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 cartidges 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.
The splices in a 4-track tape can break due to age, handling, or poor manufacturing quality. This problem also affects other infinite-loop tapes, such as 8-tracks.
George Eash, also of Toledo, an inventor who had rented space in Cousino's building in the fifties, later revised Cousino's design (1954, receiving a patent in January 1957) and marketed it under the name Fidelipac. These cartridges were first used in radio stations (broadcast cartridges) from 1959 on to program commercials & single song hits.
Entrepreneur Earl "Madman" Muntz of Los Angeles, California saw a potential in these broadcast carts for an automobile music tape system, and in 1962 introduced his "Stereo-Pak 4-Track Stereo Tape Cartridge System" and prerecorded tapes, initially in California and Florida. He licensed popular music albums from the major record companies and duplicated them on these 4-track cartridges, or CARtridges, as they were first advertised.
Previously, music in the car had been restricted mostly to radios. Records, due to their methods of operation and size, were not practical for use in a car, although several companies tried to market an automobile record player: that is where the Motorola Corporation derives its name (motor + Victrola = Motorola).
Notable celebrities such as Frank Sinatra had 4-track players installed in their cars, and music was released on 4-track tape for automobile enjoyment and later, home use.
Muntz manufactured 4-track tape players and pre-recorded 4-track cartridges until approximately late 1970, by which time the Stereo 8 8-track tape had become the dominant format. Columbia Records was one of the few major record labels to release music recorded on 4-track cartridges themselves on a widespread basis.
The Stereo-Pak cartridge had four monaural or two pairs of stereo tracks. To switch back and forth between the two program tracks, a manual lever is engaged, which physically moves the head up and down mechanically. The Stereo-Pak did not switch tracks automatically like the later Stereo 8 cartridges.
The tape was coated with a slippery backing material patented by Cousino, usually graphite, to ease the continuous slip between the tape layers. This coating sometimes also caused the pinch roller to slip, leading to poor speed control and tape flutter. Due to these problems, 4-track cartridges were never popular with audiophiles. While the design allowed simple and cheap players, unlike a two-reel system it didn't permit winding of the tape in either direction. Some players offered a limited fast-forward by speeding up the motor while cutting off the audio but rewinding was impossible.
After taking a ride with Muntz in a 4-track player-outfitted car, Bill Lear, maker of the Lear Jet, modified the 4-track technology to create the Stereo 8 cartridge, widely known as the 8-track. Most notably, eight tracks were squeezed onto the same ¼" tape, reducing potential audio quality, but allowing twice as much music to be put onto the same length of tape. The pinch roller was also an integrated part of the 8-track cartridge, although many early rubber rollers would suffer from deterioration because the rubber had not been fully cured. Once this was discovered all later rubber pinch rollers were "fully cured" (hard) rubber, or plastic rollers (introduced by RCA in 1970) were used instead. Thanks to his connection to Motorola, which made radios for Ford Motors cars, Lear was able to ensure that 8-track players would be included in many Ford cars, and they became popular mainly during the early- to mid-1970s. 4-track tapes gradually faded away and were gone by late 1970, as most people switched to 8-tracks, although players compatible with both 4-track and 8-track tapes were sometimes made. 4-track tapes are still in-demand by Collectors.
The main difference in 4-track cartridge design from 8-tracks is that 4-tracks lack a built-in pinch roller (usually made out of rubber or plastic) which would grip and help move the tape; a hole is left in the cartridge for a pinch roller to be inserted from inside the 4-track player itself. The large opening in the bottom of the 4-track cartridge, for admission of the pinch roller, leaves 4-track tapes more susceptible to trapping dirt and other substances besides those normally found inside cartridges, and requires a greater level of mechanical complexity in the player, as the pinch roller must be inserted and retracted vertically through the bottom of the cartridge.