Cassette tapes are made of a polyester type plastic film with a magnetic coating. The original magnetic material was based on gamma ferric oxide (Fe2O3). Circa 1970, 3M Company developed a cobalt volume-doping process combined with a double-coating technique to enhance overall tape output levels. This product was marketed as "High Energy" under its Scotch brand of recording tapes. Inexpensive cassettes are commonly labeled "low-noise," but typically are not optimized for high frequency response.
At about the same time, chromium dioxide (CrO2) was introduced by DuPont, the inventor of the particle, and BASF, the inventor of magnetic recording, and then coatings using magnetite(Fe3O4) such as TDK's Audua were produced in an attempt to approach the sound quality of vinyl records. Cobalt-absorbed iron oxide (Avilyn) was introduced by TDK in 1974 and proved very successful. Finally pure metal particles (as opposed to oxide formulations) were introduced in 1979 by 3M under the trade name Metafine. The tape coating on most Cassettes sold today as either "Normal" or "Chrome" consists of Ferric Oxide and Cobalt mixed in varying ratios (and using various processes); there are very few cassettes on the market that use a pure (CrO2) coating.
Simple voice recorders are designed to work with standard ferric formulations. High fidelity tape decks are usually built with switches or detectors for the different bias and equalizationrequirements for high performance tapes. The most common, iron oxide tapes (defined by an IECstandard as "Type I"), use 120 µs playback equalization, while chrome and cobalt-absorbed tapes (IEC Type II) require 70 µs playback equalization. The recording "bias" equalizations were also different (and had a much longer time constant). BASF and Sony tried a dual layer tape with both ferric oxide and chrome dioxide known as 'ferrichrome' (FeCr) (IEC Type III), but these were only available for a short time in the 1970s. Metal Cassettes (IEC Type IV) also use 70 µs playback equalization, and provide still further improvements in sound quality. The quality is normally reflected in the price; Type I cassettes are generally cheapest, and Type IV usually the most expensive. BASF chrome tape used in commercially pre-recorded cassettes used 120 µs (type I) playback equalization to allow greater high frequency dynamic range for better sound quality, but the greater selling point for the music labels was that the same Type I cassette shell could be used for both ferric and for chrome music cassettes.
Notches on top of the cassette shell indicate the type of tape within. Type I cassettes only have write-protect notches, Type II have an additional pair next to the write protection ones, and Type IV (metal) have a third set in the middle of the cassette shell. These allow cassette decks to automatically detect the tape type and select the proper bias and equalization. Virtually all recent hi-fi systems (with cassette decks) lack this feature; only a small niche of cassette decks (hi-fi separates) have the tape type selector. Playback of Type II and IV tapes on such a player will produce exaggerated treble, but it may not be noticeable because typically such devices have amplifiers that lack extended high frequency output. Recording on these units, however, results in very low sound reproduction and sometimes distortion and hiss is heard. Also, these cheaper units cannot erase high bias or metal bias tapes. Attempting to do so will result in "print-through".