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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). I have often heard that The Beatles invented ADT 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.

Can someone please explain the tape delay setup used to achieve automatic double tracking?

[Note: It is likely that a sound engineer, working with The Beatles, invented the ADT technique -- instead of it being an invention of one of John, Paul, George or Ringo.]

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The way double tracking works is to develop width and depth in a track by introducing slight differences in the phase and amplitude of a signal. When you do this with normal double tracking (i.e. recording it twice) this happens as part of the process: you're not going to have the exact same wave from even two near-identical takes.

With ADT you simply recreate this by literally varying the phase and amplitude of the wave while playing it in tandem with a non-alterwed wave.

To do this using tapes you need two tape machines. You firstly record on one of them, and copy to the second. Then you play both in sync, but vary the speed of one recording using some sort of oscillator (possibly your hand). This introduces a slight delay to one of the tracks, and varies the pitch a little, meaning that when the two are combined in an external mixer you end up with an ADT effect.

I would assume the machine that was used with the oscillator was custom-built for this very task (this was a common approach before the audio-digital revolution: many studios would have a team of guys available to go out and literally build an effect when requested by a producer).