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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

TV Freeview picture breaking up, New 4G transmitter and Aeroplanes



 
 
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  #21  
Old September 25th 15, 11:17 AM posted to uk.tech.digital-tv,uk.d-i-y
Jim Lesurf[_2_]
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Default TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

In article , Martin
wrote:

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.


Which would be a very different frequency to DVB-T/T2 and coming from a
very different azimuth into a more directional RX antenna.

Jim

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  #22  
Old September 25th 15, 11:33 AM posted to uk.tech.digital-tv,uk.d-i-y
Bill Wright[_2_]
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Default TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

Michael Chare wrote:
On 24/09/2015 18:32, john west wrote:

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.

Any suggestions please.


Wait for the wind to change, the planes will then take a different path.


That's another one. "The picture goes off when the wind blows from the
north." Often true when the aerial's loose!

When I was doing the 'wind turbine amelioration' work there was one
woman who had made a really good detailed diary who showed the close
correlation between the behaviour of the turbines (which were clearly
visible from her living room) and her TV reception. When the turbines
were turning fast her satellite reception would break up. Therefore, she
said, the wind turbine company must pay to fix her reception, and if
they couldn't do it they would jolly well have to pay for her to move
house to the bungalow that she had her eye on in the next village.

Her dish had been fixed (for no good reason) at the top of a ten foot
aerial mast, and when the wind blew (and the turbines whizzed round like
billio) the mast deflected and the dish went momentarily off beam. Thus
we must always remember that correlation is not causation.

Bill
  #23  
Old September 25th 15, 11:45 AM posted to uk.tech.digital-tv,uk.d-i-y
john west
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Default TV Freeview picture breaking up, New 4G transmitter andAeroplanes


Many thanks to all. I followed the suggestion to recheck the Aerial
lead connection going into the Humax digi-box after remembering the
visiting at800 engineer pulled it apart and seemed to re-assemble it a
bit too quickly for my liking.

After re-doing this connection very carefully it now seems things are a
bit better this morning. (Although mild weather and clear skies might be
connected?).

Is it possible that the 4G phone companies although giving us a ' new
filter' might be degrading our freeview reception in ways they are not
very willing to talk about?

Our post code is NW7 1NE and the Bigger Aeroplanes which are at a higher
altitude and pass between our house and the Crystal Palace Transmitter
are now *not* breaking up our freeview reception as before.

But the smaller millionaire type planes like the 'Lear Jets' which are
traveling at a lower altitude and which are traveling from east to west
quite close to our house (presumably going to Heathrow and are at a
lower altitude than the big planes) are *still* breaking up our
reception for a few seconds 'every-time' one passes over.

Given that the much of the consensus in this group seems to be saying
this kind of plane interference is very unlikely, should i have myself
and my whole family enter the local Whittington Psychiatric Unit.....

Seriously though we are not imagining this, grateful for any further
thoughts. Thanks.


  #24  
Old September 25th 15, 11:55 AM posted to uk.tech.digital-tv,uk.d-i-y
Dave Farrance
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Default TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

Jim Lesurf wrote:

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.


Hmm, yes. I recall reading that Doppler effects are especially
problematic for OFDM modulated signals, and is a limiting factor on the
use of OFDM broadcasts to high-speed road vehicles.

Presumably, given the huge number of subcarriers of DVB-T2 (do we use
32k in the UK?) and the speed of aircraft, then it would no longer be a
simple case of multi-path interference suppressing a few carriers, but
would be significantly destructive interference.
  #25  
Old September 25th 15, 12:03 PM posted to uk.tech.digital-tv,uk.d-i-y
Mark Carver
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Default TV Freeview picture breaking up, New 4G transmitter andAeroplanes

On 25/09/2015 12:55, Dave Farrance wrote:

Presumably, given the huge number of subcarriers of DVB-T2 (do we use
32k in the UK?) and the speed of aircraft, then it would no longer be a
simple case of multi-path interference suppressing a few carriers, but
would be significantly destructive interference.


T1 uses 8k, T2 uses 32k. Be interesting (in view of the new info the
OP has just posted) whether the T1 and T2 muxes are affected or just T2 ?





--
Mark
Please replace invalid and invalid with gmx and net to reply.
  #26  
Old September 25th 15, 12:04 PM posted to uk.tech.digital-tv,uk.d-i-y
Mark Carver
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Default TV Freeview picture breaking up, New 4G transmitter andAeroplanes

On 25/09/2015 12:45, john west wrote:

Our post code is NW7 1NE and the Bigger Aeroplanes which are at a higher
altitude and pass between our house and the Crystal Palace Transmitter
are now *not* breaking up our freeview reception as before.

But the smaller millionaire type planes like the 'Lear Jets' which are
traveling at a lower altitude and which are traveling from east to west
quite close to our house (presumably going to Heathrow and are at a
lower altitude than the big planes) are *still* breaking up our
reception for a few seconds 'every-time' one passes over.


Is your TV able to receive HD channels, if so, are these channels
the ones that break up, or just the SD channels, or both ?



--
Mark
Please replace invalid and invalid with gmx and net to reply.
  #27  
Old September 25th 15, 12:26 PM posted to uk.tech.digital-tv,uk.d-i-y
Bill Wright[_2_]
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Default TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

Big Les Wade wrote:

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.


So it could be the planes then?


No, you've not read what I put carefully enough. 'Pretty well owt'
referred to the phenomena people blame for their poor reception, not to
real causes of same.

Planes take a while to pass over when they're high up.


But they aren't, he's only 15 miles from Heathrow. They're probably only
a few thousand feet up.


He said they were so high he couldn't read their markings. In any case a
'few thousand feet' would be enough for everything I said.


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


They're not so high up.

See above.

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


They're not high.

See above.


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!


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?


Again, you've not read what I put carefully enough. Or you don't
understand why I carefully used the word 'generated'.


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?


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.


There are many other reasons why a long period of good reception can
end. Many of them are far from obvious.


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 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.


An interesting assertion. How many times have such people as Charles
Hope (ex BBC) heard people blame 'the transmitter'?
Let's take your words above, "The picture was fine for a couple of
years, then for a few months it was awful, and now it's fine again."
Without thinking too deeply, I can remember all of the following
occurrences:
1. A kite attached a bit of itself to the aerial. Eventually this blew off.
2. Yes it really was the transmitter! Or actually it was one in another
town on the same channel which had gone ever so slightly off tune, and
thus was putting coarse horizontal lines across the picture.
3. A bloke moved in at the top of the street and parked his big van in
the back alley every night, and it just happened to obstruct the signal
of someone at the bottom the street. After a while he was promoted so he
got a car, or maybe he died, or moved away, I dunno. But the van
disappeared.
4. The aerial was slightly loose and swung slightly off beam, then back
again.
5. Trees. Over and over again I've seen this sort of thing caused by
trees. No rhyme or reason quite often. They don't need to be in the
signal path.
6. A connection behind the TV was disturbed, then disturbed again a few
weeks later.
7. Ditto under the carpet.
8. Ditto in the loft.
9. Ditto on the roof.

Bill


  #28  
Old September 25th 15, 12:33 PM posted to uk.tech.digital-tv,uk.d-i-y
Bill Wright[_2_]
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Default TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

Mark Carver wrote:

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.

http://www.wrightsaerials.tv/services/interference-studies/television_interference_studies_for_wind_turbine_i nstallations.pdf


Yes I'd think a plane really close would have much the same effect as a
wind turbine similarly close. It's interesting to watch the effect on an
analyser. A great 'orrible notch of infinite depth moves across the mux,
taking a good proportion of the carriers down far enough that the AGC
can't cope and/or the carrier's s/n ration is inadequate.

Bill
  #29  
Old September 25th 15, 12:37 PM posted to uk.tech.digital-tv,uk.d-i-y
Bill Wright[_2_]
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Posts: 9,381
Default TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

Martin wrote:

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.


That of course is screening not reflections.

Bill
  #30  
Old September 25th 15, 12:43 PM posted to uk.tech.digital-tv,uk.d-i-y
Dave Farrance
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Posts: 1,551
Default TV Freeview picture breaking up, New 4G transmitter and Aeroplanes

Mark Carver wrote:

On 25/09/2015 12:55, Dave Farrance wrote:

Presumably, given the huge number of subcarriers of DVB-T2 (do we use
32k in the UK?) and the speed of aircraft, then it would no longer be a
simple case of multi-path interference suppressing a few carriers, but
would be significantly destructive interference.


T1 uses 8k, T2 uses 32k. Be interesting (in view of the new info the
OP has just posted) whether the T1 and T2 muxes are affected or just T2 ?


Thanks. 32k it is then.

The UHF channel bandwidth is 8MHz, so each subcarrier is just 250Hz.

Crystal Palace PSB3 is on 545800000Hz, so shifting it by just half the
above to 545800125Hz would be destructive.

Doppler shift: f' = f * sqrt((1 + v/c) / 1 -v/c))

I find that I can rearrange that to:

v = c * ((g - 1)/(g + 1)) where g = (f'/f)^2

Which gives me a speed of 68.6 m/s or 153.6 mph, which *is* aircraft
landing speed, close enough.
 




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