When the Beatles were recording Revolver, engineers at the Abbey Road studio invented a new technique called ADT — Automatic Double Tracking:
[Ken] Townsend came up with a system using tape delay…. Townsend’s system added a second tape recorder to the regular setup. When mixing a song, its vocal track was routed from the recording head of the multitrack tape, located before the playback head, and fed to the record head of the second tape recorder. An oscillator was used to vary the speed of the second machine, providing variation in delay and pitch depending on the change in the second machine speed. This signal was then routed from the playback head of the second machine to a separate channel on the mixer. This allowed the vocal delayed by a few milliseconds to be combined with the normal vocal, creating the double-tracked effect.
This made it sound like two almost identical voices were singing. Most of the songs on Revolver used it, and not just on vocals: for instance, the lead guitar on “Taxman” is ADT’d.
But a very similar effect was used decades earlier … in Citizen Kane. When near the beginning of the movie we hear the dying Kane whisper “Rosebud,” the sound of the voice was produced by doubling Orson Welles’s voice using two different periods of reverberation. Double tracking — just not automatic.
