Automatic double tracking

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Automatic double-tracking or artificial double-tracking (ADT) is an analogue recording technique designed to enhance the sound of voices or instruments during the mixing process. It uses tape delay to create a delayed copy of an audio signal which is then played back at slightly varying speed controlled by an oscillator and combined with the original. The effect is intended to simulate the sound of the natural doubling of voices or instruments achieved by double tracking. The technique was developed in 1966 by engineers at Abbey Road Studios in London^[1] at the request of the Beatles.