Digital subchannel
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Digital television in the United States supports multiple digital subchannels if the 19.4 Megabits-per-second (Mbit/s) bitstream is divided. Full quality 1080i HDTV (High Definition Television) requires 18 Mbit/s[citation needed]; however, many stations compress this down to 11 Mbit/s[citation needed]. A 720p HDTV signal can be compressed to 8 Mbit/s[citation needed], and an SD (Standard Definition) signal compressed to 6 Mbit/s without perceivable loss of quality.[citation needed] 4 Mbit/s or less yields acceptable results if the subject in the video moves very little, such as a slideshow-type automated weather channel. (Note that the actual bitrate moves up and down, due to usage of variable bit rate encoding.)
Therefore, station managers could run any of the following scenarios:
| Scenario | Picture Format |
|---|---|
| Scenario A | 1 1080i or 720p HDTV (19 Mbit/s) |
| Scenario B | 1 1080i HDTV (11 Mbit/s) + 1 720p HDTV (8 Mbit/s) Subchannel |
| Scenario C | 1 1080i or 720p HDTV + 1 480p or 480i SD subchannel |
| Scenario D | 1 1080i or 720p HDTV + 1 Slideshow-type subchannel (such as NBC Weather Plus) |
| Scenario E | 1 1080i HDTV + 1 480p or 480i SD Subchannel + 1 Slideshow type subchannel |
| Scenario F | 3 480p or 480i SD Channels (~6 Mbit/s each) |
| Scenario G | 4 480p or 480i SD Channels (~4.2 Mbit/s each) |
| Scenario H | 5 480p or 480i SD Channels (~3.8 Mbit/s each) |
Many PBS stations around the country broadcast 4 SD channels during the daytime, and 1 HD + 1 SD channel at night. PBS offers the following channels: PBS DT (analog station), PBS HD (PBS-XP), PBS Kids & Family (PBS Kids Channel), PBS Select (Create), & PBS Learner (The Annenberg Channel).
Another more common use for digital subchannels is for CBS affiliates to show all of the early round games of the NCAA Basketball Tournament on their digital channel, which is subdivided into four different subchannels.
With the launches of The CW and MyNetworkTV, many television stations have been launching subchannels affiliating with The CW or MyNetworkTV. Other stations have launched separate independent stations on their DT-2 signals, such as WOWT and WDJT (which carries its broadcast Class A station WMLW, and full power station WBME-TV (which has a tower move in progress) as a digital signal).
[edit] See also
- In-band on-channel (IBOC), digital radio technology allowing digital subchannels on FM stations
- The AccuWeather Channel
- The CW Plus
- ION Television
- NBC Weather Plus
- Retro Television Network
- The Tube (TV channel)
[edit] References
- How Digital Television Works. HowStuffWorks. Retrieved on 2007-01-29.
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