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. The practice is sometimes called multicasting.
ATSC-standard digital television in the United States supports multiple program streams over-the-air, using a virtual channel numbering scheme in which the television channel number is suffixed with ".2" or ".3" to indicate a second or third television program carried by the same television 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:
Trinity Broadcasting Network stations use five SDTV channels, the most of any large broadcaster in the country. Smaller stations, willing to compromise on broadcast quality, have been able to fit many more channels into a space. KAXT-CD in San Francisco is believed to have the most feeds of any individual over-the-air broadcaster, with twelve video feeds (all SDTV) and several audio feeds (WANN-CD in Atlanta, Georgia, with ten video and six audio feeds, comes in close second); several cable-to-air broadcasters have (such as one in Cortez, Colorado) multiplex more than five separate cable television channels into subchannels of one signal.
In the realm of digital cable TV (which traditionally does not use subchannels), Music Choice has all of its nearly 50 music channels (including SWRV) packaged as digital subchannels on one channel. This is possible as the only information sent in each channel is the audio, along with a still slide which rotates every twenty seconds displaying artist/title/album information and a still ad, freeing up significant bandwidth for more subchannels.
This would represent a significant cost, power and bandwidth savings for a broadcaster in comparison to the cost of operating additional analog TV 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.
Most ATSC tuners will automatically add a new digital subchannel to the internal channel map, once it is tuned to the station carrying the new channel. However, some of these will not delete the channel if it is removed by the station.
The FCC considers all subchannels of a single station to have the same call letters for legal ID purposes. However, within the broadcast sales industry, to differentiate subchannels, the initial letter of a call sign changes per subchannel. The typical pattern is, if subchannel 0.1 is WXMP, then 0.2 is EXMP, 0.3 is GXMP, and 0.4 is DXMP; if subchannel 0.1 is KXMP, then 0.2 is NXMP, 0.3 is OXMP, ans 0.4 is PXMP. (These pseudo-call signs do not reflect actual call signs, as they conflict with the internationally-recognized ITU prefixes issued to other countries.)
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. On August 17, 2012, the Canadian Radio-television and Telecommunications Commission gave approval to community station CFTV-TV in Leamington, Ontario to broadcast four local subchannels on its digital signal, making it the first station in Canada to launch original content on its multiplex channels.
Starting in 2012, Mexican state broadcaster Organismo Promotor de Medios Audiovisuales uses ATSC subchannels of the TV Once México network to transmit 3 national public non profit networks (Canal 30, Canal 22 and TV Unam) to several cities. In Mexico City several stations make use of subchannels: XEIPN TV has alternate programming for younger audiences at 11.2, XEIMT has the same programming at 22.1 and 22.2, 22.1 is for HD, while 22.2 is in standard definition with a 16:9 ratio and Cadena Tres uses 27.2 for Imagen Radio TV. Channels 7, 13 and 40 (real channels 24.x, 25.x and 26.x, respectively)owned by AZTECA, have several subchannels each, which video encoding is not possible to watch on a regular Digital TV set, only through the "Hi-TV" tuner sold also by AZTECA. This use of subchannels as pseudo-restricted signals in non-restricted channels is under investigation and litigation with the COFETEL authorities, involving a 4,453,150 Mexican pesos fine. Up to this date (April 2013), despite the orders from COFETEL, these subchannels are still on the air.
Currently, SBS, ABC, Seven, Nine and Ten show one high-definition subchannel each (SBS HD, ABC News 24, 7mate, GEM and One respectively); except for SBS One, these notably do not consist of the same content as the primary television channel.
Inclusive of their primary standard definition channels (ignoring HD):
SBS offers two unique SD subchannels;
ABC offers three unique SD subchannels;
Seven offers two unique SD subchannels;
Nine offers two unique SD subchannels;
Ten offers two unique SD subchannels.
Community television stations also broadcast digital television stations in Sydney (TVS), Melbourne and Geelong (C31), Adelaide (44 Adelaide) and Brisbane (31 Digital) however they typically only broadcast a single SD subchannel which consists of the primary television channel.
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.
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 (ITV, ITV2, ITV3 and ITV4) broadcasting on one multiplex, while two others (ITV2 +1 and CITV) are each broadcast on another, separate, multiplex.
As the amount of data which can be carried on one digital television channel at one time is limited, the addition of multiple channels of programming as digital subchannels comes at the expense of having less available bandwidth 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.
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:
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 currently broadcasting four HD services over terrestrial frequencies including BBC One HD, BBC HD, ITV HD, and Channel 4 HD (S4C Clirlun is broadcast in Wales instead of Channel 4 HD).
Many Public Broadcasting Service (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 television 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.
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.
Digital subchannels also help expand the traditional coverage areas of a station beyond their former analog boundaries. In the Upper Peninsula of Michigan and northern Minnesota, many of those stations are on duplicate frequencies to cover a large market area. This is used to duplicate network service for stations which are part of duopolies, where transmitters scattered through a large geographical area allow multiple networks and channels to be carried.
The most prominent example is Duluth, Minnesota's Northland's NewsCenter quadopoly of stations from Granite Broadcasting, which consists of two traditional broadcasters, NBC station KBJR and CBS affiliate KDLH, and three subchannels, CW Plus affiliate Northland CW 2, My9, the area's MyNetworkTV station, and a local weather station. While KDLH carries CW2 on their DT2 signal and My9 airs on KBJR-DT2 on their traditional Duluth signal, all five channels are carried on Chisholm-based sister station KRII to the north for the Iron Range region, providing that area stations which were previously unavailable over-the-air.
This is also the case with Upper Michigan station WPBN/WTOM, which also airs the signals of sister station WGTU/WGTQ, providing WGTU/WGTQ's ABC programming across a wider coverage area, while WWTV/WWUP carries their sister Fox station WFQX/WFUP on their DT2 subchannel to expand their coverage area further north into the eastern portion of the Upper Peninsula.
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.
Because of the nature of transmissions in the low VHF band (channels 2 through 6), some stations using these frequencies are relayed on the subchannels of stations that use more advantageous frequencies. An example of this is WRGB, the CBS affiliate of Albany, New York. While WRGB broadcasts in high-definition on channel 6, a standard-definition feed of WRGB can also be seen as a subchannel of CW affiliate (and duopoly partner) WCWN, on channel 45.
Other stations have launched separate independent stations on their DT-2 signals.
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 was 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. (Most of the major professional sports leagues, however, have strict prohibitions against this and only allow one game to be aired in a market at one time; all four of the major leagues have out-of-market sports packages that require a paid subscription and generate significant revenue for the leagues.)
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.
Networks dedicated to re-running TV shows and movies (such as Retro Television, Antenna TV, This TV, and MeTV) have been launched for TV stations digital subchannels. Some TV stations (such as K38IZ-D in Phoenix, Arizona) broadcast classic TV shows and music videos on their digital subchannels most recently The Country Network and The Cool TV. Music-video formats have also been tried as digital subchannels on terrestrial stations.
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.
During the Station Fire in 2009, Los Angeles station KNBC temporarily replaced NBC Plus and Universal Sports on its subchannels with standard-definition feeds from sister stations KWHY-TV (independent) and KVEA (Telemundo) as an emergency measure in the event those stations' transmitters were destroyed or disabled as the fire reached the Mount Wilson transmitter site for most Los Angeles area stations.
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 KCSM FM on its DT3 signal). WANN-CD in Atlanta offers six radio stations owned by Clear Channel Communications, in addition to ten TV channels. This type of service is known as DTV radio.
Non-broadcast content, subscription TV channels or datacasting operations unrelated to the main TV programming are also permitted by the DTV standards but are less-commonly used. USDTV was an over-the-air pay TV service that used H.264 compression instead of standard MPEG-2. Mobile DTV now uses MPEG-4 compression, which like H.264 yields a much lower bitrate for the same video quality. For example, the Sezmi TV/DVR service uses broadcast digital sub-channels (not in the clear) in selected cities to stream a limited number of "cable" channels to its subscribers for an additional fee to supplement its otherwise free Digital video recorder (DVR) service allowing recordings of local broadcast channels and free and subscription internet content.
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):
|1× 1080i or 720p HDTV (19 Mbit/s)||No additional subchannels.|
|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.
|60 Hz||50 Hz|
Various forms of digital radio also allow for multiple program streams.
|This section requires expansion. (April 2010)|
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