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Digital Audio Tape (DAT) (part 3)

Since DAT was intended to be a consumer product right from the start, the cassette is very small, roughly three inches by two inches and just under half an inch thick. For professional users, this is rather TOO small...


DAT Format

Since DAT was intended to be a consumer product right from the start, the cassette is very small, roughly three inches by two inches and just under half an inch thick. For professional users, this is rather TOO small, not just because it makes the cassette easier to lose, but because there will always be some doubt that DAT could have been a better system if there had been a bit more elbow room. The problem is not so much that DAT is unreliable, but that it has been found that a tape recorded on one machine will not always play back correctly on another. This is, I would imagine, a problem that will be solved as manufacturing tolerances tighten. Professional users want this problem solved quickly and absolutely, and I imagine that domestic users will too. It has always been a nuisance with the Compact Cassette that a cassette might play perfectly on the machine on which it was recorded, but try and play it on another machine and it will sound dreadful - all due to the alignment of the tape heads. For the home recordist, this is a problem because giving tapes to other people is something you do all the time, and they will not be very impressed if the sound is mangled due to a tiny misadjustment.

Having said that DAT’s size is a disadvantage for professional users, it really is amazing how it achieves what it does working at microscopic dimensions. As I said earlier, not only does the tape move, the tape heads move too. DAT’s full title, R-DAT, indicates that the system uses a rotary head like a video recorder. Unlike analogue tape which records the signal along a track parallel to the edge of the tape, a rotary head recorder lays tracks DIAGONALLY across the width of the tape. So even though the tape speed is a miniscule 8.15 millimetres/second, the actual writing speed is a massive 3.133 metres/second. Figure 1 shows the pattern of the tracks recorded across the tape. The width of each track is 13.591 MILLIONTHS of a metre, or to give an imperial measurement comparison, you could fit ??? tracks into just one inch. Unlike an analogue tape, the tracks are recorded without any guard band between them. In fact, the tracks are recorded by heads which are around 50% wider than the final track width and each new track partially overlaps the one before, erasing that section. Since the same heads are used for recording and playback, this may seem to present a problem because if the head is centred on the track it is meant to be reading, then it will also see part of the preceding track and part of the next track. Won’t this result in utter confusion? Well it doesn’t, of course, because a system originally developed for video recording is used, known as AZIMUTH RECORDING. The ‘azimuth’ of a tape head refers to the angle between the head gap, where recording takes place, and the tape track itself. In an analogue recorder the azimuth is always adjusted to 90 degrees, so that the head gap is at right angles to the track. In DAT, which uses two heads, one head is set at -20 degrees and the other to +20 degrees, and they lay down tracks alternately. So on playback, each head receives a strong signal from the tracks that it recorded, and the adjacent tracks, which are misaligned by 40 degrees, give such a weak signal that it can be rejected totally. Damn clever.

By David Mellor Thursday January 1, 2004
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