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Stereo-Pak logo
Media type magnetic tape
Encoding analog signal
Readmechanism tape head
Writemechanism magnetic recording head
Developedby Earl Muntz
Usage Car audio playback

The Muntz Stereo-Pak, commonly known as the 4-track cartridge,[1] 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 with Muntz partnering with Eash, as a way to play prerecorded tapes in cars.[2]

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 endless loop tapes, such as 8-tracks.


The endless loop tape cartridge was designed in 1952 by Bernard Cousino of Toledo, Ohio, around a single reel carrying a continuous loop of standard ¼ inch plastic oxide-coated recording tape running at 3¾ inches/second (9.5 cm/s). Program starts and stops were signalled either by a conductive foil splice or sub-audible tones. The tape was pulled from the center of the reel, passed across the opening at the end of the cartridge and wound back onto the outside of the same reel. The spool itself was freewheeling and the tape was driven only by tension from the capstan. George Eash, also of Toledo, an inventor who had rented space in Cousino's building in the 1950s, 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 and 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 including the Highway Hi-Fi.

Notable celebrities such as Frank Sinatra had 4-track players installed in their cars. Music was released on 4-track tape for automobile enjoyment and later for 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 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, unlike 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 integral 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. Inexpensive adapters were available that permitted 4-track tapes to be played in 8-track players. The adapter was a rubber pinch roller attached to a small metal plate that would clip into the opening in the 4-track cartridge. 4-track tapes are still in-demand by collectors.

Differences between 4-track and 8-track cartridges

The size "A" [4 inch wide] Fidelipac cartridge (or NAB cartridge), often used in radio broadcasting, was also used for Stereo-Pak recordings.

The Stereo-Pak differs from Stereo 8 in its ¼" magnetic tape contains four data (music) tracks, whereas 8-tracks have twice the tracks in the same amount of space. Thus, 4-track tapes have the potential for higher audio fidelity.

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.

Other uses

In the 1970s through the mid-1980s burglar alarms could (in some jurisdictions) be equipped with a tape dialer which would dial a number and repeat a recorded message when the alarm was tripped. Many of these tape dialers used the 4-track transport described here. They have been largely replaced with digital technology, e.g., Ademco SESCOA format dialer.

They were also popular with radio broadcasting and were used for playing commercials and songs, but were eventually replaced by CDs and computers.


  1. ^ "Archived copy". Archived from the original on July 16, 2004. Retrieved 2004. 
  2. ^ "Archived copy". Archived from the original on November 2, 2011. Retrieved 2011. 

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