In broadcasting, digital subchannels are a means to transmit more than one independent program at the same time from the same digital radio or digital television station on the same frequency. This is done by using data compression techniques to reduce the size of each individual program stream.

[edit] US digital television subchannels

Digital television in the United States supports multiple program streams over-the-air, using a virtual channel numbering scheme in which the channel number is suffixed with ".2" or ".3" to indicate a second or third program carried by the same station at the same time.

(By convention, ".1" is normally used to refer to the digital version of the station's main signal and the ".0" position is reserved for analogue channels.)

For instance, a PBS station could use one digital signal to carry three or more subchannels in a format such as:

This would represent a significant cost, power and bandwidth savings for a broadcaster in comparison to the cost of operating additional analogue stations to accommodate the extra programming.

[edit] Tradeoffs

As the amount of data which can be carried on one digital television channel at one time is finite, the addition of multiple channels of programming as digital subchannels comes at the expense of having less available space 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.

[edit] Applications

[edit] Educational programming

Many PBS stations around the country broadcast 4 SD channels during the daytime, and 1 HD + 1 SD channel at night. PBS stations often carry additional national channels such as PBS HD (PBS Satellite Service), PBS Kids Sprout, PBS World, Create, PBS Learner (The Annenberg Channel) or the Spanish-language V-me network.

In some US states, state-wide educational, cultural or public-affairs services are carried by existing PBS member stations on digital subchannels; these include regional services such as the Minnesota Channel, Wisconsin Channel or New York State broadcaster ThinkBright TV. The use of subchannels has also allowed educational broadcasters to sell off former secondary PBS analogue stations (such as WNEQ in Buffalo to LIN TV to become CW affiliate WNLO) to commercial broadcasters, as the additional educational content these once provided can now be carried by multiple subchannels of the one main station. Subchannels also allow stations to devote an entire channel devoted to telecourses which are recorded by instructors and students for later use, freeing the original channel to air a more general schedule in the morning and overnight hours.

[edit] Commercial networks

In small markets, there is often an incomplete set of national networks available from local over-the-air sources. A small city with only one or two commercial stations would not be able to carry the full programming lineup from all of the four largest commercial networks (CBS, NBC, ABC, FOX) in analogue as one station could only broadcast one program at a time. This limitation could be overcome by adding the additional networks as digital subchannels of existing local commercial stations. One prime example of this is in the Wheeling, West Virginia/Steubenville, Ohio market, which for decades has had only two television stations (CBS affiliate WTRF-TV and NBC affiliate WTOV-TV, as well as cable-only CW affiliate WBWO) and had to mostly rely on stations in Pittsburgh (and to a lesser extent Columbus and Youngstown, Ohio) for other networks. However, with the advent of digital television, it has allowed WTRF to launch two digital channels (one for Fox with MyNetworkTV as a secondary affiliate, the other for ABC) while still airing the full CBS schedule on its main signal.

In many cases, these "new" channels are existing secondary channels which were carried by a low-power or Class A station or by a cable television channel. Often, a full-power TV station acquires or already owns a low-power secondary station in the same market to carry a second network. The use of a digital subchannel on a full power TV station as a replacement for LPTV greatly increases coverage areas for this programming.

With the launches of The CW and MyNetworkTV, many television stations have been launching subchannels affiliating with these secondary networks.

Other stations have launched separate independent stations on their DT-2 signals.

[edit] Sports programming

A common use for digital subchannels is for CBS affiliates to show all of the early round games of the NCAA Basketball Tournament on their digital channel, which is subdivided into four different subchannels.

[edit] Local and informational channels

Many local broadcasters are using subchannels to carry continuous news or local weather; secondary networks such as AccuWeather Channel or NBC Weather Plus have been created to serve this audience.

[edit] Speciality and niche programming

Channels dedicated to re-running nostalgia programming have been launched by broadcasters such as Newport Television (Variety TV) and Equity Broadcasting (RTN). Music video formats have also been tried as digital subchannels on terrestrial stations.

[edit] Temporary installations

A digital subchannel can be used to restore service from a station which has been knocked off-the-air due to an antenna tower collapse; the affected signal would be made available in standard-definition on a subchannel of another local broadcaster, most often a competitor. KATV (ABC, Little Rock) was forced to follow this path in 2008, moving its digital signal to a subchannel of local My Network TV affiliate KWBF after a tower collapse knocked its main signal off-air. The virtual channel numbering scheme allows an existing licensed broadcaster to keep its displayed channel number unchanged (KATV ABC 7 in this case) even if the signal is carried physically as a subchannel of some other local station.

[edit] Data, radio and non-broadcast signals

In rare cases, digital TV broadcasters have included the audio of a commonly-owned broadcast radio station among their subchannel offerings (which is the case of KCSM-TV in San Mateo, California. They put their radio station on their DT3 signal). Non-broadcast content, subscription TV channels or datacasting operations unrelated to the main TV programming are also permitted by the ATSC standard but are less commonly used.

[edit] Technical considerations

ATSC digital television supports multiple digital subchannels if the 19.4 Megabits-per-second (Mbit/s) bitstream is divided. Therefore, station managers could run any of the following scenarios using one 6MHz ATSC channel (note that the actual bitrate moves up and down, due to usage of variable bit rate encoding):

[edit] See also

• In-band on-channel (IBOC), digital radio technology allowing digital subchannels on FM stations
• DAB and DVB, international digital radio-television standards in use in Europe and Australia

[edit] References

• "How Digital Television Works". HowStuffWorks. Retrieved on 2007-01-29.