Magneto-optical drive

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Magneto-optical disc
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A Magneto-optical disc and the numerous rectangles on its surface

A magneto-optical drive is a kind of optical disc drive capable of writing and rewriting data upon magneto-optical discs. Both 5.25" and 3.5" 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.

Initially the drives were 5.25" and had the size of full-height 5.25" hard-drives (like in IBM PC XT). Today a 3.5" drive has the size of 1.44 megabyte diskette drive. 5.25" media looks a lot like a CD-ROM enclosed in an old-style cartridge while 3.5" 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-optical 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 site 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 3.5" 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 standard CD-RW, ie one without Mount Rainier support.

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.

Contents

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 3.5" magneto-optical drives. Today's capacities exceed 2 gigabytes (Gigamo).

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 3.5" 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 3.5" diskettes, although not the 2.88 megabyte variety. Flopticals were manufactured by Insite Peripherals, a company founded by Jim Shugart.

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 players can also double the capacity of regular minidiscs.

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