In article ,
Lucifer wrote:
On Tue, 20 Apr 2021 18:09:04 -0700, (Dave
Platt) wrote:

In article ,
Lucifer wrote:
I have a Denon DMD-M10 minidisk recorder that will not play disks
recorded on other machines nor will it record on a disk that has been
used before. I was able to record on an unused disk which does play.

At a guess, I'd suspect that it might be any of the typical faults
which can develop in any laser-optical-based disc recording and
playback system:

Thank you for your reply.
However minidisk uses magnetic recording and playback similar
to a floppy disk. The laser is only used when recording to heat
the disk and thus allow it to be altered by the magnetic write head.

All of the information I've found on Minidisc, says that it works the
other way around.

During recording, the laser operates at high power, heating up the
magneto-optic layer on the disc above its Curie point. The
fluctuating magnetic field from the write head is "captured" by the MO
layer as it cools back below the Curie point.

During playback, the laser is used (at low power) to sense the
magnetic orientation of the MO layer by sensing changes in the
polarization of the reflected laser beam (via the Faraday effect).
When playing pre-recorded discs (made more like CDs) the laser is
sensing reflectivity differences rather than polarization.

References: https://www.minidisc.org/minidisc_faq.html (item 9) as
well as The Great Font Of Dubious Knowledge (a.k.a. Wikipedia).

For more detail, you can take a look at the original Minidisc patent:
http://www.minidisc.org/patents/pdfs/US05244705.pdf - the basic
descriptions of the recording and playback processes are at the bottom
of column 9 and the first part of column 9, on pages 15 and 16.

The technical advantage of this dual approach is that the magnetic
head doesn't have to "fly" at an extremely low height over the surface
of the disc. In a hard drive, the gap between head and media is truly
tiny... much smaller than the diameter of a human hair, and often
smaller than a particle of tobacco smoke. It has to be this close, in
order to be able to read and write very-tiny bits. That would be very
hard to pull off successfuly for a removable medium like MD.

With MD, the write head can be larger, and further away from the
surface. It produces a fairly broad area of magnetic field, but this
affects the material only in the tiny area being heated by the laser
(and cooling down immediately thereafter).