ADT was invented specially for The Beatles on April 6, 1966 by Ken Townsend, a recording engineer employed at EMI's Abbey Road Studios, mainly at the instigation of John Lennon. Lennon hated the tedium of doubletracking during sessions, and regularly expressed a desire for a technical alternative.[1]
In essence, Townsend's system used two studio tape decks which were connected to the recording console, and to each other. As a vocal was being recorded onto the first tape machine, specially installed connections simultaneously fed the signal from the record head of the first deck into the record head of the second deck, onto the tape, out from the playback head of the second deck and back into the record head of the first. If the playback heads of the two decks were precisely the same distance from their respective record heads, the voices would be recorded in perfect unison.