Originally an 8-track digital recorder, now shorthand for a digital transmission protocol
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ADAT (Alesis Digital Audio Tape)
An eight track digital recorder developed and manufactured by the Alesis Corporation, employing modified videotape recording technology. The ADAT uses a standard VHS video cassette for information storage.
Alesis Digital Audio Tape or ADAT, first introduced in 1991, was used for simultaneously recording eight tracks of digital audio at once, onto Super VHS magnetic tape - a tape format similar to that used by consumer VCRs. Greater numbers of audio tracks could be recorded by synchronizing several ADAT machines together. While this had been available in earlier machines, ADAT machines were the first to do so with sample-accurate timing - which in effect allowed a studio owner to purchase a 24-track tape machine eight tracks at a time. This capability and its comparatively low cost were largely responsible for the rise of project studios in the 1990s.
Several versions of the ADAT machine were produced. The original ADAT (also known as "Blackface") and the ADAT XT recorded 16 bits per sample (ADAT Type I). A later generation of machines - the XT-20, LX-20 and M-20 - supports 20 bits per sample (ADAT Type II). All ADAT's use the same high quality S-VHS tape media. Tapes formatted in the older Type I style can be read and written in the more modern machines, but not the other way around. Later generations record at two sample rates, 44.1 kHz and 48 kHz, although the original Blackface could only do 48 kHz. Most models allow pitch control by varying the sample rate slightly (and tape speed at the same time).
With locate points it was possible to store sample exact positions on tape, making it easy to find specific parts of recordings. Using Auto Play and Auto Record functions made it possible to drop in recording at exact points, providing the first automated punch-in / punch-out system (citation needed).
Although it is a tape based format, the term ADAT now refers to its successor, the Alesis ADAT HD24. This is the next step in stand alone digital multitrack audio recorders, and features hard disk recording as opposed to the traditional tape based ADAT, which is now considered obsolete—although computer based recording makes devices such as the HD24 arguably equally obselete, many engineers still use the HD24 system for redundant backup or fallback, with the newer HD24XR which features much better converters, a possible 96kHz sample rate, and is preferred to computer recording by some.
ADAT Lightpipe Protocol
"ADAT" is also used as an abbreviation for the ADAT Lightpipe protocol, which transfers 8 tracks in a single fiber optic cable. The ADAT cable standard is no longer strictly tied to ADAT tape machines, and is now utilized by analog-to-digital converters, input cards for digital audio workstations, effects machines, etc. One of the original benefits of utilizing ADAT versus S/PDIF or AES/EBU was that a single cable could carry up to eight channels of audio. (AES10 (MADI) can now carry up to 64 channels.)
Many audio interfaces will feature ADAT in/out ports as a low-footprint option for expandability and connectivity.