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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 digital switchover, 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.
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.
Yea will do , am going to try it, will be later in the week as I don't have a laptop with a com port, and will be getting a local pc shop to do the transfer for me, rather than using a usb to rs232 adaptor.
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.