Optical Storage Mediums

James Ng

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The most common way of storing data in a computer is magnetic. We have hard

drives and floppy disks (soon making way to the CD-ROM), both of which can store

some amount of data. In a disk drive, a read/write head (usually a coil of

wire) passes over a spinning disk, generating an electrical current, which

defines a bit as either a 1 or a 0. There are limitations to this though, and

that is that we can only make the head so small, and the tracks and sectors so

close, before the drive starts to suffer from interference from nearby tracks

and sectors. What other option do we have to store massive amount of data? We

can use light. Light has its advantages. It is of a short wavelength, so we

can place tracks very close together, and the size of the track we use is

dependent only on one thing - the color of the light we use. An optical medium

typically involves some sort of laser, for laser light does not diverge, so we

can pinpoint it to a specific place on the disk. By moving the laser a little

bit, we can change tracks on a disk, and this movement is very small, usually

less than a hairA•s width. This allows one to store an immense amount of data on

one disk. The light does not touch the disk surface, thereby not creating

friction, which leads to wear, so the life of an average optical disk is far

longer than that of a magnetic medium. Also, it is impossible to A’crashA“ an

optical disk (in the same sense as crashing a hard drive), since there is a

protective layer covering the data areas, and that the A’headA“ of the drive can

be quite far away from the disk surface (a few millimeters compared to

micrometers for a hard drive). If this medium is so superior, then why is it

not standard equipment? It is. Most of the new computers have a CD-ROM drive

that comes with it. Also, it is only recently that prices have come low enough

to actually make them affordable. However, as the acronym states, one cannot

write to a CD-ROM disk (unless one gets a CD-Recordable disk and drive). There

are products however, that allows one to store and retrieve data on a optical

medium. Some of those products are shown in table 1. However, the cost of this

is quite high, so it doesnA•t usually make much sense for consumer use yet,

unless one loves to transfers 20 megabyte pictures between friends. One will

notice on the table that there are some items labled A’MOA“ or magnet-optical.

This is a special type of drive and disk that get written by magnetic fields,

and read by lasers. The disk itself is based on magnetism, that affects the

reflective surface. Unlike floppy disks, to erase such a disk at room

temperature requires a very strong magnetic field, much stronger than what

ordinary disk erasers provide. To aid in writing to this MO disks, a high-power

laser heats up part of the disk to about 150 oC (or the Curie temperature),

which reduces the ability for the disk to withstand magnetic fields. Thus, the

disk is ready to be rewritten. The disk needs to passes to change the bits

though. The first pass A’renewsA“ the surface to what it was before it was used.

The second pass writes the new data on. The magnetic fields then alters the

crystal structure below it, thereby creating places in which the laser beam

would not reflect to the photodetector. Another type of recordable

medium, is the one-shot deal. The disk is shipped from the factory with nothing

on it. As you go and use it, a high-power laser turns the transparent layer

below the reflective layer opaque. The normal surface becomes the islands (on a

normal CD) and the opaque surface the pits (pits on a normal CD do not reflect

light back). These CDs, once recorded, cannot be re-recorded, unless saved in a

special format that allows a new table of contents to be used. These CDs are

the CD-Recordable, and the Photo CD. The Photo CD is in a format that allows

one to have a new table of contents, that tell where the pictures are. It is

this that distinguishes between A’single-sessionA“ drives (drives that con only

read photos recorded the first time the disk was used) and A’multi-sessionA“

drives (that can read all the photos on a Photo CD). To read an optical

medium, a low-power laser (one that cannot write to the disk) is aimed at the

disk, and data is read back, by seeing if the laser light passes to the

photodetector. The photodetector returns signals telling if there is or is not

light bouncing back from the disk. To illustrate this process, see Figure 1.

Optical data storage is the future of storage technology. However, it will take

some time before prices are low enough for the general public. Applications get

bigger, data files get bigger, games get bigger, etc. The humble floppy disk,

with its tiny 1.44 megabyte (actually, 1.40 megabytes... since disk companies

like to call 1 megabyte 1,024,000 bytes, when it is actually 1,048,576 bytes, or

220 bytes) capacity will be no match for the latest and greatest game, requiring

2+ gigabytes of space (and such games to exist now... in 4 CD-ROMs), the hard

drive will reach its capacity, while the optical drives get smaller, faster, and

cheaper. The speed of optical drives today is appalling to say the least. Also

in the future would be hard drives based on optical technology, since nowadays a

51/4 inch disk can contain as much as 1 gigabytes of data. Optical drives, with

their high-bit densities are in the near future...Sources Used:UMI - May 1992

BYTE MagazineTOM - June 1992 PC Magazine (64J2528)CD-ROMs - GrolierA•s

multimediaPrinted - Various BYTE, ComputerCraft, MacUser and MacWorld

magazinesInternet - Figure 1: http://www.byte.com/art9502img411016E2.htm

Table 1: http://www.byte.com/art9502img411016Z2.htm