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Freeview picture & sound breaking up

Discussion in 'Freeview Forum' started by akpak, Dec 4, 2010.

  1. akpak

    akpak Member

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    I'm getting Freeview from Waltham (32km away), and the Digital UK predictor gives a signal strength of 95+. However, I've been having problems with the pictures and sound breaking up. This has been going on intermittently for some time, on all channels, and I originally suspected the problem was with my set-top box, but I've just installed a new Freeview recorder and the problem is still there.

    Old Set-up:
    RF loop: Basic Freeview box > analogue DVD/HD recorder (Pioneer DVR-420H) > VCR > TV (Sony Bravia KDL-20S3020); Freeview box connected by SCART to both VCR and DVD recorder, these two then connected by SCART to TV.

    New Set-up:
    RF loop: Freeview HD recorder (Sagem DTR67250T) > DVD recorder (as above) > TV (as above); SCARTs from Sagem to DVD recorder, and DVD recorder to TV; Sagem currently connected to TV by HDMI (but could be SCART instead).

    With the old set-up we mostly used the Freeview box for recording from digital-only channels. Everything was fine at first, but after several months we had a problem with one recording where the picture was going blocky and/or pixellated and the sound was dropping. The best solution we could find in the manual was to disconnect the power for 2 minutes, and this solved the problem for some time.

    Recently the problem recurred, and we found by trial and error that adjusting the SCART lead in the set-top box output to the DVD recorder sorted it for a couple of weeks. However, we later had further problems, although sometimes it worked perfectly. (Although we mostly used the box with the DVD recorder, exactly the same problem occurred when we used it with the VCR, so we are confident that the DVD recorder itself is not the problem.)

    When this problem has been happening the picture and sound on the TV have been absolutely fine. Moreover, although we don't watch that much live TV, in the 2 1/2 years we have had the TV, always in this old set-up, we have only once seen the tiniest hint of this break-up, and the sound was not affected. Also, the TV has always reported a good signal strength (I think always at maximum), while the set-top box varied between about 40-70%. Neither gives a signal quality.

    With the new set-up we are getting exactly the same problem, but again when the signal through the Sagem is breaking up, the TV is fine. The Sagem reports signal strength around 40-50%, and if we check when there are problems, the signal quality jumps rapidly between about 2% and 98%.

    We haven't had the new set-up long enough to be sure of any patterns, but one thing that has happened so far is that in the first 10 minutes or so after switching the Sagem on, everything seems fine, and then suddenly it deteriorates. We have tried disconnecting the SCART between the Sagem and the DVD recorder, but this made no difference.

    Please can anyone suggest what the problem is and how it might be fixed? Is it something that we'll have to live with until switchover, when hopefully the signal will improve? Would a different Freeview recorder have a better tuner that can cope with the current signal? Or is the problem something else altogether? And I don't understand how we're getting a better signal at the TV at the end of the RF loop than at whatever box is at the beginning of it.

    Thanks for any help.
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  2. akpak

    akpak Member

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    Just to add to the above:

    Last night I tried recording a 2 hour programme on the Sagem, and at the same time recorded the same programme in analogue on the DVD recorder. I started chase play on the Sagem part way through, so all 3 devices were on and working at the same time. There was no problem at all in the digital recording.

    I know that it's advised not to have different leads close together, but because of the layout of the sockets etc this is very hard to avoid. However, since the problem is intermittent, and sometimes there are no problems, I find it hard to believe that my particular set-up is what's causing the problem. It also makes it very hard to identify the main cause, because if I make any changes and then things are OK, I don't know if what I did has made a difference, or if it would have been OK anyway.
  3. akpak

    akpak Member

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    I started a thread on the Freeview board (here), but have been advised it would have been better here, so I'm trying again.

    Moderator note - I tolerated this even though it is against forum rules, but really it would have been better if you had asked me to move the thread.
    The issue has been developing on parallel fronts, in two forum sections, with the risk that contributors were duplicating their efforts unnecessarily.
    I have now brought all the posts back into this one thread to keep the issue concentrated.
    End of Moderator note.


    I've recently bought a Sagem DTR67250T but I'm having frequent (but intermittent) problems with picture and sound breaking up/pixellation/freezing. (See here for a fuller description of the problem.) I'm about 20 miles from the transmitter (Waltham) and my aerial was installed specifically for Freeview about 3 years ago. Strangely, the picture and sound on the TV are fine, even though the RF signal passes through the Sagem (and a DVD recorder) before it gets to the TV.

    I'm considering returning the Sagem as not fit for purpose and possibly getting a Humax PVR 9150T or 9300T - but is it likely to be any better? Or are there any other twin-tuner recorders worth considering?

    Alternatively, as I'm getting a bit fed up with Freeview, would Freesat be subject to the same kind of reception problems?

    Thanks for any help.
  4. Gavtech

    Gavtech Administrator

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    One thing you need to eliminate as a possible cause - I cannot see that you have mentioned it - but if you have the unit connected to your TV via HDMI, you need try temporarily connecting by scart only.

    HDMI leads can radiate at RF and cause problems with reception, so if this applies to you, you need to exclude this possibility.
  5. MarkPax

    MarkPax Member

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    Reading your linked posts, your signal strength is probably not adequate at 40-50%. If your signal quality is jumping up and down so much, that's probably the cause of the problem. It may be that your TV has a better receiver than your STB.

    Digital signal quality is a funny thing, it can be fine at one signal strength, and then go off a cliff as soon as signal strength drops even slightly.
  6. Gavtech

    Gavtech Administrator

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    I had not seen that. - I would agree. That is right in the danger zone.
  7. grahamlthompson

    grahamlthompson Well-Known Member

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    The signal is slightly amplified in each loop through so this suggests your signal is right on the margin for reception. A variable output masthead amplifier fitted as close to the aerial as possible may sort it out. If the cable goes into the loft you can fit the amplifier there.
    Last edited by a moderator: Dec 8, 2010
  8. Rodders53

    Rodders53 Active Member

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    Thanks for a very comprehensive and detailed description of your setup and problems.

    1) There is some anecdotal evidence that hdmi cables can cauuse problems with DTT reception. It might be worth trying SCART only to see if things change for the better (check strength & quality again).

    2) To me, 40-50% signal strength is a bit on the low side (my Humax 9200T is always happiest above 50%, preferably nearer 60% or above). A small booster amplifier might help?
    Signal strength meters on devices can be measured before or after the automatic gain controls within the tuners and signal quality can be indicated before or after error correction. (This makes comparisons between different units a tad difficult). I suspect the Sagem is 'before' and the TV 'after'.

    3) The Sagem unit will have a small aerial amplifier built in - with two outputs for its own tuners and a third to feed on to the RF 'loop out socket. So will the DVD-Recorder. Thus the TV will be seeing a signal that has been amplified by the devices in the chain before it. {Note that amplifiers do introduce additional 'noise' into the signal as well as amplifying the wanted}.
    It might be worth trying the DVD-R as first in the chain to see if the signal level indicated on the Sagem increases? If this helps, but doesn't quite cure it, a small aerial amplifier (booster) might be a solution... ideally one should amplify as close to the aerial as possible (masthead amplifiers, with a power supply inside connected via the aerial cable) but you may be able to get away with a 'set back' type.

    4) The other causes of 'breakup' or a sharp reduction in quality is interference. Up to a point the receivers' digital error correction systems can compensate, but then you get failure (especially when the input RF signal is low and near the 'noise floor' ... Electrical interference from poorly suppressed vehicles, faulty (central heating) thermostat switches , faulty street lamps etc.,. can cause problems. {Even electric fences in the country have been known to give problems}. Thermostat interference often gets worse in the autumn as heating starts to be used more. Taxis or emergency services transmitting close by can also cause intermittent problems. Faulty electrical equipment may be in your home or that of a near neighbour.
    Does the problem only happen at certain times of day, or when something is happening nearby?
  9. Gavtech

    Gavtech Administrator

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    This thread has been blended from two threads into one.

    Please see moderator note in Post no. 3
  10. akpak

    akpak Member

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    Sorry about the duplicate threads, and thanks for sorting it out.

    Thanks too for all the input so far. I'm going to try some of the suggestions, and will report back later.
  11. akpak

    akpak Member

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    OK, time for an update...

    Because of the possibility of interference from other leads into the RF feed I've rearranged the boxes a bit (before I read today's messages), so the RF into the Sagem is now well away from everything else. Now that it's been moved, I don't have a long enough HDMI cable, so the Sagem and TV are now connected by SCART. Still having problems...

    I've checked again, and most Muxes are giving around 60%, but Mux 1 (BBC1 etc) is under 50%. (I don't understand this - info on the Waltham transmitter suggests to me that Mux1 is output at a higher power than the others.) It's BBC1 where I've been having most problems over the last day or two, but I've also had some problems on other Muxes - a few weeks ago I noticed it on ITV3 particularly.

    I haven't tried this yet - the reorganisation I did earlier makes these inputs a bit tricky to access, so I need a bit more time available to do this.

    Where do these amps take their power from? I don't have power in my loft, so if I need to get it up there, it could prove quite expensive. However, if the problem is caused by an insufficient signal, will this be rectified after switchover? - in which case if I need a booster I may be better taking the simpler option of a set back amp for now.

    I've been wondering if the problem could be a combination of interference and low signal, which might explain why it's intermittent and to some extent random. As I said in an earlier post, we had no problems at all for some time, and it's only in the past few months that the problem seems to have been getting more frequent. I've been racking my brains, but can't think of anything significant, either inside or outside the house, that has changed between when Freeview was working fine and now.

    Finally, there are few other things I've noticed while using the new Sagem which may or may not be relevant:
    1. Although the aerial points to Waltham, the Sagem also managed to tune in to BBC1 and BBC2 from Belmont, which is in the opposite direction - only with a very weak signal, though. However, these are assigned to 800 & 801, so the usual 1 & 2 are the Waltham ones.
    2. We tried to record something on ITV1 using Programme Mode, but it didn't record. We assume this is because it didn't detect the start signal - could this be related?
    3. When putting it into standby this afternoon, the Sagem commpletely froze and I had to disconnect the power to get it working again.
    4. The RF input plug doesn't seems to sit as tightly in the Sagem's socket as in other things. (The RF in lead is quite thick and not very flexible, so it has needed quite careful positioning to make sure it was in good alignment.) However, the analogue signal passed through it seems fine, and if there was a loose connection I'd expect it to be snowy etc.

    Thanks for any further help.
  12. grahamlthompson

    grahamlthompson Well-Known Member

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    Masthead amplifiers get there power from a power unit downstairs and sends the power up the coax lead to the amplifier.

    One here

    Vision 1 Way Masthead (15-27dB) + PSU (Vision V20-1127 KIT)

    Shop around you may well find cheaper.

    Your aerial will have a different gain at the frequency used by each mux. This is particulary noticeable if you have a yagi wideband as the gain of these is very poor at the low end of the uhf spectrum, more than enough to make a more powerfull mux the worst

    Delete the 800 plus channels on a Freeview+ box they can stop the recordings working properly
  13. akpak

    akpak Member

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    Thanks - so are you saying the power unit can sit behind the TV etc?

    You've lost me here, and googling hasn't provided many answers. Yes, it's a yagi wideband aerial - but what do you mean by "low end of the uhf spectrum"? The Mux with problems is Ch49, and the others range from Ch23 to Ch45 - or is that irrelevant?

    Noted - will get on to that.
  14. grahamlthompson

    grahamlthompson Well-Known Member

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    Yes the power supply can sit behind the TV

    Look at the gain curves on the graph here

    TV Aerial Tests

    The wideband yagi is shown in Black, you can see how aerials have a different gain at different uhf frequencies (lowest channel nos 21 is lowest frequency)
  15. akpak

    akpak Member

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    Thanks, but I'm still confused. If I've read the graph correctly, Ch49 should have a higher gain than all the others I'm getting. And it's also supposedly being output at 10k, whereas the others are only 8k or 5k. Surely all of that should mean that Mux1 gives me the strongest signal, whereas the Sagem reports it as the weakest. :confused::confused:
  16. grahamlthompson

    grahamlthompson Well-Known Member

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    You have read it correctly, uhf is funny it can suffer from all sorts of standing wave problems that can attenuate a specific frequency more than others. Moving the aerial by as little as 1 ft can make a big difference. Funnily enough I have a wideband yagi (DAT75) on Sutton Coldfield and the highest frequency is the worst, in my case it's caused by trees. The one mux is unwatcheable in the Summer till the leaves fall off.

    Try the amplifier it should help. Make sure you get a variable output one as post dso you may need to turn the wick down :D
  17. akpak

    akpak Member

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    OK, thanks.
    This morning I tried routing the RF through the DVD recorder first, then into the Sagem, and finally the TV. As far as I could tell there was no difference in the signal strength at the Sagem.

    Next, as I already had one, I tried a set-back booster (stated gain 14dB at 600MHz, noise <3dB). This seemed to have varying effects: mostly it boosted Mux1 to around 45%, but with all the other muxes the reading dropped a little (eg from average 65% to average 61%). However, at one point, and we're not sure why, the level on BBC1 (Mux1) jumped to about 74% - but after switching muxes and back we couldn't repeat this.

    But then it started pixellating and freezing again, so as the booster obviously wasn't solving the problem I've removed it.

    I don't have a masthead booster, and although I might be able to fit one myself in the loft, if it involves working on the roof I'd need to get someone in to do it. Anyway, thanks for the help.
  18. Rodders53

    Rodders53 Active Member

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    Interesting.
    This suggests that it is not low signal per se that is the 'problem' for the Sagem, more one of noise (and interference) as the amplifier cannot improve the signal to noise ratio.

    In your earliest posts you stated that the TV's DTT reception is fine when the Sagem is having problems. I think we've eliminated any 'RF loop through' amplifiers as improving the signal for the TV (you said it was fine direct as well, iirc)....

    This leaves me to conclude that the Sagem's tuner internals are just not up to the job.

    If I were you I'd consider asking friends, neighbours and work-colleagues if they have another brand of PVR you could try in your home to see if they behave the same. Even a different sample of the Sagem might tell you something!

    Alternatively trying your Sagem on a different aerial installation would also be informative (does it still misbehave or is it perfect?).
  19. grahamlthompson

    grahamlthompson Well-Known Member

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    That's not entirely accurate. An amplifier can't improve the signal to noise ratio if the noise is already present. It can if the amplification is used before the noise gets into the system, hence the reason for a masthead amplifier.
  20. akpak

    akpak Member

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    Maybe - but this post suggests it's more sensitive than the Humax. Or are you referring to things like error correction and noise filtration rather than simple sensitivity?

    (As an aside, I was looking into the TV's signal strength data, and Google led me to this page. Mine's a Sony too, so presumably has similar error correction - and maybe it's just better at this than the Sagem. And whereas that post reports a CODFM AGC of over 1000, my TV generally gives a figure of under 100 on Mux1 and around 200 on the others.)

    Meanwhile I've been talking to some of the neighbours, and only one is getting Freeview from the same transmitter as me, but he seems to be getting a decent signal so maybe it's something to do with my aerial or RF cable. I think I've probably reached the stage now where I need to call in an installer to troubleshoot this - which realistically won't be till the new year now - but thanks for all the ideas anyway.
  21. Gavtech

    Gavtech Administrator

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    I wonder if your neighbour would be forbearing enough to let you try the box on his aerial?

    The results of such a test may be revealing.
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  22. mrfreeview

    mrfreeview Guest

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    I've read all your very long first post. & this is the bit: signal quality that jumps out at me and the bit I would concentrate on.

    Improving the signal strength by amplification may actually do nothing as this may also amplify the noise that is causing the signal quality issue. . . . And amplifiers do themselves introduce noise.

    From your post it sounds to me as if this is a problem with impulsive noise.

    This could be directly coupled. . . . coming up the mains or be radiated. . . . being "received" by either your antenna or feeder cable.

    From my own experience radiated noise can be caused by pressure sodium lamps reaching end of life, motors in CCTV cameras, thermostats in fish tank heaters. But it is a known fact CB and Taxis radios can cause interference. As can the thermostats in boilers, when the contacts open.

    What I can't find in your post is if this problem is only with 1 MUX or more.

    What you might consider doing is looking at an analogue picture during a period of signal quality issues being reported on the Sagem. Sparkles and lines on an analogue picture can give a clue as to the problem.

    Also if your using an HDMI cable, perhaps remove this and see what happens. There are problems with some of these cables . . .. lack of proper screening.

    Telling it as I find it &#8211; I hope my post has been of use :)
    Last edited by a moderator: Dec 11, 2010
  23. akpak

    akpak Member

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    Thanks for the reply. I've thought about interference, but I can't work out what it might be, or where it might be coming from. One of the things is that I had no problem with the digital signal for around 18 months or so, and even after that, only occasional problems until recently, and I can't think of anything that's changed which might be causing interference now, but wasn't a year ago.

    This was one of the reasons why I thought my original set-top box was the source of the problem, but I've now changed this and the problem remains. However, it does seem with the new box that the problem is mainly with Mux1 (every day this last week), but I have once seen it to a lesser extent on the other muxes - but it's an intermittent problem, and as I've only had the new box set up for a week, I haven't really had long enough to get a full picture.

    I've tried swapping the HDMI lead for a SCART, but it didn't make any difference. However, next time I notice the break-up, I'll switch to analogue and see what's happening there.

    Added after posting: I've just remembered that because of the problems with digital, this last week I've been recording programmes in both digital and analogue; some of the digital programmes (from BBC1) were almost unwatchable, but the analogue versions were fine.
    Last edited: Dec 11, 2010
  24. mrfreeview

    mrfreeview Guest

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    Mux 1 = channel 49 ? UK digital TV reception predictor

    Which if so may indicate that a low signal problem can be taken out of the equation, as channel 49 is at 10kw ERP, the highest of the 6 MUXs.

    I am beginning to wonder if you are also now RX channel 49 from another transmitter. As DSO proceeds southwards channel numbers used by other transmitters for DVB-T are changing and their powers (ERP) are being increased. If that is the case no amount of filtering is going to help.

    This thought is based on my current experience with Rx DVB-T from Crystal Palace for which I am on the very fringe of reception.

    My set up: I have a large GROUP A antenna pointing south and a masthead amp and before the Amp a A-K diplexer to act as a band pass filter to remove "noise" above group A. Then suddenly some months back just ONE of the MUXs became unwatchable. I fished out of the cupboard my old Nokia 221T STB because it measures both RX power in dBm and BER. The RX power on all MUXs was still sufficent and the BER was Zero on all the other 5 MUXs. . . .but jumping around continually on the problem MUX.

    I've also looked at the analogue channels also in Group A from both Sandy Heath and Crystal Palace and I could see nothing amiss with the pictures or teletext.

    So at the moment for my problem I've ruled out impulsive noise as the cause and put it down to interference in the single channel. . . .and done no more as I still have DVB-T from Sandy Heath.

    Telling it as I find it - I hope my post has been of use :)
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    Last edited by a moderator: Dec 12, 2010

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