Compact Cassette tape types and formulations

Audio compact cassettes use magnetic tape of three major types which differ in fundamental magnetic properties, the level of bias applied during recording, and the optimal time constant of replay equalization. Specifications of each type were set in 1979 by the International Electrotechnical Commission (IEC): Type I (IEC I, 'ferric' or 'normal' tapes), Type II (IEC II, or 'chrome' tapes), Type III (IEC III, ferrichrome or ferrochrome), and Type IV (IEC IV, or 'metal' tapes).

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