Sony MINIDISC - Manual (Page 5)

What it is and How it works
A A D D C C D D
A U S T R I A
3
+
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Cross sectional view
of recording track
Laser Laser
High intensity Low intensity
0 1
Recordable disc Premastered disc
0 1
+
+
MD
Objective Lens
Polarized
beam splitter
Photo Detector A
Photo Detector B
Laser Beam
MO signal
Pit signal
Recording track
1.2. What it is and How it Works
Two Kinds of MiniDiscs are Available:
Premastered MiniDiscs for
Music Software
Premastered MiniDiscs are used most commonly
for music and are sold in record stores just the
same as compact cassettes and CDs are.
MiniDiscs, just like CDs, are manufactured in
large volumes by high-speed injection molders,
and the music signals are recorded during repli-
cation in the form of pits.
Forget the times when you would worry about
cassette tapes becoming worn out or wrinkled.
Moreover, the discs are encased in a cartridge, so
there is no worry about their being scratched.
The design of the premastered MiniDisc cartridges
Optical Pick-up of MD Player:
is special. Prerecorded music packages require a
label, featuring the artist’s picture or other infor-
mation – for it is the artwork that most often
captures the buyer’s eye. Therefore the top face
of the cartridge is left completely free for the
label. A window for the laser beam to read the
disc is only necessary on the bottom face.
Recordable MiniDiscs
Magneto-optical (MO) technology forms the heart
of the recordable MiniDisc technology. Not only
are MO discs durable enough to withstand being
rerecorded about 1 million times, they also have a
life expectancy approaching that of CDs.
When recording the MO discs, a laser beam must
be focused onto the magnetic layer from behind
while a magnetic field is applied to the front. For
this reason, the cartridges for recordable MiniDiscs
have a read/write window on both sides.
The optical pick-up developed for the MD system
has the remarkable ability to read both recorda-
ble and premastered MiniDiscs. For recordable
discs, the pick-up detects the magnetic structure
along the track. For premastered optical discs, it
detects the geometric structure along the track.
The MD system is based on the design of the
standard CD pick-up with the addition of a MO
signal readout analyzer and two photodiodes.
During playback of recordable discs a 0.5mW laser is focused onto the magnetic layer. The magnetic
signal on the disc affects the polarization of the
reflected beam. The direction of polarization is
converted into light intensity by the MO signal
readout analyzer. Depending on the direction of
polarization one of the two photodiodes will
detect more light. The electrical signals from the
photodiodes are subtracted and depending on
whether the difference is positive or negative, a
“1” or “0” signal is read.
MiniDisc Pick-up Technology