1.  Physical Parameters
 

Diameter: 12cm Thickness: 1.2 mm Pit size: width: 0.5 microns, length: 0.8-3 microns, Depth: .15 microns Scan Velocity: 1.3 m/s Laser Wavelength: typically 780 nm Play-time: 74 minutes 44 seconds, some have 80 minutes Maximum number of Tracks: 99 Modulation: EFM ( Eight to Fourteen bits) Speed at which data is read: Raw data read at 4.32 MB/sec.  After converted to 14bit, read at 1.4 MB/sec	Image Courtesy of Octave Systems Inc.
2.  Why are CD's needed?

Compact Discs (more commonly known as CD's) are similar to their previous counterparts, the laser disc.  CD's, however, are much smaller in size and more cost effective.  CD's hold more information than previous hard disks and floppy disks.  CD's are especially useful in audio recordings and digital video.  CD's can be used to store audio, video, data, and mixed media information.  CD's allow for easy retrieval of this information, and come in the forms of CD-ROMS, CD-R's, and CD-RW's.

3. How are CD's read?
 

Image from Sony Electronics	CD's are made of a series of what are called "pits" and "lands."  These pits are read using a small laser diode that emits infrared light.  The light must be focused enough to read the small pits.  The CD reads from the center out , and if stretched out creates a long "track " of pits and lands.  This track for a regular CD, if stretched out, is 5800 meters long.  5800 meters is about 3.5 miles of data programmed onto a single CD!
The series of pits and lands encode the data as a series of "0" and "1".  The jump from the land to the pit or vice-versa is what makes the "1".  All "1" must have at least two "0" in-between them.
The focused laser light reads the CD as shown in the diagram (created by Prof. Watson, Univ. of Delaware). The laser light has a built in adjustment mechanism so that even if the CD should become bent, the laser can adjust to continue to read the encoded data.	Image courtesy of George Watson, University of Delaware

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Last edited on May 14, 2000