Compact audio cassette: Encyclopedia II - Compact audio cassette - Features of the cassette

Compact audio cassette - Features of the cassette

The cassette was a great step forward in convenience from reel-to-reel audio tape recording, though because of the limitations of the cassette's size and speed, it initially compared poorly in quality. Unlike the open reel format, the two stereo tracks lie adjacent to each other rather than a 1/3 and 2/4 arrangement. This permitted monaural cassette players to play stereo recordings "summed" as mono tracks and permitted stereo players to play mono recordings through both speakers. The tape is 3.18 mm wide (nominally ¹⁄₈ inch), with each stereo track being 0.79 mm wide (¹⁄₃₂ inch) and moves at 4.76 cm/s (1⁷⁄₈ inches/sec) from left to right. For comparison, the typical open reel format in consumer use was ¼ inch (6.35 mm) wide, each stereo track being ¹⁄₁₆ inch (1.59 mm) wide, and running at either 3¾ or 7½ inches/sec (9.5 or 19 cm/s).

Compact audio cassette - Cassette types

The original magnetic material was based on ferrite (Fe₂O₃), but then chromium dioxide (CrO₂) and more exotic materials were used in order to improve sound quality to try to approach that of vinyl records. Cobalt doped ferrite was introduced by TDK and proved very successful. Sony tried a dual layer tape with both ferrite and chrome dioxide. Finally pure metal particles (as opposed to oxide formulations) were used. These each had different bias and equalization requirements requiring specialized settings. The most common, Ferrite tapes (known as Type I) use 120 µS equalization, while chrome and cobalt-doped tapes (Type II) require 70 µS equalization. Metal Cassettes (Type IV), also use 70 µS equalization, and provide still further improvements in sound quality, as well as improved resistance to wear. The quality is normally reflected in the price; Type I cassettes are generally cheapest, and Type IV usually the most expensive. BASF developed a chrome cassette designed for use with 120 microsecond equalisation but this idea only really caught on for commercial pre-recorded cassettes.

Notches (indents) 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.

Compact audio cassette - Noise reduction and fidelity

A variety of noise reduction schemes are used to increase fidelity, Dolby B being almost universal for both prerecorded tapes and home recording. Later enhancements included Dolby C and Dolby S noise reduction, Dolby HX-Pro headroom extension, the dbx noise reduction system, automatic bias setting and automatic head azimuth adjustment. Dolby HX-Pro, which was developed by Bang & Olufsen, is a system that dynamically alters the bias during recording to yield a substantial improvement in high-frequency response during playback, regardless of the playback system.

By the late 1980s, thanks to such improvements in the electronics, the tape material and manufacturing techniques, as well as dramatic improvements to the precision of the cassette shell, tape heads and transport mechanics, sound fidelity on equipment from the top manufacturers far surpassed the levels originally expected of the medium. On suitable audio equipment, cassettes could produce a very pleasant listening experience. The best home decks could achieve 20-20 kHz frequency response with wow and flutter below 0.05%, and 70 dB of signal-to-noise ratio using Dolby C and up to 80 dB of signal-to-noise ratio using Dolby S.

In the 1990's, Dolby S noise reduction was introduced to the compact cassette, derived from Dolby SR, a professional system. Whereas the Dolby B and C Noise Reduction would dynamically adjust the preemphasis to promote a lower noise floor for high frequencies, Dolby S worked across the frequency range. However, licensing restrictions meant that Dolby S was only available on higher end tape decks that included dual motors, triple heads and other refinements. This system, according to a double blind study conducted by Stereo Review, yielded more accurate results than MiniDisc.

Prior to the mid 1980's pre-recorded cassettes were often of abysmal sound quality mainly due to the high speed duplication process used to produce them. However, technologies such as XDR (a quality control system), Dolby B Noise Reduction, Dolby HX pro and chrome dioxide tape meant that pre-recorded cassettes were soon approaching vinyl records in terms of audio quality and outselling LP records in some markets.

Compact audio cassette - Playback length

Tape length is usually measured in minutes total playing time. The most popular varieties are C46 (23 minutes per side) and C60 (30 minutes per side), C90, and C120. The C46 and C60 lengths are typically 15-16 µm thick, but C90s are 10-11 µm and C120s are just 9 µm thick rendering them more susceptible to stretching or breakage. Some vendors are more generous than others, providing 132 meters or 135 meters rather than 129 meters of tape for a C90 cassette. C180 and even C240 tapes were available at one time, but these were extremely thin and fragile and suffered badly from effects such as print-through which made them unsuitable for general use. Other lengths are (or were) also available from some vendors, including C15 (useful for saving data from early home computers), C50, C70, C74, C80, C100 and C110. Except for C74 and C100, such non-standard lengths have always been hard to find, and tend to be more expensive than the more popular lengths. Home taping enthusiasts may have found them useful for fitting an album neatly on one or both sides of a tape. For instance, the initial maximum playback time of compact discs was 74 minutes, explaining the relative popularity of C74 cassettes. See also audio tape length and thickness.

Compact audio cassette - Write-protection

All cassettes include a write protection mechanism to prevent re-recording and accidental erasure of important program material. Each side of the cassette has a plastic tab on the top that may be broken off, leaving a small indentation in the shell. This indentation allows the entry of a sensing lever which prevents the operation of the recording function when the cassette is inserted into a cassette deck. If the cassette is held with one of the labels facing the user and the tape opening at the bottom, the write-protect tab for the corresponding side is at the top-left.

If later required, a piece of adhesive tape can be placed over the indention to record over the "protected" material, or (on some decks), the lever can be manually depressed to record on a protected tape. Extra care is required when doing this with high bias tape cassettes; the additional indents (adjacent to the write-protect tabs) used to differentiate them from normal bias cassettes should not be inadvertently covered up. One manufacturer, Bib, even made small plastic inserts to fit into the record tab indent, and a special tool for removing them.

Adapted from the Wikipedia article "Features of the cassette", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki