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 |
|---|---|
|
|
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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