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. An audio recording technique, in which a performer sings or plays along with their own prerecorded part, for dramatic effect or to produce a stronger sound than with a single voice or instrument. ... Multitrack recording (multitracking or just tracking for short) is a method of sound recording that allows for the separate recording of multiple sound sources to create a cohesive whole. ...

However, until the invention of ADT it was necessary to record the vocal tracks twice, with the second vocal in synchronisation with the first -- a process which was both tedious and exacting, and might require several takes and rewinds.

ADT was invented specially for The Beatles on April 6, 1966 by Ken Townshend a recording engineer employed at the EMI Abbey Road Studios in St John's Wood, London. He developed it mainly at the instigation of John Lennon, who hated the tedious doubletracking sessions and regularly expressed a desire for a technical solution to the problem. The Beatles were a British rock music group from Liverpool, England held in very high regard for both their artistic achievements and their considerable commercial success, and have amassed an enormous worldwide fanbase that continues to exist to this day. ... April 6 is the 96th day of the year in the Gregorian calendar (97th in leap years). ... 1966 (MCMLXVI) was a common year starting on Saturday (link goes to calendar) // Events January January 1 - In a coup, Colonel Jean-Bédel Bokassa ousts president David Dacko and takes over the Central African Republic. ... The EMI Group is a major record label, based in Hammersmith in London, in the United Kingdom. ... Abbey Road Studios, created in November of 1931 by EMI in London, England, is best known as the legendary recording studio used by the rock bands Cliff Richard and The Shadows and The Beatles. ... St Johns Wood is a district in the City of Westminster in London near Regents Park. ... The Houses of Parliament and the clock tower containing Big Ben Part of the London skyline viewed from the South Bank London is the capital city of England and the United Kingdom. ... John Winston Lennon (later John Ono Lennon) (October 9, 1940 – December 8, 1980) was best known as a singer, songwriter, poet and guitarist for the British rock band The Beatles. ...

In essence, Townshend's system used two studio tape decks which were connected to the recording console, and to each other. As a vocal was being recorded onto the first tape machine, specially installed connections simultaneously fed the signal from the record head of the first deck into the record head of the second deck, onto the tape, out from the playback head of the second deck and back into the record head of the first. If the playback heads of the two decks were precisely the same distance from their respective record heads, the voices would be recorded in perfect unison.

However, the doubletracking effect relied on the almost inaudible millisecond delays between the guide vocal and the doubletracked vocal. This was achieved naturally in the old system, because it was in practice impossible for even the best singer to precisely duplicate a previous vocal.

Geoff Emerick, an EMI balance engineer who began working with the Beatles during the recording of the LP Revolver, was able to introduce the fractional delay required by adjusting the variable speed oscillator (VSO) that controlled the speed of the motor on the second tape deck, so that the tape ran slightly slower than on the first deck. With this slight delay now introduced, the signal coming out of the playback head on the first deck would be audibly 'doubled', but the delay was not enough to cause the vocals to be noticeably out of sync. Geoff Emerick (born 1946) is a recording studio audio engineer, who is best known for his work with the Beatles albums Revolver, Sgt. ... The Beatles appeared on The Ed Sullivan Show in 1964 as part of their first tour of the United States, promoting their first hit single there, I Want To Hold Your Hand. ... For the album by The Haunted, see rEVOLVEr. ...

An alternate method of creating the required delay, if the second deck did not have a variable speed motor, was to simply apply pressure to the rim (or 'flange') of the feed reel on the second tape deck to slow down the tape speed. This led to the invention being dubbed 'flanging' by The Beatles, who were thrilled by Townshend's invention and used it throughout the Revolver LP and on all their subsequent recordings. The invention of ADT soon led to the development of other related studio effects, including chorus, phasing and flanging. Flanging is a time-domain based audio effect that occurs when two identical signals are mixed together, but with one signal time-delayed by a small and gradually changing amount, usually smaller than 20 ms (milliseconds). ... For the album by The Haunted, see rEVOLVEr. ... The chorus effect is a condition in the way people perceive nearly the same sound coming from more than one source. ... This article is about the musical technique. ... Flanging is a time-domain based audio effect that occurs when two identical signals are mixed together, but with one signal time-delayed by a small and gradually changing amount, usually smaller than 20 ms (milliseconds). ...

ADT quickly became a universal practice in popular music and since its invention it has become rare to hear popular music recordings that do not use it, especially on vocal tracks. Although the tape-based ADT system has since been superseded by digital sound processing technology, doubletracking is still a common production technique.