Automatic double-tracking. A tape delay is used to simulate double-tracking using only one recording.
Automatic double tracking imitates the effect of doubling voices or instruments using double tracking, without the need for a musician to overdub their part (they perform their part only once).
Abbey Road studio engineers working on the The Beatles sessions invented ADT during the late 1960s using tape delay. For example, the guitar solo on “Taxman” and the vocals on “I’m Only Sleeping” are said to be double-tracked using tape delay.
