SJ-MD100 (serv.man2) - Panasonic Audio Service Manual Supplement (repair manual). Page 5

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Y

The signals recorded on an MD are rewritten using a new process called “magnetic field modulation overwriting”*.

Y

In this process, a laser beam spot of about 5 mW is focused on the location on the disc to be rewritten, heating that
location to the Curie temperature (180

p

C) and thus canceling the magnetization.

Y

At the same time, current flows to the optical pickup and to the magnetic head opposite it, between the two of which
the disc is held, thus generating a magnetic field.

Y

When the disc revolves so the laser spot moves from the location to be rewritten, the temperature drops below the
Curie temperature and the magnetic field generated by the magnetic head re-magnetizes that location.

Y

At this time, if the direction of the current flowing to the magnetic head is reversed in accordance with whether the
data being recorded is “1” or “0”, the direction of the magnetic field also changes between north and south, and
accordingly, the direction of the magnetization of the recording film changes between upward-facing and downward-
facing.  Thus, it is possible to directly magnetize the recording film on the disc in accordance with the “0” and “1”
digital signals.

Y

Thus, the new recording data is overwritten regardless of the direction of the previously recorded magnetization,
eliminating the need for an erasing head.

Y

This process is called “magnetic field modulation overwriting”.

Y

Because this “magnetic field modulation overwriting” makes it possible to directly overwrite the new signals on top
of the old signals in a single process, re-recording on a MD is just as easy as with a magnetic tape, making the MD
ideally suited for use in personal audio equipment.

D

 Rewriting action of a magnet optic disc

U

MD rewriting process

U

No need for a erasing head

*Overwrite means to write new data while erasing the old data.