For all types of tape, MOL reaches a maximum in the 125–800 Hz area, while dropping off below 125 Hz and above 800 Hz. Further magnetization of the tape is technically possible, but at the cost of unacceptable compression and distortion. It is usually specified at 315 Hz (MOL 315) or 400 Hz (MOL 400), and its value marks the point when the third harmonic coefficient reaches 3%. Maximum output level (MOL) is relevant at low and middle frequencies.Performance at low and middle, and at treble frequencies was traditionally characterized by two related but different parameters: Often incorrectly called recording levels, these are always expressed in terms of the tape's output, thus taking its sensitivity out of the equation. Maximum output levels, usually specified in dB relative to the nominal zero reference level of 250 nWb/m or the 'Dolby level' of 200 nWb/m.From the end user viewpoint, the most important electromagnetic properties of the tape are: Manufacturers of bulk tape provided extremely detailed technical descriptions of their product, with numerous charts and dozens of numeric parameters. The horizontal span shows coercivity ― how much flux it takes to magnetize the tapes On this chart, the vertical span is remanence (remaining magnetism), a rough indicator of maximum recording output level. Electromagnetic properties Hysteresis curves of Type I, CrO 2 Type II and Type IV tapes. The squareness ratio of basic ferric tapes rarely exceeds 0.75, and the squareness ratio of the best tapes exceeds 0.9. An increase in the squareness ratio defers the onset of compression and distortion, and allows fuller utilization of the tape's dynamic range within the limits of remanence. It is an indicator of tape uniformity and its linearity in analogue recording. Ī useful figure of merit of tape technology is the squareness ratio of the hysteresis curve. High-coercivity particles are more difficult to erase, bias and record, but also less prone to high-frequency losses during recording, and to external interference and self-demagnetization during storage. The coercivity of audio tapes varies from 350 Oe to 1200 Oe. Coercivity is a measure of the external magnetic flux required to magnetize the tape, and an indicator of the necessary bias level.Remanence of audio tapes, referred to quarter-inch tape width, varies from around 1100 G for basic ferric tapes to 3500 G for Type IV tapes advertised remanence of the 1986 JVC Type IV cassette reached 4800 G. Saturation remanence limits maximum output level and, indirectly, dynamic range of audio recordings.Two fundamental magnetic properties, relevant for audio recording, are: These require strong external magnetic fields to be magnetized, and retain substantial residual magnetization after the magnetizing field is removed. Magnetic recording relies on the use of hard ferrimagnetic or ferromagnetic materials. The values approach the limits of ferric tape technology Frequency response curves of a typical cassette tape showing the effects of different bias settings (after Roberson ) Magnetic properties MOL, SOL, bias noise and dynamic range marked on the wrapper of a superferric cassette (TDK AR, 1990s). By the end of the decade performance of the best Type I ferricobalt tapes (superferrics) approached that of Type IV tapes performance of entry-level Type I tapes gradually improved until the very end of compact cassette production. Metal particle tapes migrated to Type II and Type I, ferricobalt formulations migrated to Type I. Panasonic developed evaporated metal tapes that could be made to match any of the three IEC types. In the 1980s the lines between three types blurred. Double-layer Type III tape formulations, advanced by Sony and BASF in the 1970s, never gained substantial market presence. 'Type 0' was a non-standard designation for early compact cassettes that did not conform to IEC specification.īy the time the specifications were introduced, Type I included pure gamma ferric oxide formulations, Type II included ferricobalt and chromium(IV) oxide formulations, and Type IV included metal particle tapes-the best-performing, but also the most expensive. 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). Top to bottom: Type I (and Type III), Type II, Type IVĪudio 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. Standardized notches for automatic tape selection.
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