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Thursday, 16 June 2011
N
Nick Battle
8:09 AM

OK, thanks for the responses. It looks like the power will dramatically increase at switchover - presumably this will make an enormous difference?

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Nick Battle's 2 posts GB
Friday, 17 June 2011
S
Steve
sentiment_satisfiedGold

2:09 AM

Nick - with analogue TV there is a gradual fall-off from perfect reception to worse and worse; though even after it is too bad to watch you can still make it out.

Digital TV sends a lot of yes/no blips; along with a check system to see if it got through right.

The received signal either mends any arrors from the "redundancy" (~=duplication) in what it receives; or judges that it cannot get a perfect signal so gives up. So you go promptly from good to nbone. This is called the digital cliff - that you drop off.

So if you have a mrginal signal now the odds are that a strengthening will give you a perfect one.

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Steve's 1,172 posts GB
Tuesday, 21 June 2011
R
Rose Vernon
8:43 AM
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?

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Rose Vernon's 1 post GB
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Steve
sentiment_satisfiedGold

12:15 PM

... 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.

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Steve's 1,172 posts GB
R
Ray Reed
1:32 PM
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


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Ray Reed's 4 posts GB
S
Steve
sentiment_satisfiedGold

3:12 PM

Ray - you have a very strong signal, unless you are blocked by something.

UK digital TV reception predictor

Apparently TOO STRONG a signal can be a problem. Try removing all your amps. - or a set-top ae or "Sketchley Loop" (wire coathanger)

Compare notes here

Freeview on Crystal Palace TV transmitter | ukfree.tv - independent free digital TV advice

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Steve's 1,172 posts GB
M
Mike Dimmick
sentiment_very_satisfiedPlatinum

3:14 PM

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.

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Mike Dimmick's 2,486 posts GB
R
Ray Reed
5:10 PM

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?

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Ray Reed's 4 posts GB
S
Steve
sentiment_satisfiedGold

7:01 PM

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.

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Steve's 1,172 posts GB
J
Jm F
sentiment_satisfiedSilver

8:15 PM

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?

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Jm F's 141 posts GB
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