13.3 Optical storage
A CD-ROM (Compact Disk Read Only Memory) uses a laser-based optical form of storage. This type of disk has been used for many years to distribute music and computer software. A CD-ROM drive is needed to read the disks. Data is locked into the disk during manufacture, and cannot afterwards be changed.
There are two other types of CD device for computers: CD-R (CD-recordable) and CD-RW (CD-rewritable). With the right sort of CD drive in your computer, you can 'burn' data (that is, write data) to either type. Data written to a CD-R disk cannot be changed afterwards, although further data can be added. Data written to a CD-RW disk can be erased and the disk reused.
CD-ROMs can store up to 800 MB of data, 90 minutes of audio or 60 minutes of video. CD-R and CD-RW disks vary in their capacity, but typically it is in the region of 700 MB.
DVD (Digital Versatile Disk) works in a similar way to CDs, but the data is held in a more compact format. In addition, DVDs can have more than one layer of data. A single data layer can hold about 4.4 GB. At the highest quality, roughly two hours of video can be held in a layer, and with reduced quality about three hours. As with CDs, there are 'R' and 'RW versions to which the user can write data. The 'Blu Ray' system, using a blue laser (instead of a red one), stores data even more compactly. This system promises to increase the capacity of a layer to 27 GB.