Sony MINIDISC - Manual (Page 6)

Laser Beam Move direction of Disc
New Old
Recording Head
Head Drive signal
0
1
S S N N S S N N
Heated area by Laser
above Curie Point
(erased magnetic pattern)
Magnetic field of
Recording Head
Cross section of
MO-Layer with
magnetic pattern
Top view
of one Track
Frozen magnetic pattern
after cooling below
Curie Point
What it is and How it Works
A A D D C C D D
A U S T R I A
4
The same 0.5mW laser is used for the playback of
premastered optical discs. The amount of light
reflected depends on whether or not a pit exists
on the surface of the disc. If there is no pit, a
high proportion of the light is reflected back
through the beam splitter and analyzer into the
photodiodes. If a pit does exist, some of the light
is diffracted and less light reaches the photodi-
odes. The electrical signals from the photodiodes
are added up in this case and depending on the
sum, a “1” or “0” is read.
Overwrite Technology
In order to meet the requirements of a compact,
lightweight, recordable audio system for perso-
nal use, the MD system employs a newly develo-
ped Sony magneto-optical (MO) disc that utilizes
magnetic field modulation with direct “overwri-
te” capability. With MO disc technology, the
recording of data requires the use of a laser and
a polarizing magnetic field. When the magnetic
layer in the disc is heated by the laser to a tem-
perature above the Curie point (approximately
200° C), it temporarily looses its coercive force.
As the disc rotates and the irradiated domain
returns to normal temperature, its magnetic ori-
entation is determined by an externally applied
magnetic field. Polarities of “N” and “S” can
thus be recorded, corresponding to digital data
“1” and “0”.
Unlike conventional MO rewrite mechanisms, the
”Sony MD overwrite system” positions a magne-
tic head directly accross from the laser source on
the opposite side of the disc. A magnetic field
corresponding to the input signal is generated
over the laser spot. The rotation of the disc then
displaces the area to be recorded, allowing the
temperature at the spot to drop back below the
Curie point. At that point, the domain takes on
the polarity of the applied magnetic field regard-
less of the polarity that previously existed.
Overwrite Mechanism