Subpictures

  • run-length compressed bitmaps that are overlayed ontop of the MPEG reconstructed video.
  • Applications include: Menus, sub-titles, karaoke, and simple animation.
  • Pixels are divided into four types: 1. background 2. Foregound 3. Empahsis-1 4. Emphasis-2
  • 4 colors out of 16 color palette (4 colors are determined once per PGC).
  • 4 out of 16 contrast values
  • up to a maximum of 32 sub-picture bitstreams. Each subpicture stream could, for example, could contain text from a particular language.
  • subpicture buffer size is restricted to 62 Kbytes. This means a maximum of 62 KB per GOP/cell. 32 Kbytes of this is control data.
  • Maximum number of bits per run-length coded line is 1440 bits.
  • Display area maximum: 720x480 (525/60) and 720x576 (625/50)
  • area, content, color, and contrast can be changed every video field
  • Sub-Picture Display Control Sequences (SP_DCSQ) control the presentation of Sub-pictures.
  • Presentation effects include: scroll up/down, fade in/out, etc.
  •   Structure of Sub-picture Decoding Unit (SPU):
    
        [ SPUH   ][        PXD        ][ DCSQT  ]
    
       SPUH:        Sub-picture Unit Header (size of SPU, start address of DCSQT)
    
       PXD:         Pixel Data (variable length run-length coded)
    
       DCSQT:       Display Control Sequence Table (one or more display control
                    command sequences).
       
    
       DCSQT:   [DCSQ 0][DCSQ 1][DCSQ 2] ... [DCSQ n]
    
       DCSQ:  [Start time] [ Pointer to next DCSQ] [Command Sequence]
    
       Command Sequence:  [DCC 0][DCC 1]... [DCC m]
    
    

    Display Control Commands (DCC):

  • Set start address in PXD
  • Set colors
  • Set contrast
  • Set SP screen position
  • Start/stop display
  • Set CHG_COLCON areas.