Two frequency interference
Brian Butterworth published on UK Free TV There are several different type of interference that causes problems. This page deals with the situation where:
- Freeview reception is perfect or near-perfect most of the time
- Several channels are missing, and they are all on the just two 'multiplexes'
Transmission mode problem
If you are unable to receive the channels on BOTH multiplex 2 and multiplex A, this will be down to the transmission mode. See Freeview modes.This is a particular problem for people using Loft aerials and Indoor aerials .
Non-wideband aerial in some areas
In some places you will need a wideband aerial to receive all the Freeview multiplexes. To find out if you need an aerial upgrade, see My Freeview reception.Help with Freeview, aerials?
Sunday, 15 May 2011
Rudy Walker: I note that Bisdale also uses C29, it is possible that you are getting periodic interference from there.
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Tuesday, 19 July 2011
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Ed1:10 AM
Watford
I have tried everything now. I still can't get any channels on the same mux as itv. I get all the others but on those channels it's no signal. If I'm getting all other channels, and those missing were working last week, I can't see it being my aerial, which is new anyway. I've checked everything. Now I've rescanned, the channels are totally gone. I have great signal on other channels so aerial alignment shouldn't be an issue. My postcode is WD19 7RA and I'm fresh outta ideas now :(
H E L P
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Ed's: mapE's Freeview map terrainE's terrain plot wavesE's frequency data E's Freeview Detailed Coverage
J
jb387:23 AM
Ed: I note that your first posting was under the Crystal Palace heading, but as your reception possibilities indicate that you are capable of receiving signals from Hemel Hempstead (less HD) and Sandy (first three Mux's) as well as Crystal palace, you will really have to check what the Mux's channel numbers are that you are receiving, this done in the signal strength/quality section within your tuning menu
If you are receiving from Crystal Palace the two Mux channels you complain about should be shown as Ch22(ITV1 etc) & Ch32(ITV3 etc) and it would be of help to know what these channels are showing level wise.
There is also a remote possibility that your box has a fault in its memory store, although this would be more likely to show up if the channels didn't appear when the box was first brought out of its standby position, rather than go off whilst viewing the channels concerned.
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Ed1:08 PM
yes channel 22 is a definite issue, but it isn't down to one box. The digi box in one room has the problem, but so too does the kitchen tv, which has built in digi. I have tried to pick up hemel, but I can not receive hardly anything and I did switch the aerial polarity.
I'm at a loss here :(
Even though the channels are all from the same transmitter, 22 in particular has always been very weak compared to others. For example, channel 34 is 100/90. The best I could ever get on 22 was 100/62 and that was on a good day. Channel 22 was also often prone to fluctuation, going from say 100/62 to 80/30.
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J
jb389:17 PM
Ed: Noting everything that's been said so far really only suggests something I think that you must have suspected yourself by now, that is of the problem being caused by factors that are totally out of your control and as such having no real option but to wait until switchover in April of 2012.
However, that said, I don't know if you already have one fitted as you haven't mentioned it, but the addition of an aerial amplifier mounted as near as possible to where the down lead enters the property usually always helps in situations such as yours where problems are caused by signal level fluctuations due to external reasons, as the higher the starting level of the signal, the less likely that any drop would let it dip under the reception threshold, the reason for sudden cut outs.
Of course this purely depends on just exactly how low your signal has dropped to when the picture has cut off, because if the indicator has dropped down to under about 20% or so then there is no real point in trying anything, but if its dropped to around 40% then an aerial amp could well help the situation for the time being. By the way the upper figure mentioned just being approximate, as it depends entirely the sensitivity of your receiving equipment.
One thing I will say is, that when you do manage to get a period where all channels are being received do NOT be tempted to carry out another scan if they vanish again, as they are STILL in your box, and when the signal rises will come back again themselves, whereas not if you have carried out another scan.
To finalise, as soon as the signal vanishes on Ch22 carry out a signal strength check, using what has been said as a guide in deciding your actions.
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Wednesday, 20 July 2011
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Ed3:07 AM
yes already using a booster. So when switch over happens, the signals will be much stronger? Why they haven't done that from the start seems a bit silly, most people will want to be ready well before that time.
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Friday, 22 July 2011
Ed: You wouldn't need to do the switchover if the signals were already strong...
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Sunday, 24 July 2011
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Ed3:04 PM
OK done a bit more investigation to my problem. Layout = roof aerial, feeding into amp in loft - feeding to two televisions. One TV uses external freeview, other is built in and has its own amp in the set I believe. The tv with built in freeview was seeing channel 22 yesterday but weak, freeview box nothing, no signal displayed. I disconnected the aerial lead, and fed it straight into the co-ax going to the freeview box. BINGO, signal time. Only getting 98/58 - 98/62 but its there. BBC is 100/68. The tv with built in freeview doesn't get 22 at all now, but this one did yesterday, very weak tho. So at the moment have aerial running into 2 TV's directly and the co-ax are direct connections, no components splitting them etc. Anyway, Does a tv with built in amp overload the loft amp? damaging it. Got to buy a new amp, but don't want it to burn out again. This one has lasted a year.
Many many thanks
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jb386:54 PM
Ed: the fact that the TV with built in Freeview was giving a picture on Ch22 but the separate Freeview box wasn't only means that the TV is more sensitive at picking up signals, and not that any problems exists with the Freeview box, the only reason for it (Freeview box) working when the aerial lead was fed directly into it being because the signal was higher going into it whereby it took it above the cut off threshold level of the box.
However that raises another point, as "if" when you said you fed the aerial lead straight into the Freeview boxes co-ax you are meaning the lead directly from the aerial itself? as if you are then that's inclined to indicate that the loft amp is faulty, as the signal should always be better through it than direct from the aerial, the only exception to that being where a signal is too strong, something I don't think is likely to apply at your distance from the transmitter.
The other thing is that TV's don't have a built in amplifier that can damage anything, but what I did wonder was whether or not your loft distribution amplifier is of the self powered type? and if it is make sure its not a type that is able to power a mast head amp, as some can! and if these type are used directly into a normal aerial they will be running into a short circuit.
I still think though its just a case of you being in a bad location for that particular Mux channel, and as aforementioned in previous reply the fluctuating signal levels you receive on Ch22 is out with your control, although sometimes experimenting with different aerial mounting positions can help no end, like gutter mounts being moved from one end of a roof to the other, as well as a signal sometimes being better at that level than high up on a chimney, as the usual rule of "the higher the better" doesn't apply in most difficult reception area's.
The latter being said though fully appreciating that aerial installers cannot really be expected to cater for this type of experimenting because of the time factor Vs financial costs involved.
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Ed8:24 PM
Thanks. Yes the aerial lead was fed directly from the aerial into the freeview box.
The amp is a mains powered box in the loft with an aerial-in port and 4 output ports, but just two are used. I don't think it is too strong because it has an adjustable pot, and turning it down weakens the signal to the no-signal point, whereas right up gives around 100/80 on channel 22, with no fluctuations or interference. I have a philex 48 element on the roof which I hope is adequate, it's the biggest in the street. No mast amp but says it has a 15db gain.
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