CD-ROM

Today, CD-ROM (or CD for short) is the most widespread storage medium for large amounts of data due to the

High capacity:

A CD can hold up to 650 megabyte of user data. The data rate of current ('95) quadruple speed CD-ROM drives is approx. 600 KByte/s of user data.

Low price:

The cost of a pressed CD in larger quantities is less than 2 $, and CD drives are inexpensive.

Standardisation:

The physical structure of a CD is defined in a small number of industry standards, and is independent of the logical organisation of the data.

Longevity:

Pressed CDs may hold data without loss for 40 years and longer.

Robustness:

CDs are virtually unbreakable, tolerate small scratches or stains on the medium, and can be cleaned easily.

Visual impact:

Copyright notices, title and author information, and decorative labels can be applied to the medium directly.

CD drives can either hold a single CD or have a CD changer subsystem which holds up to 7 CDs (changing CDs in a multi-CD system requires about 5 secs).

The major restriction of CDs is that they are a read-only storage medium. A CD is mastered on a hard disk the contents of which are then copied to a tape. This tape is sent to a production site which produces a glass master CD which then serves to produce metal master CDs. These metal master CDs are used to press the final CD copies.

CD-R is a write once/read many (WORM) storage medium. In contrast to pressed CDs the data is recorded on special virgin CD-R disks by burning a pattern into the reflective substrate of the CD-R. This process is irreversible. A CD-R can be read by any CD-ROM drive.

Producing CDs individually has two important consequences:

• CD-R contents can be customised to user demands, and
• CD-Rs can be given unique serial numbers, e.g. to track unauthorised distribution.

The major drawback of CD-R is that their longevity is reduced. CD-Rs come with a one year limited warranty only. However, one can expect data to be safe on CD-R for approx. 10 years if the medium is not exposed to high temperatures or ultraviolet radiation (which may affect the reflective stratum).

 
• Physical CD formats
• Logical data organisation
• CD-R software
• CD-R hardware
• Future developments