Cartridge tape questions

[Michael H. Warfield Mike]

In article <9638 at june.cs.washington.edu> witold at june.cs.washington.edu (Witold Paluszynski) writes:

>Oh, pardon me for not making myself more clear.  When I said the
>9 track drives were compatible I meant that the COMPATIBLE drives
>were compatible.  Sure you can buy an incompatible drive.  But
>you can also buy a compatible one.  No such luck with QIC.

	The example with the 9 track drives though is not a good one.  The
difference in density was in the frpi (flux reversals per inch).  The
difference in the QIC formats is in tpi (tracks per inch).  The newer format
involves a higher track density that effectively doubles the data on the
tape (60 Meg tapes record 120Meg).  In order to get to 150Meg, different tapes
are used that also support a higher frpi as well as tpi.

	The problem is that the technology, in this case, is not really
downward compatible when you change the tpi.  To increase the tpi you have to
use thinner heads.  Now thinner heads can read the wide tracks of the old
format just fine.  However, if you were to record the old track density with
the newer head geometry, you get thinner tracks spaced out with wide gaps
between them that have not been erased or recorded on.  The thinner tracks
result in a MUCH lower signal level on the older heads and the gaps between
the tracks introduce noise (or worse extranious unerased data) into the
playback system.  The result is that the old heads cannot read the thinner
tracks.  This is a physical limitation resulting from the thinner tracks.
The only way around it is not to use the thinner tracks.  Which means you
don't use this technology.  So to get the advantage of THIS technology
you sacrifice some compatibility.

	A closer analogy with the large tapes would not be the difference
between 800 bpi and 1600 bpi but rather between 7 track tapes (How old are
you?  Am I showing MY age :-) :-) ) and 9 track tapes.  Yes there were some
strange animals that could support both, but only by having two complete head
assemblies.  Most of the time it was wiser to just keep one old clunker around
for the 7 track tapes after upgrading to 9 track tapes.  Some places couldn't
even justify that after a while and just farmed out their 7 track tapes to
outside vendors who specialized in conversions.

	A comparible situation currently exists in the MS-DOS world.  The
1.2Meg 5 1/4" floppy disks are records at 96tpi.  The older 360K disk are
recorded at 48tpi.  It is a simple matter for the 1.2Meg drives to read the
360K format.  You can format and record the 360K format on the 1.2 Meg drives
but the chances of it being readable on a 360K drive is very slim.  In contrast
the 3 1/2" floppy drives use a different recording density between the 1.44Meg
and the 720K formats.  For this reason the 1.44Meg drives are fully capable
of reading and writing the 720K disks just by changing the data transfer
rate.  The resulting disks are identical to a disk recorded on a 720K drive.

	The difference in the head geometry on the new high capacity tape
drives means it is physically incapable of recording a tape that is identical
to a tape recorded by an old drive.  It is then difficult to expect the
old drive to be able to read that tape.  The fact that the manufactures do
not enable this type of action is purely defensive.  I certainly would not
want to deal with the volume of customer complaints that would result when
the old drives fail miserably trying to read a tape from a new drive.

Michael H. Warfield  (The Mad Wizard)	| gatech.edu!galbp!wittsend!mhw
  (404)  270-2123 / 270-2098		| mhw at wittsend.LBP.HARRIS.COM
An optimist believes we live in the best of all possible worlds.
A pessimist is sure of it!