Vocoder - History

History

Analog vocoders typically analyze an incoming signal by splitting the signal into a number of tuned frequency bands or ranges. A modulator and carrier signal are sent through a series of these tuned band pass filters. In the example of a typical robot voice the modulator is a microphone and the carrier is noise or a sawtooth waveform. There are usually between 8 and 20 bands.

The amplitude of the modulator for each of the individual analysis bands generates a voltage that is used to control amplifiers for each of the corresponding carrier bands. The result is that frequency components of the modulating signal are mapped onto the carrier signal as discrete amplitude changes in each of the frequency bands.

Often there is an unvoiced band or sibilance channel. This is for frequencies outside of analysis bands for typical speech but still important in speech. Examples are words that start with the letters s, f, ch or any other sibilant sound. These can be mixed with the carrier output to increase clarity. The result is recognizable speech, although somewhat "mechanical" sounding. Vocoders also often include a second system for generating unvoiced sounds, using a noise generator instead of the fundamental frequency.

The first experiments with a vocoder were conducted in 1928 by Bell Labs engineer Homer Dudley, who was granted a patent for it on March 21, 1939. The Voder (Voice Operating Demonstrator), was introduced to the public at the AT&T building at the 1939-1940 New York World's Fair. The Voder consisted of a series of manually-controlled oscillators, filters, and a noise source. The filters were controlled by a set of keys and a foot pedal to convert the hisses and tones into vowels, consonants, and inflections. This was a complex machine to operate, but with a skilled operator could produce recognizable speech.

Dudley's vocoder was used in the SIGSALY system, which was built by Bell Labs engineers in 1943. SIGSALY was used for encrypted high-level voice communications during World War II. Later work in this field has been conducted by James Flanagan.