Optical Disks

Optical disks are produced using laser technology (like CDs), and as a result offer much higher density and reliability than magnetic disks. Disks are made of plastic or metal with highly reflective coating, usually aluminum. Like magnetic disks, they allow direct access to data but access time is much longer than that of a magnetic disk drive, as much as half a second.

Technology.

Two laser beams are used: one - recording data by burning pits in the surface, another - reading data. Presence of a pit or absence (called land) represents 1 or 0.

There are different types of optical disks available on the market.

CD-ROM (Compact Disk Read-Only Memory) is the most popular and the least expensive type of optical disks. As the name indicates they come prerecorded with data which cannot be altered. It is an offshot of the CD audio disk. The compact disk is a result of a joint venture by Sony and Philips. The CD audio specification was announced in 1980. It describes the physical characteristics concerning the size of the pits and lands and their arrangement in a spiral. The CD-ROM specification is an extension of the CD audio specification. The advantage of CD-ROM over other optical disks is that the introduced standard ensures that any CD-ROM drive will read any CD-ROM disk.

They can store up to 650M (larger CDs can store up to 20 G) and are used for distribution of massive quantities of data (e.g., encyclopedias, document archiving, manuals, statistics, software packages) at relatively low cost. Today when you buy a software package, you have a choice between one CD-ROM a bundle of floppy disks. For example, in case of Visual Basic 4, it is more than 30 floppies.

A CD-ROM has one track that spirals from the center to the outside edge. The track is divided into sectors of equal length and density. Files are stored on contiguous sectors. Fragmentation of files cannot happen because data can be written only once.

CD-ROM drives are characterised by the spin rate. The spin rate is rotation speed of the disk and it influences the information retrieval speed (access time). Currently quad, hex and octal-speed CD-ROM drives are available where quad means 4x, hex means 6x and octal means 8x. Thus, octal-speed drive will be the fastest and as a result the most expensive.

Caring for CD-ROM disks.

Although CD-ROM disks are sturdy, they can become scratched and unreadable. A scratch on the label side on the disk can be more damaging than on the data side because the protective coating on the label side is thinner than that on the data side. A scratch on a data side can often be compensated by an error correction whereas a scratch on the label side can damage pits and lands with no hope for recovery.

So protect your disks from scratches, strong sunlight, heat, humidity, and extreme cold. Store your disks in the cases in which you purchase them.

To avoid fingerprints, always handle your disks by the outside edges.

CD-R (Compact Disk -Recordable) is similar to CD-ROM and can be produced by anyone who has a CD-writer. A CD-writer is a peripheral device that can record data on a CD-R disk and let you produce an audio CD or CD-ROM. Data can be recorded only once. At the time of creating of this page, CD-writers cost around $1,500.

Today the term optical disk is used to refer to disks other than CD-ROM. Optical disks come in several sizes and formats:

WORM (Write Once Read Many) is an optical disk that can be written to but data cannot be erased from it. The reason is in the technology used for recording: a laser actually burns microscopic pits into the disk's surface.

Even the early versions could hold up to 200M and modern ones can store up to one terabyte. But the PC version of WORM cartridge can store only 600M. Because the storage area is so vast that you can afford to record the same file on the disk each time you modify it. Although the old version can't be erased, it is just ignored.

Rewritable optical disks, also called MO (Magneto-Optical) disks

Although WORM and MO mediums use laser optical technology, neither has universal standards. WORM and MO drives from one manufacturer will not necessarily read data from a cartridge written by another manufacturer's drive, even though in many cases the optical cartridge is identical.

Laser videodisks come in 12" size and can also be used at home in videodisk players. The quality of the image is very high.

Digital Video Disks are expected to replace CD-ROMs, laser disks, audio CDs, and VHS in the future.