Adventures In Audio
Digital Audio Tape (DAT) (part 4)

David Mellor

David Mellor is CEO and Course Director of Audio Masterclass. David has designed courses in audio education and training since 1986 and is the publisher and principal writer of Adventures In Audio.

Thursday January 1, 2004
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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.

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