VBI Decoding

The Vertical Blanking Interval (VBI) packet (multiplexed at the Cell level along with Navigation, Video, and Audio packets) contains information which is directly inserted into the reconstructed video signal, sans level adjustments (16 levels into a, e.g. 256 nominal level video signal).

  • only 1 VBI channel per program (sub-pictures have up to 32)
  • Line range is from 10 to 23 NTSC and 6 to 23.5 for PAL.
  • Separate palette (16 Y values, Cr=Cb=128) from subpictures.
  • No highlight
  • Restricted DCSQ command set
  • closed captioning for NTSC-rate (525/60) is coded exclusively in the user_data() field of the group_of_pictures_header()

    • VBI information is losslessly represented as a waveform, and coded into packets. The 525/60 player uses a far more efficient alternative: the source character stream is coded in the MPEG video user_data() field. The NTSC/PAL modulator chip then creates the VBI closed caption (Line 21) signal from this character stream.

    This brings our tally of closed caption representations to THREE ways!!

  • as packets of 16-level sampled VBI waveforms (PAL)
  • as user_data() character streams (NTSC)
  • as rendered subpictures (NTSC and PAL)