Home > Technology > CD-ROM > CD-ROM Overview

CD-ROM  Overview Find out how the CD can be used for games, multimedia and computer data as well as music.

Since compact discs store audio in a digital format, they are suitable for storing other information that can be represented in a digital form. In 1984, Philips and Sony released the Compact Disc Read Only Memory (CD-ROM) specification, known as the Yellow Book. This defines the necessary additions to the Red Book for the storage of computer data. The CD-ROM disc is therefore like a pre-recorded floppy disk with a very large capacity. Any computer data can be pre-recorded on a CD-ROM.

The physical parameters of CD-ROMs are identical to those defined in the Red Book. However CD-ROM discs differ from CD audio discs in two important ways.

• The data on a CD-ROM disc are divided into sectors containing user data and additional error correction codes.
• The data are contained in files and so a file system is needed so that the required files can be accessed easily and quickly.

The user normally will not need to be bothered with the sector structure but will be aware of the file structure of files on a CD-ROM. There are now several formats based on the CD-ROM specification covering a range of applications.

CD-ROM Drives

CD-ROM discs are read by CD-ROM drives, which have been standard components of personal computers and some games consoles for a number of years (DVD-ROM drives, which are now replacing CD-ROM drives will also read CD-ROM discs). A CD-ROM has several advantages over other forms of data storage, and a few disadvantages.

• Capacity of a CD-ROM is nearly 700 megabytes (MB) of data, the equivalent of nearly 500 high-density floppy disks.
• The data on a CD-ROM can be accessed much faster than a tape, but CD-ROMs are slower than hard discs.
• Like audio CDs you cannot write to a pre-recorded CD-ROM but only to recordable versions.

The data on a CD-ROM can be accessed much faster than a tape, particularly using the latest high-speed drives (52x is now common). To reduce the maximum angular velocity these faster drives use CAV (constant angular velocity) rather than CLV (constant linear velocity). Therefore the data rate for data near the inside is less than the data rate at the outside of the disc. For example a 40x drive gives a maximum data rate of between 2.8 and 6 MB/s, depending where on the disc the data is being read. Faster drives can create problems so some drives make use of multiple laser beams to increase the data rate without increasing the angular velocity.