1 Oct
1998
1 Oct
'98
8:17 p.m.
I wrote:
Note that this doesn't deal with all of the weird
Apple ][ copy protection
schemes (like spiral tracks), but it will deal with some of them.
Sam Ismail <dastar(a)ncal.verio.com> wrote:
Theoretically you could simply scan half-tracks and
quarter tracks and
just look for strong signals (something above a certain threshold).
I'm not sure that would work for the protection schemes that actually
wrote the sectors while the stepper was in motion. However, fortunately
that technique wasn't very common since the mechanical characteristics
of the stepper et al tended to vary somewhat from drive to drive.
The other copy protection problem that simple attempts at raw copying won't
solve is where the software expects arbitrary features of arbitrary tracks
to be time-synchronized. It's easy to build hardware that synchronizes
the index pulse (which the Apple doesn't even use), but that's not actually
sufficient.
I wrote:
I have a twiggy disks for the Lisa 1 with a bad
sector. I've always wondered
whether hacking the drive electronics to allow software control over the
read amplifier gain and/or data slice threshold would let me recover that
sector.
Sam wrote:
I suppose it depends on what the problem is. I
don't know how similar the
Twiggy encoding was to the Disk ][ encoding,
Almost exactly the same. Every floppy Apple did through the 800K uses
the same technique.
but on the Apple ][, if I had
bad sectors on a disk, one way to attempt a repair would be to try to read
the sector over and over with the checksum disabled until the data looked
"right" and then writing it back to disk. This works a good percentage of
the time, but not always. Sometimes the disk is physically damaged, and
any bytes after the damaged portion of the sector get skewed and will
never translate correctly. I know there's some way to correct this but I
never sat down to analyze the problem and come up with a solution.
That's exactly the problem I have and what I am speculating that tweaking
the gain might help with.