Automatic double tracking

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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. Until the invention of ADT it was necessary to record the vocal tracks twice; 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 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.

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. By adjusting a variable speed oscillator (VSO) that controlled the speed of the motor on the second tape deck, the tape would run 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.

The Beatles were thrilled by Townshend's invention and used it throughout the Revolver album, and on many of their subsequent recordings. Lennon dubbed the technique 'flanging' after producer George Martin jokingly told him it was produced using a "double-bifurcated sploshing flange". Only years later did Martin learn that another technique, also called flanging, was already in use; the term referred to an engineer's pressing the flange of a tape spool as it ran, causing the recording to be delayed.

A similar technique to ADT is doubling echo, which uses short delays to mimic the doubletracking effect. With the advent of digital recording, tape- and analog-based delay methods are largely obsolete.