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Thread: Intermittent pixilation

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    Default Intermittent pixilation

    Hi All,
    I'm hoping to get some help with an issue with intermittent picture breakup in a block of 6 units. The building is tucked up against an escarpment so cannot receive signal from the closest FTA transmitter. Instead there is a VHF antenna (vertically polarised) pointing south to the transmitter that is 122kms away. Signal at antenna is: MERs 26db-29db and DCPs all around 50db, no errors in the pre or post BER. After amplification and distribution I am getting MERs of 30db and DCPs 55 - 70db. Great picture. However the customers (all units) are complaining of intermittent pixilation. This issue has been going on for as long as anyone in the building can remember.

    It is a simple system with an 8 way splitter and Kingray DW42 amp. I have relocated the antenna for optimal signal on the roof, replaced the antenna, replaced the splitter, replaced the amp and replaced the main cable from the antenna to the amp. Nothing has helped! Still intermittent pixilation! Of course whenever I am onsite the picture and signal is perfect. I have replaced the DW42 with a masthead amp as well. Still no joy. Any ideas would be appreciated.



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    Any strong out of band broadcast (including FM radio) signals?

    Any 4G LTE signals/towers nearby or in the signal path?

    Have you checked with a spectrum analyser to see what other RF sources are present?

    Any paging or 2-way radio signals?

    Have you fitted band pass filters for the channels in use?

    Any electrical impulse noise in the building or nearby?

    Is the timing of the pixelation random, or at particular hours?

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    Thanks MTV,
    I'm heading back there today to fit a 6-12 VHF band pass filter. I'll check the spectrum to see if I can see anything else. The pixelation seems to be at random hours of the day. It can go weeks without happening then happen every day for a week. I'm also wondering if a noisy GPO where the amp power supply is plugged in, could cause the problem.
    Now that you mention it, there would be a mobile phone tower directly in the signal path. Do you think LTE signal could interfere with the VHF spectrum?

    I'll let you know how I go today. Apartment building though so I need the tenants on the top floor to be home in order to access the roof.

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    Quote Originally Posted by OraraTV View Post
    Now that you mention it, there would be a mobile phone tower directly in the signal path. Do you think LTE signal could interfere with the VHF spectrum?
    It would mostly affect UHF, but if out-of-band signals are strong enough, especially if the transmitter is in the signal path and you are using an amp (which you are) then yes, it's possible.

    Paging and VHF comms are also well-known culprits.

    The random issue sounds more like electrical impulse noise, industrial, farms, welders, electric fences, solar inverters, a faulty appliance in the building, etc.

    The bandpass filter will knock out the out-of-band signals, unless they are very strong and close to the receive location.

    How good is the coax shielding in the building signal distribution? Old 'air-spaced' coax is a huge problem with impulse noise.

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    Quote Originally Posted by OraraTV View Post
    I'm heading back there today to fit a 6-12 VHF band pass filter.
    Worth a try, the VHF masthead amplifiers I use are already band limited, if the one you used was one of these, then I wouldn't be expecting an improvement.

    The installations I come across suffering from LTE interference are usually the result of the masthead amplifier being overloaded & going non-linear, or the LTE signal being so high out of the masthead it overloads the front end of the tuner.


    Quote Originally Posted by OraraTV View Post
    pointing south to the transmitter that is 122kms away
    That would probably be beyond the radio horizon, which means anything could make the signals flaky.


    Quote Originally Posted by OraraTV View Post
    However the customers (all units) are complaining of intermittent pixilation. This issue has been going on for as long as anyone in the building can remember.
    Which makes me think there is co-channel interference (ducting) happening from another VHF TV transmitter a long way away. In my experience, it doesn't take much interfering signal to make this happen. If your measuring device can show echoes, this is one way I observe this.

    Another way I use is to monitor RS uncorrected errors (packet errors) because this will show me transient occurrences of interference that may be so quick, the latency of my instrument doesn't necessarily show a drop in the pre/post Viterbi error reading.

    Made all the more difficult if the interference is not present when you are.
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    I've been back today and fitted the band pass filter at the antenna. Checked the spectrum and there is about 60db LTE coming in around the UHF channel 63 mark, plus a lot of other lower level noise lower down the spectrum. There is no other VHF TV in these parts so I don't think that should be an issue. Anyway the filter has killed everything else and the spectrum looks clean now. The amp does already have LTE filtering in it but hey I'm running out of ideas. Hopefully it helps.

    When I look on google maps there is a transmitter site (mobile phone, not sure what else) about 20kms away directly in line with the TV transmitter. I reckon that must be causing the issue.

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    It was all RG6 dual shield. I have replaced the cable between the antenna and the splitter with quad shield. The RG6 dual shield laterals are in conduit in the slab so I won't be replacing those any time soon.

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    Quote Originally Posted by OraraTV View Post
    There is no other VHF TV in these parts so I don't think that should be an issue.
    I was thinking Brisbane.....


    Hopefully it helps.
    Hope so too.

    I use this to find mobile phone towers in the area I'm working.
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    A thought came to mind upon reading this post about a intermittant on a rf site I service and the fault turned out to be the outer braid of the coax as it had copper and aluminium sheath and was producing volts over its length any way cured by using rg11 coaxwith no alli in it good luck Don
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    One of the tenants texted me last night to say all channels are pixellating again. Ahhhhhhhhhhhhhhhh. If it is internal electrical impulse noise or external VHF interference, any ideas how I would begin to track it down?

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    spectrum analyser is the only correct way but have you tried reducing the incoming signal with attenuators
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    You have to be there when it's happening, if that's practical.
    Never stand under a shadow that's getting bigger

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    Quote Originally Posted by hinekadon View Post
    spectrum analyser is the only correct way but have you tried reducing the incoming signal with attenuators

    I have a spectrum analyser and the spectrum is now clean due to the bandpass filter I have fitted. The DCP of the incoming signal is only around 50db.

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    If you look using a spec-an there will be some sort of direction given to follow other wise you are pissing into the wind with guesses , different there have been several "guesses " by people who have experienced scenarios and all good advice but hardly more than a check list so to that end place attenuators in the supply line and see if the pixelation disappears by increasing or reducing the signal
    If you can make a effective change you are on the right path , " this can be done by using lengths of lossy coax ,rg59 if its a frequency related problem it will show up at some stage but this is the oldmans cure and will take time , For what it worth the theory is if you have a good signal and its suffering from interference it will be able to be attenuated until it wont be noticeable therefore the interference will be attenuated at the same time and rate as the original signal regardless of frequency , or you can keep on pissing into the wind hahaha don
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