CS 2734 Computer Organization II

• To write a CD:
  Information is recorded on a CD ROM by burning holes (pits) in the disk. The unburned areas are called the lands. The process uses etching to produce a master.

• To read a CD:
  • A small laser diode (about 850 nm wavelength) is shown through a beam splitter and relected off the movable mirror.
  • When beam strikes a land it gets reflected back through the mirror and the beam splitter.
  • When it hits a land, it is reflected all over.
  • The beam gets reflected back off the mirror and into a photodetector which produces a bit pattern.

• Recording information:
  • Data is recording in a spiral.
  • CD does not spin at constant rate, but rather traverses its spiral at a constant rate.
  • Most CD ROMS now use a SCSI interface. (This will be a future D'vice.)

• Other types of CD's:
  • CD-R: special CD's are coated with a special film. The laser burns the coating. This is slow.
  • WORM (write once read many): has a second, more intense laser called a write laser which is intense. It burns through a coating.
  • Magneto-optical drives: use polarization by magnetic fields on electromagnetic waves (read and write).

• Other links of interest:
  • CD-ROM Technical Summary: From Plastic Pits to Fantasia:
  • How compact disks are made
  • How to repair CD Rom drives (This page has lots of interesting links.)