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The DTT conundrum

The problem with DTT is that needs a much higher minimum performance than analogue needed just to work at all. 40 years of analogue TV has spoiled us rotten - you can easily get a signal which you can watch even if it isn't perfect. There are people who use a set-top aerial - on analogue they can follow their soaps through the snow and haze. With digital, they'll be watching maddening  picture freezes and sound gaps for more than half the time likely as not. I have news for you

they lied when they told us digital terrestrial was going 
to lead to better and more reliable reception than analogue

Two things were not properly understood, it seems, at the design stage of DTT. One is just how poor most people's analogue signals were. In the late 1990s, I would guess less than half the viewers used a rooftop aerial. They may have one, but it usually fed the main TV in the living room and the proliferation of sets elsewhere in the house meant it was often not the one is use. 

The second was just how much impulse interference there was out there, and how maddening the effect was going to be on the DTT signal. Impulse interference is what you get when you turn off a light switch, your boiler cuts in, you neighbour uses his electric lawnmower or the kid across the street fires up his cheap motorbike. All of these have the power to give you a half-second sound and vision freeze on DTT, where on analogue they gave you a fleeting  bright horizontal line on a frame or so.

The third problem, that will go when analogue TV is ceased in your area, is that the presence of the analogue signal makes it much harder to receive DTT as it is a lot stronger, and if you go putting boosters on a loft aerial to get the signal up for the DTT receiver, it gets blinded by the analogue signal and you get freezes again.

To qualify the above 'they lied' statement - 

DTT, done right, gives overall better reception that most people's analogue

but they still lied about reliability... The DIYer is also faced with the problem that DTT gives you much less feedback about what you are doing. Every analogue DIYer lined his aerial up by panning it and watching the picture. The professionals did too - they might have known where to start and optimise by meter, but you had to check for ghosting. And the picture changed immediately - no half-second lag or waiting to count the number of picture freezes. You need to take this on board before you go the DIY route. With analogue, you could get a signal that was 20dB below the minimum spec signal level and still watch it - thats 10% of what it should have been. With DTT you have to get it 150% of minimum spec - the 50% to account for variations in level with weather etc. Or you'll be driven up the wall with picture freezes and dropouts.

The DIY install quality bar's a lot higher for DTT than for analogue

With analogue you could immediately see what you were doing when you tinkered with the aerial. With digital, when you are in a marginal situation, you are trying to reduce the number of random picture freezes. Say you have got things down to one freeze every half hour. Without test gear, to know you have improved that to one freeze every 90 minutes, a 50:50 chance of watching a movie uninterrupted, you'd have to watch for about ten 90 minute periods - 15 hours! That's the tinkerer's curse - it is why so many DIYers have a hard time if things don't work right first time - it takes so long to see if you are improving things by tinkering if you have just the TV and DTT box to monitor your progress. An aerial contractor should measure the signal level/bit error rate so he can see things change almost instantaneously as he adjusts things. But there again, his digital terrestrial signal level meter probably spoiled the look of £2000...

Selecting and siting your TV aerial

Aerial system components

 

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  1. Source: CAI website, July 2005, typical range £80-£190
  2. Set-top aerials are not usually balanced, so you will pick up more interference on the cable than you would with one designed for the job. It'll still be a lot better that on top of your TV!
  3. Run, do not walk, from anybody trying to sell you a UHF TV aerial with a parabolic dish on it. To be effective the diameter of the dish needs to be at least one wavelength, which is 0.5m at 600Mhz for the middle of the UHF TV band. That is one BIG set-top aerial. Any dish at UHF TV frequencies is marketing frippery, not engineering.
  4. 60dBuV minimum, 80dBuV maximum into 75 ohms if you want to be pedantic.

M E g A L i T h i A
Text and photographs RM 2005 unless otherwise credited