Compact Cassette (also known simply as cassette tape or tape) was a magnetic tape cassette format for audio, introduced by Philips in 1963. Compact Cassettes had two or four tracks (for stereophonic sound) and could be played in both directions.
On stereo cassettes, the two channels were adjacent to each other, making them compatible with mono-players and vice versa. Cassette tape was 3.81mm wide (often given as 4mm or ⅛ inch), and moved at 4.76 cm/s (1⅞ ips).
Different magnetic coatings were used, the original being ferric oxide (usually referred to as ‘normal’ or Type I). Chromium dioxide was introduced soon afterwards (usually referred to as ‘chrome’ or Type II), followed for a short time by a mixture of ferric-oxide and chromium dioxide (ferro-chrome or Type III) and later, pure metal particles were used (referred to as ‘metal’ or Type IV).
Notches on top of the cassette shell indicated the type of tape within. Type I cassettes had only write-protect notches, Type II had an additional pair next to the write protection ones, and Type IV (metal) had a third set in the middle of the cassette shell. These allowed later high-end cassette decks to detect the tape type automatically and select the proper bias and equalization.
All cassettes had a write protection tab that could be broken off to prevent accidental re-recording. By using a piece of adhesive tape, these could be covered to allow later recording.
Tape length usually was measured in minutes of total playing time. The most popular varieties of blank tape were C60 (30 minutes per side), C90, and C120. C90, and especially C120, used thinner tape which was more prone to stretching and breakage. Many other lengths were available, for example C15 for use for data recording.
Most cassettes used a tape leader, but for certain applications such as dictation, these were removed. There were also endless loop tapes available for applications such as answerphones.
It was originally designed for voice dictation use, but pre-recorded music cassettes were available from 1965, and as fidelity improved it became one of the predominant formats for pre-recorded music between the late 1970s and early 1990s.
Its success was partly due to the decision to freely license the format, and also due to its convenience over open reel tape.
It was additionally used for data storage for microcomputers in the late 1970s and 1980s.