Vocoder

A vocoder ( /ˈvoʊkoʊdər/, short for voice encoder) is an analysis/synthesis system, used to reproduce human 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 synthesizer. Since the control signals change only slowly compared to the original speech waveform, the bandwidth required to transmit speech can be reduced. This allows more speech channels to share a radio circuit or submarine cable. By encoding the control signals, voice transmission can be secured against interception.

The vocoder was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code speech for transmission. Transmitting the parameters of a speech model instead of a digitized representation of the speech waveform saves bandwidth in the communication channel; the parameters of the model change relatively slowly, compared to the changes in the speech waveform that they describe. 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.

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 pedal, basically consisting of the "second half" of the vocoder, but with manual filter controls, needing a highly trained operator.