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A magneto-optical drive is a kind of optical disc drive capable of writing
and rewriting data upon a magneto-optical disc. Both 130 mm and 90 mm form
factors exist. The technology was introduced at the end of the 1980s. Although
optical, they appear as hard drives to the operating system and do not require a
special filesystem (they can be formatted as FAT, HPFS, NTFS, etc.).
Technical aspects
A 130 mm 2.6GB Magneto optical disc.
A 90 mm 640MB Magneto-optical disc.
A 230 MB Fujitsu 90 mm magneto-optical disc.Initially the drives were 130 mm and
had the size of full-height 130 mm hard-drives (like in IBM PC XT). 130 mm media
looks a lot like a CD-ROM enclosed in an old-style cartridge while 90 mm media
is about the size of a regular 1.44MB floppy disc, but twice the thickness. The
cases provide dust resistance, and the drives themselves have slots constructed
in such a way that they always appear to be closed.
The disc consists of a ferromagnetic material sealed beneath a plastic coating.
There is never any physical contact during reading or recording. During reading,
a laser projects a beam on the disk and according to the magnetic state of the
surface, the reflected light varies due to the Magneto-optic Kerr effect. During
recording, the light becomes stronger so it can heat the material up to the
Curie point in a single spot. This allows an electromagnet positioned on the
opposite side of the disc to change the local magnetic polarization, and the
polarization is retained when temperature drops.
Each write cycle requires both a pass for the laser to erase the surface, and
another pass for the magnet to write the information, and as a result it takes
twice as long to write data as it does to read it. In 1996, a Direct Overwrite
technology was introduced for 90 mm discs, to avoid the initial erase pass when
writing. This requires special media.
Magneto-optical drives by default check information after writing it to the
disc, and are able to immediately report any problems to the operating system.
This means that writing can actually take three times longer than reading, but
it makes the media extremely reliable, unlike the CD-R or DVD-R technologies
upon which data is written to media without any concurrent data integrity
checking. Using a magneto-optical disc is a lot more like using a diskette drive
than using a CD-RW.
Progress in magneto-optical technology received a boost in the spring of 1997
with the launch of Plasmon’s DW260 drive. This used Light Intensity Modulated
Direct OverWrite technology to achieve an increased level of performance over
previous magneto-optical drives.
Vendors
The NeXT computer was the first to offer this technology, but Canon eventually
provided it to other customers.
Sony MiniDiscs are magneto-optical, and Sony produces many other formats of
magneto-optical media.
Fujitsu is a major manufacturer of 90 mm magneto-optical drives. Today's
capacities exceed 2 gigabytes (Gigamo).
The Nintendo 64DD used magneto-optical discs.
Floptical drives
Magneto-optical drives should not be confused with Floptical drives, which
likewise combine ferromagnetic and optical technologies, albeit in a different
manner. Flopticals are 21 megabyte 90 mm magnetic diskettes using optical tracks
to increase the tracking precision of the magnetic head; from the usual 135
tracks per inch to 15,000 tracks per inch. No laser or heating was involved; a
simple infrared LED was used to follow the optical tracks, while a magnetic head
touched the recording surface. The drives could also read and write traditional
90 mm diskettes, although not the 2.88 megabyte variety. Flopticals were
manufactured by Insite Peripherals, a company founded by Jim Burke.
Recent progress
At the Consumer Electronics Show in January 2004, Sony revealed a 1 gigabyte
capacity MiniDisc known as "Hi-MD." This sixfold increase in capacity is
performed using a magneto-optical trick. To record a standard MiniDisc the
writer uses an infrared laser to heat a spot of ferromagnetic material on the
disc to above its Curie point, then it is magnetised by a recording head as it
cools. In contrast a high capacity MiniDisc uses tracks that are six times
smaller than those used in standard MiniDiscs. By employing three magnetic
layers, when a high capacity MiniDisc is read, the track expands to readable
size. Specifically the three layers are, from read-face to print-face: a
displacement layer, a switching layer, and a memory layer. When it isn't being
read, the magnetic field in the memory layer is the same as those in the
displacement and switching layers. When a laser shines on the track, the
switching layer, which has a lower Curie point than the other layers,
demagnetises. It decouples from the displacement layer, whose "magnetic fence"
around the track weakens, temporarily causing the track to swell to a readable
size. Hi-MD recorders can also double the capacity of regular minidiscs with
special formatting that renders the disc unreadable (or writable) by non-Hi-MD
minidisc recorders.
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