16 Feb
2008
16 Feb
'08
10:51 a.m.
Philip Pemberton wrote:
The problem is, you'd have to modify the drive to
do that. The read
head amplifier on just about all floppy drives (certainly all modern
drives) does some pulse-shaping on the signal as well.
What you'd have to do is find the amplified signal, then bring it into
the range an A/D controller would accept, and digitise it. You'll
probably want at least 20 megasamples/second to get a good amount of
resolution out of the data, and for a revolution time of a second,
you're talking 20 megabytes of data per track (assuming you use an
8-bit converter).
Sounds like it might be a very useful thing to build. (I don't even
have my hands on the disks at this point, he's resorting to using
CopyIIMac right now, which hopefully will be able to read a bit past the
errors.)
So such a setup would need a dedicated machine... at 20MB/track it would
need something like 3.2GB for a whole floppy, maybe double that if we
wanted to include half-tracks and extra tracks, and whatever custom
software to try and clean up the signal. Sounds like a machine with a
DSP would help speed up the post processing too. Ouch... Would still
be worthwhile assuming that the media itself hasn't been physically worn
off and just degraded... I guess something like this is what the data
recovery guys use...
What you can do with the pulse-timed data is read it
out, then look at
it and analyse it based on the encoding rules. So you could throw away
data pulses that are obviously too close to others, or adjust the
timing window dynamically to compensate for motor speed variations.
One of the ideas I had for the Ext I/O inputs was a motor
tachometer/position detector that constantly tracks the instantaneous
speed or position of the motor spindle. Then you can look at the data
from that, and adjust the timing values to get rid of motor speed
induced errors. After that, all you're left with is the comparatively
small error from the clock source (usually the crystal). This sort of
thing probably wouldn't be too hard to add to the average 3.5" or
5.25" disc drive... the hard part would be getting the encoder wheel
exactly centred on the spindle hub.
I suppose you could also slow down the motor speed, as to increase
resolution, but all of these would be pretty invasive drive mods.
(you'd still need some way to precisely measure the distance traveled by
the motor.)
I'm hoping he can get some data off these disks. It's a bit more
complicated, there's an ebay auction involved too. He's promised to at
least try to convince the buyer to let us recover the data off these
disks, so we may have access to these for a while after the auction
ends. If not, I'm afraid this OS will be lost depending on the buyer's
luck/willingness to share... :-(
Any chance someone else on this list has a copy of UniPlus Unix?