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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?

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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, 2019 at 11:00
  • Does it work by looking at where your hand is on the frets, or by parsing the sound coming from the guitar?
    – eric_kernfeld
    Jul 25, 2020 at 19:47
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    @eric_kernfeld By analyzing the sound, they don’t have optical sensors to “see” the frets. Also, as pointed out, they can analyze the sound from other instruments besides guitar. It’s a little bit like how autotune works, except instead of the pitch detection being used to change a pitch, it’s used to generate new pitches.
    – Todd Wilcox
    Jul 26, 2020 at 5:30
  • That makes sense. Are there two separate microphones then? One to fetch the chord from the guitar/piano and another to fetch the main note from the vocalist?
    – eric_kernfeld
    Jul 26, 2020 at 14:34
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    @eric_kernfeld The harmonizer includes no microphones, only two separate inputs. One input is designed to accept the microphone for the voice, so it is usually an XLR input (for a mic cable). The other input is usually a 1/4” TS input to accept a guitar cable or cable from a keyboard. If you want to use a guitar that has no electronics, you could put up a mic and then plug that into a mic preamp and the plug that into the harmonizer, but there’s a problem. The guitar mic will pick up other sounds that may confuse the harmonizer, so it’s not recommended to use a guitar without a pickup.
    – Todd Wilcox
    Jul 26, 2020 at 14:38
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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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