Introduction to CD-ROM

Since compact discs store audio in a digital format they are suitable for storing other digital information. 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 physical parameters of CD-ROMs are identical to those defined in the Red Book. The use of the data it contains is different. While audio CDs can be played at only one speed, CD-ROM drives exist with a range of speed options from normal (1x) to four times normal speed (4x) and, now, 12x. As the speed increases the access time decreases.

CD-ROM discs differ from CD audio discs in two important ways.

The data on a CD-ROM disc are divided into sectors which contain both user data and other data for control and (additional) error protection. The data on a CD-ROM are contained in files. All CD-ROMs therefore, need a file system to enable the computer to access the required file easily and quickly.

Generally, the user need not bother with the sector structure but should be aware of the file structure.

In order to use CD-ROMs you need to buy a CD-ROM drive (sometimes called a CD reader) and connect it to your computer. A CD-ROM has several advantages over other forms of data storage, and a few disadvantages. It can hold 700 megabytes (MB) of data, the equivalent of nearly 500 high-density floppy disks. The data on a CD-ROM can be accessed much more quickly than data on a tape, but CD-ROMs are considerably slower than hard discs. Like audio CDs you cannot write to a CD-ROM (but CD recordable discs do exist), so they are only used for prerecorded data. In addition CD-RW (Compact Disc Re-Writable) is now available, which enables you to re-record data on the CD.

Do not use CD-RW blanks to record your data master. These special re-writable discs are not compatible with glass mastering.