Multiplex (television)

A multiplex or mux, also known as a bouquet, is a grouping of program services as interleaved data packets for broadcast over a network or modulated multiplexed medium, particularly terrestrial broadcasting. The program services are broadcast as part of one transmission and split out at the receiving end.

The conversion from analog to digital television made it possible to transmit more than one video service, in addition to audio and data, within a fixed space previously used to transmit one analog TV service (varying between six and eight megahertz depending on the system used and bandplan). The capacity of a multiplex depends on several factors, including the video resolution and broadcast quality, compression method, bitrate permitted by the transmission standard, and allocated bandwidth; statistical time-division multiplexing is often used to dynamically allocate bandwidth in accordance with the needs of each individual service. Each service in a multiplex has a separate virtual channel (also known as a logical channel number) for identification and tuning. Depending on the country, a multiplex may be controlled by one broadcaster offering multiple subchannels or may feature services from multiple broadcasters with separate licenses.

Multiplexing has made it possible for many new free-to-air television services to be introduced, some of them expressly designed for carriage as additional channels. In the United States, such services are called digital multicast television networks or diginets. The term is also used in an otherwise unrelated sense to refer to additional channels offered by premium pay television services, such as HBO, similar to its meaning with regard to movie theaters.

Tuning
Depending on the type of transmission system, individual services are either numbered with whole numbers (e.g. 36) or a two-part channel number (e.g. 20.1) consisting of a major and minor channel number.

New services
The digital conversion in countries where broadcasters retained control of their entire multiplex after switchover permitted broadcasters to introduce new supplemental and ancillary services, many of them national in scope. In Australia, Mexico, and the United States, new TV channels were introduced with national coverage.

The licensing of such additional services varies according to national broadcasting regulations. In the United States, a broadcast license covers the full 6 MHz channel and any services broadcast within it. The United Kingdom frequency plan includes three "universal" multiplexes for the national public service broadcasters and three commercial multiplexes broadcast from a total of 80 transmitter sites.

The ISDB-T specification includes 1seg, a mobile media and data broadcasting service utilizing a portion of the spectrum in each multiplex.

Brazil
In 2009, the Brazilian government ruled that only federally-owned television channels—namely TV Brasil, TV Senado, TV Câmara, TV Justiça, and TV Globo—could offer multiple channels of programming. The decision was made, per an advisor to the Brazilian communications ministry, to prevent the leasing of channels to broadcast infomercials and church services. The effect of the decision was to bar new entrants without their own stations from increasing commercial competition. During the COVID-19 pandemic in Brazil, educational and commercial broadcasters were allowed to introduce subchannels, primarily to carry educational programming, which was extended first to April 2022 and then December 2023.

Canada
The Canadian Radio-television and Telecommunications Commission (CRTC) requires stations to file licence amendments in order to be considered for permission to carry digital subchannels. On August 17, 2012, the CRTC gave approval to Leamington, Ontario, community station CFTV-TV to broadcast four local subchannels on its digital signal, making it the first station in Canada to launch original content on its multiplex channels.

Mexico
In 2015, the Federal Telecommunications Institute (IFT) formally authorized broadcasters to apply for permission to add subchannels.

In 2016, the IFT began assigning virtual channels to stations based on their programming, grouping transmitters of the same national network.

Spain
In 2010, multiplex licensees in Spain were permitted to add two new channels to their services. The Supreme Court of Spain ruled in December 2012 to void this action, stating that the move was illegal as it did not award the channels by way of a public bidding process. As a result, a total of nine channels closed down on May 6, 2014.

United States
Commercial and non-commercial broadcasters began experimenting with additional subchannels over the course of the 2000s. After the digital television transition in 2009, a new crop of national services, known as digital multicast television networks or diginets, began to emerge, taking advantage of the additional capacity available on many converted stations. Diginets affiliate with individual stations in each market and are generally genre-specific in their programming. Diginets have continued to grow as more advanced encoders enable stations to add additional, income-generating ancillary services. Subchannels have also been used in smaller "short markets" with few full-power stations to provide in-market affiliates of additional major networks; by 2011, Fox had 35 such subchannel affiliates.

Multiplexing is also used in ATSC 3.0 (NextGen TV) rollout plans. To allow multiple stations to begin 3.0 broadcasting without loss of ATSC 3.0 service, one station will typically become a "lighthouse", airing the main channels of several participating stations, while the lighthouse station's channels are broadcast in ATSC 1.0 format on the multiplexes of the other participating stations.