| | Storage : Optical Disks | 
|
|
Optical disks store data by changing the reflective properties of a plastic disk. Binary computer data (0s and 1s) are represented by the way the disk reflects light when a low power laser is shone at it. A 0 stored on a disk reflects light differently to a 1 stored on a disk.
Like floppy disks, optical disks can be moved from one computer to another. They have much larger storage capacities than floppy disks but can not store as much data as a hard disk. Data can be read from an optical disk more quickly than from a floppy disk but hard disks are much quicker. As with a hard disk the drive head in an optical drive can move directly to any file on the disk so optical disks are direct access.
There are five types of optical disks that are currently in use. They are all the same physical size. It is possible to purchase a Combo drive that can read and write most of these different types of disks.
The five types of optical disk are :
CD-ROM (Compact Disk - Read Only Memory)
This is by far the most widely used type of optical disk. A CD-ROM disk can store up to 800Mb of data. The data is written onto the CD-ROM disk before it is sold and can not be changed by the user. Because of this CD-ROMs are often described as Write Once Read Many times (WORM) disks. CD-ROMs are used for applications such as distributing software, digital videos or multimedia products.CD-R (Compact Disc - Recordable)
A CD-R disk can store up to 800Mb of data. A CD-R disk is blank when it is supplied. The user can write data to it just once. After data has been written to the disk it can not be changed. A special CD-R drive is required to write to the disk. CD-Rs are often used for making permanent backups of data and distributing software when only a small number of copies are required.CD-RW (Compact Disc - Rewriteable)
A CD-RW disk can store up to 800Mb of data. CD-RW disks can be read from and written to just like a hard disk. CD-RWs can be used for any application that a hard disk can be used for but the time taken to access data is much longer than that for a hard disk.DVD-ROM (Digital Versatile Disk - Read Only Memory)
DVD is the new standard for optical disks. By using a shorter wavelength laser, storing data on both sides of the disk and having more than one layer of data on each side of a disk DVD disks are able to store much more data than CD disks. The DVD standard includes disk capacities up to 18Gb. Current DVD disks store far less than this. DVD-ROM disks can be read from but can not be written to.Because of their high capacity, DVD-ROM disks are used to store high quality video such as complete movies. Often extra data such as information about the making of the film or the actors and actresses who star in it are also stored on the disk. Unlike movies recorded on video tape, DVD-ROM movies can be interactive. The user can make selections on the screen and change what they see.
DVD-RAM (Digital Versatile Disk - Random Access Memory)
DVD-RAM disks have all of the benefits of DVD-ROM disks and can be written to as well. These very high capacity disks are ideal for producing backups. They are starting to replace video tapes for recording television programmes. DVD-RAM disks typically store about 5Gb of data.
GCSE ICT Companion 04 - (C) P Meakin 2004