PC Hardware —

CD-ROM Technology

CD-ROMs store data as a series of 1s and 0s, just like a floppy disk or a hard disk drive. However, instead of using magnetic energy to read and write data, CD readers and writers use laser energy. There are two major advantages to using lasers:

  • There is no physical contact between the surface of the CD and the reading device.

  • The diameter of the laser beam is so small that storage tracks can be written very close together, allowing more data to be stored in a smaller space.

Hard Disk Drives vs. CD-ROMs

With the cost of hard disk drives falling and the amount of available data storage rising, the hard drive is still king of the storage media. Optical data-storage devices hold their place as removable media and as the media of choice for archival data storage.

A CD platter is composed of a reflective layer of aluminum applied to a synthetic base that is composed of polymers. A layer of transparent polycarbonate covers the aluminum. A protective coating of lacquer is applied to the surface to protect it from dust, dirt, and scratches.

NOTE
CD-recordable (CD-R) discs use materials other than aluminum. They often have a yellow or green cast on the data side. Not all CD-ROM readers are able to read these discs-some older readers based on IDE (Integrated Drive Electronics) are incompatible with CD-R technology.

Data is written by creating pits and lands on the CD's surface. A pit is a depression on the surface, and a land is the height of the original surface. The transition from a land to a pit, or a pit to a land, represents a binary character of 1. Lands and pits represent binary 0. The reading of data is based on timing-the speed at which the CD is rotating-and the reflection of light. If no data is on the disk, the reflectivity will not change and the CD will read a series of binary 0s. There are approximately 4 to 5 million pits per CD. They are arranged in a single outward-running spiral (track) approximately 3.75 miles (6 kilometers) long. The distance between each element is 1.6 thousandths of a millimeter.