All posts by Mike Dimmick
Below are all of Mike Dimmick's postings, with the most recent are at the bottom of the page.
Rod Roberts: The overall signal levels have been increased this week as well as the changes to channels. I suppose it's possible that you could have too much signal, although I would have expected that to affect the more powerful multiplexes, not the weakest. See Freeview signals: too much of a good thing is bad for you | ukfree.tv - independent free digital TV advice
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Mick Lemon: Correcting Briantist: Waltham is HORIZONTAL, Nottingham is VERTICAL. Nearly all main transmitters use horizontal polarization, nearly all relays use VP.
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John Daly: Since you don't mention problems on Pick TV (LCN 11) or Yesterday (LCN 12), which also require a wideband aerial, I'm going to guess that you have some device interfering with C41. If you have a Sky box from which you distribute channels around your home, you will need to retune it. See Single Frequency Interference for more information.
Signal levels are also generally increased, it's possible you could now have too much signal. See Freeview signals: too much of a good thing is bad for you | ukfree.tv - independent free digital TV advice .
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Simon: See Single Frequency Interference - it could be that there is some other device blocking that channel.
This is also the only channel adjacent to another channel, and is the weaker of the pair. It could be a problem of too much signal. See Freeview signals: too much of a good thing is bad for you | ukfree.tv - independent free digital TV advice .
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Andrew McPartland: There's a manual for this box at TV Re-tune productmanuals (or at least STBHDIS2010 which I assume is very similar). Have you tried using the Factory Reset feature, to see if that will allow you to select a different transmitter?
You should always select whichever transmitter the aerial is pointing to, not the nearest to you, as the aerial is directional - it is designed to reject signals from other directions. (This is not perfect rejection, some signal from other directions is still picked up.) Crystal Palace is slightly east of due south, while Sandy Heath is slightly west of due north.
For best results when feeding multiple rooms, you should split the cable as close to the aerial as possible. Try to avoid using amplification if you can; if you can't, add just enough to offset the loss in the splitter.
Digital UK do predict that the chance of reliable reception from Crystal Palace is poorer on Multiplex 2 than on the other multiplexes, at present. A long cable run will make this more difficult. It's expected to be better after switchover.
If using Crystal Palace you will need to retune on 4 and 18 April 2012, for switchover; if using Sandy Heath there are retunes on 23 November 2011 and 9 May 2012. On both occasions it's recommended that you do a factory reset.
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Paddy: At that postcode it looks like the problem is that Sutton Coldfield now (since 21 September) uses C42 and C45, and those transmissions are powerful enough to knock out LYM.
You're not really in the planned service area for this transmitter, the radiation pattern shows a -7 dB (80%) reduction in your direction.
The alternative Welsh relay listed by Brian's predictor, Llanfyllin, uses the same channels as Fenton's commercial multiplexes, so again, Digital UK offer no prediction for either Llanfyllin or for Fenton's commercial multiplexes.
The mast owners are Arqiva, but you can only blame the planners and the regulator for this. The easiest option may be Freesat. It's certainly possible to build antenna arrays that are more directional than a single antenna, and to get more directional/higher-gain aerials, but honestly, a satellite installation is cheaper.
If there is a large population that's affected, you could talk to Ofcom about a self-help relay: Ofcom | Self-help TV Relays and Digital Switchover
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Steve E: Over the last few days, the weather conditions have been right for signals to travel a lot further than normal. That's probably the most likely explanation.
Rowridge Mux 1 and 2 are now co-channel with Crystal Palace Mux A and Mux C; a digital channel - even at lower power - does more damage to another digital channel than an analogue channel does. Or, given your aerial points roughly due south, it could be interference from France.
At switchover, Rowridge will be much more powerful so this should not happen as often; if it still does, you could consider changing the aerial to vertical polarization - all services will transmit on VP from 18 April 2012, and the commercial multiplexes will be more powerful on VP than on horizontal polarization.
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John: If you had reliable reception before switchover, chances are you now have too much signal. See Freeview signals: too much of a good thing is bad for you | ukfree.tv - independent free digital TV advice .
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Phil: It may be co-incidental, the hot weather is helping distant signals arrive much more strongly.
In Windsor I would expect you to be using the Crystal Palace transmitter, but I can't be sure without a full postcode.
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Friday 30 September 2011 3:10PM
Stuart O.: The voltages we're talking about are very low. The recommended signal level for analogue was 60 to 80 dBuV - which is 1 millivolt to 10 millivolts. The normal level for the signal on a cable from another piece of equipment, such as SCART, is 1 volt peak-to-peak, 50 to 500 times the size of the signal arriving at the TV.
The problem is that most boosters are simply one transistor, as is the mixer circuit in the tuner. (The mixer downconverts from the frequency arriving on the aerial to a much lower intermediate frequency; it does this by controlling the gain of an amplifier with a local oscillator circuit, that outputs a pure tone at the tuned frequency.) Digital TV requires that any amplification is linear. A transistor's transfer curve is actually exponential - Bipolar Transistor - Characteristic Curves , second diagram down - but if you use a small enough region of it, it's approximately enough linear. If the input is too large, the range of the curve covered becomes larger and it's no longer linear.
For analogue transmissions this didn't actually matter too much - it was noticeable if you knew what to look for, but the transmissions were spaced far enough apart that it didn't really matter. For digital it causes problems both within one multiplex and for adjacent multiplexes - it's a problem called intermodulation.
At extreme high levels it can cause clipping as the amplified signal reaches the upper supply voltage - the maximum that the transistor can switch - or the ground voltage - no current flowing at all. Neither of these cause damage either, and you would notice intermodulation long before clipping was a problem.