Admittedly, this topic is somewhat distant from recording on Compact Disc, but questions of format arise often enough that they seem to be worth addressing. The following summary simplifies the topic and gives short shrift to variant formats, particularly those which are obsolete today and which never had broad success in the marketplace. More information can be found at http://www.richardhess.com/tape/tips.htm
Modern tape recording originated in Germany just before World War II. Magnetic recording on wire antedated tape, but both mechanical and magnetic problems kept it from taking off; in practice, recordings to 78-rpm discs were used for transcription both in studios and in the home beginning in the mid-1930s and continuing into the 1950s when open-reel tape recording began to take over.
The standards for magnetic tape use English units. Standard tape speeds are 30 inches per second (ips), 15, 7.5, 3.75, 1.875 (1 7/8), 0.9375 (15/16). Tape width is quarter-inch for open reel and cartridges and 0.150-inch for cassette. Other speeds and widths have been used for professional applications and in various obsolete configurations. The original base or backing of the tape was paper, but that was soon replaced by acetate, followed by Mylar and by tensilized Mylar, which last is the onlysubstrate in common use today. The recordable medium is an iron oxide compound, though chromium dioxide had a brief heyday for high-end cassettes. The oxide is suspended in a binder which holds it firmly (one hopes) to the backing. Different oxides have different magnetic properties and three varieties are used commonly in cassettes. Note that the properties needed for digital recording differ from those for analogue; in general, digital tape is a poor medium for recording audio and audio formulations do not give good results for digital data.
The history of tape recording has been paced by the technologies of media and heads. There has been a consistent tradeoff between recorded quality and cost; in general, the wider the track and the higher the speed, the higher the fidelity and the greater the cost. For serious recording, three heads are involved, one each to erase, record and play back. Fewer heads may be used for compactness and economy, but then the performance penalty is great. The erase head must handle high currents to do its job, the record head needs precise properties for the edges of a relatively wide magnetic gap, and the playback head needs a narrow gap and high sensitivity.
Recording is done by 'permanently' magnetizing the tape as it runs across the record head. Each head is like a U-shaped electromagnet where the gap is the space between the two ends at the top of the U. The "azimuth" of the head is the angle between the gap and the path taken by the tape; ideally, the tape runs exactly perpendicular to the gap. Erase and playback heads are similarly aligned and also require control of the azimuth so that what is erased or played back is the same information that was originally recorded. Note that the heads have some properties of permanent magnets and that it is necessary occasionally to reduce persistent magnetism to minimize unwanted erasure when the tape passes over an unused head. To that end, degaussers are available in various forms. In most modern tape decks, the residual magnetism is low and degaussing is rarely needed.
Initially, the heads extended over the full width of the tape resulting in full-track recording. As technology evolved, two-track recording was introduced. The first application was half-track monaural where the head gap extended a bit less than half the width of the tape. A narrow guard band was left unrecorded so that when the second track was recorded (by flipping the tape over), there would be little chance of crosstalk between the two directions. At that stage, correct height adjustment for the head became necessary so that the full signal was read but there was no leakage from the second track.
Stereo recording was now possible using two heads for each of recording and playback. However, that required precise positioning of the pairs so that the phase relationship between the two tracks (now running in the same direction, of course) was preserved. Such "staggered" heads were needed until the technology permitted combining the two into a single "stacked" head. That is the configuration used today.
The next step was to offer four tracks on each tape to yield stereo recording in each direction. The tape was effectively divided into four strips, each a bit less than a quarter of the tape's width, again leaving a guard band. "Four-track stereo" uses the first and third tracks for stereo in each direction. Heads were then developed to record four tracks simultaneously, providing both four independent tracks for mixdown and discrete quadrophonic recording. Technology continued to advance, allowing still more tracks across the width of the tape; wider tapes were used to allow more tracks without substantial loss of signal quality.
Philips introduced and patented a format for cassettes based on open-reel but using narrower tape held in a shell. Conceptually, cassette and open-reel media are the same except that stereo cassettes have the two tracks adjacent to one another to ensure acceptable playback on a monaural machine and vice versa. One advantage of the cassette is that it is covered by patents controlling format and configuration so that any cassette is playable to some extent on any player so long as both conform to the standard. There is only one tape speed - 1 7/8 ips, one layout of tracks and one configuration of shell. Since the tape is narrower, control of azimuth and height is even more critical than on an open-reel deck, but again technologic advances made that practical.
All things being equal, a wider track and a higher transport speed produce higher audio quality: better high-frequency response and greater signal-to-noise ratio (SNR). In the real world, the generalization loses out to technology, so better tape formulations and tools such as Dolby B allow quality recording on tape which is too narrow, using too many tracks, and moving too slowly.
E-mail me at cdrecording@mrichter.com
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