Magnetic tape and tape recorders have been with us since the 1930s. Although magnetic tape has seen dozens of changes over the years, the basic ideas remain the same.
A magnetic tape has three basic components: a fine metal oxide powder, a binder and the backing. The powder is iron oxide, or some related material, such as chromium dioxide. The binder glues the metal powder to the backing in a thin, uniform layer. Polyester film, sold under the trade name Mylar, is used for the backing---the actual tape. Mylar works well because it's strong, stable and resists stretching. In the 1950s, acetate was used for tape backing, but Mylar replaced acetate, as it was more durable.
Many different metal powder formulations have been used over the years. Iron oxide---common rust---has had the most use because of its low cost and good magnetic characteristics. A major concern is powder particle size and uniformity. Particles that are too large will have poor sound quality.
A tape recorder works by moving the tape past a set of small metal blocks, called heads, at a steady speed. A recorder has erase, record and playback heads. On some units, record and playback functions are combined into the same block. A tape player is simpler, having only a playback head. Each head's role is straightforward; the erase head erases old signals from the tape, the record head writes new signals to the tape and playback reads signals from the tape.
The heads are small, specialized electromagnets. Electric currents pass through them, creating magnetic fields that read or write patterns on the passing tape. Electronic circuits connected to the heads convert these signals into into sound, video or computer data.
A tape recorder has a mechanical aspect also. The tape unwinds, passing from a supply (full) reel to a take-up (empty) reel. The tape speed must be tightly regulated; any changes in speed show up as pitch distortion. The recorder's mechanical parts, called the transport, must also rewind, fast-forward and stop the tape without stretching or stressing it.
Tape has been packaged in dozens of different ways over the years. Early on, tape was available on open reels, ranging from 3.5-inch to 14-inch diameter. Most tape was 1/4-inch wide, with professional grades ranging up to 2 inches. The compact cassette was introduced by Phillips in 1963 and offered convenient, easier handling in a small package. The eight-track tape cartridge was introduced in 1964, containing a long, endless loop of tape that could play continuously. Dictation and telephone answering machines used specialized mini cassettes of their own.
Videotape has also been available on open reels, being the preferred format for professional use. In 1975, Sony introduced the Beta videocassette, and JVC offered the VHS format soon afterward. Although larger than audio cassettes, they used the same idea: tape in a convenient, self-contained package.
Computer tape also was initially offered on open reels, later being sold as a variety of cassettes and cartridges.
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