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From: sienkiew@UDEL-LOUIE.ARPA
Newsgroups: net.micro
Subject: Re: confused about disk drive specifications
Message-ID: <11527@brl-tgr.ARPA>
Date: Fri, 12-Jul-85 02:11:15 EDT
Article-I.D.: brl-tgr.11527
Posted: Fri Jul 12 02:11:15 1985
Date-Received: Wed, 17-Jul-85 03:46:43 EDT
Sender: news@brl-tgr.ARPA
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Well, this isn't a thesis, but it may help:


A 35 track drive has the tracks in the same place as a 40 track drive does.
You can read disks from one on the other if you have to.  (Of course, the 35
track drive can't get the last few tracks from a 40 track disk...)
In practice, I have used 36 tracks on 35 track drives and 41 tracks on 40 track
drives with no problems. It seems to me that a lot of 40 track drives
only ever get to the first 35 tracks.  I've hacked Apple DOS 3.3 to use 41
tracks with no problem, but I had to do it myself--it didn't come with the
drive.

From what I've read about 96 TPI drives, they have twice as many tracks in the
same space as 48 tpi drives.  Thus, if you only read every other track, you
can read a 48 tpi disk.  You can't write one because the signal will be too
weak for a 48 tpi disk to pick up.  [has anybody ever heard of a 96 tpi
disk that adjusts the head current to write 48 tpi disks?]


Disks have to be approached differently from tapes, because a tape can be made
to move the media at a fixed rate.  A disk changes the rate each time you go to
a different track.  Your controller
isn't really writing Bits Per Inch--it's writing Bits Per Second.  I'm sure
some creative engineering could get around that, but I'm not aware of any.
(I have heard rumors that some disk systems vary the timing based on the track
number so they can get more data on the outside tracks--can anybody verify 
this?)