All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Ryan: A well-designed communal system shouldn't allow signals injected downstream of the split point to affect your reception. It's likely that it's coincidental, and reflects a problem with the system as a whole.
Digital UK rate the chances of reliable reception on Mux 2 as about the same as Mux 1 from both Rowridge (same probability, 82%) and Hannington (99% for Mux 1, 98% for Mux 2).
You can see a few suggested channels below the list of transmitters under 'My Settings' in the top-right corner of the page.
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Dave Clarke: You could be using any of three different transmitters: Nottingham, Sutton Coldfield or Waltham. Nottingham requires a vertically-polarized aerial (elements going up-and-down) while Waltham is horizontally polarized (rods going side-to-side). Both are to the south-east (121° and 123° respectively). Sutton Coldfield is horizontal and to the south-west.
If the aerial is oriented for Waltham, it's possible that your box has still tuned in Nottingham PSB multiplexes as the frequencies are lower. Check the frequencies match those at the Nottingham page and if not, delete the channels that are wrong and manually tune in the right ones. See also Digital Region Overlap.
Digital UK's predictor shows average results for Mux A from Waltham at the moment and poor for Mux 2. This will improve at switchover in August.
Or, if oriented for Nottingham, it may not be compatible with the 8K mode used after switchover - check the list of unsupported equipment at http://www.digitaluk.co.uk/2kequipment. In this case, it may be tuning into Waltham instead.
If the aerial is not the right way round for the transmitter you're tuned into, the signal level received is much weaker, and this could explain why it breaks up.
If it's oriented for Sutton Coldfield, you're actually not expected to get reliable results from that transmitter until September.
If oriented for Nottingham and the frequencies are correct, try removing the booster - the level could be too high, although I don't think this is very likely as the level in your direction is 7 dB below maximum and it's not that powerful to start with.
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Stuart: The problem is that the final channel allocations for Wenvoe COM multiplexes clash with the allocations for the PSB multiplexes at Trefechan. The change to the final channels for SDN and ArqA multiplexes happened on the 27th of April. The ArqB multiplex moved at switchover at the end of March 2010.
You could see how well it works without the amplifiers first. The COM multiplexes are now in adjacent channels to the PSB multiplexes, and if the signal levels are too high it causes intermodulation, which causes errors in other channels but the worst errors occur in adjacent channels.
Otherwise, there's really nothing you can do without spending a lot of money on an antenna array that rejects more signal from Trefechan, or getting a satellite subscription. Freesat doesn't carry many of the channels carried by the Freeview COM multiplexes.
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Scott, Ian: STV provide a 'West' (Glasgow) and 'East' (Edinburgh) service on satellite. These match the advertising sales macro-regions. On Freeview, the 'West' advertising micro-region transmits from Black Hill, Darvel and Torosay while 'East' transmits from Craigkelly. STV also transmit different versions of News at Six for Glasgow and Edinburgh (according to STV Player).
I don't know whether Craigkelly transmits a different region ID, though.
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Grant Franklin: To the south of Emley Moor, you can fall into the overlap area of Emley Moor and Sutton Coldfield. This is a problem at present as each uses the other's analogue channels for low-power digital.
It's why they're switching over on the same day. At switchover, high-power digital signals at each transmitter replace the analogue channels, so they no longer overlap, but if you did either one first you'd wreck low-power digital at the other.
Digital UK predicts a low chance of reliable or even 50% reliable results on Multiplex 1 at the moment (Sutton Coldfield uses this channel for Mux C). It shows no prediction for Mux 2, A, B and C which are the channels clashing with Sutton Coldfield analogue services.
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Andrew: The early HD broadcasts really had to be crammed in, so the radiation pattern is restricted. Based on the position of the transmitting aerials - half-way down the south face of the Lichfield mast - the intent was to serve Birmingham only.
The full service available after switchover is expected to be transmitted in all directions, and you should be able to get it then.
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Colin Rees: Briantist is wrong; while the amount of signal received from directly behind is not zero, it is far less than you would receive if the aerial was pointing at that transmitter. On most aerials the level from directly behind is no greater than that received from either side.
Aerial Polar Reponse Diagrams
On those examples, the lobe to the rear is -24 dB or 1/250th of the power (0.4%).
Digital UK's predictor only allows for a maximum of 16 dB of front/back ratio, 1/40th or 2.5%. Its predictions can be a bit pessimistic as a result.
In terms of incorrect polarization, again, some signal from the wrong polarization is picked up. I don't have figures handy, again DUK's model allows for 16dB and I would expect real aerials to better that.
When signals reflect off buildings and terrain, some changes in polarization do occur, so the actual amount received could be larger than the specifications of the aerial would lead you to believe.
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Briantist: Ofcom have today published a new version of the Central DSO Details document, which now says ArqA and ArqB are at 50W, not 25.
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David: effective power level for Mux 2 and A is always 'low'. It's a consequence of the fact that they never changed mode after ITV Digital failed in 2002. Mux 1, B, C and D changed to a mode roughly equivalent to increasing the power by 2-2.5x, without causing any increase in interference to analogue transmissions (primarily those from Emley Moor). The cost was about 25% of the capacity on each multiplex.
The bars shown against each multiplex indicate the normal situation. When there is engineering work, Digital UK only report 'possible weak signal' and don't tell us exactly when and by how much the signal is to be reduced.
At switchover, power levels are 25 times higher and there is no longer a clash with Emley Moor. All SD multiplexes will use the same mode as Mux 2 and A do now.
The BBC did recommend that all six multiplexes changed mode, but ITV and Channel 4, who run Mux 2/D3&4, and S4C, who then ran Mux A, were not required to do so and decided to maintain the higher capacity at the cost of coverage. (ITV Digital had only operated Mux B, C and D. Freeview Ltd doesn't operate anything, it's a PR organization only.).
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Monday 23 May 2011 1:18PM
Hugh Froggatt: At this time of year, 7pm is a bit early in the day for it to be down to tropospheric enhancement (see Tropospheric DX Modes for the meaning of this term). I'd look for sources of impulse interference, such as thermostats switching on and off. When is your central heating or hot water controller set to turn on? Try adjusting the temperature on the thermostat to see if clicking it on or off affects the TV reception.
If it does, there are two things you can do. First, replace the thermostat to reduce the amount of electrical noise it produces when it switches. Second, replace the cabling to the aerial with 'satellite-grade' cable. This has much better screening than the 'low-loss coax' typically used for household aerial cabling before digital services started.
The channels used by the Chalford transmitter are also used by the Ridge Hill transmitter, which is beyond a few hills. In the right weather conditions - causing enhancement or ducting - they could travel far enough to interfere. In ducting conditions you might also get some interference from Sandy Heath, and particularly Rowridge once it starts high-power transmissions next March. Digital UK show a drop in probability for reliable reception from 86% now to 28% next March.
Digital UK's predictor reckons you'd have a higher probability of reliable service from Oxford once it switches over in September.
If you had Freeview before switchover, you could be using Mendip. DUK are showing this as poor for the PSB multiplexes at present, with the COM muxes as good but going poor in March. It's not clear to me why that should be, as you appear to have line-of-sight to Mendip.