Hauke Fath <hauke@Espresso.Rhein-Neckar.DE> wrote:

>As to DDS drives: At work, we have about a dozen defective DAT streamers
>(mostly HP) from customers lying in the corners. They tend to
>degrade/deadjust silently, and when the heads finally die, we all too often
>discover that the backup tapes are not readable by other streamers. When
>the DAT drive of our fileserver died, we got a Tandberg 5G QIC drive - no
>problems so far, and the cartridges simply look safer than the toyish DDS
>media. They are more expensive, though. But then - why doing backups when
>the data is not worth the effort, anyway?

Hopefully the drive will be landing this week, so I'll get a chance to play
around.  Ended up going with a refurbished HP35480A.  (Three hundred plus
bucks cheaper than the APS, with a warranty that lasted three times as long
(!!)).

In real life, I'm a recording engineer, where DAT is very commonly used as
a means of exchanging audio material, storing audio data in a 16-bit linear
PCM format (the same way an audio CD does).  Having 7-8 years to come to
grips with the medium, I have my own set of prejudices and gripes, as well
as a set of 'safe practices'.

I don't know how much of this will translate into the DDS world, but they
are (essentially) the same physical tapes, running in 4 mm helical scan
transports.

DDS tapes tend to be more reliable than audio grade--more stringent
testing.  Note that audio grade are sold in 'minutes' and DDS are sold in
meters.  In an audio player, a tape moves at 0.5 meters/minute: a 60 meter
tape lasts two hours.

60 meters is also a line of demarcation with respect to tape thickness.  In
order to physically fit more than 60 meters in the cassette shell, a
thinner (and consequently, more fragile) tape stock is needed.   There are
exceptions, but as a general rule of thumb, audio decks aren't meant to use
anything longer than 120 minutes.  (125's are the longest audio tapes sold).

In theory, a 120 meter tape *could* yield 4 hours of recording time.   In
reality, when breaking the '2-hour rule'--some machines handle the extended
lengths fine.  Some machines will spit out error messages, refusing to play
tapes longer than 2 hours.  And then there are the problem childs--I've
seen a few  decks that have had the nasty habit of turning extended length
DDS tapes into confetti.  If you've invested a few thousand bucks in mixes
contained on that tape---'Ouch.. I hope you had a backup!'

Which brings me to safe data practices--

If you are recording on DAT--make 2 copies.  No excuses, no exceptions.  In
Audioland, that translates into using 2 machines, recording on one while
simultaneously making a digital clone on the other.  Then listening back to
the recording to verify its integrity.  In Computerland, I'd equate that
into rotating copies and verification after write.

Readability from one machine to another is greatly influenced by transport
alignment.  It needs to be done regularly (on audio machines), as well as
the cleaning of the heads.  (I have to find out more about the DAT drive
aspects of transport alignment).

Have you ever opened up a DAT cassette (slide the botom plate back and flip
up the door?)  There's a small section of tape that's directly exposed to
atmospheric contaminants, and thus susceptible to degredation much more
quickyly than tape wound up inside the shell.  The solution in audioland is
to record absolutely nothing during the first minute or two of the tape,
and then completely rewind after use.  That way the part that goes first is
the head..and you don't have anything important there anyway.

When I first started using DATs I didn't realize that.  Have a few old
tapes that are 7-8 years old, where I start recording right from the
beginning.  Yep, that stuff is trashed...

In computer use, does anyone know if most archival utilites will do the
same thing (skip the first few minutes)?  Or if there is a provision for
accomplishing something similar?

Tapes wear out.  After thirty passes, I start getting suspicious.  After 50
passes, I chuck the tape.  Tapes are cheaper than what you put on them.

Again, I realize that despite the surface similarities, these are two
different creatures, if anyone out there has a real nuts and bolt knowledge
in Data DAT innards, I'd be curious to hear some thoughts/ feedback.

Steve Revilak
revilak@umbsky.cc.umb.edu