15 Nov
2007
15 Nov
'07
10:49 p.m.
Subject: Re: Tarbell is making me insane
From: ard at p850ug1.demon.co.uk (Tony Duell)
Date: Thu, 15 Nov 2007 20:50:34 +0000 (GMT)
To: cctalk at classiccmp.org
As I understand it, FM at 125bps has (only) pulse spacings that can occur
in MFM at 250kbps. The latter has some other spacing too, of course, but
the point is that unless you're trying to be awkward, it's impossible to
make a drive that will correctly handle any MFM data at 250kbps that will
not also correctly handle FM data at 125kbps. (About the only way you
could do it is if the drive tries to decode the data back to user bytes
and then re-encodes them onto the disk in a totally different way. I know
of know floppy drive that ever tried something like that!).
There should be no problem at all in using a 3.5" DD floppy drive with
the FM data stream that uou'd have sent to an SA400.
Doesn't completely work that way. If it were bandwidth only maybe.
The problem is it's timing and that causes peak shifts that increase
with jamming those bits in a more confined space (flux tranistions
per inch) and that makes 3.5" track 000 an approximation of SA400L
track 41ish. That doesnt' factor the lower S/S+N ratio of 135 tpi
against 48TPI. Will it work, I may but if it doesn't work well
it may not be broken. Reason is the DD FDCs (both 765 and 1793/2793)
have additional hardware to add a timing shift based on pattern written
and better feature extraction for read. This is not present on 1771
based boards.
Allison
This
faster signal might be better for the disk drive?
Potentially. Remember FM floppy recording was passe' by the early
80s with every one needed far greater space. It want even a
consideration buy time PC 3.5" floppies hit the street. -tony