In his book on ambient music, "The Ambient Century: From Mahler to Moby", Mark Prendergast views "Sgt. Pepper" as the Beatles' "homage" to Stockhausen and Cage, adding that its "rich, tape-manipulated sound" shows the influence of electronic and experimental composer Pierre Schaeffer. Martin recalled that "Sgt. Pepper" "grew naturally out of "Revolver"", marking "an era of almost continuous technological experimentation". The album was recorded using four-track equipment, since eight-track tape recorders were not operational in commercial studios in London until late 1967. As with previous Beatles albums, the "Sgt. Pepper" recordings made extensive use of reduction mixing, a technique in which one to four tracks from one recorder are mixed and dubbed down onto a master four-track machine, enabling the engineers to give the group a virtual multitrack studio. EMI's Studer J37 four-track machines were well suited to reduction mixing, as the high quality of the recordings that they produced minimised the increased noise associated with the process. When recording the orchestra for "A Day in the Life", Martin synchronised a four-track recorder playing the Beatles' backing track to another one taping the orchestral overdub. The engineer Ken Townsend devised a method for accomplishing this by using a 50 Hz control signal between the two machines.