SJ-MD100 (serv.man2) - Panasonic Audio Service Manual Supplement (repair manual). Page 3

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D

U

Can be recorded and played
back repeatedly.

U

Recording principle

U

Ve r t i c a l   m a g n e t i z a t i o n
system

U

N u m b e r   o f   r e c o r d i n g s
possible

Y

By using a magnet optic disc, digital signals can be recorded and played back over and over again.

Y

To record on a magnet optic disc, a laser beams momentarily heats "pin spots" on the magnetic film on the back of
the disc and a magnetic field is applied from the other side of the disc.  Thus, both sides of the disc must be
accessed in order to record.

Y

To explain the recording principle, we will assume that the directions of the magnetism on an unrecorded disc are
all facing downward (south-north = "0 0 0 0 ...").  (Actually on an unrecorded disc the directions are random.)

Y

Thus, to record the signals "1 0 1 1 0 1 0", the direction of the magnetism at the locations where "1" is to be
recorded must be changed to face upward (north-south).  Because the magnetic film is strongly magnetic, once a
downward-facing magnetism is recorded, it is not easy to change it to an upward-facing magnetism.

Y

By directing a laser beam at the magnetic film, the temperature of the location that the laser beam strikes rises to
the Curie temperature (recordable MD; about 180

p

C), eliminating the magnetic force (retention force). (Because

the magnetic film is strongly magnetic, similar to a permanent magnet, once it is magnetized it has a strong
retention force.  In order to eliminate that retention force, it is irradiated with a laser beam so that the temperature
rises to the Curie temperature.)

Y

After the magnetism of the specific location is eliminated, an external magnetic field with an upward direction
(north-south) is applied, thus changing the direction of the magnetism at that location to face upward (north-south).

Y

Conversely, if a downward-facing (south-north) external magnetic field is applied, the direction of the magnetism at
that location is changed to face downward (south-north).

Y

Then, when the disc rotates and the location which has been changed to upward-facing magnetism leaves the
laser spot, the temperature of the magnetic film drops, and the upward-facing magnetism recorded at that location
is retained.

Y

In this way, digital signals of "1" (upward-facing magnetism) and "0" (downward-facing magnetism) are recorded
on the tracks on the disc.

Y

With a conventional magnetic recording tape, the magnetic material is magnetized parallel (horizontal) to the
surface of the tape.  A magnet optic disc, however, uses a vertical magnetization system in which the magnetic
poles are recorded perpendicular (vertical) to the disc surface.  Because the magnetism is recorded vertically
rather than horizontally, much more data can be recorded in a smaller area.

Y

A magnet optic disc can be recorded more than 1 million times, so it can virtually last forever.

w w w w w w w w w w w w w w w

S

i

w w w w w w w w w w w w w w w

N

Only locations struck by a

powerful laser beam

(reach the Curie temperature)

lose their magnetism

retention force.

B

Assuming that the directions of

the magnetism on the unrecorded

disc are all facing downward

(they may also all face upward). 

A

Disk rotation direction

Disk rotation

direction

Laser spot

i

w w w q w t w w q q q w w q

i

w w w w w t       w w w w w w w

N

An external magnetic 

field is applied to the 

demagnetized location, 

creating upword-facing

magnetism.

C

Recorded signals.

Directions of magnetism differ

according to whether a "1" or

a "0" digital signal is recorded.

D

Disk rotation direction

Disk rotation

direction

Laser spot

S

0 0 0 1 0 1 0 0 1 1 1 0 0 1

B

A

C

D

Upword     : "1"
Downword : "0"

Surface of the disk is a magnetic thin film made of terbium-cobalt alloy.

Heated

External magnetic field is applied.