Multiplex (TV)
A multiplex or mux (called virtual sub-channel in the United States and Canada, and bouquet in France) is the erroneous term given to the grouping of program services that are sub-grouped as interleaved data packets for broadcast over a network or modulated multiplexed medium, which are split out at the receiving end. There are two different types of groupings, which are closely related but not identical.
In the UK a terrestrial multiplex (usually abbreviated mux) has a fixed bandwidth of 8 MHz CODFM of interleaved H.222 packets containing a number of channels. In the US, a similar arrangement using 6 MHz 8VSB is often described as a channel with virtual sub-channels.
Pay TV multiplexes
Many pay TV services on cable television offer program service packages, in which a single provider offers a number of separate channels to its subscribers. The channels may be distinct, be timeshifted, or offer different views of the same event like overviews of a car race, the view from different drivers' cars, or a view of the pits. Some pay TV program services offer picture in picture (PIP) capabilities to follow the various feeds simultaneously on one screen. If the hardware allows, several channels in one multiplex can be viewed simultaneously.
Pay TV may be on a subscription or a pay-per-view basis.
Digital subchannels
Analog television channels, whether terrestrial, cable, or satellite, are transmitted as a single program service uncompressed at the same fixed bandwidth, which fills the entire bandwidth available. Digital television channels can be interleved and are in a highly compressed format, so that the bandwidth they require varies due to the bit-rate provided to each channel; it is more cost efficient to transmit several channels together so that they share the same bandwidth, each channel have carefully constrained bit-rates allocated, so that the sum of all channels fill the fixed bandwidth provided. A group of program services transmitted within a particular bandwidth allocation is erroneously known as a multiplex; or the channels may be called sub-channels. Sometimes, when analog transmissions are replaced by digital, the fixed bandwidth of one analog broadcast is allocated to the program services; the bandwidth of one analog broadcast is sufficient for several compressed channels.
A set top box or Integrated Digital Television is required to tune in, receive, and split a channel for viewing. A H.222 transport can contain (depending on the bit-rate available) half a dozen TV channels yet only uses the same space of one analog broadcast.
Any program services, not necessarily from the same broadcaster, can be interleaved on the same transport. Programming from a commercial broadcaster that would not otherwise be available in the station's broadcast area can be transmitted.
Digital television transports 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 transport may carry conventional TV channels, radio, teletext, and sometimes hidden channels carrying metadata or interactive services.
US
In the US the standard for over-the-air digital transmissions is ATSC, digital cable is based on the worldwide DVB standard, DVB-C (C for cable) and transmission via satellite are based on the DVB-S (S for satellite) standard, all use multiplexes to deliver various channels to the viewer. Smaller and newer commercial networks, such as The CW and MyNetworkTV, are available in some markets as digital sub-channels of other network affiliates rather than as standalone stations. A transport can also carry radio and interactive TV content.
The ATSC standard and its early adoption of HDTV is the best quality available of HDTV programming since the cable and satellite providers in the US use heavy compression to fit as many channels into their limited number of transports as possible. ATSC and free-to-air satellite TV is free of charge, while digital cable and direct broadcast satellite TV do not offer any free content. A typical American ATSC transport offers 3 to 4 channels, in most cases one of them is broadcast in HDTV (main TV network channel) and the rest of the channels are broadcast in SDTV, due to the limited 6 MHz 8VSB bandwidth used.
Europe
In Europe transports are used on CODFM 64-QAM/256-QAM modulated DVB-T and DVB-T2 (T for Terrestrial) digital terrestrial standards, on CODFM 256-QAM modulated DVB-C digital cable and on CODFM QPSK/8PSK modulated DVB-S digital satellite. Publicly and privately owned TV broadcasters use interleaving to broadcast many digital channels over a few transports using the various digital broadcast standards. In Europe a typical DVB-T transport offers 4 or more SDTV channels transmitted simultaneously; if some channels transmit only for part of the day (e.g., a children's channel during daytime, a channel with programs for adults in the evening), many channels may share the same transport.