The engineers who designed the services created digital broadcasting systems that would allow the reliable distribution of bits using the same infrastructure of cables and masts (or satellites) that already carried the analogue services to millions of homes.
Part of the design process allowed for more than one television channel to be carried by the digital system using a long-established data-processing technique called "multiplexing", a word that had, by another route, also come to mean a cinema complex where several films are shown at once on different screens.
It was back in 1996 that Parliament decided that the UK should have a six-multiplex service.
The current coverage figures range from around 90 per cent of the UK population for the largest of the multiplexes down to around 70 per cent for the smallest.
Of these six multiplexes, two are wholly reserved for existing broadcasters who are guaranteed places on the multiplexes under the Broadcasting Act 1996. The first will be used by the BBC to transmit existing programmes in digital form and develop new digital services. The second is reserved for Channel 3, Channel 4 and Teletext Ltd whose existing services will be reproduced in digital form, and who can use the additional capacity available for new services. This multiplex will be licensed and regulated by the ITC.
Licences to operate the remaining four were advertised and three have now been awarded to British Digital Broadcasting plc (BDB). The fourth multiplex, the third largest in terms of UK coverage, has been awarded to S4C Digital Networks Ltd (SDN). It will carry the new Channel 5, S4C in Wales, and a certain amount of Gaelic programming during peak hours in Scotland.
The ITC named the "wholly reserved" multiplexes 1 and 2, and the others, by the level of coverage: A, B, C and D.
British Digital Broadcasting plc eventually launched their service as ONdigital on 15th November 1998, re-launched as ITV Digital on 11th July 2001 and went out of business on 1st May 2002.
ITVDigital handed back B, C and Dmultiplex licences to the regulator, who awarded them to the promise of a free-to-air service the BBC and Crown Castle International, a company formed by the privatization of the BBC's transmitternetwork.
When Freeview started on 30th October 2002, the BBC realised that the existing broadcasting "mode" used for the digitalservices was causing problems for some viewers. For this reason, some multiplexes changed from "mode 1" to "mode 2" (see the diagram at the end of the article), which provided a more stable signal, but with the loss of some channel capacity. See Freeview modes - a simplified explanation for a longer discussion about this.
This was not a problem for the BBC with six full-time television channels to fit onto two whole multiplexes. So, multiplexes 1, B, C and D changed to the robust mode, but 2 and A did not.
Ofcom plans PSBs and COMs
During the planning for the digitalswitchover, where the existing analoguechannels are turned off over several years, Ofcom, now the regulator used the names PSB1, PSB2, COM4, PSB3, COM5 and COM6 to refer to the six multiplexes.
The PSB multiplexes have almost total coverage of UK homes - 98.5%, and are broadcast from all television masts in the UK, including 1,036 smaller ones that only carry the PSB services.
COM is an abbreviation of "commercial", referring to the two multiplexes held by Arqiva, the company that owns and operates all of the UK television transmitters, and SDN, a company owned by ITV plc.
The COM multiplexes have coverage of around 90%, and are broadcast from the main 81 transmitters.
Because this planning required that all multiplexes move to "mode 3" the post-switchover multiplexes these new descriptions have been widely used to represent the "final" multiplexes, even though legally they are named 1, 2, A, B, C and D - see Ofcom - Multiplex licensees.
Digital UK now refers to the multiplex by the name of the ower, usually four letters, but sometimes the written in full, so we now have:
Multiplex 1 is also known as PSB1 and also BBCA
BBCA denotes the first BBC multiplex when in "mode 3", with all the services from "mode 2" multiplexes 1 and B. This multiplex has many regional versions to supply BBC regional news on BBC One.
Multiplex 2 is also known as PSB2 and also D3+4
The legal owner is "Digital 3 and 4 Ltd", which is half-owned by the Channel 3 licensee, and half-owned by the channels/channel/C4">Channel 4 Corporation. This multiplex has more than 20 regional versions to supply regional news on ITV 1/STV/UTV, plus advert regions for that station, Channel 4, Channel 5 and the channels 3 and 4 "plus one" services.
Multiplex A is also known as COM4 and also SDN
SDN, originally "S4CDigital Networks" was sold to ITV plc in April 2005 for £134m. The SDN multiplex has two versions, one for Wales, where it carries E4 and one for the rest of the UK.
The above diagram illustrates the capacity of different ONdigital/ITVdigital/Freeview transmissions over the years. The longer the bar, the more bits are transmitted per second. Mode 2 is 16QAM, mode 7 is QSPK, the DVB-T2 modes are 256QAM, the rest 64QAM.
lee: Good news then! although I would have liked the serial number associated with your model of box so that I could tag it as being from a serial number onwards that the update works on, as subtle changes to a devices circuitry always takes place during long batch runs, and with this in many cases being why an update will work on some models but not others even although the model to all intents is the same.
Ian: If your CCTV system is of the type that sends the image picked up by the camera to the TV via its aerial input socket? then this signal is from the control units "analogue" RF modulator, therefore in order to view the signal on your TV you will have to set the TV's "input select" button (on remote control) to "analogue", followed by carrying out a scan to pick up the signal sent out by the control units modulator, which its assumed is on C50 on your device, that is unless the analogue modulator offers the choice of more than one outputchannel.
Many CCTV devices do to avoid the possibly of the signal clashing with a channel used for Freeviewreception, although C50 is OK in your particular case "if" you are receiving Freeview from Blackhill.
However, if on the other hand your CCTV device is of the "movement sensor" variety that couples into your TV via a scart lead connected into the TV's AV1 input? then that said with regards to analogue does not apply, in other words you do not require to alter anything on the TV, because as soon as movement is detected the control unit sends a signal (via scart lead) into the TV that switches it over to the CCTV device no matter what programme you happen to be viewing.