Digital Audio Tape
|
|
Dat_cartridge.jpg
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 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.
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, as ProDigi, DASH, or analogue tape recordings can be.
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. The tapes themselves are not physically editable in the cut-and-splice manner of analogue tapes. 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.
The format was designed for audio use, but through the ISO DDS 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 integrity in data backups, a computer-grade DAT was introduced.
DAT was not the first digital audio tape; an early form known as 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 it was not developed into a consumer product. Decca also had a similar PCM system for mastering records in the 1970s. Likewise, DCC was not a success either. 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.
Flaws on the tape or heads can cause the signal to mute briefly on playback, which can be frustrating when attempting to copy material.
See also
- Tape_editing about the analogue variantde:Digital Audio Tape