SJ-MD100 (serv.man2) - Panasonic Audio Service Manual Supplement (repair manual). Page 2

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U

Construction of an MD

Y

The playback-only MD and the recordable MD are exactly identical in size and shape.

Y

The figure below is a cross-sectional diagram of an MD disc.  The diameter of an MD disc is 6.4 cm, approximately
half that of a CD, and the thickness is 1.2 mm, the same as a CD.  Similar to a CD, only one side of an MD disc is
used to store data.

U

Materials used in an MD

Y

The MD disc is made of polycarbonate, the same material that is used for a CD.  Polycarbonate is a type of
engineering plastic that is highly resistant to temperature and humidity, as well as having excellent wear and
impact resistant.

Y

A clamping plate is mounted in the center of the MD disc, and when the MD is loaded into a player, a magnet in the
player attracts that metal plate to secure the disc in place.  If the MD disc were to be secured by clamping it from
above and below similar to a CD, it would be necessary to have a hole pass through the center of the MD cartridge,
which would reduce the amount of space available for attaching a label.  By using this magnetic method of securing
the disc, the entire front side of the MD cartridge can be used as a label area.

Y

Because of the metal plate mounted at the center of the MD disc, the center of the cartridge is 2 mm thick, slightly
thicker than the rest of the cartridge.

Y

To protect the MD disc from dust, fingerprints, and other things that might hinder the reading of the recorded
signals, the disc is stored inside a cartridge similar to that of a floppy disc.  When the MD is loaded into a player, the
shutter on the cartridge is opened and the disc is ready to be played.

Y

For a recordable MD, because there is no need for a recording head and it is only necessary for a laser beam to be
directed at the underside of the disc, the shutter is located only on the back of the cartridge.

Y

For a playback/record magnet optic disc MD, because it is necessary for the recording head and the laser beam to
be able to access both sides of the disc, the shutter is located on both sides (upper shell and lower shell) of the
cartridge.

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T

Y

"MD" stands for "mini disc".

Y

Similar to a music CD, an MD is also a small disc capable of recording and playing back digital sound.

Y

There are 2 types of MDs, a optical disc for playback-only MD and a magnet optic disc for recordable MD that is
capable of both recording and playback.

Y

A playback-only MD is merely a smaller-diameter version of a CD.  Just like a CD, the signals are read by light
striking pits on the surface of the disc.

Y

With a magnet optic disc MD(Recordable MD) that is capable of both recording and playback, recording is per-
formed by a vertical magnetization system in which a magnetic thin film on the surface of the disc is heated by a
laser beam, and magnetism is applied in accordance with the data (audio signal) being recorded.

Y

When a recordable MD is played back, a laser beam weaker than that used during recording strikes the disc and
is reflected back, and the reflected light is twisted (polarized) in accordance with whether the magnetized direction
is upward or downward, causing the reflected light to rotate very slightly clockwise or counterclockwise.  Those
subtle differences in the reflected light are picked up by two light-receiving elements and detected as either a "1" or
a "0" by reading whether there is electrical current or no electrical current.

Y

If an MD were recorded in the same way as a CD, it would only have about 15 minutes of recording time.  However,
by using a new signal compression technology called ATRAC that was specifically developed for MDs, the signals
are compressed to approximately one-fifth, making it possible to record for an extended time of 74 minutes, the
same as with a CD.  (Blank MDs are currently marketed in two recording times, 74 minutes and 60 minutes.)

Y

Although MDs were originally developed for use in recording and playing back music, in July 1993 the "MD data"
standard was established.  By using an MD data+music player, MDs can be used as external memory storage
media for computers, and a single MD has a storage capacity of 140 MB, equivalent to about 100 floppy discs.

U

What is an MD?

U

Two types of MDs

U

Playback-only MD is same
as a CD.

U

R e c o r d a b l e   M D   u s e s
magnet optic recording.

U

Playback of a Recordable
MD

U

74-minute recording time

U

Can also be used on a
computer.

T