Digital Audio Tape
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Digital Audio Tape (DAT or R-DAT) is a signal recording and playback medium developed by Sony and Philips in the mid 1980s. In appearance it is similar to a compact audio cassette, using 4 mm magnetic tape enclosed in a protective shell, but is roughly half the size at 73 mm × 54 mm × 10.5 mm. As the name suggests the recording is digital rather than analog, DAT converting and recording at higher, equal or lower sampling rates than a CD (48, 44.1 or 32 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 Digital Compact Cassette or MiniDisc, both of which use lossy data compression.
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History
Development
The technology of DAT is closely based on that of video recorders, using a rotating head and helical scan to record data. This prevents DATs from being physically edited in the cut-and-splice manner of analogue tapes, or open-reel digital tapes like ProDigi or DASH. 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). Some machines aimed at the domestic market did not operate at 44.1 kHz when recording from analog sources. 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. Two channel stereo recording is supported under all sampling rates and bit depths, but the R-DAT standard does support 4-channel recording at 32 kHz. DAT tapes are between 15 and 180 minutes in length, a 120 minute tape being 60 meters in length. DAT tapes longer than 60 meters tend to be problematic in DAT recorders due to the thinner media.
Predecessor Formats
DAT was not the first digital audio tape; pulse-code modulation (PCM) was used in Japan to produce analog phonograph records in the early 1970s, using a videotape recorder for its transport, but this was not developed into a consumer product.
Later in 1976, the first commercially successful digital audio tape format was developed by Soundstream, using 1" (2.54 cm) wide reel-to-reel tape loaded on a instrumentation recorder manufactured by Honeywell acting as a transport, which in turn was connected to outboard digital audio encoding & decoding hardware of Soundstream's own design. Several major record labels like RCA and Telarc used Soundstream's system to record some of the first commercially-released digital audio recordings.
Soon after Soundstream, 3M starting in 1978 introduced their own line (and format) of digital audio tape recorders, with one of the first prototypes being installed in the studios of Sound 80 in Minneapolis, Minnesota.
Professional systems using a PCM adaptor, which digitized an analog audio signal and then encoded this resulting digital stream into an analog video signal so that a conventional VCR could be used as a storage medium, were also common as mastering formats starting in the late 1970s.
dbx, Inc.'s Model 700 system, notable for using high sample-rate delta-sigma modulation (similar to modern SACDs) rather than PCM, and Decca's PCM system in the 1970s (using a videotape recorder manufactured by IVC for a transport), are two more examples.
Uses of DAT
Computer Data Storage Medium
The format was designed for audio use, but through the ISO Digital Data Storage standard it has been adopted for general data storage, storing from 1.3 to 72 GB on a 60 to 170 meter tape depending on the standard and compression. It is, naturally, sequential-access media and is commonly used for backups. Due to the higher requirements for capacity and integrity in data backups, a computer-grade DAT was introduced, called DDS for Digital Data Storage. Although functionally similar to audio DATs, only a few DDS and DAT drives (in particular, those manufactured by Archive for SGI workstations [1]) are capable of reading the audio data from a DAT cassette.
Home Use
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 the Serial Copy Management System in DAT recorders, to prevent digital copying for more than a single generation, was another response. Flaws on the tape or heads can cause the signal to mute briefly on playback, which can be frustrating when attempting to copy material. DAT format initially quite popular for trading recordings of live music as available DAT players predated affordable CD recorders.
The Future of DAT
On November 25th 2005, Sony announced that the final DAT machines would be discontinued in December 2005, since sales had dropped to around 100 per month. This makes the format obsolete. However, the DAT format still finds regular use in film and television recording, principally due to the support in some recorders for SMPTE time code synchronization, although it is slowly being superseded by modern hard disk recording equipment which offers much more flexibility and storage.
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