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.
ADT was invented specially for the Beatles[2] during the spring of 1966 by Ken Townsend, a recording engineer employed at EMI's Abbey Road Studios,[2] mainly at the request of John Lennon, who despised the tedium of double tracking during sessions and regularly expressed a desire for a technical alternative.
[footnotes 1] Most of the double-tracked vocals heard on the rest of the album were created using ADT, while the group also used the technique on a number of the instrumental parts to colour the sounds – there is factually more use of ADT on the mono version of the album than on the more widely known stereo version, with the lead guitar on "Taxman" and the backwards guitar on "I'm Only Sleeping" treated with the effect.
Over the years, many artists, including the Beatles, continued to use both manual double tracking, ADT, or a combination of both in different circumstances depending on the effects they wished to achieve, with each technique thought to have certain unique qualities of its own.
Former Beatles engineer Norman Smith used ADT extensively on Pink Floyd's debut album The Piper at the Gates of Dawn, recorded at Abbey Road in 1967.
As well as using it for more conventional simulated double tracking, Smith made much use of the technique to split Syd Barrett's vocals between the stereo channels.
In some cases, Smith (or possibly Barrett himself) used such extraordinarily wide ADT in this way as to give the slightly disorientating impression of not so much double tracking but two quite separate voices on either channel wildly out of time with each other – the best example of this is perhaps on "Bike".
In the US, Simon and Garfunkel began to use ADT on stereo mixes of their songs to split vocal tracks between the channels, examples of which include "Mrs. Robinson" and "Cecilia".
There has since been a thriving market among guitarists and other musicians for guitar pedals, or effects units, reproducing chorus and delay that owe their development to ADT.
A notable example of this technique is "Itchycoo Park" by the Small Faces, where the effect is prominent almost throughout the entire track, particularly on the vocals, drums and cymbals during the chorus.
Some musicians and engineers may casually use the term ADT to refer to any form of simulated double tracking, including digital delay used in this manner.
[7] One of the very few examples of the original ADT technique being used in recent times is on the Beatles' Anthology albums from the mid-1990s, on which George Martin and Geoff Emerick decided to revive the analogue technique rather than simply use the modern digital alternatives to achieve a more authentic sound, feeling that ADT produced a warmer, less synthetic sound than digital delay and the latter would be inappropriate for use on recordings made on analogue equipment in the 1960s.