Digital Audio Tape

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Digital Audio Tape

A 90-minute DAT cartridge, size compared to a AAA (LR03) battery.
Media type	Magnetic tape
Capacity	120 minutes
Read mechanism	Rotating head
Write mechanism	Rotating head, helical scan
Developed by	Sony
Usage	Audio storage

Digital Audio Tape (DAT or R-DAT) is a signal recording and playback medium developed by Sony and introduced in 1987.^[1] 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 has the ability to record at higher, equal or lower sampling rates than a CD (48, 44.1 or 32 kHz sampling rate respectively) at 16 bits quantization. If a digital source is copied then the DAT will produce an exact clone, unlike other digital media such as Digital Compact Cassette or non-Hi-MD MiniDisc, both of which use lossy data compression.

Like most formats of videocassette, a DAT cassette may only be recorded on one side, unlike an analog compact audio cassette.

[edit] History

[edit] 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 analog 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 the same 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. DAT machines running at 48 kHz and 44.1 kHz sample rates transport the tape at 8.15 mm/s. DAT machines running at 32 kHz sample rate transport the tape at 4.075 mm/s.

[edit] Predecessor formats

DAT was not the first digital audio tape; pulse-code modulation (PCM) was used in Japan by Denon in 1972 for the mastering and production of analogue phonograph records, using a 2-inch Quadruplex-format 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" (25.4 mm) wide reel-to-reel tape loaded on an instrumentation recorder manufactured by Honeywell acting as a transport, which in turn was connected to outboard digital audio encoding and 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 for use in a recording studio, 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.

One of the most significant examples of a PCM adaptor-based system was the Sony PCM-1600 digital audio mastering system, introduced around 1979. The PCM-1600 used a special U-Matic-format VCR for its transport, connected to external digital audio processing hardware. It (and it's later versions such as the PCM-1610 and 1630) was widely used for the mastering of some of the first Audio CDs in the early 1980s, with tapes recorded on the PCM-1600 being sent to the CD pressing plant to be used to make the glass master disc for CD replication.

dbx, Inc.'s Model 700 system, notable for using high sample-rate delta-sigma modulation (similar to modern Super Audio CDs) rather than PCM, and Decca's PCM system in the 1970s^[2] (using a videotape recorder manufactured by IVC for a transport), are two more examples.

Mitsubishi's X-80 digital recorder was another 6.4 mm (¼") open reel digital mastering format that used a very unusual sampling rate of 50.4 kHz.

For high-quality studio recording, effectively all of these formats were made obsolete in the early 1980s by two competing reel-to-reel formats with stationary heads: Sony's DASH format and Mitsubishi's continuation of the X-80 recorder, which was improved upon to become the ProDigi format. (In fact, the first ProDigi-format recorder, the Mitsubishi X-86, was playback-compatible with tapes recorded on an X-80.) Both of these formats remained popular as an analog alternative until the early 1990s, when hard disk recorders rendered them obsolete.

[edit] R-DAT and S-DAT

For a while, the DAT format was produced in two physically incompatible formats: one with helical scaning heads, called R-DAT, and one with a stationary head block, called S-DAT. S-DAT failed to gain market share ^[3] as it required more expensive technlogy in the machine, compared to the relatively simple (and much cheaper) spinning head approach of R-DAT.

Philips later recycled the basic mechanical design of S-DAT into DCC.^{[citation needed]}

[edit] Anti-DAT lobbying

In the late 1980s, the Recording Industry Association of America unsuccessfully lobbied against the introduction of DAT devices into the U.S. Initially, the organization threatened legal action against any manufacturer attempting to sell DAT machines in the country. It later sought to impose restrictions on DAT recorders to prevent them from being used to copy LPs, CDs, and prerecorded cassettes. One of these efforts, the Digital Audio Recorder Copycode Act of 1987 (introduced by Sen. Al Gore and Rep. Waxman), instigated by CBS Records president Walter Yetnikoff, involved a technology called CopyCode and required DAT machines to include a chip to detect attempts to copy material recorded with a notch filter, meaning that copyrighted prerecorded music, whether analog or digital, would have distorted sound. A National Bureau of Standards study showed that not only were the effects plainly audible, but that it wasn't even effective at preventing copying. Thus the audible pollution of prerecorded music was averted.

This opposition by CBS softened after Sony, a DAT manufacturer, bought CBS Records in January 1988. By June 1989, an agreement was reached, and the only concession the RIAA would receive was a more practical recommendation from manufacturers to Congress that legislation be enacted to require that recorders have a Serial Copy Management System to prevent digital copying for more than a single generation.^[4] This requirement was enacted as part of the Audio Home Recording Act of 1992, which also imposed "royalty" taxes on DAT recorders and blank media.

[edit] Uses of DAT

[edit] Professional recording industry

DAT was widely used in the professional audio recording industry in the 1990s, and is still used to some extent today,^[when?] as the archives created in the '90s are still widely used, although most labels have a program in place to transfer these tapes to a computer-based database. DAT was used professionally due to its lossless encoding, which allowed a master tape to be created that was more secure and did not induce yet more tape noise (hiss) onto the recording. In the correct setup, a DAT recording could be created without even having to be decoded to analogue until the final output stage, since digital multi-track recorders and digital mixing consoles could be used to create a fully digital chain. In this configuration, it is possible for the audio to remain digital from the first AD converter after the mic preamp until it is in a CD player.

DATs were also frequently used by radio broadcasters. Until recently, they were still used by the BBC as an emergency broadcast that would initiate if the player detected a lack of noise continued for more than a pre-determined time. This would mean that if for any reason the broadcast from the studio stopped, the DAT would continue broadcast until normal service could be resumed.^{[citation needed]}

This section requires expansion.

[edit] Amateur and home use

DAT was envisaged by proponents as the successor format to analogue audio cassettes in the way that the compact disc was the successor to vinyl-based recordings; however, the technology was never as commercially popular as CD. DAT recorders remained relatively expensive, and commercial recordings were generally not made available on the format. However, DAT was, for a time, popular for making and trading recordings of live music, since available DAT recorders predated affordable CD recorders.

In the U.S., the RIAA and music publishers continued to lobby against DAT, arguing that consumers' ability to make perfect digital copies of music would destroy the market for commercial audio recordings. The opposition to DAT culminated in the passage of the resulting Audio Home Recording Act of 1992, which, among other things, effectively imposed a tax on DAT devices and blank media.

[edit] 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 80 GB on a 60 to 180 meter tape depending on the standard and compression. It is sequential-access medium 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 (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)^[5] are capable of reading the audio data from a DAT cassette. SGI DDS4 drives no longer have audio support; SGI removed the feature due to "lack of demand".^[6]

[edit] Future of DAT

In November 2005, Sony announced that the final DAT machines would be discontinued the following month.[1] 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. In 2004, Sony introduced the Hi-MD Walkman with the ability to record in linear PCM. Hi-MD has found some favour as a disc-based DAT alternative for field recordings and general portable playback.

[edit] Archived audio problem

The discontinuation of DAT player production lead to a significant problem regarding audio archives, since a tremendous amount of recordings from the mid-80s until about 2000 exist solely on DAT.

Even if some larger broadcasting facilities or studios still have some DAT recorder/players in their internal stock or could find a handful of second hand models, each unit can inevitably suffer from wear-out in the spinning drum heads, winding mechanisms, brakes, etc. The best solution would be to transfer the information off the DAT "tapes" and into computer-based hard drive systems via an AES/EBU digital connection. Even though these "transfers" have to happen in real-time which can make this process tedious and time consuming, an exact clone can be made using this process, where no digital information will be lost or compromised. This is unlike running the DAT through an analog soundboard and then into the hard drive system, which would defeat the purpose (since it would cause loss of data through the digital-analog and analog-digital conversions).

[edit] See also

• ADAT
• Magnetic tape
• PCM adaptor
• Digital Audio Stationary Head
• ProDigi

[edit] References