DVD capacity
  Allows up to four layers.
     Single sided, single layer - 4.7 GB (133 minutes -mpeg 2)
       Data layer near middle of disc (top to bottom).

     Single sided, double layer - 9 GB
       * Long movies.

     Double sided, single layer - 9.4 GB
       * Two versions or Widescreen/TV width

     Double sided, double layer - 18 GB
       Not implemented - cost of accurate of equipment not justified.
 
DVD 2 layer technologies
  For dual layering
     Bottom layer has a highly reflective base

     This is topped with a semi-reflective surface for the upper layer.

     Bits on lower layer are slightly larger to be easily recognized.
       This results in the not quite doubling of storage.

     Second layer has track read from outside in. This allows for a movie
     longer than 133 minutes to be read continuously with only a minor time
     delay for laser refocusing.

Data layout.
  DVDs use a bit-level data layout that is just like CD only completely
    different.

  Reflective/non-reflective

  NRZI encoding.

  RLL 8/16 coding known as EFMPlus modulation.
    6% storage improvement over EFM used by CDs. 
    Uses 2 more bits but doesn't need the 3 bit merge(sync) between
      bytes so actually uses fewer bits overall.

  Each sector has 2048 bytes data, 12 byte header, and 4 byte EDC.
    Additionally there is a sync code between each 91 byte sequence.

    Header 
      ID (4 bytes)
        Sector Info (1 byte)
          sector format type
          tracking method
          reflectivity
          reserved
          area type
          layer number
          sector info
        Sector number (3 bytes) 2^24 * 2^11 = 2^35 or 16GB
      IED - id check bytes. (2 bytes)
      CPR_MAI Copyright management info. (6 bytes)
      
    Data
      2048 bytes arranged in 172 bytes 'rows'.
        * 1st and last row smaller because id and check included.

    EDC (4 bytes ) - error detection code.
      * Advanced version of ECC.
       
    Actual byte count of sector is 2418 bytes.

    This just delivers a valid stream of bits. Additional decoders
      must analyze data coming off for audio, video, or binary data.

    Data is further decoded by software known as CODECs.