Reference sheet prepared by Jolene M. Morris, District Technology Director, Grand Co. School District
Disk or Disc?
CD-ROM is an acronym which stands for Compact Disc, Read Only Memory. These discs are approximately five inches across and are made of metal; the information on the disc cannot be changed.
Data is stored as 3D pits on the disc -- about two billion pits on a three-mile track. The surface is then sealed with a protective coating. Compact disc drives use a laser beam to retrieve stored data, while both floppy and hard disk drives use a magnetic head. As such, compact discs are spelled with the letter-c (disc) which represents light in science; and magnetic disks are spelled with the letter-k (disk) representing magnetism in science.
The CD-ROM was announced in 1982 for audio (music/sound) only. In 1985, Philips/Sony launched data CD-ROMs for computers.
CD-ROMs in Education
Because of its large storage capacity, the CD-ROM holds great potential for education. Using a print-based encyclopedia, a student may have to pull out the index, look up articles related to the desired topic, retrieve the appropriate volume of the encyclopedia, and find the right page. With a CD-ROM-based encyclopedia, the student simply types in the desired search word and the program may present the titles of a dozen articles that contain references to that topic.
One side of a single CD-ROM disc can store between 250,000 and 2.5 million pages of text, so there is plenty of room to store several thousand books, or several sets of encyclopedias. This may be viewed in better perspective when you count the number of titles in an average school library -- 15,000.
Data Transfer Speed
Original CD-ROM drives transferred data at a speed of 150KB per second. This standard speed came from the fact that audio CD-ROM drives play music at 150KB/sec. Since virtually all CD-ROM drives also have the ability to play audio CDs, it is necessary to preserve the 150KB/sec speed.
But 150 KB/second is just too slow for today's multimedia applications. A speed-doubling mechanism was added to disc drives which allows them to "switch gears" from 150KB/sec to double or four times that speed. These newer CD-ROM drives are called double-speed or quad-speed drives.
Access Time
Access time is how long it takes from the time the "fetch data" instruction is given to when the location of the data is actually found on the disc and the transfer begins. This access time is always represented in milliseconds (1/1000 of a second).
The access time of a standard (single-speed) drive is over 500 ms while the access time of a double-speed drive is about 300 ms. This compares to average hard drive access time of about 20 ms.
Buffer Size
Think of a buffer as a temporary holding area, where data is stored until it's needed. All CD-ROM drives are equipped with built-in buffers that are used for data streaming to keep a smooth and continuous flow of data going to the host device at a constant rate of speed. Today, the minimum buffer size is recommended at 64KB, although many drives offer 128KB, 256KB, or higher.
Interface
CD-ROM drives must be connected to the computer, this connection is called the interface. Some CD-ROM drives require a specific interface, but most use a standard SCSI (small computer systems interface) interface. The interface kit is an additional purchase. All SCSI drives have interface kits which allow them to work with ISA-EISA based PCs, Macs, and MicroChannel machines; but the specific-interface drives are installable only on ISA-EISA PCs or MicroChannel machines. Since all Macintoshes have a built-in SCSI interface, special-interface models won't work with them.
Caddies
Another point to consider before you buy a CD-ROM drive is how the disc inserts into the drive. Many drives use caddies, which offer a high degree of protection; but cost about $10 each. Some drives have a slide-out drawer which accepts the disc, and others have lift-lids which require placing the disc directly on the drive spindle. These drives offer some economy over caddies, but you have to handle the discs very carefully or you'll scratch them. As such, CD-ROM drives that function without caddies are not advisable for schools.
Internal or External
When buying a CD-ROM drive, you need to decide whether you want an internal or an external drive. Internal drives are similar in form and mounting to floppy, hard, or tape drives. They are less expensive. They rely on the host PC to supply the required operating voltage, so your computer must have adequate power.
External drives come housed in a cabinet with their own power supplies, which makes them both easy to position and to move from computer to computer (even from an IBM to a Mac). Because of the extra casing and power supply, external drives are more expensive. Some external drives are battery operated so they can be considered portable drives.
CD-ROMs for Schools
More and more educational software is available on CD-ROM discs. Schools will eventually need to purchase CD-ROM drives. The information in this sheet should help with making purchasing decisions.
