The vocoder
The quintessential ‘robot voice’ effect, vocoders transform normal speech into a ‘talking synth’, with results ranging from menacing monotone commands and choir-esque pads to melodic vocal lines. The vocoder was never originally intended for music-making, though – it was invented in the 1930s as a voice encoder (hence the name: VOice enCODER) for transmission of speech within a limited bandwidth. A form of audio compression, in other words (though we wouldn’t recommend running your mixdowns through a vocoder – stick with MP3 for now).
The vocoder’s musical possibilities were explored by synth music producers in the 60s and 70s, and before long, singing robot vocals were ‘a thing’. Examples include Kraftwerk’s We Are The Robots, Styx’ Mr Roboto, and the Beastie Boys’ Intergalactic – and yes, sci-fi themes are mandatory when you go full vocoder.
Vocoders work like this: a modulator signal (speech) and a carrier (a synth sound, ideally with plenty of harmonics) are both split into a number of frequency bands, and envelope followers on the modulator bands are used to control the level of corresponding carrier bands, which we hear. So you could either say that the vocoder makes your synth sounds sing, or that it gives your voice the timbre of a synthesiser.
The number of bands varies from vocoder to vocoder, but can range from single digits to three figures. The type of filters used also affects the outcome, and some vocoders may also offer options to map the modulator and carrier filter frequencies differently, for more unusual effects. Another common vocoder feature is to allow some of the modulator (voice) signal to pass through, usually high-passed so we hear just the top-end elements such as sibilance, to aid intelligibility.