What does it do?
It used to be that when a singer fluffed a note in the studio, a punch-in or re-recording was necessary to correct the mistake. Thanks to a modern miracle of software technology, however, producers are now blessed (or cursed) with pitch correction effects that can snap any wayward notes to a specific, preordained pitch.
When used as intended such processes are all but inaudible, but when they’re pushed to extremes, the ubiquitous ‘Auto-Tune effect’ is achieved. Not to be confused (though it invariably is) with a vocoder, the sound is synthetic, gargling and, frankly, done to death.
How does it work?
Originally made famous by Antares, there are loads of variations on the original Auto-Tune processor. There are even freeware variants on the theme.
Most of these allow the user to specify a musical scale or play one in via MIDI. Any notes that fall outside of the scale will be shifted to the nearest note in that scale.
Some pitch correction takes place offline, not as you play the audio. Celemony’s Melodyne is like that - it allows you to change not only the pitch but also the duration of individual notes by moving them around a grid. Impressively, it even works on chords.
Use it on...
Uses for pitch correction depend on what you want to achieve. If you need to correct a few notes of a voice or other monophonic instrument, it can save you time, and Melodyne makes a great tool for writing vocal harmonies. If you want that sound, though, you’ll slather pitch correction all over every vocal in sight.