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 : Type I, Type II, Type III, and Type IV. '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 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 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 for basic ferric tapes to for Type IV tapes; advertised remanence of the 1986 JVC Type IV cassette reached.
  • 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 to. 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 or the 'Dolby level' of. 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 is relevant at low and middle frequencies. It is usually specified at 315Hz or 400Hz, 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 125800Hz area, while dropping off below and above. The maximum output of Type I tape at is 35dB lower than MOL400, while in Type IV tapes it is 67dB lower. As a result, ferric tapes handle bass-heavy music with apparent ease compared to expensive metal tapes. Double-layer Type III 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, usually specified at . 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 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 6065dB; 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 and.
  • 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 all quality tapes can reliably reproduce frequencies from to, 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 for a typical chromium dioxide tape, and for metal tapes; for chromium dioxide tapes, this is partially offset by lower hiss levels. In practice, the extent of the high-level frequency range is not as important as the smoothness of the midrange and treble frequency response.

Standards

The original specification for Compact Cassette was set by Philips in 1962–1963. Of the three then available tape formulations that matched the company's requirements, the BASF PES-18 tape became the original reference. Other chemical companies followed with tapes of varying quality, often incompatible with the BASF reference. By 1970, a new, improved generation of tapes firmly established themselves on the market, and became the de facto reference for aligning tape recorders — thus the compatibility issue worsened even further. In 1971 it was tackled by the Deutsches Institut für Normung, which set the standard for chromium dioxide tapes. In 1978 the International Electrotechnical Commission enacted the comprehensive standard on cassette tapes. One year later the IEC mandated the use of notches for automatic tape type recognition. Since then, the four cassette tape types were known as IEC I, IEC II, IEC III and IEC IV. The numerals follow the historical sequence in which these tape types were commercialized, and do not imply their relative quality or intended purpose.
An integral part of the IEC 60094 standard family is the set of four IEC reference tapes. Type I and Type II reference tapes were manufactured by BASF, Type III reference tapes by Sony, and Type IV reference tapes by TDK. Unlike consumer tapes, which were manufactured continuously over the years, each reference tape was made in a single production batch by the IEC-approved factory. These batches were made large enough to fill the need of the industry for many years. A second run was impossible, because chemists were unable to replicate the reference tape type formulation with proper precision. From time to time, the IEC revised the set of references; the final revision took place in April 1994. The choice of reference tapes, and the role of the IEC in general, has been debated. Meinrad Liebert, designer of Studer and Revox cassette decks, criticized the IEC for failing to enforce the standards and lagging behind the constantly changing market. In 1987, Liebert wrote that while the market clearly branched into distinct, incompatible "premium" and "budget" subtypes, the IEC tried in vain to select an elusive "market average"; meanwhile, the industry moved forward, disregarding outdated references. This, according to Liebert, explained sudden demand for built-in tape calibration tools that were almost unheard-of in the 1970s.
From the end user viewpoint, the IEC 60094 defined two principal properties of each tape type:
  • Bias level for each type was set equal to the optimal bias of the relevant IEC reference tape, and sometimes changed when the IEC changed the reference tapes, though the BASF datasheet for the Y348M tape, approved as the IEC Type I reference in 1994, says that its optimal bias is exactly 0.0 dB from the previous reference. The IEC reference tape bias definition is: Using the relevant IEC reference tape and heads according to Ref. 1.1, the bias current providing the minimum third harmonic distortion ratio for a 1 kHz signal recorded at the reference level is the reference bias setting. Type II bias equals around 150% of Type I bias, Type IV bias equals around 250% of Type I bias. Real cassette tapes invariably deviate from the references and require fine tuning of bias; recording a tape with improper bias increases distortion and alters frequency response. A 1990 comparative test of 35 Type I tapes showed that their optimal bias levels were within of the Type I reference, while Type IV tapes deviated from the Type IV reference by up to. Some typical cassette deck frequency response curves showing the effects of different bias settings are provided in the relevant figure.
  • Time constant of replay equalization for Type I tapes equals, as per the Philips specification. The time constant for Type II, III and IV tapes is set at a lower value of. The purpose of replay equalization is to compensate for high-frequency losses during recording, which, in case of ferric cassettes, usually start at around 11.5kHz. The choice of time constant is a somewhat arbitrary decision, seeking the best combination of conflicting parameters — extended treble response, maximum output, minimum noise and minimum distortion. High-frequency roll-off that is not fully compensated in the replay channel may be offset by pre-emphasis during recording. Lower replay time constants decrease the apparent level of hiss, but also decrease apparent high-frequency saturation level, so the choice of time constants was a matter of compromise and debate. "Hard" maximum and saturation levels, in terms of voltage output of playback head, remain unchanged. However, the high-frequency voltage level at the output of the replay equalizer decreases with a decrease in time constant. The industry and the IEC decided that it would be safe to decrease the time constant of Type II, III and IV tapes to, because they are less prone to high-frequency saturation than contemporary ferric tapes. Many disagreed, arguing that the risk of saturation at is unacceptably high. Nakamichi and Studer complied with the IEC, but provided an option for playing Type II and Type IV tapes using the setting and matching pre-emphasis filters in the recording path. A similar pre-emphasis was applied by duplicators of prerecorded chromium dioxide cassettes; although loaded with Type II tape, these cassettes were packaged in Type I cassette shells and were intended to be replayed as Type I tapes.