R-DAT (Rotating-head Digital Audio Tape)
recorder or DAT is a digital audio recorder that makes use of tape transport
and rotating head technology originally developed for video recorders.
The DAT system houses the tape in small cassettes roughly half the
size of a standard analog cassette and is protected from dust and handling
by the same kind of hinged door used for video tapes. As with analog cassettes
a sliding write protect tab is provided to prevent accidental erasure.
Tape recording times of up to 2 hours are possible with DAT.
The tape is drawn out of the cassette housing by the tape transport
and wrapped around the rotating record/playback head. As in helical scan
video systems the drum rotates at high speed and lays the digital audio
information on the tape in a series of closely spaced diagonal lines with
a resulting high information density on the tape. Fast forward and rewind
times are very fast with a two hour tape rewinding in about 45 seconds.
Most DATs house the tape transport and digital audio processors in
one enclosure although some manufacturers separate these functions. The
audio processing functions include both analog-to-digital conversion (ADC)
and digital-to-analog conversion (DAC) plus anti-aliasing filters. Most
professional DATs are capable of recording and playback at a number of
sampling rates with the most usual being 44.1 kHz and 48 kHz.
Program numbers are added in numerical order and can be used to locate
Start ID marks can be added either manually or automatically. If manually
set they can occur anywhere in the recorded program. If automatically set
the machine uses a level threshold to decide where to place marks. If the
program level has been below the threshold for more than two or three seconds
a mark is placed at the next place which exceeds the threshold. Start ID
marks can be erased at any time and in any order.
Skip ID marks allow a portion of the recorded program to be marked
so that when the machine senses one of these during playback it will stop
and enter fast forward mode until it reaches the next Start ID where it
will continue playing.
Most DATs provide both analog and digital I/O. By using the digital
I/O it is possible to limit the number of times the digital signal is passed
through the D/A-A/D converters until final playback when the digital signal
must be converted to analog for amplification and loudspeaker dissemination.
By keeping the signal in the digital domain signal loss and noise which
might be introduced by analog components is kept to a minimum.
Several types of digital connectors are used with DATs. The most common
are RCA, BNC, and XLR type connectors. Analog connectors are usually of
the unbalanced RCA or balanced XLR type.
DAT machines provide peak meters using bar graph type level indicators.
not VU meters and a reading of 0 is the absolute maximum
level the machine can achieve without distortion.
If peaks are reaching or exceeding 0 the level is too high and input level
should be reduced.
Some DAT machines provide a headroom indicator which keeps track of
the highest signal peak since last being reset. This provides a handy way
to tell if your program has reached or exceeded the 0 level maximum of
the machine. This is a very useful feature as it is not always possible
to pay such close attention to levels when recording, mixing, or mastering.