Suddenly in the last month or so our Freeview television picture is suddenly breaking up.
We got a visit from the 'at800' Engineer who fitted a filter in-line before our television aerial amplifier. Apparently we have a new phone
4G transmitter installed quite near.
The engineer said his readings were within accepted limits.
We are about 15 miles north east of Heathrow Airport and get a lot of planes, they are sufficiently high so that we cannot read the markings on the planes. We are about thirteen miles north west of the crystal palace transmitter.
Now every time a plane goes over the picture and sound break up and that's very often.
Everything worked fine before until about a month or so ago, and we have carefully checked all connections and leads and have bought a new Aerial.
I can't believe reflections from aircraft (as high as that) could punch out enough of the 8 MHz worth of COFDM 'lump' to cause such a severe and noticeable effect ?
Has the set "helpfully" retuned itself to a different transmitter. You'll need to find the channel numbers each MUX is transmitted on from near by transmitters and dig through the sets menus to find out what it is using.
This sounds like a classic case of 'misleading fault reporting', to be honest. I think you'll find that there's no connection between the planes and the fault. Please don't be offended, but customers often assign irrelevant 'causes' to their reception faults. It can be anything from the weather, 'the number of aerials sucking on it', the radio ham across the road coming home from work, next door's dog barking and the noise affecting the waves, pretty well owt really.
Planes take a while to pass over when they're high up. How does that fit in with the picture break up? What about the planes you don't see? Any break up when you can't see any planes?
But assuming you aren't simply having a laugh it might be interesting to discuss the scenario you describe.
Planes so high up will probably be in an area of relatively low field strength, so any reflections (which might have a rapid additive and subtractive effect on the signal output of your aerial) will be similarly reduced in strength. Beam tilt at the tx is the reason for this. Apart from the obvious reasons for beam tilt there's the fact that such high powered tx's aren't allowed to radiate much above the horizon because of the possibility of EMI in aeroplanes.
Yes, 'aeroplane flutter' was an issue in the analogue days, very rare under circumstances such as yours for UHF but quite common for VHF. The main reason for this is that UHF aerials are far more directional than VHF ones. Indeed some types of VHF aerial had no discrimination whatsoever against signals from above (anyone like to name two examples, one of each polarisation?)
Any reflection from a high plane would arrive sufficiently delayed for the modulation system in use by digital TV to ignore it (it's expressly designed to do just that), unless it was so strong that it confused the receiver's AGC. And that's impossible, really, because your aerial will be discriminating against the reflection by 10 or 20dB (which is a lot.)
It isn't possible that any interference actually generated by the plane will affect your reception. Think what a fuss there's be near airports if that was the case!
When you say you've bought a new aerial, well, buying the thing is only a small part of the process! Is it the right type, pointing the right way? Any obstructions? And there are other things. Just because you've had good reception until now doesn't mean thing really. I'm guessing that the signal from your aerial is marginal, and something is causing occasional break up.
I must apologise for the length of this reply but it's either skulk in here or talk to a damnfool friend of my wife who's called round seemingly to get us to join the Green Party. I just don't want to cause an upset. . .
yes, are you using the old downlead? It might have got damaged. interesting I responded to a cry of "picture break up" from some neighbours and found the fault was due to badly fitted plugs on the aerial downlead behind the set. A drop of solder cure that! Give all the lead behind the set a good shake. That might show up something.
Has somebody built a new high rise or put a crane up somewhere quite close? Normally the direct signal should more than compensate for any weird reflections from aircraft. the flutter effect on signals is normally a problem when its a significant part of the signal.
Well, in his area the signal strength normally is very high, which was why I did wonder about new buildings or cranes, as just down the road from my bit of west London, a bloody great tower block has meant a lot of people becoming Virgin customers due to apparently poor signal quality, not strength.
I suppose if he has not replaced the downlead, it could well be that he is just using a water filled resistor as an aerial. We really need hard figures etc to be close to a diagnosis. The channels in use down here seem pretty well away from the 4g stuff and thought removing said filter as a test might be a valid thing to do. Most seem to be attenuators at other frequencies as well. Brian
But they aren't, he's only 15 miles from Heathrow. They're probably only a few thousand feet up.
They're not so high up.
They're not high.
You seem to be saying here that interference from planes can't happen because it would cause too much fuss from householders. Yet above, you say "Yes, 'aeroplane flutter' was an issue in the analogue days"? Which is it; interference from planes can happen, or it can't? Why would householder fussing prevent interference to the digital signal but not to the analogue signal?
He said "Everything worked fine before until about a month or so ago". So unless somebody's knocked the aerial it must be pointing the right way.
I have had exactly the same problem as him, and my set is showing 100% signal strength from the nearby transmitter. The picture was fine for a couple of years, then for a few months it was awful, and now it's fine again. No LOS obstructions have appeared or disappeared and we've changed nothing in the setup. It can only be the transmitter.
Because of the way COFDM modulation (used for digital TV and DAB) works, you effectively have interleaved data (with forward error correction), spread over a few thousand separate carriers occupying almost 8 MHz (for TV) of spectrum. Any reflection (at any given moment) will only cancel a small selection of those carriers.
Aircraft and indeed everything that moves causes interference with TV signals.
The point is that modulation techniques are designed, in the case of digital, to instead of 'slowly degradewith interference' to have enough error correction so that signals don't degrade at all under light interference, but degrade massively once the error correction is overwhelmed.
So its perfectly reasonable to say that light moving ghosting from aircraft on an analogue TV signal would be no obstacle to decent digital transmissions.
We have a boat moored at Gallions Point Marina, right under the flight path into London City Airport. OK, we *are* very close to the aircraft when they land but they certainly do break up the signal (both vision and sound) on our Freeview TV. It's not just a minor hiccough either, the sound goes off and the picture breaks up and freezes for the few seconds while an aeroplane goes over. It is only a few seconds though.
Yes, that doesn't surprise me, a plane very low will be reflecting enough stuff back with a large aperture to 'punch out' enough of a mux for a few seconds as it passes over.
As ever with all digital reception issues the only viable method to see what's going on is with a spectrum analyser.
Mr Wright of both these parishes has published work on the effect on reception of windfarms, which is a related subject of course.
We are about the same distance from Schiphol Airport. Planes landing at Schiphol are between 600 metres and 3,000 metres altitude, when they pass directly over us. On rare occasions a plane very briefly interrupts Freesat.
You and others may be correct to dismiss the idea. But I'm not so sure as I've not yet done the maths. However...
It occurred to me that the large path difference combined with the high velocity might phase and amplitude fluctuate the received signal fast enough to confuse any mpx correction. The situation could combine large dopplers with large delays that change fast.
It may depend on where the planes are, and what their velocity is if the above speculation is relevant. No idea if it is at present.
Another possible problem is this group's old friend, "Digital Cliff".
So 4G might be enough to tip something over such a cliff for all I know. Given millions of receivers someone somewhere could draw the short straw.
8-]
Anyone done any estimates on the effects I outline?
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