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.Come on the FREE COURSE TOUR
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