Disc Areas

Recording order on the disc (Track Structure)
===============================================

Legend:
  I     Lead-in area (leader space near edge of disc)
  D     Data area     (contains actual data)
  O     Lead-out area (leader space near edge of disc)
  X     un-usable area (edge or donut hole)
  M     Middle area (interlayer lead-in/out)
  B     Dummy bonded layer (to make disc 1.2 mm thick instead of 0.6mm)

Single layer disc:
  direction: continuous spiral from inside to outside of disc.

       |   ----------------------->
       |BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB     outer edge 
       |XXIIIDDDDDDDDDDDDDDDDDDDDDOOOXX     of disc
       |

      reference
      axis

Dual layer disc:

  (A) Parallel track path (for computer CD-ROM use)
    Direction: same for both layers.                     
    
          ----------------------->
        XXIIIDDDDDDDDDDDDDDDDDDDDDOOOXX    Layer 1
        XXIIIDDDDDDDDDDDDDDDDDDDDDOOOXX    Layer 0
          ----------------------->
                         

  (B) Opposite track path (for movies)
    Direction: opposite directions 
     Since the reference beam and angular velocities are the same
     at the layer transition point, the delay comes from refocusing.  
     This permits seamless transition for movie playback.
    
          <----------------------
      XXOOOODDDDDDDDDDDDDDDDDDDDDMMMXX    Layer 1
      XXIIDDDDDDDDDDDDDDDDDDDDDDDMMMXX    Layer 0
          ----------------------->