Vocoder

Also in the 1930s, the vocoder (``voice coder'') was developed by Homer Dudley at Bell Telephone Laboratories as a means of reducing the bandwidth required to transmit speech [60]. The vocoder can be regarded as a sinusoidal or noise model, in that it switches between a tonal and a noise signal depending on whether the speech is voiced or unvoiced. The amplitude envelopes for the vocoder's sinusoidal oscillators were measured by means of amplitude followers at the output of a uniform filter bank. A simplified version of Dudley's system, called the ``voder'', was demonstrated at the 1939 World's Fair, being manually operated by trained technicians. The name ``vocoder'', however, applies to automatic systems which synthesize speech based on the results of analysis, i.e., coder-driven synthesis. A surprisingly complete collection of sound examples spanning the history of speech synthesis can be heard on the CD-ROM accompanying a JASA-87 review article by Dennis Klatt [113].