A DTT multiplex is a bundle of TV services that have been digitised, compressed and combined into a data-stream for transmission to the consumer over a single channel. The receiver separates each service from this compressed data-stream and turns it into a form which can be viewed.
Anything that can be digitised can be contained in a multiplex. This can include: sound, video, text, computer applications, electronic programme guide information, receiver upgrades and conditional access (descrambling).
The UK has eight national terrestrial multiplexes. There are three public service broadcaster (PSB) multiplexes - BBC A, BBC B and D3&4 - and five commercial multiplexes - SDN, ARQA, ARQB, COM7 and COM8.
Public service multiplexes provide coverage to 98.5% of households, while the main commercial multiplexes reach 90%. COM7 and COM8 are interim multiplexes and reach around 76% of homes.
For information on launching a multiplex, click here.
DTT is divided into regions enabling the BBC and ITV to broadcast relevant local programming to each region of England. Separate national programming is provided by the BBC, ITV, STV, UTV and S4/C to Scotland, Wales and Northern Ireland. For each version of BBC One, BBC Two, ITV and its national alternatives, a different version of the multiplexes containing these services is required.
Televisions and set-top boxes identify all of the different multiplexes they can receive and then present the available TV, radio and data services. When all eight national multiplexes can be received this is relatively straightforward. When two or more different versions of regional multiplexes are received from transmitting stations in different regions the viewer can choose which of the regional services they would like to watch.
The following table gives the transmission characteristics of the multiplexes on the platform.
|BBC A||D3&4||BBC B||SDN||ARQ A||ARQ B||COM7/8||RNI_1||Local||G-Man|
|Useful bitrate (Mbit/s)||24||24||40||27||27||27||40||9.8||9||18|
As well as video and audio for each service carried in the multiplex there is also some data which is carried on each multiplex in the region.
This data, called Service Information, comes in two parts:
The LCN is the number on the viewer's remote which relates to the service, therefore, BBC One can be found on channel 1 on the viewer's TV or set-top box. Without this information the TV or set-top box would not know what order to place the TV, radio and data services within the electronic programme guide or channel listing.
Whenever a change is made to a multiplex or a service this data is updated. Some TVs and set-top boxes automatically update themselves when a change occurs. Others require the viewer to perform a retune to update and take the change into account. An example would be when a new service launches.
This data updates several times a day, perhaps even several times an hour. TVs and set-top boxes are designed to deal with these updates without the need for a retune. If they weren't, watching TV would be almost impossible.
As well as basic information like the title and timing of the programme and which channel it can found on a broadcaster can choose to provide a wealth of other information such as whether the programme is also available in high definition or whether it forms part of a series.
TVs and set-top boxes can then offer the viewer the choice of what to do - perhaps to view the HD version or to set up a recording for the whole series. Whether or not these options are made available to the viewer will depend on the capability of the TV or set-top box itself. Freeview+ products will allow the viewer to make recordings. Freeview HD products allow high-definition viewing with Freeview+HD products allowing both.
Freeview Play TVs and set-top boxes connect to the internet to acquire enhanced metadata for channels and programmes including logos, images and additional information. Connected Freeview Play devices also present information for on-demand programmes, which be be accessed directly.