CD-ROM

CD-ROM
Name	Compact Disc Read-Only Memory
Type	Optical disc
Encoding	Various
Capacity	Typically 650–700 MB
Read	780 nm wavelength semiconductor laser
Standard	Rainbow Books (ISO/IEC 10149)
Developed by	Philips and Sony
Usage	Data storage, software distribution
Released	1985

CD-ROM. A Compact Disc Read-Only Memory is an optical disc format used for the distribution of software, multimedia, and data. Developed from the audio Compact Disc, it became a dominant medium for software distribution and reference works throughout the 1990s and early 2000s. The technology stores digital data read by a low-power laser and is governed by standards set by the International Organization for Standardization.

History

The development of the format was a natural evolution from the successful Compact Disc Digital Audio system introduced by Philips and Sony in 1982. The Yellow Book standard, established in 1983, formally defined the specifications for storing computer data. The first commercial drive, the Philips CM100, was demonstrated in 1984, with the first commercially available title being Grolier's Academic American Encyclopedia in 1985. Adoption accelerated with its inclusion in systems like the Commodore CDTV and the IBM PC compatible market, where it was championed by companies such as Microsoft with its Microsoft Office suite and the Encarta multimedia encyclopedia. The ISO 9660 file system, created in 1988, ensured cross-platform compatibility, cementing its role in the personal computer revolution.

Technology

A standard disc is a polycarbonate plastic substrate, 120 mm in diameter, with a thin reflective layer of aluminium and a protective lacquer coating. Data is stored as a series of microscopic pits and lands arranged in a continuous spiral track from the center outward. A drive uses a 780 nanometre wavelength semiconductor laser diode to read this pattern; light reflected from the lands is detected by a photodiode, while light scattered by the pits is not, translating into digital ones and zeros. Critical components include the optical pickup unit, a precision servomechanism for tracking, and error-correction systems like Cross-interleaved Reed-Solomon coding to ensure data integrity despite minor scratches or dust.

Formats and standards

The core physical and logical format is defined by the Rainbow Books, specifically the Yellow Book. The ISO 9660 file system, later extended by the Joliet and Rock Ridge extensions, governs volume and file structure for universal readability. Related formats include CD-R for recordable discs, CD-RW for rewritable media, and multimedia extensions like CD-i and Photo CD. The Video CD standard, based on MPEG-1 compression, was popular in parts of Asia. All these standards were maintained by industry consortia to ensure interoperability across hardware from manufacturers like Apple, Toshiba, and Hewlett-Packard.

Capacity and data storage

A standard disc has a nominal capacity of 650 MB (74 minutes of audio-equivalent storage), which was later extended to 700 MB (80 minutes). This capacity is achieved through constant linear velocity recording, where the disc's rotational speed varies to maintain a fixed data rate. The data is divided into sectors, each containing 2,352 bytes of user data, with additional bytes allocated for synchronization, header information, and error-correcting code. This structure allows for the storage of diverse content, from the text of the King James Bible to the installation files for major software titles like Adobe Photoshop or operating systems such as Microsoft Windows 95.

Use and applications

The primary application was the mass distribution of software and games, replacing numerous floppy disks. It revolutionized the distribution of reference material, with encyclopedias like Encarta and the Britannica becoming commonplace. The format was essential for multimedia titles, educational software, and complex applications from companies like Autodesk and Oracle Corporation. It also served as a common medium for device drivers, firmware updates, and archival data storage. In corporate and government settings, it was used for distributing large databases, tax return software, and documentation for products like the Boeing 777.

Decline and legacy

The rise of high-speed Internet access, facilitated by technologies like DSL and cable modems, enabled efficient software downloads and streaming, diminishing the need for physical media. Higher-capacity formats like DVD and later Blu-ray offered significantly more storage. The proliferation of USB flash drives provided more convenient portable storage. By the late 2000s, major software vendors, including Microsoft and Adobe Systems, shifted to digital distribution. Its legacy persists in the ISO image file format used for software distribution, the enduring use of optical drives in some contexts, and its foundational role in the development of all subsequent optical storage technologies. The Museum of Science and Industry and the Computer History Museum hold examples in their collections.

Category:Computer storage media Category:Optical disc authoring Category:1985 introductions