A vocoder (pronounced /ˈvoʊkoʊdər/, a combination of the words voice and encoder) is an analysis/synthesis system, mostly used for speech. In the encoder, the input is passed through a multiband filter, each band is passed through an envelope follower, and the control signals from the envelope followers are communicated to the decoder. The decoder applies these (amplitude) control signals to corresponding filters in the (re)synthesizer.
It was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code speech for transmission. Its primary use in this fashion is for secure radio communication, where voice has to be encrypted and then transmitted. The advantage of this method of "encryption" is that no 'signal' is sent, but rather envelopes of the bandpass filters. The receiving unit needs to be set up in the same channel configuration to resynthesize a version of the original signal spectrum. The vocoder as both hardware and software has also been used extensively as an electronic musical instrument.
The vocoder is related to, but essentially different from, the computer algorithm known as the "phase vocoder".
Whereas the vocoder analyzes speech, transforms it into electronically transmitted information, and recreates it, the voder (from Voice Operating Demonstrator) generates synthesized speech by means of a console with fifteen touch-sensitive keys and a foot pedal, basically consisting of the "second half" of the vocoder, but with manual filter controls, needing a highly trained operator.[1]
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