Compact Disc

In 1978, Philips and Sony together launched an effort to produce an audio compact disc (CD) as a method of delivering digital sound and music to consumers. The two companies continued to cooperate through the 1980s and eventually worked out standards for using the CD technology to store computer data. These recommendations evolved into the CD-ROM technology of today.

The CD-ROM (compact disc-read only memory) is a read-only optical storage medium capable of holding 600 megabytes of data (approximately 500,000 pages of text), 70 minutes of high fidelity audio, or some combination of the two. Legend has it that the size of the compact disc was chosen so that it could contain a slow-tempo rendition of Beethoven's Ninth Symphony. As can be seen from Table 1, compact discs offer a high volume of data storage at a lower cost than other media.

The first users of CD-ROMs were owners of large databases: library catalogs, reference systems, and parts lists. Typical applications of CD-ROMs as storage media now include storage of such information as the following:

• every listing from all of the Yellow Pages in the United States
• maps of every street in the country
• facsimile numbers for all publicly held businesses and government institutions
• a 21-volume encyclopedia

The extremely thin metal layer of the CD, usually pure aluminum, reflects light from a tiny infrared laser as the disc spins in the CD player. The reflections are transformed into electrical signals and then further converted into meaningful data for use in digital equipment. © Kelly A. Quin. Reproduced by permission.

CD-ROMs are expected to achieve significant impact in storage of the following kinds of documents:

• business reference materials
• interactive educational materials for schools
• scholarly publications
• government archives
• home-based reference materials

Each CD-ROM drive for a personal computer (PC) may be characterized according to the following:

• drive specifications
• formats the drive can read
• interface the drive requires to connect the computer

The data on compact discs need to be organized if the CD-ROM drive and computer are to make sense of the data. The data are therefore encoded to conform to certain standards. Although advanced CD-ROM standards are still evolving, most drives today comply with earlier CD-ROM formats.

The CD-ROM interface is the physical connection of the drive to the PC's expansion bus. The three typical interfaces are SCSI Standard, SCSI-2, and ASPI, and nonstandard SCSI.

SCSI standard interfaces

Small Computer System Interface (SCSI) refers to a group of adapter cards that conform to a set of common commands. These adapter cards allow a chain of devices to be strung from a single adapter. Consequently, SCSI interfaces are preferred for connecting a CD-ROM drive to a personal computer.

SCSI-2 and ASPI interfaces

SCSI-2 and Advanced SCSI Programming Interfaces (ASPI) take into account rapid enhancements of computer interface technology. SCSI-2 incorporates several enhancements, including greater data throughput and improved read and write technologies. ASPI provides a standard software interface to the host adapter hardware.

Nonstandard SCSI interfaces may not accept installation of multiple SCSI devices; in cases where this is not a problem, they may prove acceptable.