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.