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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). 'Type 0' was a non-standard designation for early compact cassettes that did not conform to IEC specification. By 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. Double-layer Type III tape formulations, advanced by Sony and BASF in the 1970s, never gained substantial market presence. In the 1980s the lines between three types blurred. Panasonic developed evaporated metal tapes that could be made to match any of the three IEC types. Metal particle tapes migrated to Type II and Type I, ferricobalt formulations migrated to Type I. 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. Specifications Magnetic properties Magnetic recording relies on the use of hard ferrimagnetic or ferromagnetic materials. These require strong external magnetic fields to be magnetized, and retain substantial residual magnetization after the magnetizing field is removed. Two fundamental magnetic properties, relevant for audio recording, are: Saturation remanence limits maximum output level and, indirectly, dynamic range of audio recordings. 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. Coercivity is a measure of the external magnetic flux required to magnetize the tape, and an indicator of the necessary bias level. The coercivity of audio tapes varies from 350 Oe to 1200 Oe. 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. A useful figure of merit of tape technology is the squareness ratio of the hysteresis curve. It is an indicator of tape uniformity and its linearity in analogue recording. 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. The squareness ratio of basic ferric tapes rarely exceeds 0.75, and the squareness ratio of the best tapes exceeds 0.9. Electromagnetic properties Manufacturers of bulk tape provided extremely detailed technical descriptions of their product, with numerous charts and dozens of numeric parameters. From the end user viewpoint, the most important electromagnetic properties of the tape are: 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. Often incorrectly called recording levels, these are always expressed in terms of the tape's output, thus taking its sensitivity out of the equation. Performance at low and middle, and at treble frequencies was traditionally characterized by two related but different parameters: Maximum output level (MOL) is relevant at low and middle frequencies. It is usually specified at 315 Hz (MOL315) or 400 Hz (MOL400), and its value marks the point when the third harmonic coefficient reaches 3%. Further magnetization of the tape is technically possible, but at the cost of unacceptable compression and distortion. 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. The maximum output of Type I tape at 40 Hz is 3–5 dB lower than MOL400, while in Type IV tapes it is 6–7 dB lower. As a result, ferric tapes handle bass-heavy music with apparent ease compared to expensive metal tapes. Double-layer Type III (IEC III, ferrichrome or ferrochrome) tape formulations were supposed to allow bass frequencies to be recorded deeper into the ferric layer, while keeping the high frequencies in the upper chromium oxide layer. At treble frequencies the playback head cannot reliably reproduce harmonics of the recorded signal. This makes distortion measurements impossible; instead of MOL, high-frequency performance is characterized by saturation output level (SOL), usually specified at 10 kHz (SOL10k). Once the tape reaches saturation point, any further increase in recording flux actually decreases output to below SOL. Noise level, usually understood as bias noise (hiss) of a tape recorded with zero input signal, replayed without noise reduction, A-weighted and referred to the same level as MOL and SOL. The difference between bias noise and the noise of virgin tape is an indicator of tape uniformity. Another important but rarely quantified type of noise is modulation noise, which appears only in the presence of a recorded signal, and which cannot be reduced by Dolby or dbx noise reduction systems. Dynamic range, or signal-to-noise ratio, was usually understood as the ratio between MOL and A-weighted bias noise level. High fidelity audio requires a dynamic range of at least 60–65 dB; the best cassettes tapes reached this threshold in the 1980s, at least partially eliminating the need for the use of noise reduction systems. Dynamic range is the most important property of the tape. The higher the dynamic range of music, the more demanding it is of tape quality; alternatively, heavily compressed music sources can do well even with basic, inexpensive tapes. Sensitivity of the tape, referred to that of an IEC reference tape and expressed in dB, was usually measured at 315 Hz and 10 kHz. Stability of playback in time. Low-quality or damaged cassette tape is notoriously prone to signal dropouts, which are absolutely unacceptable in high fidelity audio. For high quality tapes, playback stability is sometimes lumped together with modulation noise and wow and flutter into an integral smoothness parameter. Frequency range, per se, is usually unimportant. At low recording levels (−20 dB referred to nominal level) all quality tapes can reliably reproduce frequencies from 30 Hz to 16 kHz, which is sufficient for high fidelity audio. However, at high recording levels the treble output is further limited by saturation. At the Dolby recording level the upper frequency limit shrinks to a value between 8 kHz for a typical chromium dioxide tape, and 12 kHz for metal tapes; for chromium dioxide tapes, this is partially offs.... Discover the Hans Beekhuyzen popular books. Find the top 100 most popular Hans Beekhuyzen books.