Some of the mixing employed automatic double tracking (ADT), a system that uses tape recorders to create a simultaneous doubling of a sound. ADT was invented by Townsend during the "Revolver" sessions in 1966 especially for the Beatles, who regularly expressed a desire for a technical alternative to having to record doubled lead vocals. Another important effect was varispeeding, a technique that the Beatles used extensively on "Revolver". Martin cites "Lucy in the Sky with Diamonds" as having the most variations of tape speed on "Sgt. Pepper". During the recording of Lennon's vocals, the tape speed was reduced from 50 cycles per second to 45, which produced a higher and thinner-sounding track when played back at the normal speed. For the album's title track, the recording of Starr's drum kit was enhanced by the use of damping and close-miking. MacDonald credits the new recording technique with creating a "three-dimensional" sound that, along with other Beatles innovations, engineers in the US would soon adopt as standard practice.