Stereo-Pak

The endless loop tape cartridge was designed in 1952 by Bernard Cousino of Toledo, Ohio.[6][7]

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 and the Auto-Com flexidisc[8]

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.[8]

Music came in four cartridge sizes. 1 AA (single) size was an inch wide by two inches long and carried the same amount of time per track (6 minutes) as a 45 RPM EP per side. 2 A size, 4 inches wide by 5 inches long the most common was the same size as the vast majority of NAB (Fidelipac) carts and able to carry a 3-inch reel. 3 B size, six inches wide by 7 inches long was used very infrequently for 2-LP sets and other extended programs and able to carry a 5-inch reel. 4 C size, the largest, able to carry a full 1800-foot 7-inch reel of one-mil tape, used also very infrequently for extremely extended 4-LP sets.

Muntz also developed and marketed a variety of mobile and stationary players and recorders for his 4-track tapes.[9]

The B-and C-size carts would have their stereo sound split to mono and be used for background music systems all the way up to the early `90s when digital took over. In the last part of that period, a last-ditch effort to reduce cost came in the form of reducing the tape speed first to 1-7/8 IPS and then to 15/16 IPS while reducing the cartridge size first back to the standard widely available A-size and then to a hybrid size between the AA (single) size and the standard A-size. First Chromium high-bias tape was used to offset the loss of fidelity from the lower speed, and then when that proved too expensive, cobalt-based tape was substituted.

After riding in Muntz's car, listening to his 4-track cartridge system, electronics and aerospace entrepreneur Bill Lear had the a staffer at Lear Jet Corporation create a modified derivative, resulting in the more convenient and long-playing 8-track cartridge system, which quickly supplanted and surpassed the 4-track in the market -- until being surpassed, itself, by the cassette tape system.[2]