Digital Subchannel - Technical Considerations

Technical Considerations

Digital television supports multiple digital subchannels if the approximately 19.4Mbit/s (megabits per second) bitstream is divided. Therefore, station managers and broadcast engineers could run any of the following scenarios using one 6 MHz channel (note that the actual bitrate moves up and down, due to usage of variable bitrate encoding):

HDTV channels Subchannels
1× 1080i or 720p HDTV (19 Mbit/s) No additional subchannels, unless HD is transmitted at 15Mbit/s or less.
1× 1080i or 720p HDTV (15 Mbit/s) + 1 480p or 480i SD subchannel (~3.8 Mbit/s)
1× 1080i or 720p HDTV (11 Mbit/s) + 1 720p HDTV (8 Mbit/s) subchannel
1× 1080i or 720p HDTV (11 Mbit/s) + 2 480p or 480i SD subchannels (~3.8 Mbit/s each)
1× 720p HDTV channel (8 Mbit/s) + 3 480p or 480i SD subchannels (~3.8 Mbit/s each)
2× 720p HDTV channels (9.6 Mbit/s each) No SD subchannels
2× 720p HDTV channels (7.8 Mbit/s each) + 1 480p or 480i SD subchannel (~3.8 Mbit/s)
No HDTV channels + 2 480p or 480i SD subchannels (~6 Mbit/s each)
No HDTV channels + 3 480p or 480i SD subchannels (~6 Mbit/s each)
No HDTV channels + 4 480p or 480i SD subchannels (~4.2 Mbit/s each)
No HDTV channels + 5 480p or 480i SD subchannels (~3.8 Mbit/s each)
No HDTV channels + 6 480p or 480i SD subchannels (~3.1 Mbit/s each)
No HDTV channels + 7 480p or 480i SD subchannels (~2.7 Mbit/s each)
No HDTV channels + 8 480p or 480i SD subchannels (~2.4 Mbit/s each)
No HDTV channels + 9 480p or 480i SD subchannels (~2.1 Mbit/s each)
No HDTV channels + 10 480p or 480i SD subchannels (~1.9 Mbit/s each)
No HDTV channels +120 mono radio subchannels (~0.2 Mbit/s each)

With improvements in MPEG encoding, and tighter VBR encoding, more subchannels can be combined. 1×720p + 4×480i is becoming more common.

For a frame rate of 30p or 60i, uncompressed DTV channels have the following data rates in megapixels per second:

60 Hz 50 Hz
  • 640×480i60/p30 (NTSC SDTV): 9.216
  • 704×480i60/p30 (ATSC wide SDTV): 10.1376
  • 720×480i60/p30 (NTSC DVD): 10.368
  • 1280×720p30: 27.648
  • 1280×720p60: 55.296
  • 1920×1080i60/p30: 62.208

  • 704×576i50/p25 (PAL SDTV): 10.1376
  • 720×576i50/p25 (PAL DVD): 10.368
  • 1280×720p25: 23.04
  • 1280×720p50: 46.08
  • 1920×1080i50/p25: 51.84

For ATSC, these must be compressed into 19.4Mbit/s total per physical 6 MHz RF channel over the air, and 38.8Mbit/s for cable.

Read more about this topic:  Digital Subchannel

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