                  |
 Net World
Directory Your personal directory for the internet |
||
| Net World Directory-Computers-Hardware-CD-ROM | ||
| Net World Directory: Computer hardware-CD-ROM |
An average CD contains about 783 megabytes of data. To fit this much data on disc, which is only 4.8 inches in diameter, requires that the individual bytes be very small. Just examine a CD and you will appreciate how a CD is made. Now you can begin to understand how small are the bytes encoded in a CD.
CD is generally made of plastic material, and it has a thickness of 1.2 mm. Your computer is capable of converting the bytes encoded on a CD into understandable data blocks and sends it to the computer for processing.
A CD player sheds laser beam on the track of bumps. The laser beam passes through the polycarbonate layer, reflects off the aluminum layer and hits an opto-electronic device that detects changes in light. The bumps that are present on the CD reflect light differently than the places that do not contain bumps. This produces a variation in th electrical signal that corresponds to the data recorded on your CD. The electronics in the drive interpret the changes in reflectivity in order to read the bits that make up the bytes.
The encoding standard on CD/ROM information is governed by IEC 10149. The error-detection and correction strategies employed in CD/ROM (IEC 10149) are even more elaborate than those employed in conventional CD-audio! The reasons for this are relatively simple. An error in a CD-audio disk might result in an audible "click". However an error in a CD/ROM disk might mean the failure of operation of a piece of valuable software. Thus, a second layer of error detection and correction encoding is employed in computer CDs.
The fundamental data structure on the CD/ROM is organized differently than with the CD-audio disk. CD-audio disk has a fundamental frame of 588 channel bits.