Digital Switchover - UTV Region (DigitalUK)

At the RRC-06 conference in Geneva, Divis was allocated frequencies E21, E23, E24, E26, E27, E29, E31 and E33 with an ERP of 100kW while Brougher Mountain was allocated E21, E22, E24, E25, E27, E28, E31 and E32 with an ERP of 20kW.

The transmission configuration for the BBC A & D3&4 multiplexes (as well as the BBC B mux for HD) means that the set Guard Interval of 1/32 (1/128 for BBC leaves only an 8.4km distance before the same signal starts to interfere, wherever it is another transmitter as part of a single frequency network or the same signal from the wanted transmitter being reflected off a distant object, with such reception causing quality loss to the point of it being completely corrupted and unusable.

The decision was taken that at Brougher Mountain and Divis to reuse analogue allocations for the PSB multiplexes, hence E21, E24 and E27 from Divis and E22, E25 and E28 from Brougher Mountain. For the SDN and Arqiva multiplexes at Divis, E23, E26 and E29 were available and are run at 50kW ERP - this is normal for these multiplexes to run at half the power of the PSB multiplexes as done so by the operators. This is also the case from the Limavady transmitter. There are a few exceptions where these three commercial multiplexes are run at the same power as the PSBs - Crystal Palace in London being one. Brougher Mountain however is one of the very small exceptions at the other end with the SDN and Arqiva multiplexes being broadcast with less than 1/2 the power of the PSB multiplexes.

The simple reason for this power discrepancy is that as the PSB frequencies are those which are deemed to be the freest from co-channel interference, interference to Divis' PSB multiplex transmissions from Brougher Mountain must be limited. Hence the compromise of 2kW for the Arqiva and SDN multiplexes.

The power difference between these three multiplexes and the PSB ones being ten times higher should not normally cause a problem of the PSB multiplexes causing adjacent channel interference unless the reception of the PSB multiplexes is unusually strong e.g. using a Blake DMX16A with a powerful masthead amp around 5km from Brougher with a clear LOS. What the SDN and Arqiva multiplexes are more sensitive to is that they need a higher carrier-to-noise or signal-to-noise value compared to the PSB multiplexes as they use a higher FEC value. Depending on how the C/N or S/N value is measured, this demands about 1.5 to 2.3db higher value.

OTOH the NImm multiplex on E30 from Brougher Mountain, while only has an ERP of 1kW has a transmission configuration requiring a significantly lower C/N or S/N ratio than all the other multiplexes. In the absence of co-channel interference from the likes of Holywell Hill and Three Rock, it's coverage is in my experience so far matching that of the three PSB multiplexes.

Could anything have been done differently? Well they could have run the SDN and Arqiva multiplexes from Brougher with 10kW, but this would have started to significantly impact on PSB reception from Divis in many parts of Northern Ireland and also some parts of the Republic. In particular parts of Eastern Co.Tyrone, Northern and Western Co. Armagh, a few parts of South Co. Derry, a few parts of Southern Co. Fermanagh (where normally reception from Divis is better than that from Brougher, like Roslea), a good bit of Co. Monaghan and maybe even bits of Co. Cavan and Co. Meath.

The next option was to put the PSB multiplexes on E21, E24 and E27 (the same as Divis) with the SDN and Arqiva multiplexes on E22, E25 and E28. It would have been in terms of regulation difficult to have the PSB muxs operate on less power than the commercial muxs to start with. Secondly this set up could only really work as an SFN to ensure that anyone receiving a PSB mux from both Brougher and Divis had a reduced chance of their reception being corrupted. But this would have been severely hampered and compromised - first the guard interval would have needed to be significantly increased to 1/4 and with it, a > 4MB/s decrease in available data rate multiplex (meaning something would have to be sacrificed) for the BBC A and D3&4 muxs. Also reducing the guard interval to 1/4 would only increase the distance limit to 67.2km, meaning that if there was a distance difference of more than 67.2km at the location of reception between the two transmission sites and the two signals were still strong enough together, a picture blackout would still take place. The LOS distance between Divis and Brougher is nearly 96km, meaning that even this gap would be impossible to bridge - not to mention that large parts of northern and western Fermanagh would be affected as many aerials there aimed towards Brougher would also have Divis within the beamwidth of the receiving aerial, or in say Enniskillen where one of the major sidelobes of a yagi aerial aimed at Brougher would 'see' Divis.

Next would be using alternative frequencies for the SDN and Arqiva multiplexes at Brougher. E23, E26 and E29 was actually thought about, but then rejected. According to a report by Arqiva, a 10kW ERP of these multiplexes would have been needed to reach Omagh - why this was I don't know. Maybe they were taking into account possible co-channel interference from Holywell Hill (who have allocations on these frequencies not currently in use, but they were for previous analogue transmissions)? As these frequencies are in use from Divis for the same multiplexes at 50kW rather than 100kW, a 4-5kW ERP might have been possible without affecting the core service area of these multiplexes from Divis - fringe reception might have been affected, but it is taken by the multiplex operators that their broadcasts are slightly more liable to interference. In the end, this hasn't happened. Another alternative would have been to look for other possible allocations in the UHF band - this would have resulted in nearly all with TV aerials aimed at Brougher Mountain having to use either Group K or wideband aerials. It would have been unpopular as it would have made it likely more difficult to receive cross-border Saorview transmissions without using either frequency shifters or manual switches. Traditionally all UHF TV broadcasts at Brougher have been in Group A frequencies including DTT multiplexes pre-DSO.

The last option I could think of would be the "Rowridge" option. With all physical transmission infrastructure now in place this would be impractical for the amount of people served for this to now be implemented. Rowridge is a main transmitter site on the Isle of Wight that serves a significant area of the south coast of England - the main problem being that co-channel interference from France is a common occurrence. The solution? Introduce vertically polarised transmissions alongside horizontally polarised transmissions. On both polarisations the PSB muxs are 200kW ERP each, but for the SDN and Arqiva multiplexes, they are 50kW ERP horiz and 200kW vertical. This encourages any new aerial installs to use vertical polarisation not only to help equalise all multiplex signal strengths but also help reject co-channel interference of the SDN and Arqiva multiplexes - not only from French transmitters but also from Crystal Palace and Stockland Hill, two near-neighbouring transmitter sites hence another reason why the SDN and Arqiva muxs horizontal ERP is limited to 50kW. In the contect of Brougher Mountain, this could have meant that the SDN and Arqiva multiplex there could, alongside the PSB multiplexes, transmitted with both horizontal and vertical polarisation with the horizontal ERP for SDN and ARQ A and ARQ B limited to 2kW but the vertical component of the signal having a higher ERP, for example 10kW - enough when using the cross-polar rejection of aerials not to have major effects on the PSB muxs from Divis and vice versa. However the additional expense of having such a transmission system set up at Brougher was not likely to be cost effective especially for SDN and Arqiva - Brougher only serves (within its designated service area) about 36,000 homes - this is actually slightly less than the Derry relay on Sherrif's Mountain and a lot less than the 620,000 homes served by Rowridge. Incidentally Divis serves ~440,000 homes and Limavady about 45,000 homes.

As you can see, with UHF broadcast spectrum being squeezed for space, compromises have to take place somewhere.