CD-ROM stands for compact disc-read-only memory. A CD-ROM is a CD designed to be "played" by a computer. Although CD-ROMs look just like audio CDs, they only work in a computer and, more specifically, in a CD-ROM drive that is either installed inside your system unit or attached to your system unit with a special cable. You can think of a CD-ROM drive as a computer-friendly version of a CD player. Unlike music CDs, which contain only audio information, CD-ROMs can hold graphics, text, sound, and full-motion video (movies). With today's CD-RW (compact disc rewritable) drives, you can make your own CDs easily and inexpensively.
CD-ROMs Can Store Lots of Information
CD-ROMs have become increasingly popular in the last few years for a simple reason: they pack a huge amount of information into a small space at a low cost. A single CD-ROM can contain up to 660MB of data, enough to comfortably house an entire encyclopedia.
What CD-ROMs Are Good For
Because they can accommodate so much data, CD-ROMs are ideal for storing multimedia applications, which tend to take up a lot of spaceoften more space than you might have to spare on your hard disk. CD-ROMs are also a popular way of distributing large applications that you will actually copy to your hard disk. An application that might fill 10 or even 20 floppy disks will fit easily on a single CD-ROM, and installing it from a CD-ROM will take a lot less time (not to mention sparing you the bother of changing disks 10 or 20 times).
What Read-Only Means
As their name implies, standard CD-ROMs are read-onlyyou can copy files from CD-ROMs to your hard disk, but not the other way around. You can also load programs from a CD-ROM into your computer's memory, but not save the contents of memory back to the disc. (Certain specialized CD-ROM drives allow you to write information to CDs; you'll learn about these in Chapter 7, "More About Hardware.")
Internal and External CD-ROM Drives
CD-ROM drives come in two basic flavors: internal and external. The only difference between the two is their location: Internal drives fit into your computer's system unit and external ones sit in their own little boxes, which are connected to the system unit with cables. Internal drives are more common and slightly less expensive.
CD-ROM drives are generally categorized by speed: quad-speed, 6X, 8X, 10X, 12X, 24X, 32X and 48X. The first CD-ROM drives spun exactly as fast as a stereo's CD player. This tempo turned out to be a bit slow for running programs, however, so manufacturers created double-speed CD-ROM drives, which transfer data from the CD-ROM to your computer twice as fast. As of this writing, they've started manufacturing 48X drives, which transfer data 48 times as fast as the original CD-ROM drives.
Choosing a CD-ROM Speed
Which CD-ROM speed is right for you depends on your needs and budget. If you're buying a drive mainly to install new software and to look up things in an encyclopedia on occasion, 16X may be sufficient. Playing games is another story, however. If you spend hours on your computer playing graphics-intensive games, go for at least a 24X drive.
Handle CD-ROMs with Care
Because information is recorded on CD-ROMs using light rather than magnetic read/write heads, CD-ROMs, unlike floppy disks, are not sensitive to magnets or magnetic fields. They are susceptible to other hazards, however, such as dust, fingerprints, and soda pop. This means you need to take special care not to touch the surface of the disc, especially the part near the middle that contains the data and programs. Be particularly leery of touching the unlabeled side of the disc; that's where the CD-ROM drive reads the data. Instead, hold the disc by the edges or put your finger through the hole in the middle.
Handling Dirty or Damaged CDs
If one of your CD-ROMs becomes dusty, wipe the disc from the center out to the sides using a clean, soft cotton cloth. If the disc is actually dirty or you've spilled something on it, try either plain water or CD-ROM cleaner. If your CD-ROM gets scratched, try one of the products designed to repair scratches on audio CDs. Make sure the disc dries completely before you insert it in your CD-ROM drive. If you can't repair the disc, contact the company that produced it. Many CD-ROM publishers will send you a replacement disc in exchange for the damaged CD and a small fee.
Using a CD-ROM Drive
To use your CD-ROM drive, you press the open/close button on the front of the drive to elicit a plastic or metal tray. Place the disc on the tray and press the button again to cause both disc and plate to withdraw into the drive. To eject the disc, press the open/eject button again.
Using a CD-RW Drive
CD-RW drives have come down in cost so that they are now accessible to the average consumer. Many people use them to create their own music CDs, but they are also valuable as a means to back up data or exchange large quantities of data with others. Because the discs themselves are so inexpensive, CD-RWs are now in mainstream use.
The Data Transfer Rate
The speed at which the CD-ROM drive can read information from the disc and transfer it to your computer is known as its data transfer rate. The data transfer rate is 150KB per second for single-speed drives, 300KB for double-speed drives, 450KB for triple-speed drives, and so on. The other important benchmark for CD-ROM drives is access speedthe number of milliseconds (ms) it takes for the drive to locate a piece of data on a disc. When shopping for a CD-ROM drive, don't confuse the access speed with the data transfer rate. Data transfer rate is always measured in kilobytes or megabytes, because it refers to the amount of data that can be transferred in a second; access speed is measured in milliseconds.
What to Do If Your Disc Gets Stuck in the Drive
If your disc gets stuck in the drive (nothing happens when you press the eject button), check the drive's documentation to see whether your drive has an emergency eject hole. If it does, turn off the power to the drive. (If you have an internal drive, this means turning off the computer.) Then insert the tip of a paper clip into the emergency eject hole. If that doesn't work, contact a computer repair person or at least a technical-minded friend. If you do manage to get the tray out of the drive, make sure there's nothing wrong with it before you use it again.
Using Multimedia CD-ROMs
Most multimedia CD-ROMs feature sound as well as pictures. To fully enjoy such CD-ROMs, you'll need a sound card and a set of speakers as well as a CD-ROM drive. You'll learn more about sound cards, speakers, and multimedia in Chapter 7.