Format

Alesis have made a very sensible decision to use an existing tape format, namely
S-VHS, rather than reinvent the wheel (yet again). S-VHS tapes are easy to come
by and they are not likely to become outmoded for years to come. Like other
rotary head digital recorders, ADAT works mechanically pretty much like a video
recorder. The tape is wrapped around a head drum which carries, in this case,
four heads. As the drum spins and the tape is carried past at a constant seed,
the heads will trace out a pattern of diagonal tracks on the tape. This allows
a very high relative speed between the head and the tape. Although it may seem
like a marvellous feat to record eight tracks of digital audio onto a video
tape in this way, the technology is already well established. The professional
digital video format known as D2 records eight bit samples onto tape at a sampling
rate of 17.7MHz. Compare this to 16 bit digital audio sampled at 48kHz and work
out how many digital audio tracks D2 could theoretically hold if re-designed
for audio. Alesis are probably playing safe in recording just eight tracks onto
S-VHS, which is a good thing because the last thing we want is a format which
is unreliable when not working at its peak fitness.

Alesis are keeping silent about the way the tracks are recorded onto tape.
I would like to have been able to tell you, for instance, whether the tracks
are recorded with a guard band between them, or whether the heads have different
azimuth settings as in DAT, which can cause problems due to narrow tracks being
created at drop ins. All I can say however is what I see with the top panel
removed: the tape is wrapped approximately three quarters of the way round around
a drum just under 60mm in diameter, carrying four heads. There is a stationary
head with two elements at the edges of the tape, which I assume are used for
control tracks of some sort. The tape moves at three times the speed a normal
S-VHS recorder would use. The mechanism, by the way, appears very solid and
well built. Compared to DAT, which most users will agree is far smaller than
common sense ought to have allowed, this transport is heavy engineering. I can’t
give any guarantee of course, but it does certainly promise a high degree of
reliability.