Digital Subchannels
Analog television channels, whether terrestrial, cable, or satellite, are transmitted uncompressed, and always require the same bandwidth, so there is no reason not to transmit each one separately. Digital television stations are transmitted in a compressed format, so that the bandwidth they require varies from instant to instant; it is more efficient to transmit several channels together so that they share the same bandwidth, each channel using the instantaneous bandwidth it needs, with channels that are currently not transmitting giving up their bandwidth to those that are. A group of channels transmitted within a particular bandwidth allocation is known as a multiplex; or the channels may be called subchannels. Sometimes, when analog transmissions are replaced by digital, the fixed bandwidth of one analog station is allocated to a multiplex; the bandwidth of one analog station is sufficient for several compressed channels.
A set top box or Integrated Digital Television is required to tune in, receive, and demultiplex a channel for viewing. A multiplex can contain half a dozen TV channels yet only uses the same space of one analog channel.
Any programs, not necessarily from the same network, can be multiplexed. Programming from a commercial network that would not otherwise be available in the station's broadcast area can be transmitted.
Digital television multiplexes vary in the number of channels that can be transmitted, based on the bandwidth of the multiplex and the broadcast quality specified for each channel. Digital terrestrial offers the least, digital cable and satellite the most bandwidth.
A single multiplex may carry conventional TV channels, radio, teletext, and sometimes hidden channels carrying data.
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