Stereo-Pak

The Muntz Stereo-Pak, commonly known as the 4-track cartridge, is a magnetic tape sound recording cartridge technology. The in-car tape player that played the Stereo-Pak cartridges was called the Autostereo, but it was generally marketed under the common Stereo-Pak trade name.

The Stereo-Pak cartridge was inspired by the Fidelipac 3-track tape cartridge system invented by George Eash in 1954 and used by radio broadcasters for commercials and jingles beginning in 1959. The Stereo-Pak was adapted from the basic Fidelipac cartridge design by Earl "Madman" Muntz in 1962, as a way to play prerecorded tapes in cars.

The tape is arranged in an infinite loop which traverses a central hub and crosses a tape head, usually over a pressure pad to assure proper tape contact. The tape moves at 3¾ inches per second, pulled by tension, but this tension is dampened by a lubricant, usually graphite, on the back of the tape to prevent a tape's tension from damaging the tape and/or player. The tape ends in a Stereo-Pak are not connected by a splice made of a conductive material — as are the later "automatic" switching 8-track cartridges. 4-track cartridge players had to be switched manually between programs 1 & 2 by a lever on the machine. Due to the method by which the tape is moved, it is impossible to rewind and often risky to fast forward a 4-track tape.