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.
Several cable-to-air[clarification needed] broadcasters, such as those in Willmar, Minnesota and Cortez, Colorado, have multiplexed more than five separate cable television channels into subchannels of one signal.
The Federal Communications Commission (FCC) considers all subchannels carried by a single station to have the same call letters for legal identification purposes.
For example, if the call letters are WFRC and the station broadcasts on channel 10, then the 10.2 subchannel is identified as EFRC, 10.3 is GFRC, 10.4 is HFRC, 10.5 is IFRC and 10.6 is JFRC.
If the call letters are KFRC and the station broadcasts on channel 10, then 10.2 is identified as NFRC, 10.3 is OFRC, 10.4 is QFRC, 10.5 is RFRC and 10.6 is SFRC.
[citation needed] 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.
[citation needed] Televisa and TV Azteca use subchannels in rural areas in order to ensure national network service.
[citation needed] 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.
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.
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.
[7] In the United States, digital subchannels have been used to provide programming from multiple major networks on a single television station.
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.
In the Upper Peninsula of Michigan and northern Minnesota, many of these stations are on duplicate frequencies to cover a large market area.
In the Traverse City-Cheboygan market in Upper Michigan, NBC affiliate WPBN/WTOM also simulcasts sister station WGTU/WGTQ, providing that station's ABC programming to the entire market; CBS affiliate WWTV/WWUP carries its Fox-affiliated sister WFQX/WFUP on their DT2 subchannel to expand their coverage area further north into the eastern portion of the Upper Peninsula.
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.
Until 2010, CBS affiliates often subdivided four temporary subchannels in order to show all of the early round games of the NCAA men's basketball tournament in addition to those broadcast on the main digital channel (this was superseded as a result of a new television agreement with the NCAA that took effect in 2011, which gave cable networks TBS, TNT and TruTV partial rights to the tournament).
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.
Station groups also increased their presence in the space, most notably the E. W. Scripps Company's 2017 acquisition of Katz Broadcasting, which was seen as giving the sector legitimacy.
Ratings and coverage have increased as these channels seek to reach cord cutters who still use antennas to receive broadcast signals.
Diginets generally are reliant on national advertising revenue and, in some cases, pay stations to be carried on their subchannels, prizing lower channel numbers.
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 11 September attacks, which destroyed the primary TV transmission site in New York City, WWOR-TV was broadcast as a subchannel of co-owned WNYW-DT.
[13][14] For five months from October 2019 to 2020, commonly operated WBBH-TV and WZVN-TV in Fort Myers, broadcast from the former's facility on one multiplex to allow for an overhaul of the latter's antenna.
For example, the Sezmi TV/DVR service uses broadcast digital subchannels (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.
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.
[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(.