Vocoder

A vocoder is an analysis and synthesis system, used to reproduce human speech. The vocoder was originally developed as a speech coder for telecommunications applications in the 1930s, the idea being to code speech for transmission. 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 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 encrypting the control signals, voice transmission can be secured against interception. Its primary use in this fashion is for secure radio communication. The advantage of this method of encryption is that none of the original signal is sent, but rather envelopes of the bandpass filters. The receiving unit needs to be set up in the same filter configuration to resynthesize a version of the original signal spectrum.