What is a multiplex
For TV, in the past we broadcast BBC One and BBC Two from each transmitter site on separate frequencies and you had to tune your TV into to each individual frequency to receive each service. In the digital world, we now broadcast a multiplex of services (BBC One, BBC Two, CBBC, CBeebies, BBC Four etc) on an individual frequency from each transmitter site and your TV or set top box extracts all the services from the multiplex and allows you to access them via the EPG. This is the same for satellite and cable.
This is also true for radio. FM and AM radio services are broadcast from each transmitter site on separate frequencies. So taking FM radio as an example, at a transmitter each radio services (Radio 1, Radio 2 etc) is broadcast on individual frequencies. In the DAB world we now broadcast a multiplex of radio services using a single frequency and your DAB radio extracts all the services from the multiplex for you to listen too.
This allows a number of multiplexes to be transmitted from each transmitter site or satellite, allowing a wider range of stations to be broadcast.
For Freeview, the BBC runs 2 multiplexes (PSB1 and PSB3) and has a small number of services available on commercial multiplex (COM7). For Radio, the BBC has one single UK wide DAB multiplex for its network services. National and local radio services are carried on commercially run multiplexes. Click below image for our full size multiplex broadcast chain.
Why do we use multiplexes?
Because digital coding is much more efficient than analogue transmission, it is possible to fit several stations into each block of frequencies allocated to a single analogue TV or radio station. Grouping several stations together allows broadcasters to fill these spectrum allocations to the maximum extent, giving you more stations to choose from.
In addition, digital transmission and reception requires less power than analogue, so it is more cost-effective and environmentally friendly.
Using multiplexes also means that temporary services can be introduced very easily, by adding an extra datastream within the multiplex. This happens especially with DAB digital radio. This would be very difficult with analogue as an extra transmission would be required from each transmitter site, per service. In addition, software updates for receivers can be sent as ‘piggy-backs’ on existing multiplexes.
Finally, multiplexes also allow for introduction of new services (eg Red Button +) without the need for new transmitters. All that is needed is some reshuffling of stations within the multiplex; sometimes receivers also need to be re-tuned to accept the new services. This increased flexibility is of enormous value to broadcasters and audience alike.
How does this affect reception?
If you have poor reception of one multiplex, all of the stations bundled within it will be affected equally. If reception is better on one multiplex than another, the group of stations within it may be received well. If you find only one station is suffering poor reception but all others are unaffected, the problem will not be due to poor reception, but rather a fault with the decoding of that single station within your receiver. In other words, a receiver problem rather than a reception problem.
The way digital signals are processed means poor reception usually results in pixellation (break-up of pictures into small squares) and pops, cracks or warbling on TV or radio sound. All stations on the affected multiplex will show a similar effect.
If you are contacting the BBC about a reception problem, please check first whether it affects only one station, or only one multiplex, or all multiplexes. You can find out which stations are on which multiplex by clicking one of the links below.
How does digital transmission work?
Digital TV and radio work by turning the signals from the microphone or TV camera etc. into a stream of digital data which makes up part of a multiplex. Once this reaches your receiver, it is decoded to recover the original sound and pictures.
In the BBC, digital datastreams for each network are created at our studio centres, but before being transmitted they go to a central location where they are combined into broader streams, to which are added extra data. Some of this data is needed by your receiver, such as programme information, now and next, schedule information and Red Button. Other added data tells the transmission chain how to handle the digital signals, including how to relate them to what other broadcasters are sending.
These broad datastreams are distributed to land-based transmitters for Freeview, DAB and via uplink stations for satellite. At the transmission point the individual digital multiplexes are created, and sent as signals to be received and decoded by your TV set or satellite receiver. As well as the data for the broadcasts themselves, error correction* is added to make reception more robust. Cable TV systems have their own multiplexing and distribution systems which are similar to those of Freeview and satellite.
Cable TV systems have their own multiplexing and distribution systems which are similar to those of Freeview, DAB and satellite.
* Error correction is data added to the multiplex to make reception more robust when signals are weak, or there is interference. It’s a vital part of the system of digital transmission and reception.
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