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:
- KXII, the CBS affiliate in Sherman, Texas, airs Fox programming in HD on its third subchannel in addition to airing its main CBS programming in HD. The station also has a myNetworkTV affiliate in SD on subchannel 12.2.
- WGXA, Fox's affiliate in Macon, Georgia, picked up the ABC affiliation on January 1, 2009 after former ABC station WPGA-TV dropped the network in a compensation and program content dispute, and is running two 720p HD signals for Fox and ABC (the native format for both networks) over their channel space. The original Fox channel is still branded via PSIP as Fox 24 and 24.1, while the ABC subchannel is known as ABC 16, using the physical digital channel 16 for their branding (and an eventual PSIP virtual channel of 16.2; currently 24.2 as problems are ironed out balancing the bandwidth needs of both signals).
- WKBN-TV, the CBS affiliate in Youngstown, Ohio which also owns & operates low-powered Fox affiliate WYFX-LD, simulcasts WYFX on its second digital subchannel while broadcasting its main CBS feed on its first subchannel. Both subchannels are broadcasting in 720p HD, a reduction from CBS's preferred 1080i transmissions although Fox has a preference for 720p. Its sister station, ABC affiliate WYTV, is currently in the process of converting its MyNetworkTV subchannel into high-definition broadcasts. It currently also has a weather channel on its third subchannel, which currently has an uncertain future if WYTV-DT2 is converted to HD. Such a move would make the Youngstown DMA the only such market with two stations broadcasting two subchannels in HD.
- WPVI-TV, the ABC O&O in Philadelphia, simulcasts two channels in 720p HD (the regular WPVI/ABC schedule and Live Well HD Network, respectively), and then WPVI's "ABC Plus"/Local AccuWeather in 480i standard definition. WPVI is also one of the few stations broadcasting digital television in the VHF spectrum (channels 2 to 6) below the FM radio band. Other ABC-owned stations also have the same ABC/Live Well/ABC Plus-Local AccuWeather channel map setup as well, though a few have Live Well and ABC Plus-Local AccuWeather airing in regular 480i.
- WTHI-TV, the CBS affiliate in Terre Haute, Indiana, airs Fox programming in 720p HD on its second subchannel in addition to airing its main CBS programming in HD.
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, ITV1 HD, and Channel 4 HD (S4C Clirlun is broadcast in Wales instead of Channel 4 HD).
Read more about this topic: Digital Subchannel