Automatic double tracking

Automatic 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 doubletracking lead vocals in popular song recordings gave them a much stronger and more appealing sound (especially for singers with weak or light voices). First pairs of tape recorders were used, then later multitrack recording machines, to produce the effect. Until the invention of ADT it was necessary to record the vocal tracks twice; a process which was both tedious and exacting, and might require several takes and rewinds.
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