11 December 2014

Magnetic Tape

Magnetic tape is a long, thin ribbon of plastic, coated with a metal or metal oxide, which can be locally magnetised to store information. Generally used for three reasons, and while hurtling out of fashion for general consumer purposes, magnetic tape still has a finger-hold in key areas to which it's clinging with all its might. Audio, video or data - that's your basic choice for magnetic tape storage.1

Data-wise, we've all seen those films from the seventies, when computers came in sizes ranging from 'room' to 'tower block'; six-foot high cabinets with gigantic reels of tape spinning wildly back and forth. Or there's that tape-recorder you found in the attic one day with one empty reel and another one marked 'Donovan', jam-packed with Jennifer Juniper audio magic. And if you've ever owned an early 8-bit computer, say a ZX Spectrum, you'll be familiar with the beeeee-bip, beeeee-biddlebiddlebeep of Sir Clive's cheap data storage solution.


While the compact cassette is undoubtedly the most successful format for recording audio onto magnetic tape, such recording techniques were founded long before Philips ever thought of trademarking the name Compact Cassette. It's 1928. In Germany Fritz Pfleumer is working hard to modernise the 1898method of wire recording. A long spool of thin wire containing magnetic audio data, occasionally knotted and lightly welded together, gave way to magnetic tape - thinner, easier to handle and even simpler to splice together. By the 1950s quarter-inch wide tape had become popular, and plastic reels up to seven inches in diameter held the magnetic tape. Used in reel-to-reel devices, the magnetic tape would be fed from one reel, through the playback head and spooled back onto the second reel. Standard reels with quarter inch tape came in 8, 16 or 32 minute playing times, although different thicknesses of tape, and thus different lengths, allowed for other lengths, right up to a stunning sixty-four minutes with a double-sided seven inch reel, playing at seven and a half inches per second. Later innovations, like long play, double play and even triple play, extended tape sizes even further and reflected multiple improvements in the technology as a continued reduction in tape thickness allowed more and more tape to be placed on one reel, although often at the expense of robustness.

Convenience spurred further development, of course. The beginning of the end for threading reels of tape through machines and onto spools came in 1963 when Philips introduced their compact cassette design, a cartridge system which contained ready-spooled tape, safely stored away from sticky human fingers. The market, at this time, was dominated by the reel-to-reel system, and as there were other formats available, as with so many new technologies, Philips format faced an uphill struggle at first. Eventually, with devices like Sony's walkman and Philips' decision to licence the compact cassette design out for free, the system became something of a standard, replacing reel-to-reel and even vinyl. Over the years the magnetic tape used inside the compact cassettes evolved from basic ferric oxide, through chromium dioxide and the now-current mixture of ferric oxide and cobalt. Like reel-to-reel systems, cassettes come in a variety of lengths, with the quantity of magnetic tape contained within the plastic shell deciding the recording time available.

Digital methods of recording audio onto magnetic tape have also enjoyed their hey-day. Digital Compact Cassette was intended to replace analogue methods, though it was ultimately doomed to failure. DAT tape, on the other hand, still provides an acceptable alternative for storing audio data in a digital form on magnetic tape. Ultimately, however the use of magnetic tape to record audio continues to decline as other methods become cheaper, easier and more reliable. That's not to say reel-to-reel has vanished completely; it is still used for commercial recording, though digital heads have replaced the traditional analogue heads.


Although early video recorders also used the reel-to-reel system, the version most people are familiar with is the VHS-system, which employed a cartridge system very similar to the audio compact cassette format. There were a few refinements, of course; the video cassette featured a flap to keep the magnetic tape free of greasy fingerprints, and the actual magnetic tape used was thicker. Video storage, as you'd imagine, involves an awful lot more information, delivered at a much higher rate. Helical scanning, which involves moving the tape reading head as well as the magnetic tape, was the key to this technology. It is not without its disadvantages, of course; anyone who's opened up a VCR has seen how the tape has to be drawn completely out of the cassette and threaded around various parts of the machine, leading to increased wear on the tape and, on occasion, that dreadful crumpling sound as metres of magnetic tape filled the VCR, utterly destroying your copy of The Italian Job. DVD, anyone?


Magnetic tape has a rich history in the field of computing. Way back in 1951 the UNIVAC 1 used half-inch wide nickel-plated bronze tape to record 128 characters per inch on eight separate tracks, the tape speeding through the machine at a speed of 100 inches per second. Only seven of the tracks contained data, with the last being devoted to tracking, allowing UNIVAC to process about 7,200 characters per second.

Meanwhile, IBM seized upon the ferrous-oxide coated tape that had become popular for audio recordings. Thinner and more flexible, their technology soon became the industry standard. Still centering around a half-inch thickness, IBM's tape was wound onto removable reels up to ten and a half inches in diameter - approximately 30cm. The early tape drives were huge, free-standing behemoths, equipped with two reels and columns which 'buffered' long u-shaped loops of tape. The two drives would spin tape on and off the buffers, resulting in bursts of precise spinning. The movement was visually striking - one drive spinning, then the other, perhaps both at once in different directions - and the tape drives quickly became recognisable as 'the computer' in television programmes or films.

In the 1980s, compact audio cassettes were perfect for use with home computers, though access times and reliability were low. Other options making use of magnetic tape were developed, such as Sinclair's Microdrive system. Ultimately, however, such efforts were doomed to failure - two-dimensional magnetic storage offered faster access times, and ultimately drove formats like Microdrive into the ground. Meanwhile workstations moved onto digital audio tape. The size of the reels decreased accordingly, and, as in the field of audio, cartridges became common simply as a means to protect the tape as well as increasing convenience and ease of use. Most modern formats make use of compression, or may employ large memory buffers to avoid the delays necessary when accessing data on different regions of tape. Although the market share for tape storage products has steadily decreased, tape backup systems still offer cost-effective solutions.

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