CD-ROM TECHNOLOGY

A CD-ROM (Compact Disc-Read Only Memory) is a plastic disc that holds large amounts of data through the use of laser optics technology. Because the discs store data optically, CD-ROM's have a much higher data capacity than ordinary computer discs (or diskettes). One of the downfalls of CD-ROM, however, is that the devices used to access the information on CD-ROM's can only read the information, not write it. To write on CD you need to look at CD/R.

A basic CD-ROM disc is 120mm in diameter and 1.2mm thick. It is formed from three layers: a back layer of clear polycarbonate plastic, a thin sheet of aluminum and a lacquer coating to protect the disc from scratches and dust. The most common format of CD-ROM holds approximately 800 megabytes of data which is approximately equal to 80 minutes of audio. By comparison a diskette holds only 1.44 megabytes, quite a difference.

During the traditional mastering process, a polycarbonate blank is stamped with millions of tiny indentations called pits -molded is a spiral from the center of the disk outwards-, which is then coated, with a thin layer of aluminum, giving the disc its characteristic silver color. Unraveled and laid in a straight line the spiral of data would stretch four miles!

Data is read from an optical disc by sensors that detect light that is reflected off the disc from a laser located in the drive. The laser will direct a light beam at the rotating disc. Some portions of the disc will directly reflect the light, while other portions will diffuse the light. The amount of light read by the sensor will fluctuate based on whether it is detecting the reflected light or the diffused light. It is these fluctuations in intensity that are converted into the digital signals that are sent to the computer from the drive. The areas that reflect the light are known as "lands" and the areas that diffuse or diffract the light are known as "pits".

These pits and lands are created on the disc surface in several ways depending on how the disc is made. The two types of optical discs generally used are replicated discs and recordable discs. The concept of how they are read, however, is similar. A laser light is directed at the disc. When the light hits a land it is directed straight back towards a sensor. When the light hits a pit is is "scattered", thereby reducing the intensity of the light reaching the sensor.

Replicated CD's and Recorded CD's are manufactured differently and also vary in the way that the light is diffused.

As a side note, it is frequently assumed that the "pits" and "lands" represent the binary numbers "1" and "0". This is not the case. It is actually the transition from a string of pits or lands that signals the change from a 1 to a 0, or vise-versa. This whole explanation is somewhat technical and for most people extremely irrelevant!