NAB cartridge

The Fidelipac, commonly known as a "NAB cartridge" or simply "cart", is a magnetic tape sound recording format, used for radio broadcasting for playback of material over the air such as radio commercials, jingles, station identifications, and music. Fidelipac is the official name of this industry standard audio tape cartridge. It was developed in 1954 by inventor George Eash (although the invention of the Fidelipac cartridge has also been credited to Vern Nolte of the Automatic Tape Company), and commercially introduced in 1959 by Collins Radio at the 1959 NAB Convention. The cartridge was often used at radio stations until the late 1990s, when such formats as MiniDisc and computerized broadcast automation predominated.

The Fidelipac cartridge was the first audio tape cartridge available commercially, based on the endless-loop tape cartridge design developed by Bernard Cousino in 1952, while Eash shared space in Cousino's electronics shop in the early 1950s. Instead of manufacturing the Fidelipac format himself after developing it, Eash decided to license it for manufacture to Telepro Industries, in Cherry Hill, New Jersey. Telepro then manufactured and marketed the format under the Fidelipac brand name.

Fidelipac was originally a ¹⁄₄-inch-wide (6.4 mm) analog recording tape, two-track format. One of the tracks was used for monaural program audio, and the other being used for a cue track to control the player, where either a primary cue tone was recorded to automatically stop the cart, a secondary tone was recorded to automatically re-cue the cart to the beginning of the cart's program material (in some models, two secondary tones, one after the program material, and one before it, were recorded to have the cart machine automatically fast-forward through any leftover blank tape at the end of a cart's program), or a tertiary tone, which was used by some players to trigger another cart player or another form of external equipment. Later versions used three tracks, two for stereo audio, and the third for the cue track.

...
Wikipedia