SJ-MD100 (serv.man2) - Panasonic Audio Service Manual Supplement (repair manual). Page 4

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Y

Because the signals on a magnet optic disc are recorded vertically as north-south and south-north magnetism, the
north and south magnetic poles appear on the surface of the disc's magnetic film.  These signals are played back
utilizing a phenomenon called the "Kerr effect" which occurs when a weak laser beam strikes the magnetic poles.

Y

Light has wave vibration directions called "planes of polarization". ("Polarization" refers to a light wave which
vibrates only in a fixed direction.)

Y

With normal light, because the wave vibration directions are all mixed, no planes of polarization appear.

Y

Because a laser beam is artificially generated light, it is possible to align the planes of polarization.

Y

When a laser beam strikes something that has a magnetic field, the direction of the plane of polarization of the
reflected light varies very slightly in accordance with whether the magnetism is north polarity or south polarity.
When playing back a magnet optic disc MD, these slight changes in the direction of the plane of polarization are
read.

Y

To further explain the principle used to read the signals recorded on a magnet optic disc, first a laser beam is
directed at the disc.  If the direction of the magnetism recorded on the disc is upward (north polarity), the plane of
polarization of the light reflected from the disc rotates very slightly clockwise as a result of the Kerr effect.  Conversely,
if the direction of the magnetism is downward (south polarity), the plane of polarization rotates very slightly
counterclockwise.

Y

When the reflected laser light is passed through a Wollaston prism, the light is distributed to photo detector 1 if the
direction of rotation is clockwise or to photo detector 2 if the direction of rotation is counterclockwise.

Y

The light striking the two light receiving elements is converted into electrical current and a subtraction is performed.
If the result of A-B is plus, a "1" is detected, and if the result of A-B is minus, a "0" is detected.

Y

An MD player is compatible with both optical recording and magnetic recording, changing the reading system in
accordance with the type of disc that is loaded.

D

 Playback of a recordable MD

U

Playback of a recordable
MD

U

Reading of magnetic signals