All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Bill Kett: The prediction for your postcode isn't great at the moment. It's possible that the level of interference has been steadily climbing as the switchover programme has progressed, both here and abroad, and is now too high for reliable reception.
Still, I'd normally expect break-up rather than complete failure. Check that the aerial is pointing in the correct direction and that all cables are still intact.
You say that the aerial is new. How new? Was there a guarantee? I'd recommend calling out the installer and getting them to remedy it.
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David Jackson: Presumably this was a diplexer, combining a Group A aerial pointing at Crystal Palace with a Group B or E pointing to Sudbury? There's no problem diplexing aerials together for digital services. Nothing in the chain is specifically 'digital' or 'analogue' until you get down to the receiver, and usually only the decoder stage rather than the front-end tuner.
Some boxes do get confused with a mixture of 2K and 8K mode transmissions, in the interim period between Sudbury and Crystal Palace switchovers, and it can be difficult to get some boxes to store the services you want at the preferred numbers. Most would likely store Crystal Palace services at the published channel numbers as they are on the lowest frequencies.
CP may be stronger now, but Sudbury shows a slightly higher probability of reliable reception once the switchover programme is completed. Both are ultimately good, though (except Crystal Palace Mux C/Arqiva A drops to 'variable' in 2013, possibly due to changes overseas as the UK programme should be complete by the end of 2012.)
However, you have to wait almost another year for Crystal Palace to switch over, while Sudbury starts in a couple of weeks. That said, only the PSB multiplexes change to final channels and power levels. The COM multiplexes (A/SDN, C/Arqiva A and D/Arqiva B) stay at low power from Sudbury until after Dover switches over in a year's time.
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pappyon: Usually this has something to do with the length of cable between the two boxes and the aerial.
If the aerial splitter is near the aerial itself, and the working box is on the shorter cable, signal levels may be slightly too low at the non-working box. Add a small amount of additional amplification or replace the aerial with a slightly higher-gain one, of the appropriate type. Warning: if your transmitter uses Group A, and you have a grouped aerial now, a wideband with a high quoted gain may turn out to have less gain in the place you need it.
If the working box is on the longer cable run, signal levels are probably slightly too high.
If you're splitting at the back of the living room recorder, the added drop in the cable run to the bedroom is probably too much. Try to split the cable as close to the aerial as possible. Again, an amplified splitter can result in too *much* signal - ensure that amplification only just offsets losses in the splitter. Also, amplification adds extra noise, which can be a problem if the signal-to-noise ratio is marginal.
Make sure you're using proper splitter devices, not just twisting cables together.
Ensure all cables are satellite-grade, double screened, copper braid over copper foil screen, and ensure that if you're building them yourself, that you're fitting the connectors properly.
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Ron Henshaw: Can you provide a full postcode so we can see the prediction?
You should not need boosters on any TV. It is always better to amplify as close to the aerial as possible if it is required, but unless you're in a particularly difficult location it shouldn't be. You should try to maximize aerial gain before adding amplification.
If you are splitting the signal, and some amplification is required to ensure that each set has enough signal after the split, just add enough to offset the insertion loss of the splitter and the losses in the cables.
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G Davies: You should be able to see the Neath Abbey relay out of the window - the centre of the grid square for the centre of your postcode is only 200 metres away from the centre of the grid square containing the transmitter. It's the phone tower behind the school.
Neath Abbey transmits on both horizontal and vertical polarization. The transmitter is due east of you, and transmits more signal on horizontal polarization in that direction. Make sure the aerial elements go left-to-right rather than up-and-down.
As you're so close, you could end up with excessive signal levels even though you're using a set-top aerial in the loft. These typically have little useful gain, but the theoretical received power level is about 10 times the recommended limit, roughly the gain you get from a good aerial. If you have any amplification in the system, such as a booster, try removing that first.
Your TV may have decided to store the signals from the main transmitter at Kilvey Hill, as they are on lower frequencies, or if it selects them based on quality and the Neath Abbey transmissions are distorted due to being too loud. Kilvey Hill is 6km south-west of you, just to the east of Swansea.
See Digital Region Overlap for thoughts on how to ensure you're getting the signals you want.
Digital UK's prediction for this transmitter is for a good chance of reliable service on the PSB multiplexes, but a variable service on the commercial multiplexes. The problem here is that the commercial muxes at Kilvey Hill use the same frequencies as the public service multiplexes from Caradon Hill in Cornwall. All multiplexes clash with services from Stockland Hill in south-east Devon. It looks like line-of-sight to Kilvey Hill is clear.
The raw field strengths could still be quite high from Kilvey Hill - theoretically still a few times the recommended upper limit.
With the expected relative signal strengths, I would recommend a grouped aerial to reduce the level from the unwanted transmitter. A Group A aerial, for Kilvey Hill, has a lot more gain and better directional properties than a wideband in the appropriate group, you can then add attenuation to reduce the overall levels. Neath Abbey uses Group C/D.
To reduce signal pick-up from Caradon Hill and Stockland Hill, you could try rotating the aerial clockwise so it points a bit to the north of Kilvey Hill. Aerials do have 'side lobes' in their response patterns, places where they have more gain than at others. Careful positioning can reduce unwanted signals to a minimum, though it's hard to quantify this without professional equipment. Higher-gain aerials have tighter directional response - you would need to add attenuation to keep levels down to what your TV can handle.
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Mark Aberfan Aerials: I assume the phone companies will want to site-share as much as possible, just as they do now.
The danger is that the combined signal level from the TV transmissions (wanted) and the 4G signals (unwanted) pushes equipment outside its linear region, causing distortion, resulting in intermodulation. That intermodulation creates frequency-shifted products, potentially splatting the 4G phone signal - an OFDM signal just like digital TV - onto one or more multiplexes. That will at least reduce the tolerance of interference and noise from other sources.
However, the 4G power is likely to be lower than digital TV power. If you make the coverage area too big, the bandwidth is shared between too many subscribers. Cells have to be fairly small. 800 MHz spectrum is in demand because of its penetration into buildings and the relatively lower power needed for the same coverage, not providing wider coverage (though some wide-area transmitters are needed for base coverage of relatively unpopulated areas).
I do think Ofcom are seriously underestimating the extent of the problem. It seems to be normal practice for newer aerial installations to have excess gain and often the owner has added amplification as well, thinking that this will correct any reception problems. Sometimes it might appear to have done so as by the time the amplifier is fitted, the root cause of the issue has gone away.
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Mary: 'Low Power' is relative. Digital UK's prediction for Emley Moor is 99-100% across the board, because 38km away isn't that much for even 4 kW transmissions. Indeed signal strength is more likely to be too high than too low.
You say you're on cable TV. Do you actually have the TV connected to an aerial, for Freeview? With the kind of signal strength expected in your area, it's possible that enough signal could be picked up just in cables for it to work some of the time.
If you only have the Virgin Media cable, and you want to stop paying for the subscription, you would need to have an aerial fitted. There is no free service on Virgin Media (there should be a basic free service of just the public service channels, i.e. the BBC channels, ITV1, C4 and C5, but VM are flouting the law). Even if there was one you would need a set-top box for cable transmissions, which are sent differently to Freeview, and Virgin Media are supposed to take the box away if you stop subscribing.
Some Virgin Media installers are careless or malicious and actually chop the aerial cable, preventing it from being used (happened to my sister). If it used to work before you got cable, and it doesn't now, you may find this has happened.
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Bryan McAlister: All power increases have already happened this morning, if your transmitter is back on-air. Some are still off-air as work is being completed. The status for each relay can be found at http://www.digitaluk.co.uk/when_do_i_switch/stv_central/black_hill/blackhillrelaytimes .
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Maggie Nicholson: Looking at the prediction for your postcode, chances are that you now have too much signal. If you have an amplifier or booster, remove it. Then you might need to add attenuation to bring the signals down to an acceptable level.
Based on the postcode, it looks like you're in a tower block, and using a communal aerial. If so, the system may need recalibrating and/or retuning. This may already be happening - check with your landlord or residents' association. See PARAS - Professional Aerial Riggers Against The Sharks for some thoughts.
Other possible problems would be 2K-only equipment, limits on internal memory in the box, failure to understand a large Network Information Table, failure to handle negative offsets on some equipment, or other equipment adding modulated output on a required channel. I don't expect any of these to apply to you. The first three would have shown up two weeks ago, Black Hill doesn't use any negative offsets, and modulators would normally only knock out one or two channels.
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Tuesday 21 June 2011 6:49PM
Mike: Different equipment has different limits on what it will accept, and behaves differently if more than one transmitter is received.
Digital UK's predictor suggests a good probability of reliable reception from three transmitters: Craigkelly, Durris and Angus. They're all in kind of the same direction - Craigkelly slightly west of due north, 341°, Angus slightly east, 6°, and Durris a bit further clockwise, 20°. Aerials do have a fairly wide acceptance angle - the larger the aerial, typically the smaller the angle - so the TVs could be picking up one of the further transmitters. It's likely to be Durris, as that has the lowest frequencies, though I would have expected it to pick PSBs from Craigkelly and commercial channels from Durris.
The Durris commercial multiplexes only moved to this location on 15 June, their temporary locations were on much higher frequencies.
If the TV offers a choice of region, ensure you select Central Scotland rather than Highlands and Islands. If you can find a status screen, check that the UHF channels match those from Craigkelly, rather than one of the others. See Digital Region Overlap for other thoughts.
Check that you do need the amplifier, and if you're adding an attenuator, add it before the amplifier. Amplifiers should normally only have just enough gain to offset the losses in the splitter and cabling, if the basic level from the aerial is correct.
On pure distance alone, I would expect the signal levels at your location to be very high, but you're in a fairly deep valley, which means that you're relying on refraction over the hills. Still, excessive amplification can still cause problems.