Page 1
SJ-MD100
-“ 2 -“
Construction of
magnet optic disc
UConstruction of an MDYThe playback-only MD and the recordable MD are exactly identical in size and shape.
YThe 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.
UMaterials used in an MDYThe 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.
YA 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.
YBecause 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.
YTo 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.
YFor 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.
YFor 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.
$ Basic Knowledge of MD
T Definition of an MD and the types of MDs
Y"MD" stands for "mini disc".
YSimilar to a music CD, an MD is also a small disc capable of recording and playing back digital sound.
YThere 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.
YA 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.
YWith 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.
YWhen 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.
YIf 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.)
YAlthough 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. UWhat is an MD?
UTwo types of MDs
UPlayback-only MD is same
as a CD.
URecordable MD uses
magnet optic recording.
UPlayback of a Recordable
MD
U74-minute recording time
UCan also be used on a
computer.
T Construction of an MD
MD disc form
Page 2
SJ-MD100
-“ 3 -“
D Recording on a magnet optic disc
UCan be recorded and played
back repeatedly.
URecording principle
UVertical magnetization
system
UNumber of recordings
possibleYBy using a magnet optic disc, digital signals can be recorded and played back over and over again.
YTo 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.
YTo 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.)
YThus, 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.
YBy 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 180pC), 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.)
YAfter 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).
YConversely, 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).
YThen, 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.
YIn this way, digital signals of "1" (upward-facing magnetism) and "0" (downward-facing magnetism) are recorded
on the tracks on the disc.
YWith 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.
YA magnet optic disc can be recorded more than 1 million times, so it can virtually last forever.
wwwwwwwwwwwwwww
S i wwwwwwwwwwwwwww
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
directionLaser spot
i
wwwqwtwwqqqwwq
i
wwwwwt wwwwwww
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
directionLaser spot
S
00010100111001B 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.
Magnet optic disc recording principle
Vertical magnetization Horizontal magnetization
