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ll current ADATs are now based on a 20-bit
recording format with three models in the range:
the professional M20, the XT20 and the cheaper
LX20. Other than a slightly different front panel finish,
the XT20 is very similar in both appearance and
features to the XT it replaces. The XT20 shares the
same transport and the same heavy cast chassis as
its predecessor. Audio connections are by means of
both phono (unbalanced, -10dBv) and EDAC
multipin (+4dBu), and of course there's the ADAT
‘light pipe’ fibre optic Toslink interface. The cheaper
LX20 has a simplified feature set, with no balanced
EDAC connector, and is built on a folded steel
chassis, but is otherwise capable of the same audio
quality. Both ADATs' power supplies are designed
to work on any mains supply, from 90 to 250V AC
at 50 or 60Hz, without the need to reset the
operating voltage.
A new recording format, ADAT Type II, has been
devised to squeeze the extra data (generated from
recording with a larger 20-bit word size) onto a regular
ADAT S-VHS tape, without shortening the
record/playback time. It is still possible to play and
record using the original 16-bit ADAT Type I format
where necessary, but the only limitation is that a Type II
(20-bit) tape can't be played on an original Type I (16-bit)
ADAT. Bear in mind that this may mean that owners of
existing ADATs may run into trouble if clients start
bringing Type II tapes into their studios to have more
work done. In all other respects, the original XT and the
XT20 offer the same facilities and are compatible with the
original BRC.
Like the XT, the XT20 offers 10 on-board autolocate
points and includes an LRC wired remote control unit,
an optical cable, a power cable and a blank S-VHS tape.
An ST-120 tape gives around 40 minutes of recording
time, while an ST-180 may be used to yield just over
one hour of recording. Rewind speed is 40 times the
play speed, so a regular length tape takes only a little
over one minute to wind completely. Sadly, Alesis have
missed the opportunity to fit an MTC output into their
new machines, so you are still unable to sync to a
sequencer without using third party hardware, such as
an Alesis BRC, a JL Cooper DataSync or a MOTU
Digital Time Piece.
Bit by bit
A 20-bit recording has 16 times the resolution of a 16-bit
recording, but for the foreseeable future the release
medium remains the 16-bit CD, so what are the benefits of
recording 20-bit? For starters, it can be useful to have
more bits at the recording stage to allow a little more
headroom and help prevent clipping. In a perfect world,
one bit is equivalent to 6dB of dynamic range, but,
because of limitations in the analogue side of the
converters, the difference in dynamic range between 16-bit
and 20-bit isn't as great as you might imagine.
Furthermore, every time a digital signal is compressed,
equalised or altered in level, some resolution is lost
Paul White digs beneath the surface of Alesis’ new generation ADATs and finds
that little bit extra.
Alesis ADAT
XT20 and LX20
Crash course on ADAT
ADAT stands for Alesis Digital Audio Tape and is a digital format providing eight
tracks of audio recording on a conventional S-VHS video tape. A pair of 9-pin
ADAT sync in and out sockets are provided so that two or more ADATs can be
synchronised to sample accuracy when more than eight tracks are required. This
requires no additional hardware. When two or more machines are used, the
machines can be offset to allow regions of audio to be copied to different parts of
the song on the second machine – so if you have that one perfect chorus, you can
use it throughout the song. Editing, including punching in and out, is gapless and
up to 128 channels may be assembled by sync'ing up to 16 machines. For
multiple machine use, a BRC (Big Remote Control), can remotely control up to 128
tracks of ADAT, and provide SMPTE/MTC and Midi Clock sync, plus a number of
powerful editing options. Different ADATs may be sync'ed in the same system,
though the chase time is limited by the slowest machine. The old black-face
ADATs wind much more slowly than the XTs, XT20s or LX20s.
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(140 pages)
(24 pages)
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