Digital subchannel
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In broadcasting, digital subchannels are a means to transmit more than one independent program at the same time from the same digital radio or digital television station on the same radio frequency channel. This is done by using data compression techniques to reduce the size of each individual program stream, and multiplexing to combine them into a single signal.
US digital television subchannels
Digital television in the United States supports multiple program streams over-the-air, using a virtual channel numbering scheme in which the channel number is suffixed with ".2" or ".3" to indicate a second or third program carried by the same station at the same time.
(By convention, ".1" is normally used to refer to the digital version of the station's main signal and the ".0" position is reserved for analog channels.)
For instance, an Ion Television station could use one digital signal to carry three or more subchannels in a format such as:
| Subchannel | Format | Program |
|---|---|---|
| 16.1 | 720p | ION |
| 16.2 | 480i | qubo |
| 16.3 | 480i | ION Life |
| 16.4 | 480i | Worship |
This would represent a significant cost, power and bandwidth savings for a broadcaster in comparison to the cost of operating additional analogue stations to accommodate the extra programming. In practice, operating extra stations is impossible, due to required channel and distance separations and the available number of channels.
Canadian digital television subchannels
Although digital services in Canada use the same technology as the United States, none of the stations currently available in digital are broadcasting any subchannel other than a possible HD service.
Salvadoran digital television subchannels
Digital television in El Salvador supports multiple program streams over-the-air, using a virtual channel numbering scheme in which the channel number is suffixed with ".2" or ".3" to indicate a second or third program carried by the same station at the same time.
(By convention, ".1" is normally used to refer to the digital version of the station's main signal and the ".0" position is reserved for analog channels.)
This would represent a significant cost, power and bandwidth savings for a broadcaster in comparison to the cost of operating additional analogue stations to accommodate the extra programming. In practice, operating extra stations is impossible, due to required channel and distance separations and the available number of channels.
Australian digital television subchannels
Currently the ABC, SBS, Nine and Ten each show one high-definition channel (ABC HD, SBS HD, Nine HD and One HD) and one standard-definition subchannel (ABC2, SBS Two, Go! and One respectively). Seven currently only have a high-definition subchannel, Seven HD, but the network will launch its own subchannel on 22 November 2009. The ABC will also launch a third SD subchannel, ABC3, on 4 December 2009.
There have been a number of issues surrounding the introduction of digital subchannels in Australia. The first subchannels launched by the ABC, ABC Kids and Fly TV, closed after less than two years in operation in 2003 due to lack of budget and lack of viewers, and commercial broadcasters could not legally air a digital subchannel other than a single high-definition service until 2009.
Digital television subchannels in Europe
As most digital services in Europe rely on more complex methods of multiplexing, where a large number of digital channels by many different broadcasters can be broadcast on one single frequency, the concept of a subchannel is instead applied to the variety of channels produced by a single company. This can vary widely depending on the country: for example, ITV currently has four of its digital channels (ITV1, ITV2, ITV3 and ITV4) broadcasting on one multiplex, while two others (ITV2 +1 and CITV) are each broadcast on another, separate, multiplex.
Tradeoffs
As the amount of data which can be carried on one digital television channel at one time is finite, the addition of multiple channels of programming as digital subchannels comes at the expense of having less available space for other purposes, such as high-definition television. A station carrying multiple subchannels will normally limit itself to one high-definition channel, with the additional channels being carried in standard definition. Because of the tradeoffs, stations owned by CBS Corporation that are a part of CBS Television Stations (which include CBS O&O's, CW O&O's, and some independent stations) are not permitted to have digital subchannels.[citation needed]
It is, however, possible for stations to carry two subchannel feeds in HD, though it is still extremely rare. Some examples of stations broadcasting in this format are:
- KXII, the CBS affiliate in Sherman, Texas, airs Fox programming in HD on its third subchannel in addition to airing its main CBS programming in HD. The station also has a myNetworkTV affiliate in SD on subchannel 12.2.
- WKBN-TV, the CBS affiliate in Youngstown, Ohio which also owns & operates low-powered Fox affiliate WYFX-LP & simulcasting sister station WFXI-CA of nearby Mercer, Pennsylvania, simulcasts WYFX/WFXI on its second digital subchannel while broadcasting its main CBS feed on its first subchannel. Both subchannels are broadcasting in 720p HD, a reduction from CBS's preferred 1080i transmissions although Fox has a preference for 720p.
- WPVI-TV, the ABC O&O in Philadelphia, simulcasts two channels in 720p HD, and a third weather channel in 480i standard definition. WPVI is also one of the few stations broadcasting digital television in the VHF spectrum (channels 2 to 6) below the FM radio band.
Outside of the United States, especially in Europe, high-definition feeds are rarer, and most countries only provide a single high-definition service for each broadcaster. For example, digital television in France only has five HD services, one each for TF1, France 2, Canal+, M6 and Arte, and the United Kingdom is not currently broadcasting any HD services over terrestrial frequencies.
Applications
Educational programming
Many PBS stations around the United States broadcast four SD channels during the daytime, and one HD and one SD channel at night. PBS stations often carry additional national channels such as PBS HD (PBS Satellite Service), PBS Kids Sprout, PBS World, Create, PBS Learner (The Annenberg Channel) or the Spanish-language V-me network. MHz Networks transmits to the Washington, D.C. area on ten subchannels transmitted by two stations, with virtual channel numbering making them appear as one station.
In some US states, state-wide educational, cultural or public-affairs services are carried by existing PBS member stations on digital subchannels; these include regional services such as the Minnesota Channel, Wisconsin Channel or New York State broadcaster ThinkBright TV. The use of subchannels has also allowed educational broadcasters to sell off former secondary PBS analogue stations (such as WNEQ in Buffalo to LIN TV to become CW affiliate WNLO) to commercial broadcasters, as the additional educational content these once provided can now be carried by multiple subchannels of the one main station. Subchannels also allow stations to devote an entire channel to telecourses which are recorded by instructors and students for later use, freeing the original channel to air a more general schedule in the morning and overnight hours.
Commercial networks
In small media markets in the United States, one or more national networks are unavailable from local over-the-air sources. A small city with only one or two commercial stations would not be able to carry the full programming lineup from all four largest commercial networks (CBS, NBC, ABC, Fox) in analogue as one station could only broadcast one program at a time. This limitation could be overcome by adding the additional networks as digital subchannels of existing local commercial stations. One prime example of this is in the Wheeling, West Virginia/Steubenville, Ohio market, which for decades has had only two television stations (CBS affiliate WTRF-TV and NBC affiliate WTOV-TV, as well as cable-only CW affiliate WBWO) and had to mostly rely on stations in Pittsburgh (and to a lesser extent Columbus and Youngstown, Ohio) for other networks. However, the advent of digital television has allowed WTRF to launch two digital channels (one for Fox with MyNetworkTV as a secondary affiliate, the other for ABC) while still airing the full CBS schedule on its main signal.
In many cases, these "new" channels are existing secondary channels which were carried by a low-power or Class A station or by a cable television channel. Often, a full-power TV station acquires or already owns a low-power secondary station in the same market to carry a second network. The use of a digital subchannel on a full power TV station as a replacement for LPTV greatly increases coverage areas for this programming.
With the launches of The CW and MyNetworkTV, many television stations have been launching subchannels affiliating with these secondary networks.
Other stations have launched separate independent stations on their DT-2 signals.
Sports programming
Networks dedicated to sports programming (such as Universal Sports and Untamed Sports TV) have been launched for TV stations digital subchannels. A 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.
Local and informational channels
Many local broadcasters are using subchannels to carry continuous news or local weather; secondary networks such as AccuWeather Channel have been created to serve this audience.
Specialty programming
Networks dedicated to re-running TV shows and movies (such as Retro Television Network, This TV and .2 Network) have been launched for TV stations digital subchannels. Some TV stations (such as K38IZ-D in Phoenix, Arizona) broadcast classics TV shows and music videos on their digital subchannels. Music video formats have also been tried as digital subchannels on terrestrial stations.
Spanish programming
Networks dedicated to Spanish programming (such as Estrella TV and Mexicanal) have been launched for TV stations digital subchannels.
Temporary installations
A digital subchannel can be used to restore service from a station which has been knocked off-the-air due to an antenna tower collapse; the affected signal would be made available in standard-definition, or even compressed high definition, on a subchannel of another local broadcaster, most often a competitor. KATV (ABC, Little Rock) was forced to follow this path in 2008, moving its digital signal to a subchannel of local MyNetworkTV affiliate KWBF after a tower collapse knocked its main signal off-air. The virtual channel numbering scheme allows an existing licensed broadcaster to keep its displayed channel number unchanged (KATV ABC 7 in this case) even if the signal is carried physically as a subchannel of some other local station.
Data, radio and non-broadcast signals
In rare cases, digital TV broadcasters have included the audio of a commonly-owned broadcast radio station among their subchannel offerings (for instance, KCSM-TV in San Mateo, California broadcasts the KCSM-FM radio station on its DT3 signal). Non-broadcast content, subscription TV channels or datacasting operations unrelated to the main TV programming are also permitted by the ATSC standard but are less commonly used.
Technical considerations
ATSC digital television supports multiple digital subchannels if the 19.39 megabits-per-second (Mbit/s) bitstream is divided. Therefore, station managers could run any of the following scenarios using one 6MHz ATSC channel (note that the actual bitrate moves up and down, due to usage of variable bit rate encoding):
| HDTV channels | Subchannels | ||
|---|---|---|---|
| 1 x 1080i or 720p HDTV (19 Mbit/s) | No additional subchannels, unless HD is transmitted at 15Mbit/s or less. | ||
| 1 x 1080i or 720p HDTV (15 Mbit/s) | + 1 | 480p or 480i SD subchannel (~3.8 Mbit/s) | |
| 1 x 1080i or 720p HDTV (11 Mbit/s) | + 1 | 720p HDTV (8 Mbit/s) subchannel | |
| 1 x 1080i or 720p HDTV (11 Mbit/s) | + 2 | 480p or 480i SD subchannels (~3.8 Mbit/s each) | |
| 1 x 720p HDTV channel (8 Mbit/s) | + 3 | 480p or 480i SD subchannels (~3.8 Mbit/s each) | |
| 2 x 720p HDTV channels (9.6 Mbit/s each) | No SD subchannels | ||
| 2 x 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 | +120 | mono radio subchannels (~0.2 Mbit/s each) | |
With improvements in MPEG encoding, and tighter VBR encoding, more subchannels can be combined. (1 x 720p) + (4 x 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:
| 60Hz | 50Hz |
|---|---|
|
|
For ATSC, these must be compressed into 19.4Mb/s total per physical 6MHz RF channel over the air, and 38.8Mb/s for cable.
See also
- In-band on-channel (IBOC), digital radio technology allowing digital subchannels on FM stations
- DAB and DVB, international digital radio-television standards in use in Europe and Australia
- QAM tuner
References
- "How Digital Television Works". HowStuffWorks. Retrieved 2007-01-29.