Vocoder

A vocoder (/ˈvoʊkoʊdər/, short for voice encoder) is a category of voice codec that analyzes and synthesizes the human voice signal for audio data compression, multiplexing, voice encryption, voice transformation, etc.

The earliest type of vocoder, the channel vocoder, was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code speech in order to reduce bandwidth (i.e. audio data compression) for multiplexing transmission. In the channel vocoder algorithm, among the two components of an analytic signal, considering only the amplitude component and simply ignoring the phase component tends to result in an unclear voice; on methods for rectifying this, see phase vocoder.

In the encoder, the input is passed through a multiband filter, then each band is passed through an envelope follower, and the control signals from the envelope followers are transmitted to the decoder. The decoder applies these (amplitude) control signals to corresponding filters for re-synthesis. Since these 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 single communication channel, such as a radio channel or a submarine cable (i.e. multiplexing).

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