Doubletracking

Doubletracking is an audio recording technique, in which a performer sings or plays along with their own prerecorded performance, usually to produce a stronger or "bigger" sound than can be obtained with a single voice or instrument. It is a form of overdubbing, similar to singing a duet with oneself, except that one attempts to sing or play in near-perfect unison rather harmonize.

Artificial or automatic double tracking|automatic doubletracking, also known as ADT, was developed at Abbey Road Studios by engineers recording the Beatles in the 1960s. It used variable speed tape recorders connected in such a way as to mimic the effect created by doubletracking. ADT produced a unique sound which could be imitated but not precisely duplicated by later analog and digital delay devices, which are capable of producing an effect called doubling echo.

John Lennon referred to his home-studio overdubbing technique as "doubletracking", but this usage isn't technically correct, since he recorded new parts. Lennon's post-Beatles albums frequently employed doubling echo on his vocals in place of the ADT. Some critics complained that the effect gave the impression that Lennon recorded all his vocals in a bathroom.