The effect sounded different in mono or stereo, as in mono, various phase-related comb-filtering effects could be heard as the speed was varied. By continually making slight adjustments to the speed of the second machine, its output could be moved around in time and pitch relative to the first machine, creating the illusion of a separate take. This was fed to a second tape machine set in record mode, so now the signal feeding the mixer from the replay head of the multitrack and also from the second machine's replay head, which was also routed back into the mixer, were pretty much coincident. When mixing a song that required ADT or Artificial Double Tracking, the track to be treated would be sourced from the recording head of the multitrack tape machine, which meant it played back before the signal reached the replay head.

Ken Townsend's setup involved connecting a primary tape machine to a second machine equipped with varispeed in such a way that two versions of the same audio source could be played back more or less simultaneously. Jimi Hendrix was also fond of the effect. The effect is very pronounced on the Beatles' more psychedelic tracks such as 'Tomorrow Never Knows' and 'Lucy In The Sky With Diamonds', while the Small Faces' 'Itchycoo Park' features a classic example of intense tape phasing on the drum fills. It transpired that an ingenious two-tape-machine setup had been dreamed up by Abbey Road engineer Ken Townsend to help automate some of the time-consuming vocal double-tracking they were doing on a Beatles recording project. I soon discovered that it involved running two signals off tape and then varying the delay time so as to give the impression of two performances being layered if the delay time was allowed to pass through zero, a psychedelic effect that we then knew as tape phasing occurred. The first time I heard automatic double-tracking and tape phasing on a record, I was in college studying electronics, and was intrigued to find out how this effect was being created. Waves have painstakingly recreated its original implementation. An effect invented by Abbey Road engineers has become a studio staple.

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