DATMAN TECHNICAL BULLETIN #010
From: Kan Yabumoto tech@datman.com
To: DATMAN user
Subject: 4mm Tapes (Audio grade vs. DDS grade DAT tapes)
Date: 1996-12-26
====================================================================
Is it safe to use audio grade DAT tape with DATMAN?
How can a DAT drive tell the difference?
Many people ask questions about tapes. This memo answers some
of the questions and offers our recommendations.
The following diagram is a rough sketch of the bottom side of
the DAT cartridge.
+------ Recognition Hole 1
| +---- Recognition Hole 2 +--- File Protect Hole
| | +-- Recognition Hole 3 | +- Recog.H. 4
| | | | |
+---------------------------------------------+
| o o o o O o o |
+---------------------------------------------+
| + + |
| |
+---------------------------------------------+
| |
| / \ / \ |
| | | | | |
| \ / \ / |
| |
| || || |
| || || |
| || || |
+---------------------------------------------+
The depths of the four recognition holes encode the
type of cartridge.
Hole # Tape
1 2 3 4 Thickness Length Coating Format
------------------------------------------------------
0 0 0 0 13 um 60m MP DDS, Audio
0 0 1 0 Head Cleaning Cartridge
0 1 0 0 9 um 90m MP DDS
1 0 0 0 6.9 um 120m MP+ DDS-2
1 0 1 0 6.9 um 125m MP++ DDS-3
1 1 0 0 4.5 um 150m ME DDS-4
0: shallow, 1: deep
All audio grade tapes are like the 60m DDS tape. You can't tell the
length from the hole pattern. The manufacturers generally say the
DDS tapes are more thoroughly tested (and certified).
There is one more criterion in 4mm tapes --- MRS which stands for
"Media Recognition System". Apparently, MRS was not in the original
4mm DAT tape specification. A MRS tape has a short zebra-pattern
region between the clear leader section and the remaining magnetic
tape section.
As far as I know, MRS is good for one specific purpose. It helps the
tape drive distinguish 60m DDS tapes from audio grade tapes of various
lengths. All 60m and 90m DDS tapes manufactured since early 1993
come with the MRS and its logo (four stripes next to the DDS logo).
All 120m DDS-2 tapes are MRS compatible (with the zebra strip).
They don't show the four-stripe MRS logo. Very old 60m and 90m
tapes did not conform to MRS and therefore certain drives (e.g.,
HP drives) treat them as if they are an audio grade tape.
Hewlett Packard's C1533A (and its big brother, C1553A with Auto-
Loader) drive is the only drive that cares about the presence
of the MRS pattern (shipped as factory standard default). When
you insert a non-MRS (audio grade or old 60/90m) tape, the C1533A
treats the tape as read-only. The drive apparently does not trust
audio tape for recording anything new, but still reads the data
for backward compatibility. This MRS-sensitive behavior can be
disabled by the DIP switch on the bottom of the HP drive
(position 3, ON to write any tape). As far as we know, there is
no mechanism for the host computer (i.e., the driver software)
to know whether or not you have a MRS tape. Other manufacturers
don't seem to care about this distinction.
-----------------------------------------------------------------
Here are our recommendations
1. Buy a DDS-2 drive. Even though the price is higher, the
difference (typically 10-15% over a DDS-DC drive) is relatively
small. The DDS-2 drives usually run faster than their DDS-DC
counterparts because of higher head speed (e.g., 5737 rpm on
C1533A vs. 2000 rpm on 35480A).
2. Buy 90m DDS tapes even if you have a DDS-2 drive, unless your storage
need exceeds the capacity. 60m tapes are not much cheaper than 90m
tapes, but avoid the 120m type. First of all, 120m tapes are priced
over twice that of 90m tapes by most vendors. That is, 90m tape has
the lowest per-bit price. Second, you don't typically fill up the
entire tape (in our case, the average capacity usage is less than 30%).
Third, 120m DDS tapes are very hard to obtain. Today, almost three
years after the DDS-2 technology came to the market, the 120m tapes
are still under "allocation" --- you have to wait for a long time
before your order will be filled. One distributor explained to us
that there is only one company in the world which supplies the ultra-
thin tape substrate to all the DDS tape manufacturers. It's hard to
believe, but that may well be the case. Last, we have not found a
single brand of 120m tapes with consistent quality AND delivery.
3. About the audio grade tapes...
While we do not encourage our users to go out and buy audio grade
tapes to use with DATMAN, we see no reason for you to throw away
audio DAT cartridges you already have. You must know the risk
factor and deal with it accordingly. The DDS drives (all models
except Exabyte) support "N-Group writing" (also called multiple
group writing). N-Group writing repeats each group of data so that
there are N consecutive copies of each group on the tape. This
feature is so automatic and transparent, you can hardly notice
it. In fact, the N-group factor of 1 (data copied twice), usually
speeds up file transfer with DATMAN! Its' too complicated to
explain the reason in this memo. In the past, we have seen 120m
DDS-2 tapes (which the manufacturer "certified") with very marginal
quality. If you use the N-group feature wisely according to your
reliability requirements, we believe audio grade cartridges can
be quite dependable.
With DATMAN, to set N-group of 1 (to copy twice) is very simple...
DATMANFE -g1 ; you can specify at installation time
DATMAN dngroup 1 ; or you can change at any time later
4. Please share your personal experience regarding DAT tape media with us.