Automatic double tracking

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 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 originally developed in 1966 by engineers at Abbey Road Studios in London at the request of The Beatles.

As early as the 1950s, it was discovered that double tracking the lead vocal in a song gave it a richer, more appealing sound, especially for singers with weak or light voices. Use of this technique became possible with the advent of magnetic tape for use in sound recording. Originally, a pair of single-track (or "mono") tape recorders were used to produce the effect; later, multitrack tape machines were used. Early pioneers of this technique were Les Paul and Buddy Holly. Before the development of ADT, it was necessary to either record the vocal track twice on two different tracks of a multitrack tape, or to record the vocal first on one tape, then again on a second tape while simultaneously copying the first to the second—a process that could be both tedious and exacting, and might require several takes. After the development of ADT, this process became known as "manual double tracking".

ADT was invented specially for the Beatles during the spring of 1966 by Ken Townsend, a recording engineer employed at EMI's Abbey Road Studios, mainly at the request of John Lennon. Lennon hated the tedium of double tracking during sessions and regularly expressed a desire for a technical alternative.

Because it is nearly impossible for a performer to sing or play the same part in exactly the same way twice, a recording and blending of two different performances of the same part will create a fuller, "chorused" effect with double tracking. But if one simply plays back two copies of the same performance in perfect sync, the two sound images become one and no double tracking effect is produced.

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