Disk hardware emulation, was Re: Grandfather system RTE6/VM? - test-drb@ccmp.vtda.org

10 Dec 2003

...
 From: "John Lawson" &lt;jpl15(a)panix.com&gt;

On Tue, 9 Dec 2003, Eric Smith wrote:

  The part of the ST-506 disk emulation that's
of most concern to me is
 whether anything needs to be done about write precompensation.  The
 controller shifts the pulses on write to compensate for the peak
 shifts that happen on magnetic media when flux changes are recorded
 close together. 

  Is it not true that the precomp circuits are driven by head position
info - thus the 'solid' version (emulation) of any drive would just be
permanently 'stuck' in one mode or the other would the controller care, or
even know, what track and cylinder the actual 'data' were coming from? 
Hi
 Some controllers used the cylinder information to turn on
the compensation for the inner tracks.

...

  I don't know if this would cause the emulator qua emulator to fail at
some perhaps subjective level - but I can't see where one would need to
actually legislate write precomp into a block of RAM.

 One can see from the timing that there is compensation being
written. My understanding is that the selected compensation
is only used for the write. The read amps do have some non-linear
response to compensate for the normal effects of megnetic
data. From what I've seen, the read is fixed for and compensation
so regardless of the track, one should be able to provide
data with the same framing as any other track.
Dwight