I've seen plenty of live musicians singing with a guitar, but recently I've been noticing that some of them were singing in harmony with themselves! Obviously, I soon realised it was the work of a stomp pedal. I assume it just took the input of the singer's voice and pitch-shifted it up or down by an interval (not the trivial intervals of octaves and fifths). My question is about how performers use those pedals.

Do they have to set it to a new key for every new song, since not all thirds are equal? How much control do the performers typically have over what notes are actually outputted? What kinds of scales can be chosen for the harmony?


The old way people use these pedals is to create a preset that specifies one key and how many harmony notes should be added and in some cases which diatonic intervals to add. So a preset might be in C major and always add a diatonic third above and fourth below (making a second inversion diatonic triad). When changing songs or changing keys in the middle of a song, you have to change presets.

The new way (made possible by newer technology) is to take a copy of your guitar signal and route it into an auxiliary input of the harmonizer that is there for exactly this purpose. The harmonizer analyzes the chord being played on the guitar and creates vocal harmonies that match the chord. This technology usually supports MIDI keyboard inputs also or instead. Some harmonizers let you play the exact harmony chord you want on the keyboard and it creates the other vocal doubles necessary to create that chord.

Of course if you prefer the preset method, you can still use that on modern harmonizers.

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    The harmonizer pedals don't really know or care if the harmony source instrument is a guitar, or if the "vocal" is really a vocal or a sax or a lawnmower. A keyboard works just as well as the harmony source and can give the harmonizer more notes to snap to. If the harmony source only plays a power chord and the "vocal" only sings the chord's root note, then the harmonizer won't be able to generate a third, because it won't know if it should be a major or minor third. It also won't be able to add a constant third (or any other interval) above/below, if/when it doesn't know what the scale is. – piiperi Reinstate Monica Aug 27 '19 at 11:00

The early machines needed the song's key to be inputted. That in itself was problematic, in that then, it would only reproduce diatonic notes. Which was fine for most harmony - thirds, fifths, octaves. I worked with a band once that used one, and at a certain place in a song it sounded off. The singer accused the other two singers of being out of tune, until it was pointed out that the song had modulated slightly, and needed a M3 which was being 'sung' by the machine as m3, as it couldn't produce that!

Newer, much more successful machines have the guitar (usually) plugged into it as well. That gives it all or any of the notes that could be used as harmony at any point. So, if the song changes key, even, the machine 'hears' the chord played, and produces appropriate harmony on that basis - once it's been told there's a need for thirds, etc. Provided the guitarist plays exactly the correct chord...

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