Coding and multiplexing
A multiplex, in simple terms, is an ensemble of television channels which are grouped together in order to fit efficiently into the channel bandwidth determined by the frequency plan.
In the UK, between 10 and 15 Standard Definition (SD) channels are grouped within each SD multiplex and around five High Definition (HD) channels in an HD multiplex.
There are currently seven national multiplexes in the UK with varying degrees of coverage (determined by the frequency plan):
- PSB 1: Operated by the BBC in SD carrying BBC TV and radio channels
- PSB 2: Operated by Digital 3&4 in SD and carrying a mix of ITV and Channel 4 channels
- PSB 3: Operated by the BBC in HD and carrying a mix of PSB HD channels
- COM 4: Operated by SDN in SD
- COM 5: Operated by Arqiva in SD
- COM 6: Operated by Arqiva in SD
- COM 7: Operated by Arqiva with mixed SD and HD services
How does a multiplex work?
In the UK DTT network a multiplex actually performs two functions: encoding and multiplexing. Hence this stage in the process is often known as “coding & multiplexing”.
Firstly, each channel to be broadcast must be fed to the multiplexer. Whether the channel is live (from studios or outside broadcasts) or from pre-recorded sources, the channel is fed from the broadcasters playout or broadcast centre to the multiplex – having first passed the broadcasters technical and quality controls. The multiplex can be co-located with the broadcast centre or remotely located.
When the channels reach the multiplex they are then compressed and multiplexed together, prior to being fed to the distribution network for onward transmission.
To broadcast 10 uncompressed video channels would take in the order of 2.7Gbps of data throughput, however the UK frequency plan only allows around 30-40Mbps of throughput depending on the transmission parameters used. So the information in these channels must be reduced by compression or coding at a ratio of around 150:1 in order for these to be broadcast on the UK DTT platform.
Compression techniques are continually being developed but we are currently operating a mix of MPEG-2 (H.263) and MPEG-4 (H.264) on the network.
Once compressed, the channels are combined together “multiplexed” into a signal format suitable for handing over to the distribution and transmission networks. The multiplexer is essentially a very large processing engine, typically using hardware based codecs and multiplexer technology for the most efficient operation. Again technology continues to develop and our technology experts continue to explore options for the next generation of coding and multiplexing.
We operate six out of the eight national UK multiplexes and work collaboratively with a variety of vendors to provide our customers with the most efficient and cost effective solutions to fit their needs.
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