A device that uses a series of bandpass filters to impose the spectral characteristics of a person's voice (or some other input signal) on a second signal. The first vocoders were developed in the 1930s at Bell Labs, which was seeking a way to encode voice signals in a way that would require less bandwidth on a phone line than actually transmitting voice. The idea was that, on a phone conversation, the speaking party's voice would be encoded at that end of the line and sent to the listening party's phone, where the voice would be reconstructed. Particularly on long distance calls, this would have saved money at a time when bandwidth was very expensive. However, the recreated voice sounded very unnatural, and the cost to build, power and maintain the tube-circuit electronics in the early 20th century would have more than offset the savings in transmission costs.
Bell Labs was still tinkering with the idea in the 1960s, when Bob Moog got wind of what they were doing, and realized the potential for musical uses. Moog constructed the first musical vocoder for Wendy Carlos, who used it for several of her late-'60s efforts including the soundtrack to the movie A Clockwork Orange. In the 1970s, Kraftwerk and the Alan Parsons Project featured vocoders in influential pieces ("Autobahn" and "The Raven" respectively), and other musicians became interested. Vocoders were widely used in the late 1970s in everything from progressive rock to disco, as the cost of the needed components came down and commercial products appeared in the market. The usual design for a musical vocoder uses a set of bandpass filters which analyze the spectrum from the microphone, and the levels measured by these filters set the levels of a second set of bandpass filters. This second set of filters acts on a waveform usually produced by a conventional voltage controlled oscillator played by a keyboard. The vocoder sound is often referred to as a "robot voice", although this depends to a considerable extent on the specific singing and playing technique.
Vocoding is a form of resynthesis.