Automatic double tracking (ADT) was an electronic system designed to augment the sound of voices and instruments during the recording process. It used linked tape recorders to create an instant and simultaneous duplication of sound which could then be captured on tape.
During the 1950s it was discovered that doubletracking lead vocals in popular song recordings gave them a much stronger and more appealing sound (especially for singers with weak or light voices). First pairs of tape recorders were used, then later multitrack recording machines, to produce the effect.
However, until the invention of ADT it was necessary to record the vocal tracks twice, with the second vocal in synchronisation with the first -- a process which was both tedious and exacting, and might require several takes and rewinds.
ADT was invented specially for The Beatles on April 6, 1966 by Ken Townshend a recording engineer employed at the EMI Abbey Road Studios in St John's Wood, London. He developed it mainly at the instigation of John Lennon, who hated the tedious doubletracking sessions and regularly expressed a desire for a technical solution to the problem.
In essence, Townshend'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.
However, the doubletracking effect relied on the almost inaudible millisecond delays between the guide vocal and the doubletracked vocal. This was achieved naturally in the old system, because it was in practice impossible for even the best singer to precisely duplicate a previous vocal.
Geoff Emerick, an EMI balance engineer who began working with the Beatles during the recording of the LP Revolver, was able to introduce the fractional delay required by adjusting the variable speed oscillator (VSO) that controlled the speed of the motor on the second tape deck, so that the tape ran slightly slower than on the first deck. With this slight delay now introduced, the signal coming out of the playback head on the first deck would be audibly 'doubled', but the delay was not enough to cause the vocals to be noticeably out of sync.
An alternate method of creating the required delay, if the second deck did not have a variable speed motor, was to simply apply pressure to the rim (or 'flange') of the feed reel on the second tape deck to slow down the tape speed. This led to the invention being dubbed 'flanging' by The Beatles, who were thrilled by Townshend's invention and used it throughout the Revolver LP and on all their subsequent recordings. The invention of ADT soon led to the development of other related studio effects, besides flanging, including chorus and phasing.
ADT quickly became a universal practice in popular music and since its invention it has become rare to hear popular music recordings that do not use it, especially on vocal tracks. Although the tape-based ADT system has since been superseded by digital sound processing technology, doubletracking is still a common production technique.