Channel Vocoder with Digital‐Pitch Extraction

The channel vocoder is a speech processor that provides a moderate amount of band compression. Unfortunately, this compression is generally accompanied by a severe degradation in quality and intelligibility. The degradation is usually attributed to both inadequate pitch extraction and poor spectrum reproduction. Work at Lincoln Laboratory during the past two years has resulted in a digitized channel vocoder with acceptable output‐speech quality and compression on the order of six or seven to one (5000–6000 bits/sec). This is due primarily to the use of a sophisticated digital pitch extractor at the vocoder analyzer and flattening of the excitation spectrum at the synthesizer. The pitch extractor is of a novel design, utilizing six elementary analog pitch extractors as inputs to a real‐time digital processor that chooses the correct pitch period with high accuracy. The excitation spectrum is flattened before modulation at the synthesizer with a filter‐bank and limiter‐bank combination. Output speech is demonstrated.