In broadcasting, digital subchannels are a method of transmitting more than one independent program stream simultaneously from the same digital radio or 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 television
United States The ATSC digital television standard used in the United States supports multiple program streams over-the-air, allowing television stations to transmit one or more subchannels over a single digital signal. A virtual channel numbering scheme distinguishes broadcast subchannels by appending the television channel number with a period digit (".xx"). Simultaneously, the suffix indicates that a television station offers additional programming streams. By convention, the suffix position ".1" is normally used to refer to the station's main digital channel and the ".0" position is reserved for analog channels. For example, most of the owned-and-operated stations/affiliates of Trinity Broadcasting Network transmit five streams in the following format: Subchannel Programming xx.1 720p Trinity Broadcasting Network xx.2 TVDEALS (infomercials) xx.3 480i TBN Inspire xx.4 OnTV4U (infomercials) (4:3) xx.5 Positiv
The most of any large broadcaster in the United States, Ion Television stations transmit eight channels (in standard definition) and the Scripps Networks subchannel services Court TV, Ion Mystery, Bounce TV, Laff, Grit, and Ion Plus. More programming streams can be fit into a single channel space at the cost of broadcast quality. Among smaller stations, KAXT-CD in San Francisco is believed to have the most feeds of any individual over-the-air broadcaster, offering twelve video and several audio feeds (all transmitted in standard definition). WANN-CD in Atlanta, Georgia, with ten video and ten audio feeds, comes at a close second. Several cable-to-air broadcasters, such as those in Willmar, Minnesota and Cortez, Colorado, have multiplexed more than five separate cable television channels into subchannels of one signal.
Operating in a sector traditionally lacking subchannels, digital cable television provider Music Choice packages its nearly 50 music channels (including Music Choice Play) as digital subchannels of one channel. This is possible as the only information sent over each channel are audio feeds and a still slide which rotates every 20 seconds, displaying an advertisement and information about the current playing song on the individual channel. The audio feed and rotating stills occupy significantly less bandwidth than video feeds, leaving space for more multiplexed content.
A broadcaster saves significant costs in power and bandwidth through multiplexing in comparison to the cost of operating additional analog television stations to accommodate the extra programming. In practice, operating extra stations is impossible due to the required channel and distance separations combined with the available number of channels.
Most ATSC tuners will automatically add a new digital subchannel to their internal channel map, once it is tuned to the station carrying the new channel. However, some of these will not delete the channel if the station removes it.
Mobile DTV is also carried on ATSC stations, but as a separate service, according to the ATSC-M/H standard.
The Federal Communications Commission (FCC) considers all subchannels carried by a single station to have the same call letters for legal identification purposes. However, within the broadcast sales industry, to differentiate subchannels, the initial letter of a call sign changes per subchannel.
Canada Although digital television services in Canada use the same ATSC technology as the United States, none of the stations currently broadcasting a digital signal transmit any subchannel other than a possible HD service or a standard definition simulcast of the main channel. Unlike the FCC in the United States, the body that governs Canadian broadcasting licenses, the Canadian Radio-television and Telecommunications Commission (CRTC), requires stations to file license amendments in order to be considered for permission to carry digital subchannels (this differs from the commission's rules for premium cable television services, which allow the addition of multiplex channels consistent with the service's license requirements without the need to amend the license). On 17 August 2012, the CRTC gave approval to Leamington, Ontario community station CFTV-TV to broadcast four local subchannels on its digital signal, making it the first station in Canada to launch original content on its multiplex channels.
Mexico Some Mexican TV stations use digital subchannels as they are used in the United States. The Sistema Público de Radiodifusión del Estado Mexicano, a public broadcaster, operates 26 multiplexed transmitters throughout Mexico carrying five to six public television services, while XHTRES-TDT carries Imagen Radio audio on a subchannel.
One notable experiment involving digital subchannels in Mexico was undertaken by TV Azteca, which used its three muxes in the Mexico City area to broadcast a service called Hi-TV, featuring several channels encoded in H.264 MPEG-4 encoding, which while available in the ATSC standard is not common on TV sets. This use of subchannels as pseudo-restricted signals within non-restricted channels was placed under investigation and litigation with authorities at COFETEL (the Federal Telecommunications Commission), involving a fine of 4,453,150 Mexican pesos. HiTV subchannels began broadcasting on an intermittent basis in 2013 and were almost completely deactivated in late 2014.
Televisa and TV Azteca use subchannels in rural areas in order to ensure national network service. As a result, since 2016, many areas that formerly had only one Azteca or Televisa network now have both from the same transmitter. Additionally, TV Azteca has two national services that are broadcast as subchannels in most areas, a+ and adn40.
In October 2016, the IFT put into effect new guidelines for the numbering of virtual channels. As a result, national networks use consistent numbers nationwide; SPR transmitters now use four or five major channel numbers (11, 14, 20, 22, and 45 in some areas). Prior to this, digital television stations usually used virtual channels corresponding to their former analog positions, still the case for certain local stations.
The IFT enforces minimum bitrates for digital television channels, and as such it is not possible for a station to broadcast two HD feeds in MPEG-2 encoding. Most HD feeds are provided in 1080i with all subchannels in 480i standard definition.
DVB television
Australia Australian digital subchannels are currently divided between high definition (HD), standard definition (SD) and radio subchannels (the latter type is only carried by the stations of non-commercial networks SBS Television and ABC Television). Each network currently has at least one HD sub channel. All networks use their HD subchannel to provide a simulcast of their primary channel or their multichannels.
Inclusive of their primary standard definition channels (ignoring HD): SBS Television offers three unique SD subchannels (SBS Food, NITV and SBS WorldWatch) and a HD simulcast of its primary channel (SBS HD); as well as its SBS Viceland and SBS World Movies channels with no SD simulcasts. ABC Television offers three unique SD subchannels (ABC TV Plus/ABC Kids, ABC Me, and ABC News) and a HD simulcast of its primary channel (ABC HD). The Seven Network offers two unique SD subchannels (7two and 7flix), a datacasting channel (Racing.com) and a HD version of its primary channel (7HD) as well as its 7two, 7mate and 7Bravo channels, with no SD simulcasts for both 7mate and 7Bravo. The Nine Network offers four unique SD subchannels (9Go!, 9Rush, 9Gem and 9Life), a datacasting channel (Extra) and a HD simulcast of its primary channel (9HD) as well as a HD simulcast of its 9Gem channel. Network 10 offers two unique SD subchannels (10 Peach, and 10 Shake), two datacasting channels (TVSN and gecko) and a HD simulcast of its primary channel (10 HD) as well as their 10 Bold channel with no SD simulcast.
Community television stations in Melbourne (C31) and Adelaide (C44) also broadcast digital signals, however they typically only broadcast a single SD subchannel which simulcasts that station's primary 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 as a reaction to budget cuts by the conservative Howard government under Communications Minister Alston and low viewership (partly due to the limited distribution of set-top boxes); and commercial broadcasters could not legally air a digital subchannel other than a single high-definition service until 2009.
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 that are produced by a single company. This can vary widely depending on the country: for example, ITV currently has six of its digital channels (ITV1, ITV1 +1, ITV2, ITV3, ITV4 and ITV Quiz) broadcasting on one multiplexed service, while three others (ITV2 +1, ITV3 +1 and ITV4 +1) are each broadcast on another, separate multiplex.
ISDB television In Japan and Latin America (except Colombia, Mexico and Panama), ISDB (similar to the DVB format) is used, and was specifically designed with physical RF segments that could be split to use for different subchannels.
In Japan, subchannel programming is used by NHK’s NHK General TV, NHK Educational TV (terrestrial) and NHK BS (satellite television channel), usually for sports broadcasts.
In Brazil, a digital subchannel was only allowed to the public and educational stations, until 2024, when commercial TV networks were allowed to have subchannels.
Tradeoffs 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 the ability to transmit high definition content. A station carrying multiple subchannels will normally limit itself to one high-definition channel (or in some cases, two HD channels), with any additional channels being carried in standard definition. Because of the tradeoffs, stations owned by CBS Corporation through its CBS Television Stations subsidiary (which include owned-and-operated stations of CBS and The CW, and some independent stations) generally opted not to carry digital subchannels and transmitted only a 1080i high definition main feed; this changed in 2013 with the addition of dedicated local news channels on CBS O&Os in New York City and Philadelphia (the company later announced the creation of Decades, a multicast network part-owned by CBS which aired on all CBS and CW owned-and-operated stations from 2015 to 2018).
It is possible for stations to carry more than two subchannel feeds in HD, at least nominally. Actual picture quality may be comparable to DVD video. Some examples of stations broadcasting in this format are: