Revolver.

It was Revolver. The four-track limitation at Abbey Road forced George Martin and the engineering team to get creative, leading directly to inventions like ADT (Automatic Double Tracking) and heavy use of varispeed. Sgt. Pepper gets the fame for innovation, but Revolver is where they had to break the rules just to fit everything onto tape.

The album was recorded on a Studer J37 four-track machine. To layer sounds, they constantly bounced tracks (mixing down to one track), which required careful planning. ADT was born because John Lennon hated doing double takes—Ken Townsend at Abbey Road built a device to copy the vocal onto a second tape machine with a slight delay. V