Differences from H.261

• Larger gaps between I and P frames, so expand motion vector search range.

• To get better encoding, allow motion vectors to be specified to fraction of a pixel (1/2 pixels).

• Bitstream syntax must allow random access, forward/backward play, etc.

• Added notion of slice for synchronization after loss/corrupt data. Example: picture with 7 slices:

  

• B frame macroblocks can specify two motion vectors (one to past and one to future), indicating result is to be averaged.

  

• Compression performance of MPEG 1

         ------------------------------
         Type      Size     Compression   
         ------------------------------
          I      18   KB        7:1       
          P       6   KB       20:1      
          B       2.5 KB       50:1      
         Avg      4.8 KB       27:1      
         ------------------------------