<- Virtual Exhibitions in Informatics

Music tape

audio compact cassette

The compact audio cassette audio storage medium was introduced by Philips in 1963. The compact cassette had originally been intended for use in dictation machines, but soon became, and remains, a popular medium for distributing prerecorded music. Starting in 1979, Sony's Walkman helped the format become widely used and popular.

The cassette was a great step forward in convenience from reel-to-reel audio tape recording, though because of the limitations of the cassette's size and speed, it compared poorly in quality. Unlike the open reel format, the two stereo tracks lie adjacent to each other rather than a 1/3 and 2/4 arrangement. This permitted monaural cassette players to play stereo recordings "summed" as mono tracks and permitted stereo players to play mono recordings through both speakers. The tape is 1/8 inch (3.175 mm) wide, with each stereo track being 1/32 inch (0.79 mm) wide and moves at 17/8 inches per second (47.625 mm/s). For comparison, the typical open reel format was ¼ inch (6.35 mm) wide, each stereo track being 1/16 inch (1.5875 mm) wide, and running at either 3¾ or 7½ inches per second (95.25 or 190.5 mm/s). Some machines did use 17/8 inches per second (47.625 mm/s) but the quality was poor.
The original magnetic material was based on ferrite (Fe2O3), but then chromium dioxide (CrO2) and more exotic materials were used in order to improve sound quality to try to match or exceed that of vinyl records. Cobalt doped ferrite was introduced by TDK and proved very successful. Sony tried a dual layer tape with both ferrite and chrome dioxide. Finally pure metal particles as opposed to oxide formulations were used. These each had different bias and equalization requirements requiring specialized settings. Ferrite tapes use 120 µS equalization (known as Type 1), while chrome and cobalt doped tape types require 70 µS equalization (Type 2). In practice the cassette shell was modified with indents to automatically select the proper bias and equalization on compatible cassette decks.

Many home computers of the 1980s, notably the TRS-80, Commodore 64, ZX Spectrum, Amstrad CPC and BBC Micro, used cassettes as a cheap alternative to floppy disks as a storage medium for programs and data. Data rates were typically 500 to 2000 bit/s, although some games used special faster loading routines, up to around 4000 bit/s. A rate of 2000 bit/s equates to a capacity of around 660 kilobytes per side of a 90 minute tape.

Related Topics:
Magnetic Tape