artificial double tracking (adt) was invented by ken townsend in 1966, during the recording of "revolver". with the advent of four-track recordings, it became possible to double track vocals whereby the performer sings along with his or her own previously recorded vocal. phil mcdonald, a member of the studio staff, recalled that lennon did not really like singing a song twice - it was obviously important to sing exactly the same words with the same phrasing - and after a particularly trying evening of double tracking vocals, townsend "had an idea" while driving home one evening hearing the sound of the car in front. adt works by taking the original recording of a vocal part and duplicating it onto a second tape machine which has a variable speed control. the manipulation of the speed of the second machine during playback introduces a delay between the original vocal and the second recording of it, giving the effect of double tracking without having to sing the part twice.