CD-ROM: The Mass Storage Media

Article excerpt

CD-ROM stands for Compact Disk Read Only Memory. It is an optical medium in that it is read by a laser beam which scans a series of tiny pits on the disk surface. The information (which is digital) stored on the disk can be text, diagrams, photographs, graphics and software. It uses the same technology as that used for the compact disks on which music is recorded. The information is stored digitally and the CD-ROM system sends it directly to the microcomputer.

The Compact Disk is 12 centimetres (4.75 inches) in diameter and is very durable as there is no contact with the surface during reading. Each disk can store 550 megabytes of data, to illustrate the volume of printed material this represents, one CD-ROM disk can hold a quarter of a million A4 pages of single spaced text, 2,000 high resolution colour images, 20 hours of recorded speech or any combination of the three. This gives it the potential to hold large databases on one CD-ROM disk which can then be accessed without incurring computer and telephone charges. It costs about 2,000 to master a CD-ROM disk and about 10 per disk to replicate them in volume. In addition to providing a vast amount of data on a single disk CD-ROM allows sophisticated searching, skipping between texts. Multi media information including text, data, music, voice, photographs and film can be supported on the same disk.

HARDWARE REQUIREMENTS

The hardware configuration required to read the disks is an IBM PC or compatible microcomputer which uses MS-DOS, interfaced to a CD-ROM drive. There are a number of CD-ROM drives available for both internal and external fitting, these currently cost around 700. The SCSI (Small Computer Systems Interface) is becoming the standard for connecting the drives to the microcomputer.

The following are some of the complete systems currently available:

Philips are offering a choice of two workstations which cost between 2,000 and 3,000 depending on the amount of memory required, the type of monitor and whether the microcomputer has a hard disk or not.

Alternatively, if the user has an Apple Macintosh the Apple CD SC drive can be connected through the Macintosh SCSI (Small Computer System Interface).

As with all applications using microcomputers, the hardware should not be chosen until the software to be used has been decided upon. CD-ROM's are supplied with a floppy disk which contains the searching software. Other hardware devices can be incorporated into a CD-ROM system, for example, a desk top publishing system with a laser printer could extract information from the CD-ROM for inclusion in its output.

Because some CD-ROM databases are on two or more disks, SilverPlatter are introducing daisychaining and data compression capabilities to its Information Retrieval System. Daisychaining allows the linking of multiple CD-ROM drives to one microcomputer. Users can access any SilverPlatter database in a daisychained system through a new exchange menu. Daisychaining allows users to access a multi-disk database without having to switch disks, and to maintain search histories across disks of the same database. Data compression allows large amounts of information to be stored on a CD-ROM disk. Multi-disk databases such as MEDLINE (four disks) will require fewer disks and more information can be placed on single disk databases. This system will be compatible with single drive workstations and non-compressed SilverPlatter disks. Other suppliers will no doubt soon supply similar systems.

For users who need mass storage there are library systems available which use WORM technology (see below) to provide permanent data storage on high capacity, removable CD-ROM disks. One product from Optimen (734-775757), can store the equivalent of 3.5 million pages of text.

STANDARDS FOR CD-ROM DISKS

Philips/Sony have created an international standard for CD-ROM. This was accepted by suppliers as the standard, at a meeting held at High Sierra Hotel near Lake Tahoe in 1986. …