Impulse interference - how do you stop it?

Discussion in 'Satellite TV, Sky TV & FreeSat' started by Sinster, Mar 20, 2006.

  1. Sinster

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    Recently had a new aerial fitted - wideband high gain with masthead amp and distribution box to send a signal into 4 rooms using twin screened cable. The aerial is now aimed at the main Winter Hill transmitter rather than the local relay and the signal is obviously superior. The other advantage is that I can now get a Freeview signal.

    However, whilst watching TV the analogue picture goes fuzzy momentarily or the digital picture and sound freezes for a second or two. Reading around I gather this is called impulse interference but it's not clear how to get rid of it other than using screened cable.

    Is there anything else I can do? Will the situation improve once analogue is switched off and the digital transmitters are powered up?

    Thanks for any help.
     
  2. Nick_UK

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    Depends on where the source of the interference is. If it's ignition interference from passing traffic, then there's not a lot you can do. If the interference is mains-borne, then feeding the aerial amplifier from a filtered outlet could help. Did the intstaller also do the room wiring ? If he did not, it might not be done with double-screened cable, which will also pick up interference. Double-screened cable has both a woven and a copper foil screen around it.

    P.S. The situation will improve if/when the power is increased.
     
  3. maldonian

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    Sounds like a description of the sort of effect you can get from a nearby transmitter, a taxi for example, when you're using a wideband booster or masthead amp in a fringe area. The effect will vary if it's a mobile transmitter.

    Impulse interference typically produces white spots on analogue pictures, a single horizontal splash of white from a switch or thermostat, broad horizontal bands of white dots from an electric motor, or a narrow horizontal white band moving up or down at a varying rate from ignition interference. Ignition interference was common on 405 line TV on VHF, it seems to be rare on UHF (or ignition leads are better these days). The other types are common.

    The effects on digital pictures depend on the tuner, the picture normally breaks up completely but this is masked in some tuners by freezing the most recent good frame.

    Identifying the cause of interference is going to be more difficult when we haven't got analogue pictures to refer to.

    Nick's suggestions are the best place to start. You could also try aiming the aerial slightly upwards, to make it less sensitive to nearby sources below the horizon. If the problem is a nearby transmitter you could try a high pass filter (e.g. from Maplin or Waters and Stanton).
     
  4. Sinster

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    Hi, thanks for the replies.

    The effect I'm getting with analogue is most like
    For a second or two there are 3 or 4 horizontal bands of snowy picture across the screen which account for ~40% of the viewing area. As far as I can tell they don't move. The picture is for the most part still visible and the sound isn't really affected. What kind of motor do you mean?
     
  5. Nick_UK

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    The interference you refer to could come from a fridge or freezer, or from the ignition circuit of a central heating boiler.
     
  6. Sinster

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    Would they have to be on the same circuits? I do have an old chest freezer on one circuit - analogue TV and a fridge/freezer and central heating boiler on another - digital TV. I assume the interference isn't going via the mains cables as the interference happens at the same time on both TVs.

    Is there anything I can do to prevent it - apart from turning everything off!
     
  7. Chris Muriel

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    It could actually be radar interference.
    I pick this up on amateur TV reception at 1240 MHz .
    It hasn't been too annoying for me to do anything about it but a bandpass filter for the channels you want combined with a bandstop (rejection) filter tuned to the radar emission frequency would minimise this.

    Chjris Muriel, Manchester
     
  8. Nick_UK

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    If the interference is mains-borne, then it doesn't have to be on the same circuit. As I said in my original reply, the mains-borne interference can be picked up by the aerial amplifier itself - try putting a mains filter in the circuit where this draws its power. Fridge and freezer interference is usually quite brief (a second or two) - if it's longer than that, then it could well be the ignition on a central heating boiler (old oil-fired boilers are the worst culprits).

    The radar suggestion from Chris should also be taken seriously - one problem with wideband aerial amplifiers is that they amplify the unwanted stuff along with the wanted.
     
  9. Sinster

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    Thanks.

    The problem is only a couple of seconds but enough to miss the occasional word on digital TV.
    Please would you point me to a link for the mains filter you mention.
     
  10. Nick_UK

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    Lots of places sell them (Maplin, many DIY stores). Here's one from Ebuyer to give you an idea of what to look for.
     
  11. Andy3

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    The periodic interference sounds familiar. Here in Manchester, analogue channels around the 35/36 area (where most video recorders output their signal) are TROUBLE as Manchester Airports ground radar system operates on this frequency (yup, 600MHz-ish) and every ten seconds any TV tuned to ch 35ish gets a massive dose of 'dashes' as the radar swings around! We had to retune loads of our customers' videos to get around this. With s-o-o-o much gain up front, I wouldn't be at all surprised to see this kind of interference spreading over several channels.
     
  12. Chris Muriel

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    It's the 2nd harmonic of this that causes my interference on the 24 cms amateur TV channels.
    Certainly the "lasting a few seconds" description sounds just like it.
    (I assume this occurs when a rotating radar transmit antenna happens to swing towards one's location so that it is beaming at you until it has swung past).

    Chris Muriel, Manchester
     
  13. maldonian

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    Can you describe how often you see this. Does it happen at regular intervals or randomly? Or at a particular time of day?

    Is it the same on all the analogue channels?

    Are all the digital channels affected?

    Any electric motor with brushes that's not properly suppressed. An electric drill, food mixer, vacuum cleaner, sewing machine, etc. But these tend to be switched on for longer periods than a second or two. The motor in a paper shredder only runs for a couple of seconds or so. The source could be in a neighbour's house, not just yours. Typically, every third house is on the same phase, so the source could be three houses away if it's mains borne.

    But your description of 3 or 4 horizontal bands doesn't quite fit. Mains powered motor interference normally produces two broad horizontal bands, though one band can be split between the bottom and the top of the picture. The bands correspond to the stronger current near the peaks of each half cycle of the mains.

    A good suggestion, but I have my doubts in this case. The radar band is 590 to 598 MHz, which is channel 36 (and only channel 36, though other channels could be affected a bit if it's strong). Sinster hasn't said how many channels are affected yet, but he did say it's affecting his analogue and digital pictures, so that's at least two channels affected, one analogue channel and one digital multiplex.

    Radar interference has a very characteristic appearance (as described by Andy3), short horizontal white lines scattered across the whole picture for a fraction of a second, repeated with a very regular interval, typically about 10 seconds (the period of rotation of the radar scanner).

    Could well be. I've no experience of this. Can it be cured at source by suppression or a replacement part?

    Faulty thermostats can arc for several seconds.
     
  14. Sinster

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    Thanks

    All the time, randomly but usually at least once every 10 minutes if not more.

    As far as I can tell.

    Again as far as I can tell

    Isn't always clearly 3 or 4 bands. Occasionally 2 large bands. They only last for a split second but are always noticeable and accompanied by a glitch of some sort on digital TV.


    Not had chance to watch much TV recently but will start taking some notes. Will get hold of a surge suppressor of some type. Have tried the Russ Andrews Silencer with no noticeable effect. Will try turning the central heating off as well (when it's a bit warmer).
     
  15. maldonian

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    Could be a thermostat. 10 minutes is fairly rapid, so perhaps there are two thermostats involved, which would also explain the apparent randomness.

    I'm not surprised. I think you plug it into a nearby socket, don't you? I'm sceptical about the effectiveness of doing that. But looking at the Russ Andrews website, they seem to be in the business of selling expensive placebos to audiophiles (mains leads at silly prices for example). If you want to try a mains filter, I suggest you try one that you connect in line with the supply to the equipment. But it's better to cure the interference at source if you can.

    No need to wait for summer, you only need to turn it off for 20 or 30 minutes while you're watching TV and see if the interference stops.

    If the interference occurs with the heating off, try turning off the fridge and freezer too. (I assume your immersion heater is off if your central heating is on.)
     

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