Automatic double tracking
From Freepedia
Artificial 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 using multitrack tape to 'double' lead vocals in popular song recordings gave them a much stronger and more appealing sound (especially for singers with weak or light voices).
However, until the invention of ADT it was necessary to record such vocal tracks twice, first recording the 'guide' vocal, then rewinding the tape and recording a second vocal in precise synchronisation with the first -- a process which was both tedious and exacting.
ADT was invented specially for The Beatles on April 6, 1966 by Ken Townshend a senior 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 double-tracking sessions and regularly expressed a desire for a technical solution to the problem.
In essence, Townshend's system used two professional-quality tape decks which were connected to the recording console, and to each other. As a vocal was being recorded onto the first tape machine, a series of specially installed connections simultaneously fed the signal from the record head of the first tape-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 machine. 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 double-tracking effect relied on the almost inaudible millisecond delays between the guide vocal and the double-tracked 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.
Townshend was able to introduce the fractional delay required by adjusting the variable speed oscillator (VSO) that controlled the pitch (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 synch.
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' (see below) by The Beatles, who were thrilled by Townshend's invention and used it throughout their next LP Revolver and on all their subsequent recordings. The invention of ADT soon led to the development of other related studio effects, including chorus, phasing and flanging.
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, double-tracking is still a common production technique.
