Autocorrelation is the simplest and worst pitch detection algorithm. It's highly prone to "octave errors" (not necessarily off by an actual octave, it's just th

Autocorrelation is the simplest and worst pitch detection algorithm. It's highly prone to "octave errors" (not necessarily off by an actual octave, it's just the term for detecting a harmonic/subharmonic of the "true" pitch). Pitch detection algorithms are a fascinating rabbit hole, and designing a good one for a given set of requirements is a real art. Edit: One thing autocorrelation is quite effective for is autotune. Here, you need to snap to the nearest (12-tone equal temperament) note, and it turns out the ratio you calculate to perform that correction is unaffected by the most common octave errors. Eg. If I detect your slightly flat D4 as a slightly flat D5, the correction to get to the nearby D is the same.