How Does a CD-ROM Work?

By Keith Evans

A CD Contains Data in Tiny "Bumps" and "Lands"

The data on a compact disc (CD) is stored on the disc as a series of tiny, almost microscopic bumps and lands. Each of these bumps represent a piece of information, though no specific information is directly stored. Instead, the bump is an indicator of data, roughly equivalent to a "1" in binary code (the lands, or flat surfaces on the CD, are considered to be the "0" in binary). A CD's bumps are so tiny that each one is only one half of one millionth of a millimeter long, far too small for a human eye or mechanical device to detect, but large enough for a laser to read.

A Laser Reads the Bumps on a CD

When data is read from a CD, a laser directs an extremely fine stream of light onto the surface of the disc. The laser follows the data stream of bumps and lands from the inside center of the disc outward in a spiral direction, allowing the size and even the shape of a data CD to vary if needed. As the one millionth of a millimeter (one micron) wide laser light shines on the CD's data track, it reflects one pattern of light off of a bump and a different pattern off of a land area. The resulting reflections equate to a series of ones and zeros, or the binary language of computers. A special tracking system in the CD-ROM drive constantly monitors the laser and adjusts its path to ensure it follows the correct data path on the CD.

Light Sensors and Processors Interpret the Data

As the laser light is reflected off of the data stream in tiny bits of data (ones and zeros), the incoming reflection is measured by a light sensor and sent to a data processor in the CD-ROM drive. This processor interprets the ones and zeros to assemble bytes of data which can then be forwarded to the computer's main processor for interpretation. The computer interprets this data as it was originally written on the CD, whether as a complex computer program, a basic collection of files or even simple music. As technology progresses and processor speeds increase, the CD can be spun at higher and higher rates, allowing the data on the disc to be read and sent to the processor, interpreted and used more quickly.