On 2/27/23 12:55, r.stricklin via cctalk wrote:
On Feb 27, 2023, at 10:21 AM, Mike Katz via
cctalk <cctalk(a)classiccmp.org> wrote:
the drive would see half the new data and half the old data.
I think that explaining it this way can easily lead to an incorrect inference on the part
of an arbitrary hypothetical neophyte that what is going on in the drive in such a case is
that the head can equally well read either the old data or the new data but the controller
can’t distinguish which is which, or might return old data, or might return new data, or
might indiscriminately return some old data and some new.
What the drive reads in such a case is noise, because the wider head picks up a
superposition of the old (wide) 48tpi track data AND the new (narrow) 96tpi track data,
Double-stepped disks written at 96 tpi are perfectly readable on a 48
tpi drive if the disks are bulk-erased first. I've done this using a
videotape eraser and later, a DC eraser made up of two ring magnets from
an old magnetron, spaced perhaps 3-4 mm apart with like poles facing
each other. Just pass the disk around the gap. Works a treat.