Digital Audio Tape (DAT) (part 4)
Mechanically, there is a strong similarity between a DAT recorder and a video cassette recorder. Both use a rotary head drum on which are mounted the record/playback heads. But there are differences. A video recorder uses a large head drum with the tape wrapped nearly all the way around. This is necessary so that there can always be a head in contact with the tape during the time that each video frame is built up on the screen. With digital audio, data can be read off the tape at any rate that is convenient and stored up in a buffer before being read out at a constant speed and converted to a conventional audio signal. The head drum in a DAT machine is a mere 30mm in diameter (and spins at 2000 revolutions per minute). The tape is wrapped only a quarter of the way around which means that at times neither of the two heads is in contact with the tape, but as I said, this can be compensated for. This 90 degree wrap has its advantages: there is only a short length of tape in contact with the drum so high speed search can be performed with the tape still wrapped; tape tension is low, giving long head and tape life; if an extra pair of heads is mounted on the drum, simultaneous off-tape monitoring can be performed during recording just like a three-head analogue tape recorder.
The signal that is recorded on the tape is of course digital, and very dissimilar to either analogue audio or video signals. As you know, the standard DAT format uses 16 bit sampling at a sampling frequency of 48kHz. This converts the original analogue audio signal to a stream of binary numbers representing the changing level of the signal. But since the dimensions of the actual recording on the tape are so small, there is a lot of scope for errors to be made during the record/replay process, and if the wrong digit comes back from the tape it is likely to be very much more audible than a drop-out would be on analogue tape. Fortunately, ERROR CORRECTION techniques exist, which pre-dating DAT because they are useful for data storage in computers. There are various ways of checking whether data is correct or not. DAT, like the Compact Disc, uses a technique called Double Reed-Solom on Encoding which duplicates much of the audio data, in fact 37.5%, in such a way that errors can be detected, then either corrected completely or concealed so that they are not obvious to the ear. If there is a really huge drop-out on the tape, then the DAT machine will simply mute the output rather than replay digital gibberish. As an extra precaution against drop-outs, another technique called INTERLEAVING is employed which scatters the data so that if one section of data is lost, then there will be enough data elsewhere - hopefully beyond the site of tape damage - which can be used to reconstruct the signal.
The pulse code modulated audio data is recorded in the centre section of each diagonal track across the tape. There is other data too: ATF signals allow for Automatic Track Finding which makes sure that the heads are always precisely positioned over the centre of the track, even if the tape is slightly distorted and the track curved. Of more interest to the actual use of DAT are the Sub Code areas of the track. These Sub Code areas allow extra data to be recorded alongside the audio information. Not all of the capacity of the Sub Code areas is in use as yet, allowing for extra expansion of the DAT system. Those at present in use include: time codes (not SMPTE/EBU timecode, apart from the professional Fostex D20 machine) which can log the total elapsed time and the time since the beginning of each item on the tape; the Start ID marks the beginning of each item; the Skip ID tells the machine to go directly to the next Start ID, thus performing an instant edit. These codes make DAT easier to use.