Digital subchannel

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. 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:

Channel	Res.	Aspect	Programming
40.1	720p	16:9	Trinity Broadcasting Network
40.2	720p	16:9	Merit Street Media
40.3	480i	4:3	TBN Inspire
40.4	480i	16:9	OnTV4U
40.5	480i	16:9	Positiv

The most of any large broadcaster in the United States, Ion Television stations transmit eight channels and the Scripps Networks subchannel services Court TV, Ion Mystery, Bounce TV, Laff, Grit, Defy TV, and Scripps News. 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. 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 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 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, requires stations to file license amendments in order to be considered for permission to carry digital subchannels. 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, 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. 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, standard definition and radio subchannels. 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 :

SBS Television offers three unique SD subchannels and a HD simulcast of its primary channel ; as well as its SBS Viceland and SBS World Movies channels with no SD simulcasts.
ABC Television offers three unique SD subchannels and a HD simulcast of its primary channel.
The Seven Network offers two unique SD subchannels, a datacasting channel and a HD version of its primary channel 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, a datacasting channel and a HD simulcast of its primary channel as well as a HD simulcast of its 9Gem channel.
Network 10 offers two unique SD subchannels, two datacasting channels and a HD simulcast of its primary channel as well as their 10 Bold channel with no SD simulcast.

Community television stations in Melbourne and Adelaide 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 ; 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 four of its digital channels broadcasting on one multiplexed service, while two others are each broadcast on another, separate multiplex.

ISDB television

In Japan and Latin America, ISDB is used, and was specifically designed with physical RF segments that could be split to use for different subchannels. 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, with any additional channels being carried in standard definition. Because of the tradeoffs, stations owned by CBS Corporation through its CBS Television Stations subsidiary 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.
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:

Callsign	Market/city of license	Description
KBMT	Beaumont, Texas	In addition to broadcasting ABC programming on its main channel in 720p, KBMT transmits NBC programming on its second subchannel and Cozi TV on its third subchannel ; the station also offers Me-TV programming in 480i 4:3 SD on its fourth subchannel.
KXII	Sherman, Texas	KXII carries Fox programming in HD on its third subchannel and MyNetworkTV programming in SD on its second subchannel, in addition to carrying CBS programming in HD on its main channel.
KTEN	Ada, Oklahoma	In addition to carrying NBC programming in HD on its main channel in 1080i, KTEN carries CW programming in 720p HD on its second subchannel and ABC programming in 720p HD on its third subchannel.
WGXA	Macon, Georgia	WGXA began carrying ABC programming on its second digital subchannel on 1 January 2010, after WPGA-TV disaffiliated from the network in a compensation and program content dispute; WGXA transmits its Fox-affiliated main channel and its ABC subchannel in 720p. The station's main channel is branded as "Fox 24" and maps via PSIP as 24.1, while the ABC subchannel brands as "ABC 16", in reference to the station's physical digital channel 16. A standard definition feed of Comet was launched on its third subchannel in October 2015.
WKBN-TV	Youngstown, Ohio	WKBN carries a simulcast of Fox-affiliated sister station WYFX-LD on its second digital subchannel in order to make its programming available to the entire market, in addition to transmitting CBS programming on its main channel. Until a later multiplexer upgrade, both subchannels transmitted in 720p HD. Comparatively, its sister station, ABC affiliate WYTV transmitted both its MyNetworkTV-affiliated second subchannel and its Bounce TV-affiliated third subchannel in standard definition. In 2017, WKBN sold its spectrum for $34 million as part of the FCC's spectrum reallocation program and merged their programming onto WYTV's, requiring the removal of the 33.3 signal in order to manage the high definition needs of three 720p channels and one in 1080i.
WTHI-TV	Terre Haute, Indiana	In addition to airing its main CBS program feed in 1080i, WTHI transmits Fox programming in 720p HD on its second subchannel and CW+ programming in 720p HD on its third subchannel, in addition to a standard definition 480i widescreen Ion Television feed on its fourth subchannel.
WTRF-TV	Wheeling, West Virginia	In addition to carrying CBS programming in 1080i on its main channel, WTRF-TV carries ABC programming in 720p on its third subchannel. Until the network moved to a subchannel of NBC affiliate WTOV-TV in September 2014, WTRF carried Fox programming in 720p on its second subchannel, which remains affiliated with MyNetworkTV.
WUVG-DT	Atlanta, Georgia	WUVG, which operates as a Univision owned-and-operated station, carries UniMás in 720p HD on its second subchannel due to a lack of a sister full-power station, and both GetTV and Escape in 480i on their respective third and fourth subchannels in addition to carrying Univision programming on its main channel in 720p HD.
WATM-TV	Johnstown, Pennsylvania	In addition to carrying ABC programming in 720p on its first subchannel, WATM-TV transmits Fox programming of its sister station, WWCP-TV, in 720p on its second subchannel. This enables the WWCP-TV signal to reach the State College area. WATM-TV further transmits This TV and Antenna TV programming in 480i on its third and fourth subchannels, respectively.
WKTV	Utica, New York	WKTV previously carried NBC programming in 1080i on its first subchannel, CBS programming on its second subchannel, and The CW on its third subchannel, both in 720p HD. All three networks are currently transmitting in 720p. The station also offers MeTV programming in 480i SD on its fourth subchannel.
WTVG	Toledo, Ohio	In addition to carrying ABC programming in 720p on its first subchannel, WTVG-TV transmits The CW programming in 720p on its second subchannel. This allows Toledo, Findlay, and surrounding areas to receive CW programming over-the-air; in the past CW and WB programming was exclusive to local cable viewers through Buckeye CableSystem via their cable-only Toledo 5 channel, requiring antenna viewers to pull in WKBD-TV/Detroit from the north for CW programming. WTVG-TV further transmits WeatherNation TV in 480i on its third subchannel.
WGEM-TV	Quincy, Illinois	The station broadcasts four subchannels in total, all in HD: the main subchannel in 1080i, and three additional subchannels – respectively affiliated with Fox, The CW and MeTV – in 720p.
WDTN	Dayton, Ohio	Through channel-sharing agreements with WBDT and WKOI, WDTN's spectrum transmits two 1080i signals: NBC and The CW. WDTN's spectrum also carries Ion Mystery, Bounce, and Ion in 480i. Through the use of PSIP, these channels appear to the viewer to be three different channels: 2, 26 and 43

Outside 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, in France, there are only five HD services: one each for TF1, France 2, Canal+, M6 and Arte; in the United Kingdom, four HD services are currently transmitted over terrestrial frequencies: BBC One HD, BBC Two HD, ITV HD and Channel 4 HD.