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Digital audio tape

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Digital Audio Tape (DAT or R-DAT) is a signal recording and playback medium introduced by Sony in 1987. In appearance it is similar to a compact audio cassette, using 1/8" magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm x 54 mm x 10.5 mm. As the name suggests the recording is digital rather than analog, DAT converting and recording at the same rate as a CD (44.1 kHz sampling rate and 16 bits quantization) without data compression. This means that the entire input signal is retained. If a digital source is copied then the DAT will produce an exact clone. Unlike other digital media such as DCC[?] or MiniDisc, both of which use data compression.

The technology of DAT is closely based on that of video recorders, using a rotating head and helical scan[?] to record data.

The DAT standard allows for four sampling modes: 32 kHz at 12 bits, and 32 kHz, 44.1 kHz or 48 kHz at 16 bits. Certain recorders operate outside the specification, allowing recording at 96 kHz and 24 bits (HHS). Since each recording standard uses the same tape the quality of the sampling has a direct relation to the duration of the recording - 32 kHz at 12 bits will allow six hours of recording onto a three hour tape while HHS will only give 90 minutes from a three hour tape. Included in the signal data are subcodes to indicate the start and end of tracks or to skip a section entirely, this allows for indexing and fast seeking. The tapes themselves are not physically editable, in the cut-and-splice manner of analogue tapes.

DAT tapes are between 15 and 180 minutes in length, a 120 minute tape being 60 metres in length.

The format was designed for audio use, but through an ISO standard it has been adopted for general data storage, storing from 4 to 40 GB on a 120 metre tape depending on the standard and compression (DDS-1 to DDS-4). It is, naturally, sequential-access media and is commonly used for backups. Due to the higher requirements for integrity in data backups a computer-grade DAT was introduced.

DAT was not the first digital audio tape standard; an early form was available in the late 1970s, but it flopped commercially. Modern DAT has not been very popular outside of professional and semi-professional music artists, although the prospect of perfect digital copies of copyrighted material was sufficient for the music industry in the US to force the passage of the Audio Home Recording Act of 1992[?], the so-called DAT Tax. The inclusion of SCMS (serial copy management system[?]) in DAT recorders, to prevent digital copying for more than a single generation, was another response..



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