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Rose VernonTuesday 21 June 2011 8:43AM
Bristol I live in Bristol and recently I have been unable to watch Sky news or any ITV channels. The BBC channels are perfect. What is the problem?SteveTuesday 21 June 2011 12:15PM
... UK digital TV reception predictor
Probably that the BBC "Mux" is more powerful.
Which transmitter are you using? You seem spoilt for choice! Once you know, find the page for it here and ask again. Ray ReedTuesday 21 June 2011 1:32PM
New Malden Hi, I’m new to your site and am hoping you may be able to offer some advice. At the beginning of April this year I began to get poor quality TV pictures on all channels via Freeview on my home TVs. An old analogue set seems to have poor quality pictures too. I am located in South West London, KT3 6LD, and have used the Crystal Palace transmitters with no problems for the last 20 years.
The problems I have include an overall ‘compressed’ look to the pictures, poor skin tone looking too pink and sometimes red or heavily ‘sun tanned’, overexposed highlights, particularly white, poor shadow detail, odd contrast, odd colours, particularly mauve and green, sometimes very soft pictures and darker scenes being too dark. I’ve also noticed that captions and news channel ‘ticker tape’ strings often stutter a little and become intermittently pixelated. The pictures often look very wrong on standard definition and similar, though less pronounced, on HD. These effects appear on two LED flat screens, one of which is a 9 month old Samsung C8000, and an old CRT TV.
I installed a new, good quality loft mounted, high gain, horizontal polarised, group A aerial when the new Samsung TV was bought. Again I have always had a loft mounted aerial with no problems. The aerial feeds a 4 way distribution amplifier. I’m a professional sound engineer with a background in radio. I have some electronics skills and carry out studio installations and first line equipment maintenance and have installed several aerial systems, amplifiers, cabling, etc. I’ve checked every connection from the loft down, remade coax and F type connections and disconnected the distribution amp all to no effect. I’ve checked signal strength with a basic DTV meter, which shows a high signal level, and have even run a coax cable direct from the aerial to one of the TVs with no improvement. I spent some time fine tuning the many picture options on the new Samsung TV and, before these recent changes, had what I considered to be a very good picture. I am unable to make any adjustments that make much difference to the poor picture quality I am now experiencing.
I’ve checked the BBC and Digital UK websites for information about reduced power or planned work at the Crystal Palace transmitters and see that there are no problems listed. I did wonder if anything had changed at the transmitters recently, lower power output, higher compression rates, bitrate reductions, any changes made to multiplexes in preparation for the digital switchover perhaps but I’m told there have been no changes.
I’m guessing the problems are caused by some sort of RF interference. I wonder if anyone is able to throw any light on the problems I’m experiencing or can explain the sudden poor quality TV pictures I now have.
Apologies for the long description. Thank you for your attention.
Ray Reed
Mike DimmickTuesday 21 June 2011 3:14PM
Ray Reed: I'll say at the outset that I'm a software engineer rather than an electronics or RF engineer, though I did cover some electronics at university (started out studying electronics but couldn't handle the maths for Semiconductors and Opto-Electronics).
You could have too high a signal level.
Digital TV requires that all amplifiers in the signal chain are linear, or as close to linear as possible. Distortion introduced by non-linearities causes intermodulation of the individual channel transmissions making up the service as a whole. At Crystal Palace there are currently 11 services, four analogue and seven digital (six SD, 1 HD), plus one from Croydon (analogue Channel 5). The intermodulation changes the amplitude and phase of the signal from what was intended.
The digital transmissions carry redundant information from which the original data can be recovered. They also carry a further checksum which can indicate when the data recovered is incorrect. This 'outer code' can detect up to 16 bytes in error and correct 8 of them. The effect of uncorrected errors depends on where the errors fall and could cause colour errors, synchronization errors, picture and sound drop-outs. The MPEG 2 video compression scheme relies heavily on encoding only changes from frame to frame, so errors in one frame can persist for several frames.
The Confederation of Aerial Installers recommend that digital signal levels are set to 45 to 65 dBuV (some sources have 60 dBuV) for each 8 MHz channel. Ensure that your meter is measuring the power across the whole channel - for analogue, most of the power is concentrated in the carrier (especially in sync pulses) so older meters may measure a narrower bandwidth. Analogue levels should be set to 60 to 80 dBuV. At Crystal Palace, digital transmissions are currently 17 dB down on analogue, which means that setting analogue correctly should give roughly correct digital levels and vice versa. The overall signal level should be less than the maximum marked on the distribution amplifier.
I wouldn't rely on a cheap meter, they are often poorly calibrated - both in the designed range and in quality control - or don't even measure the right thing.
Intermodulation between two analogue signals tended to only cause a 'herringbone' pattern, due to the channel spacing. Digital signals had to be packed in anywhere they could be fit in, but to an analogue receiver, a superimposed digital transmission just looks like random noise - this is a deliberate property of digital transmissions, which are fed through pseudo-random processes to make them look random.
Your postcode is only 12km away from Crystal Palace. If the aerial were outside, you could theoretically get 100 dBuV at the aerial terminals for analogue channels, and 83.5 dBuV for digital. 
http://www.megalithia.com/elect/fieldstr.html
Neighbouring buildings can block signals, so you often get 10-20 dB less than predicted, and the tiles or walls will reduce that further. Still, I think it's more likely that your problems are down to excess signal than insufficient.
To fix the problem, I'd go back to your old aerial. You didn't need to change it. If that still gives excess levels even without the amplifier, add an attenuator. Ensure that the amplifier is only adding as much gain as you lose through the splitter - this should be marked on it, typically a four-way splitter drops 8 dB.
I suspect the change was down to the switchover at Sandy Heath in April. Digital transmissions cause more harm to each other than an analogue signal does to a digital one. While Sandy Heath is off to the side, aerials do still pick up some signal from neighbouring transmitters, and refractions and reflections within the loft space will amplify this. Ray ReedTuesday 21 June 2011 5:10PM
Thanks for your comments so far. I did think the signal may be too strong, though I could see no reason why it should suddenly change from its previous levels, so I disconnected the DA and put 1 then 2 attenuators on the line with no effect. I changed the original aerial as I couldn't remember what the spec of the old one was and wanted to give my shiny new HD TV the best signal I could. I tried moving the new aerial around the loft and put the old one back up, again no change. Interesting possibility re the Sandy Heath Tx as it coincided with the onset of my problems. Any ideas what I could do about it if it is the problem?SteveTuesday 21 June 2011 7:01PM
Try a cheap set-top aerial! It will probably get the local signal well enough, but not the distant ones.
Or the Sketchley Loop.
Just as modern high performance cars run happily on 95 octane mogas your new TV just needs a good enough one.Jm FTuesday 21 June 2011 8:15PM
Ray - you might be getting adjacent channel interference, where the analogue signals are received and are too powerful for the digital channels that are adjacent (there's a bit of overlap from the analogue which gets worse if the analogue is much brighter than the digital signal).
The only MUX that's non-adjacent is MUX B, so are programmes such as BBC4 (9), CBeebies (71) and BBC Parliament (81) any better behaved?
Ray ReedWednesday 22 June 2011 10:57AM
Thanks again for our suggestions. I would reiterate that the problem only surfaced in recent weeks after using the current set up successfully for many months and the previous TV for many years. Jm F, same problem on the channels you mention. Can anyone suggest or recommend an RF engineer of some sort in my area who could investigate the problem perhaps with spectrum analysers, etc?SteveWednesday 22 June 2011 5:17PM
Rau - You said your problems started in April. That was when a lot of places converted to digital only, with increased power and new frequencies. So soething related to that it at minimum a serious possibility.
You have a strong signal. Wolfbane - notoriously conservative - says you need only a set-top. Have you tried one?
Have you compared notes wiith neighbours?MazbarThursday 23 June 2011 5:46PM
Ray i know nothing about your line of work, there is a lot more to fitting a aerial than most people think. Just call a local aerial rigger to come out and check everything with a proper meter this will cost you money but it will solve your problem.Ray ReedFriday 24 June 2011 1:59AM
New Malden Steve, I’ve now tried taking the front elements and reflector off the loft aerial to drastically reduce the gain with no affect. Also bought a basic set top aerial to try which just about gets a useable signal in one position. It may have reduced the problem a little but I’m not sure, if it has it’s not by much. I do still wonder if it has anything to do with the increase to full power in other areas. My immediate neighbours get their TV via Virgin cable. I haven’t canvassed any others, yet.
Mike, I tried bypassing the amplifier in the early stages of investigation and even ran a coax cable out of the loft direct to a TV in case there was a problem with the down lead, no change.
Mazbar, I think I’ll have to admit defeat and do as you suggest.
Many thanks to all for your interest and suggestions. Please feel free to comment if anything else occurs.
SteveFriday 24 June 2011 10:34AM
Ray - you have (?) tried a different TV to make sure it really IS the signal?
I often comment that what you most importantly pay a TV aerial man for is his local knowledge.jb38Friday 24 June 2011 12:21PM
Ray Reed: An aspect of what you mention is a bit puzzling, that is when you say that these effects can be seen on two LCD sets and also on an old CRT type, which I take it might be fed from a set top box.
The area I really find puzzling is, that if a problem is cause by anything connected to the signal being received, e.g: overloading or anything else if it comes to it, the symptoms seen portrayed when trying out the same signal on different pieces of equipment are never exactly the same, as various brands cope differently when dealing with a signal problem, that is of course provided they do NOT use the same chassis.
I would be interested in knowing the result of trying out what Steve has suggested, as I suspect that overly complex reasons are possibly being thought about for whatever is responsible for your difficulties. Jm FSaturday 25 June 2011 8:04PM
Ray Reed: Possible options for RF interference on digital TV include things like taxi transmitters, though your aerial would need to be fairly close to any offending transmit antenna.
Power line adapters for carrying internet signals around mains wiring are another option for interference, though I've not seen any yet that interfered directly with digital or analogue TV.
Your suggestion of a spectrum analyser test seems like the next logical step to identify what's going on.JayFriday 1 July 2011 1:00AM
Its the TV, I have removed all other tv leads, and switched off everything near.
With no cables other than the aerial lead, we still have issues.jb38Friday 1 July 2011 3:16PM
Jay: It would appear that you are virtually on the doorstep of the Whitehawk Hill transmitter, and although admittedly herringbone patters are more associated with analogue TV interference, the exceptionally powerful signal you are receiving could be causing severe instability in the tuners RF circuitry, this necessitating the use of an attenuator in line with the aerial input.
Purely for a test, try a short piece of wire in the TV's aerial socket, if the signal at your location is overpowering you might well get results with the wire.
neuzaThursday 14 July 2011 6:53PM
Bournemouth what I can do if i don't know what is the interference?
have a way to do that? kowning what is the interference?IrfanSaturday 16 July 2011 5:32PM
Hi i have a indoor aerial placed near a window i have used the aerial for amny years with three diffrent set-up-boxes up-to-date and they all worked fine - yesterday i bought a new top up tv t215 box and about every 20s it has a interference inwhich the picture and audio get scrambled - everything for the previous boxes was fine does this mean there is something wrong with the top-up-tv set-up-box - please could you help thanksSteve PSaturday 16 July 2011 6:28PM
Might be a faulty product.
Might just be that it is a bit less sensitive than the other boxes.
Plug in your postcode top right or here
UK digital TV reception predictor
to get info on signal strength where you are. Steve PSaturday 16 July 2011 6:31PM
neuza - what problem are you experiencing?jb38Saturday 16 July 2011 8:14PM
neuza: Just purely for information purposes, the trade predictor for the location provided reveals that at this present time only one single Mux (Ch30) from the Rowridge transmitter is receivable at a reasonable level, the rest being listed as variable, this situation remaining until 21st March 2012 when the situation improves slightly, but not completely until 18th April 2012 when you should then be able to receive all six Mux's.
Low signal levels such as experienced at your location, make reception susceptible to interference from all sorts of things, so this should be kept in mind.
Irfan: I would have a look at the suggestions at the top of the page. Josephine petersMonday 18 July 2011 7:24PM
Chelmsford please could you tell me why there are no subtitle's on itv3(10) I am deaf and I need subtitle's they have now seamed to have dissapeared Paul FordTuesday 2 August 2011 6:21PM
Atherstone Hi, I hope someone can help. I have a Toshiba Regza 32RV635D IDTV. I have a new ariel on my roof and it points at the Sutton Coldfield antenna. The ariel engineer reports a strong signal. However I am getting interference on most channels. The interference is more likely to happen when the weather is very fine or fine. It typically stops in the evening after 19:00'ish. This does not correspond to any usage pattern of any electrical items, so I suspect it is an outside interference source. BBC1, 2 etc are never affected, neither is Dave ???
Anyone throw me a life-line please?Jane LawrenceSunday 14 August 2011 8:47PM
Saffron Walden Yesterday channel on my digital tv has suddenly become distorted & unwatchable.
Any ideas? Sandy Heath transmitter.
Jane L (CB10 2QS)EdWednesday 17 August 2011 5:26PM
I live i a newly built (3 years)property. I use freeview set top boxes on three tv's.I loose sound, the picture distorts and freezes.It seems to happen when the gas boiler stars up, light switchs turned on, mobile text message received etc. does anyone know why this is happening and how to eliminate it.EdWednesday 17 August 2011 5:26PM
I live i a newly built (3 years)property. I use freeview set top boxes on three tv's.I loose sound, the picture distorts and freezes.It seems to happen when the gas boiler stars up, light switchs turned on, mobile text message received etc. does anyone know why this is happening and how to eliminate it.VanessaSunday 21 August 2011 2:27PM
Bournemouth i'm not sure if it's because of my freeview box but the only channels i can watch are the /BBC ones, Viva, E4+1, all the others i have mostly a black screen and little squares, it's been this way for about 5 months! Can it be because of my TV which is a very old second hand bush? I've try resetting it and also unpluging it for a while but the problem is still there! Please helpkeithTuesday 4 October 2011 7:58AM
Coventry i have re tuned my box. on channel 11 (pick tv) after a few seconds the picture disappears and all i get is a screen with info about challenge tv, but i can still hear pick tv. what is going on?Lemon FreshTuesday 4 October 2011 8:35AM
Watching Freeview in Coventry.
Pick TV and Challenge ( Ch 11 and 46 ) show program for approx. 30 seconds, and then show only a graphic - press 'red' for 'Challenge' ( games, quizzes and MUCH more ...' ) or press 'blue' for 'Holidays' ( 'fabulous holidays at fabulous prices' ). There's also a 'Press TV guide or key in channel number to return to TV' blurb at the bottom of screen.
Pressing TV guide / keying in channel number doesn't resolve issue.
Not noticed this before. Don't really tune in to either channel much, so not a huge problem in itself ... mebbe an indication of just how shoddy the switchover has been / future issues ?
Oh .. update time : Left digibox alone on ch 11 while I was typing this post, and - et voila - the graphic has disappeared, and Pick TV has returned ... uh, animal cruelty !!
Graham StoneWednesday 12 October 2011 4:29PM
Leek location ST13 8LQ - Leek.
(re-posted and update from original in 'Leek transmiiter' page)
Hi since the Digital switchover, while I can receive a signal I have been having a problem getting it to all rooms in my house.
Thesituation is this:
I still have a loft aerial - I know I need one on the roof but that will have to wait a while.
The cable from the aerial comes into my Living room and goes through a signal amplifier - shouldn't be needed (?) but it also acts as a splitter.
one output goes to the main TV, which works fine. another output goes back into the wall into the loft and is then split to feed kitchen and bedroom.
Two Problems:
1. TVs in these other two rooms will not find all channels, and will not 'lock on' to allow you to watch those they have found. Is the answer to split the original signal in the loft or might it just be bad cabling somewhere - though there was never a problem with analogue.
2. I have 3 TIVOs in the living room, they have RF out which allowed them to be viewed in Kitchen/bedroom by putting 'in line' with analogue TV signal - all use SCART to main TV. doing this with the Digital signal - ie feed digital signal through the Tivos OR combine them after Tivos - caused interference on analoge and loss of digital channels. RF channels were: 39, 60, 53.
the 'Free RF' link for our trasmitter indicateds that only 64,65 and 66 are 'free' on our transmitter, which if I change to these it causes interference between all 3, presumably because they're adjacent ? Does Freeview really use up all other available channels ? And if so what's the answer - is there one ?
Any help/advice much appreciated - I WILL get a rooftop aerial, honest !
Graham
RacundraSunday 16 October 2011 4:54PM
Hereford Our signal at HR2 9JY comes from Ridge Hill, which as you know is now digital. Reception is normally good, but with any heavy rain in the area, BBC channels go down, while Skynews etc are still OK. This morning all BBC went down for an hour or more, with no rain within 50 miles ore more (I checked on the Met Office radar). Barometric pressure is high -- around 1023mB; could that have anything to do with it? Would a different aerial help?Racundra: The problem is not your aerial, but your cables. If the signal goes bad during rain, then it most likely that water is getting into the cable. You should replace the cable, preferably with "satellite grade coaxial" cable. John OsbornTuesday 18 October 2011 5:00PM
We have a lot of recordings on our Goodman free view box with the hard drive. The reception through the box varies from good to very bad. I have rescanned the programmes, but still get intermittent signal.
If I re install the box will we lose all the recorded programmes. I believe re install is what we are supposed to do now we have changed over. we are with Sutton Coldfield transmitter jb38Tuesday 18 October 2011 8:44PM
John Osborn: Unfortunately I cannot say for sure whether or not it would lose the previously recorded channels, as on some boxes it does whereas on others it doesn't, however it would have been of assistance to know what model of Goodman's you are actually referring to.
The other point is, and with reference to the problem you mention, it would also help in assessing possible reasons for your intermittent reception if you had provided your location, (post code) as this would then enable checking on the strength of signal you are liable to be receiving,
especially when you have mentioned its from the high powered Sutton Coldfield transmitter.
AndyThursday 20 October 2011 2:20PM
Southend-on-sea We get interference from buses, scooters and some cars, from pixaltion to complete freezing, and even lock down, where we have to change channels then return. You cannot record anything, because it becomes unwatchable eventually.
Mike DimmickThursday 20 October 2011 4:13PM
Andy: Yes, impulse interference can be a problem. It can be reduced by swapping the aerial for a new one which has a balun, a device that matches the properties of the aerial better to the properties of the cable. Older aerials tended not to have a balun or other matching device.
Aerials meeting CAI Standards 1-4 (which require a balun or otherwise matching to the cable) are usually marketed as 'digital' aerials, though there's nothing inherently digital about them. The testing to actually get the CAI Standard label is quite expensive, so many perfectly good aerials meet the standard but don't get the label. If another aerial in the same family has the label, it's likely this one matches up, the manufacturer just only did the testing for
Log-periodic aerials are inherently balanced and therefore don't need a balun.
You can also improve the screening of the cable - traditional so-called 'low loss' coax cable is now considered anything but. For best results, a dense copper braid screen over copper foil is recommended. This is normally marketed as 'satellite grade'.
You might also try moving your aerial around a little so it no longer points at the road. Aerials have a reasonably wide angle of acceptance before losing a significant amount of gain, so a few degrees to one side or the other may resolve the problem.candleMonday 24 October 2011 10:27PM
Hi I wonder if you can help.
We live DH7 and are connected to Bilsdale as are all the houses here, because there are hills in the way of Pontop Pike. We have Freeview and analogue. When we are watching freeview for e.g.BBC and ITV its sometimes fineish, others not,and when it gets really bad I try the analogue channel. Often on there, there are white horizontal lines across the analogue picture, lines that move but don't seem to look like anything I've found on web pages referring to interfernce. What is it and can anything be done? I know we change to digital next year but I'm not sure we can last that long!!! How can we find out what might be casuing the interfernce if it is that, and do we need an aerial person or an electrician. One website said interfernce could be from street lights but it doesnt coincide with them coming on.
We have a rooftop aerial with digi quality cable and it was installed by a proper CAI installer in 2007.He also fitted an amplifier.ThanksJohn HaynesTuesday 25 October 2011 5:52PM
Dover We live in St. Margarets-at-Cliffe (postcode CT15 6HH) and are having major problems with receiving ITV1, Channel 4, Channel 5 etc on our Freeview television. All the other channels are perfect. THe transmitter that we are picking Freeview up from is Dover. It looks like Ch42 / mux is basically the one that is the issue. Does anybody know why this would be the case?
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Freeview on Crystal Palace TV transmitter | ukfree.tv - independent free digital TV advice 
http://www.wrightsaerials.tv/articles/whatsat-201006.pdf 
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