The syNth Degree: What The Hell Is Vocoding?

Every sound has its own sonic fingerprint. Hear a 'middle C' on a violin, and on a trumpet, and while you can tell it is the same pitch, you can also tell the sounds of the different instruments apart. In the same manner, your friend calls you up on the phone, and as you answer they say, "hey, its me", and you know exactly who it is because you recognize their voice (and because they don't make phones without caller ID anymore). Vocoding takes the sonic fingerprint of a sound, and filters another sound through it.

Yes, But What the Hell is Vocoding?

The most common application for vocoding is to make a synthesizer sound like a human voice, though there are other applications that we will explore. This is a vey important distinction, that the synth sounds like the human voice. When you hear a vocoded synth, make no mistake, you are hearing the synth being effected by the human voice, not the other way around. In this application, the synth is the carrier, and the human voice is the modulator.

Carriers and Modulators

In order to have a vocoded signal, there must be two simultaneous signals; a carrier, and a modulator. The carrier is the signal you hear, or in the vocoding application we are discussing, the synthesizer, and the modulator is the signal that effects, or modulates the carrier signal, in this case a human voice.

What Does the Modulator Modulate?

More specifically, the way that vocoding works is that the carrier signal is silent, except at the frequency bands being modulated. Vocoders take the frequency spectrum and break it up into narrow bands (like rack-mounted 31 parametric EQ racks), the narrower the frequency band, the better the resolution of the modulator signal. As the modulator signal is fed through, the amplitude of the signal at various frequencies opens up, or modulates the bands of the vocoder, letting the carrier signal through. The best way to think about the vocoders frequency bands is that they are volume gates. As the carrier signal plays, all frequencies are silent until the modulator signal hits, and opens up specific frequency bands (the very frequencies that give the modulator signal its sonic fingerprint), and carrier signal plays at just those frequencies. Essentially, the modulator signal forces the carrier signal to occupy its timbral confines.

Rhythmic Uses for Vocoding

A vocoder can use a drum groove as the modulator signal, instead of a human voice, to achieve interesting rhythmic pads and lead synths. Try experimenting with expanded frequency bands. Narrower bands from the modulator signal give you greater clarity, but expanded frequency bands may sound more interesting in this context.

Vocoding examples

Formant Shift

Most vocoder plug-ins have some sort of "shift" parameter. This allows you to change the timbre of the modulator signal, thus affecting the sound of the carrier signal. If a modulator signal affects the frequencies around 1kHz, and you shift it up an octave, than it will modulate frequency bands at 2kHz instead. Note that the perceived pitch of the carrier signal does not change, just the timbral quality from the modulator signal.

Shift automation