sampling of the unrectified spectrum channel energy. This method of identifying the speech spectral pattern is entirely feasible when certain requirements are recognized. The frequency occupancy of the complete semi-vocoded speech channel is directed toward multiplexing within a 900-cps total bandwidth inclusive of guard space.