Optical storage

  Opticial discs record data by varying the reflectivity of the media to
    represent changes in the data stream.

  Changes marked by lands (reflective) and pits (non-reflective)

  Uses NRZI (1 bits transistion from pit to land or land to pit.
    Makes it easy to detect 'edge' of a 1 without additional timing
    information. 

  Uses various versions of RLL encoding to protect against long sequences
    of zeros with out an 'edge' to resync timing.

  Data recorded on single continuous track per layer.

  DVDs and Blu-Rays support multiple layers or sides allowing more 'tracks'.
    DVD - 2 layers or 2 sides.
    BluRay - up to 8. 8 not commericially viable.  
  
  CD - compact disc
    Read with a infra-red laser.

    CD-ROM - manufactured compact disc read only memory.
      Disc stamped similar to a LP record, except with lands and pits.
        Pits 1/2 wavelength of laser which causes the reflected laser light
        to cancel itself out (dark).

    CD-R - user CD recorable one time *. WORM
      Layer of dye above the reflective material can be charred by a 
        laser stronger than the read laser, creating the 'pit'

      Current protocols allow CD-Rs to be burned in sessions and/or packets.
      but once a portion of the track is 'burned', it can not be altered.

    CD-RW - CD rewritable. Uses 3 different laser strengths.
      Standard - for reading
      1. - trigers phase change to make a spot more reflective.
      2. - trigers phase change to make a spot less reflective.
      CD-RWs proved to be rather unreliable.
 
  DVD - Digital video disc.
    DVDs use a red-orange laser.  Shorter wavelength the infra-red.
    Lands/pits closer together and track wound tighter allows for 
      greater storage density, faster data stream, and the ability
      to put 2 tracks on a disk.

  DVD-R - DVD recordable.
    DVD-RW - uses same physics as CD-RW but with different reflective 
    material that is more reliable.

  Blu-Ray
    Uses a Blue-violet laser, again increasing the density and read
      throughput of data access.