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Virtual ChannelIn telecommunications, a virtual channel is a channel designation which differs from the actual radio channel or frequency which the signal travels. This is most often applied to television, where digital TV channels are in-band adjacent-channel to analog ones. Channel 8 might use channel 32 for its ATSC or DVB transmission, but a virtual channel map or virtual channel table (VCT) allows viewers to tune to channel 8 on a digital set anyhow. DTV tuners typically scan all channels for this information while they are turned "off", and store it in memory for instant access later on. Because DTV can carry multiple programs simultaneously, virtual channels also map out subchannels. Typically, the main programme is on channel 8.1 (or 8-1), in the example for channel 8. Other programmes are on 8.2, 8.3, and so forth up to 99, with data channels from 100 to 999. In this case, 8 is the "major channel", and 1, 2, or 3 is the subchannel. There is no practical difference between a dot and a dash, though the dot looks familiar to FM listeners, and avoids confusion with a range (e.g. 2-4 is 2.4, not 2 to 4). In the ATSC standard, the virtual channel map is part of the Program and System Information Protocol (PSIP). In 2005, the US FCC has begun requiring that major channel numbers of all DTV stations be mapped with PSIP to the old numbers by February 1. Old analog channels will continue to be referred to as subchannel 0 (i.e. 8.0) when tuning, until they are no longer transmitting. Virtual channels are also used on direct broadcast satellites, such as DISH Network, DirecTV, and Astra. Rather than a few dozen channels with a few subchannels each, these services map to a range of hundreds of individually-numbered channels. Cable TV also does this on digital cable, as does satellite radio. Digital radio also uses channels and subchannels, but only for DAB. The IBOC systems (iBiquity and Digital Radio Mondiale) do not currently use any virtual channels, mainly because they are lucky to fit anything digital at all on an analog carrier. As IBOC progresses and bandwidth improves due to new technology and the discontinuation of stereo subcarriers (and later analog althogether), virtual channels may take hold there as well.
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