PHONETICS: Short for voice coders, invented in telephony. Vocoders used a bank of e.g. 10 filters that divided the speech spectrum into that numer of separate, adjacent bands. During speech, changes in the spectrum of the sound wave cause changes in the output levels of individual filter channels. Only the intensity variations in each filter are transmitted, together with the fundamental frequency. At the receiving end the speech is regenerated using a bank of filters, just like the filters at the transmitting end. The output intensity from each analysing filter is used to control the output intensity of the corresponding synthesising filter. In this way, a close replica of the original speech spectrum is obtained. This type of vocoder is called a channel vocoder, because its operation is based on a bank of filter channels.
Tutorial on Vocoder