Digital subchannel
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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
Channel | Res. | Aspect | Programming |
---|---|---|---|
40.1 | 720p | 16:9 |
Trinity Broadcasting Network |
40.2 | Merit Street Media
| ||
40.3 | 480i | 4:3 |
TBN Inspire |
40.4 | 16:9 |
Smile | |
40.5 | Positiv |
The most of any large broadcaster in the United States,
Operating in a sector traditionally lacking subchannels,
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.[citation needed]
Most
The
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 August 17, 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.[1]
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
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
Inclusive of their primary standard definition channels (ignoring HD):
- SBS Television offers three unique SD subchannels (SBS HD); as well as its SBS Viceland and SBS World Movieschannels with no SD simulcasts.
- ABC Television offers three unique SD subchannels (ABC HD).
- The Seven Network offers two unique SD subchannels ( channels, with no SD simulcasts for both 7mate and 7Bravo.
- The Nine Network offers four unique SD subchannels (9HD) as well as a HD simulcast of its 9Gem channel.
- Network 10 offers two unique SD subchannels (10 HD) as well as their 10 Boldchannel 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 –
Colombia
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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 (ITV1, ITV2, ITV3 and ITV4) broadcasting on one multiplexed service, while two others (ITV2 +1 and CITV) are each broadcast on another, separate multiplex.
ISDB television
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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 Brazil, a digital subchannel is only allowed to the public and educational stations.[2][3]
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
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 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 January 1, 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 native HD resolution format for both networks). 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 (but maps its PSIP virtual channel as 24.2). 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 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 (two former WKBN and WYTV subchannels moved onto WYFX-LD's spectrum).[5]
|
WTHI-TV | Terre Haute, Indiana | In addition to airing its main CBS program feed in 1080i, WTHI transmits 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 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 (in the case of WKTV-DT2, this differs from CBS's preferred 1080i HD format). 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 WKOI (Richmond, Indiana), WDTN's spectrum transmits two 1080i signals: NBC (channel 2) and The CW (channel 26). 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 (NBC), 26 (The CW) and 43 (Ion)
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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.[
Television applications
The examples and perspective in this section deal primarily with the United States and do not represent a worldwide view of the subject. (February 2015) |
Commercial networks
In the United States, digital subchannels have been used to provide programming from multiple major networks on a single television station. This has become prevalent since the late 2000s in smaller markets that have as few as one or two commercial stations, which during the era of analog television, would not have been able to carry the complete programming lineups of all four major commercial networks (CBS, NBC, ABC and Fox) because of the station's own local and syndicated programming commitments, and overlapping network programs that would be tough to schedule outside of regular timeslots. A prime example is the Wheeling, West Virginia/Steubenville, Ohio market, which for decades was home to only two stations (CBS affiliate WTRF-TV and NBC affiliate WTOV-TV; the cable-only WBWO also served the market as a WB and now as a CW affiliate) and had to mostly rely on stations in Pittsburgh (and to a lesser extent Columbus and Youngstown, Ohio) to view programming from other networks. However, the advent of digital television allowed WTRF to launch two digital channels (one as a primary Fox/secondary MyNetworkTV affiliate, the other affiliated with ABC) while still carrying CBS programming in full on its main signal (WTOV later took the Fox affiliation for its second subchannel in September 2014).[6]
Upon their launches in September 2006, The CW and MyNetworkTV were among the first conventional networks to actively utilize subchannel-only affiliations in markets where a standalone station is not available to affiliate with; this is particularly true of The CW's small-market feed, The CW Plus, which originally consisted mostly of cable-only affiliations (by way of inheriting the model and much of the affiliate body of predecessor The WB 100+ Station Group). Since its launch, affiliates of other major networks have taken over the operations of cable-only CW Plus affiliates (or even outright replacing WB 100+ cable channels at the launch of The CW) and began transmitting the service over subchannels to reach viewers who do not subscribe to a pay television service. Some Spanish language networks (such as Estrella TV and Telemundo) have also been carried on digital subchannels, either as subchannel-exclusive services or to provide programming to markets where a main channel affiliation may not be available. Other stations have launched subchannels with an independent station format on their DT2 signals (such as WTTV in Indianapolis, Indiana – a market with enough commercial stations able to support affiliations with all six networks and a standalone independent, although the seventh (WTTK) instead acts as a WTTV satellite – which converted its 4.2 subchannel as an independent station in January 2015 as a result of owner Tribune Media selling the local rights to the CW affiliation that was to move from its main feed on 4.1 to Media General-owned WISH-TV, whose CBS affiliation was assumed by WTTV).[8][9]
Digital subchannels are also used to relay stations beyond their traditional signal coverage areas to reach an entire market. In the
In many cases, these "new" channels are existing secondary channels that were carried by a low-power or Class A station or by a cable television channel. Often, the owner of a full-power television station acquires or already owns a low-power secondary station in the same market to carry another network. The use of a digital subchannel on a full-power television station as a replacement for low-power station greatly increases the available coverage area for its programming.
Because of interference issues that stations transmitting on the low VHF band (channels 2 to 6) often experience, some stations broadcasting on these frequencies are relayed on the subchannels of stations that are less prone to interference. An example of this is CBS affiliate WRGB in Albany, New York. While WRGB broadcasts its main digital on VHF channel 6 in high definition, CW-affiliated sister station WCWN relays a standard-definition subchannel feed of WRGB over its digital channel 45.
Since the late 2010s, some station groups have started consolidating major network affiliations onto one signal if they own the non-licensing assets of those channels. Some of this was due to the
Sports programming
Networks dedicated to sports programming have been launched specifically for use on digital subchannels. Until 2010, CBS affiliates often subdivided four temporary subchannels in order to show all of the early round games of the
Most sports programming on digital subchannel broadcasters has been relegated to low-budget content such as amateur athletics, extreme sports, and hunting and fishing programming geared toward outdoorsmen, though
In January 2016, Sinclair Broadcast Group launched a 24-hour feed of its American Sports Network sports syndication service on subchannels of ten stations owned and/or operated by the group; the ASN multicast network was subsequently replaced by Stadium in August 2017, following the formation of a multi-platform network venture with the Chicago White Sox's Silver Chalice unit and 120 Sports.
2023 saw the moves of two National Hockey League team broadcasts to digital subchannels, in at least part of the team's market. In May 2023, the Vegas Golden Knights and Scripps Sports announced plans to bring the team's broadcasts to over-the-air television in the home market; as part of the deal, Golden Knights games air on the second subchannel of KIVI in Boise, KSAW-LD in Twin Falls, and the Montana Television Network.[10] In October of that year, the Arizona Coyotes moved their broadcasts to Scripps Sports, where the games air on the second subchannel of KNXV in Phoenix and KGUN in Tucson.[11]
Local and informational channels
Although not to the same level as in the late 2000s due to the population of entertainment-based multicast services, many local stations have used or currently use subchannels to carry continuous news or local weather content; in particular, there have been at least four networks that have been created to serve this audience:
Subchannels also allow stations to air news programs without fully pre-empting normally scheduled programing on the station's main feed. During significant breaking news or severe weather events, for instance, a station may choose to air extended news coverage on either its main channel or a subchannel and air network programming on the other. Thus, the station can accommodate viewers wanting to watch either regular programming or news coverage. Some sports leagues, most notably the NFL, have strict rules against their game broadcasts airing on a subchannel.
Specialty programming
The first major nationally distributed general entertainment digital multicast television network, or diginet, for use on subchannels was Retro Television Network in 2005. Several new services launched or attempted to launch in 2008, including This TV, utilizing classic TV programming and library movies. This time period also saw the launch of some of the first services for public TV stations in the United States, such as Create.
The field of diginets grew throughout the 2010s.
Diginets generally are reliant on national advertising revenue and, in some cases, pay stations to be carried on their subchannels, prizing lower channel numbers. Some have obtained national distribution on paid and free ad-supported streaming TV services.
Educational programming
Many
In some U.S. states, statewide educational, cultural or
Temporary installations
Subchannels and transmitter reconfigurations have been used to temporarily restore service from a station that is unable to broadcast for technical, weather-related, or other reasons using the facilities of another. This use dates to the early days of digital television: in the immediate aftermath of the
Data, radio and non-public signals
In rare cases, digital television broadcasters have included a service known as
Non-broadcast content,
Technical considerations
The examples and perspective in this section deal primarily with ATSC and do not represent a worldwide view of the subject. (June 2023) |
Digital television supports multiple digital subchannels if the 19.39 Mbit/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):
HDTV channels | Subchannels | ||
---|---|---|---|
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 + 3×480i is becoming more common. [when?]
For a
60 Hz | 50 Hz |
---|---|
|
|
For
Digital radio
Various forms of digital radio also allow for multiple program streams.
HD Radio
The primary distinguishing feature of HD Radio has been its ability to multiplex an FM radio signal. As HD Radio never achieved widespread popularity in the United States (unlike television, radio is not required to turn off its analog signals due to HD Radio being in-band on-channel and thus compatible with analog, plus the greater quantity and difficulty in signal conversion of radios compared to fixed-link television sets), its use has largely been to serve as a legal fiction. Since HD Radio was introduced in the United States in the late 2000s (decade), the FCC has allowed American broadcasters to use low-powered translators to transmit HD Radio subchannels in analog FM. This has allowed broadcasters to increase the number of programming choices available in a given media market beyond FCC limits.
AM broadcasting generally lacks the bandwidth to multiplex; though in theory an AM station could transmit two separate channels using C-QUAM AM stereo, there is a limit to how far the two audio channels can be separated, and thus crosstalk is inevitable. HD Radio can be used on AM, but the bandwidth limits the digital signal to a single channel, which under FCC rules must match the analog signal. AM broadcasters have criticized the use of HD Radio on AM due to the increased adjacent-channel interference caused by the greater bandwidth it requires, with little benefit.[16] WWFD has operated as a digital-only station with no analog signal under special dispensation since 2018; in December 2019, it began testing a multiplexed digital signal with two channels. No consumer radio receivers currently have the capability to receive AM multicast signals, and thus (as with FM HD Radio) WWFD's signals have been carried on FM translators (and the Internet) to ensure continued availability.[17] The FCC, in October 2020, concluded from WWFD's experiments: "the record does not establish that an audio stream on an HD-2 subchannel is currently technically feasible(.)"[18] A proposed FCC rule would require stations that wish to multiplex their digital AM signals to request and receive permission to do so.[18]
DAB
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See also
- In-band on-channel (IBOC), digital radio technology allowing digital subchannels on FM stations
- QAM tuner
References
- ^ "Decision CRTC 12-446". Canadian Radio-television and Telecommunications Commission. 17 August 2012.
- ^ "Folha de S.Paulo - Outro Canal - Daniel Castro: Governo proíbe novos canais de TV digital - 27/02/2009".
- ^ "MULTIPROGRAMAÇÃO". Archived from the original on 10 July 2017.
- ^ "CBS Stations, Weigel Partner on Oldies Digi-Net Decades". Broadcasting & Cable. 21 October 2014. Retrieved 11 November 2014.
- ^ "Big money from FCC auction means end of some TV stations". post-gazette.com. Retrieved 15 March 2018.
- ^ TVSpy. Retrieved 18 January 2015.
- ^ "Freeview HD". Freeview. Retrieved 18 January 2015.
- ^ Malone, Michael (22 December 2014). "Tribune Sells Indianapolis CW Affiliation to Media General". Broadcasting & Cable. Retrieved 23 December 2014.
- Indianapolis Star. Retrieved 31 December 2014.
- ^ "How to Watch the Vegas Golden Knights | Vegas Golden Knights". www.nhl.com. 1 September 2023. Retrieved 21 October 2023.
- ^ "Arizona Coyotes, Scripps Sports form multi-year broadcast partnership". Scripps. Retrieved 21 October 2023.
- ^ Fybush, Scott (11 September 2002). "NERW Extra: 9/11 Plus One". fybush.com.
- ^ Dudek, Duane (25 July 2010). "Storms show local TV, radio can work for viewers, listeners". Milwaukee Journal Sentinel.
- ^ Malone, Michael (22 June 2009). "Weigel Gives WTMJ a Hand". Broadcasting & Cable. Archived from the original on 15 January 2014. Retrieved 21 January 2023.
- ^ "#82500 DTV Engineering STA Application". Licensing and Management System. Federal Communications Commission. 27 September 2019.
- ^ Why you don't need, or want, "HD-AM" radio. Stop IBOC Now Coalition (2007). Retrieved from the Wayback Machine December 17, 2019.
- ^ Fybush, Scott (6 December 2019). "HD2 arrives on AM radio". NorthEast Radio Watch. Retrieved 6 December 2019.
- ^ a b "All-Digital AM Broadcasting; Revitalization of the AM Radio Service" (PDF). fcc.gov. 6 October 2020. Retrieved 25 June 2023.
External links
- "How Digital Television Works". HowStuffWorks. 10 January 2001. Retrieved 29 January 2007.