Vocoders can be an excellent way to get interesting textures and hear sounds in a new way.
Vocoders have been used in music since the early 1970's when Moog developed a vocoder with Walter Carlos for the soundtrack to A Clockwork Orange. Since then, they have been used by influential electronic artists such as Kraftwerk, in "The Robots" (1978), legendary songwriter and drummer Phil Collins, and more recently pop performer Imogen Heap.
However, the vocoder didn’t actually start life as a musical instrument. Instead, in the 1920s, Homer Dudley at Bell Labs created a device whose function was to make it easier to transmit telephone conversations over long distances by reducing bandwidth. The design of the vocoder broke speech (as an input signal) down into multiple bands, retaining only those necessary for speech intelligibility. This lower-frequency transmission allowed telephone conversations to remain audible, while requiring significantly less copper wire than a full-frequency equivalent, thus increasing bandwidth. This proved to not be a cost effective as originally hoped, however, this method of communications was adapted for secure communications in World War II.
A vocoder works by blending two sources of audio, the first of which is usually a human voice, which is called the Modulator signal. It provides an input signal which is broken down into a number of bands (as per Bell Labs’ original design) using filters running in series. There’s also the Carrier signal, the synthesizer component of the vocoder, which substitutes a traditional Oscillator stage by using a frequency analysis of the Modulator as an audio trigger. In other words, singing into a microphone and then playing keys on the vocoder will trigger the pitches played, producing a multi-voiced, harmonised performance interpretation of the words and notes you sing.
See how this works in practice in the video below: