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Compact audio cassette

From Academic Kids

For the meaning of cassette in genetics, see cassette (genetics).

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Tdkc60cassette.jpg
typical audio Compact Cassette.

The compact audio cassette audio storage medium was introduced by Philips in 1963. It originally consisted of a length of magnetic tape from BASF inside a protective plastic shell. Four tracks are available on the tape, giving two stereo tracks – one for playing with the cassette inserted with its 'A' side up, and the other with the 'B' side up, thus mimicking gramophone records. There were other magnetic tape cartridge systems at the time, but the compact cassette succeeded through Philips's backing. The mass production of compact audio cassettes began in 1965 in Hanover, Germany, as did commercial sales of prerecorded music cassettes, known as musicassettes or MC for short.

Contents

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 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 1/8 inch (3.175 mm) wide, with each stereo track being 1/32 inch (0.79 mm) wide and moves at 17/8 inches per second (47.625 mm/s). For comparison, the typical open reel format was ¼ inch (6.35 mm) wide, each stereo track being 1/16 inch (1.5875 mm) wide, and running at either 3¾ or 7½ inches per second (95.25 or 190.5 mm/s). Some machines did use 17/8 inches per second (47.625 mm/s) but the quality was poor.

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. Ferrite tapes use 120 μS equalization (known as Type 1), while chrome and cobalt doped tape types require 70 μS equalization (Type 2). In practice the cassette shell was modified with indents to automatically select the proper bias and equalization on compatible cassette decks.

A variety of noise reduction schemes are used to increase fidelity, Dolby B being almost universal for both prerecorded tapes and home recording. By the late 1980s, sound fidelity on equipment by the top manufacturers far surpassed the levels expected of the medium by early detractors and on suitable audio equipment could produce a very pleasant listening experience.

Tape length is usually measured in minutes total playing time, and the most popular varieties are C46 (23 minutes per side), C60 (30 minutes per side), C90, and C120 (usually thinner tape, more likely to be destroyed in use). 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 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 playback time of compact discs was 74 minutes, explaining the relative popularity of C74 cassettes.

All cassettes include a mechanism to prevent re-recording and accidental erasure of important program material. Each side of the tape cassette has a plastic tab that may be broken off, leaving a small indentation in the shell. This allows space for the entry of a sensing lever which prevents the operation of the recording function when the cassette is inserted into a cassette deck.

Applications

Audio

The compact cassette was originally intended for use in dictation machines. In this capacity, some later-model casette-based dictation machines could also run the tape at half speed (15/16 IPS) as playback quality was not critical. The Compact Cassette soon became, and remained into the early 2000's, a popular medium for distributing prerecorded music – initially through Philips's record company, PolyGram. Starting in 1979, Sony's Walkman helped the format become widly used and popular. In 2005, one finds cassettes used for a variety of purposes such as journalism, oral history, meeting and interview transcripts and so on, however, they are starting to give way to compact disc and numerous lossy compression systems, such as MP3.

Home studio

In the 1980s, Tascam introduced the Portastudio line of four and eight-track cassette recorders for home studio use with emphasis on MIDI. To increase audio quality in these recorders, the tape speed is doubled in comparison to the standard, and, in addition, DBX noise reduction provides compression which yields increased dynamic range). Multi-track cassette recorders with built-in mixer and signal routing features provide a wide range of features and benefits from easy to use beginner units up to professional level recording systems.

Home dubbing

Most cassettes were sold blank and used for recording (dubbing) the owner's records (as backup or to make compilations), their friends' records or music from the radio. This practice was condemned by the music industry with such slogans as "Home taping is killing music". However, many claimed that the medium was ideal for spreading new music and would increase sales, and strongly defended at least their right to copy their own records onto tape. In the late 1970s, Sony brought out the Walkman, a small portable cassette player, which greatly increased the consumption of music in this manner. Cassettes were also a boon to people wishing to make bootlegs (unauthorized concert recordings) for sale or trade, a practice tacitly or overtly encouraged by many bands with a more counterculture bent such as the Grateful Dead.

Various legal cases arose surrounding the dubbing of cassettes. In the UK, in the case of CBS Songs vs Amstrad (1988), the House of Lords found in favour of Amstrad that producing equipment that facilitated the dubbing of cassettes, in this case a twin cassette deck that allowed one cassette to be copied directly onto another, did not authorise the infrigement of copyright.

Data Recording

Many early home computers of the 1970s and early 1980s, notably the TRS-80, Commodore PET, VIC-20, Commodore 64, TI-99/4a, ZX Spectrum, Amstrad CPC, Coleco Adam and BBC Micro, could use cassettes as a cheap alternative to floppy disks as a storage medium for programs and data. The typical encoding method was simple FSK which resulted typical data rates 500 to 2000 bit/s, although some games used special faster loading routines, up to around 4000 bit/s. A rate of 2000 bit/s equates to a capacity of around 660 kilobytes per side of a 90 minute tape.

The usage of both audio channels, better modulation techniques like QPSK or those used in modern modems, combined with the greater bandwidth and Signal to noise ratio of cassette tapes compared to a PSTN telephone line could have been used for achieving much greater capacities and speeds (several KBytes/sec for data rate, and several MBytes on each cassette), but such a solution wasn't adopted since it would require much more expensive decoding/encoding circuitry on the computers or on dedicated "datacorders", apart from good quality tapes and recorders with constant performance.

Cassette equipment

Cassettes can be played on a wide variety of different types of device. Early recorders tended to be small battery-powered portable devices, in keeping with the intention of the medium for dictation, reportage and similar low-level recording duties, but by the mid 1970s, the cassette deck became a commonplace component of home high fidelity systems, largely superseding the reel-to-reel recorder for home use. Another key element of the cassette's success was its use in in-car entertainment systems, where the small size of the tape was significantly more convenient than the competing 8-track cartridge system. Cassette players in cars and for home use were often integrated with a radio receiver, and the term "casseiver" was occasionally used for combination units for home use. In-car cassette players were the first to adopt the idea of automatic reversal ("auto-reverse") of the tape at each end, allowing a cassette to be played endlessly without manual intervention. Home cassette decks soon followed this practice as well.

Successors to the cassette

Technical development of the cassette effectively ceased when digital recordable media such as DAT and MiniDisc were introduced in 1992. Philips introduced the Digital Compact Cassette — a DAT-like tape in the same form factor as the compact audio cassette — but this attempt failed in the market. Since the rise of cheap CD-R discs, the phenomenon of "home taping" has effectively switched to compact disc. The microcassette has in many cases supplanted the full-sized audio cassette in situations where voice-level fidelity is all that is required, originally in answering machines but most commonly now in inexpensive pocket-sized tape recorders and dictation machines. Even these, in turn, are starting to give way to digital recorders of various descriptions.

Present and future of the compact cassette

Although considered obsolete by many, audio cassettes continue being produced and marketed in many countries and are still popular in some applications such as car audio and difficult environments in general, because they tend to be more rugged and more resistant to dust and heat and shocks than most digital media (notably CDs) and their lower fidelity is not a very serious drawback inside the typically noisy interior of most cars.

Cassettes (often in the form of microcassettes) are also often used in business and educational settings as adjuncts or substitutes for note-taking. While digital voice recorders are becoming available, tape recorders tend to be universally cheaper and of sufficient quality to do the job.

Also, cassettes and related equipment are still popular in undeveloped and third-world countries and in general, where digital audio technology has not thrived yet for various reasons.

In the future, most estimates claim that cassettes and related equipment will continue to be manufactured until perhaps the year 2010 or 2015, after which the fate of audio cassette will be similar to that of vinyl records or rather, of the common photographic film (limited, niche market production). As of 2005 it is common for otherwise complete audio systems to be sold with only a single cassette tape deck instead of two, with playback-only decks, or even with no cassette support at all.

Cassette in other languages

In French and in Catalan the word "cassette" is abbreviated as "K7" (ka-sept); the "K7" shorthand also works in Portuguese: c-sete. In Spanish it is known by the letters KCT (pronounced "ka-c-te").

See also

External links


cs:audiokazeta de:Musikkassette fr:musicassette lt:MC nl:Muziekcassette pt:Cassete ja:コンパクトカセット fi:C-kasetti sv:Compact cassette

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