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Freeview and the Tinkerer's Curse

Optimising a DTT aerial is a maddening trial-and-error job without tools. The correct way to optimise your DTT reception is with a DTT signal strength meter, looking at the pre-Viterbi error rate[1], and if you're in an area of really great reception you'd measure the digital channel power and be done.

Such meters are expensive, starting at about £500, so the DIYer isn't going to have one. So we optimise our systems by looking at the output. On analogue you got instant feedback - ghosting, graininess and snow meant optimising the aerial heading was simple. On DTT, once you've got yourself down to one picture freeze every 5 minutes, you have a problem going much further.

Say you tweak your aerial. How long do you wait before you say to yourself that was a good thing? The obvious answer is five minutes, and the obvious answer is wrong, because the noise is probably random. You have to count several intervals and average them, both before and after. Say you average four intervals - that's still 40 minutes for the four befores and four afters. 

We humans find it easy to convince ourselves that something we've done improved things. If the next freeze comes after 8 minutes instead of five we think 'great, sorted' - whereas we really need to average the glitches. And count the two-minute intervals in as well as the longer ones :-) How many of us are going to spend 20 minutes assessment per tweak? When you factor in the hellish unpredictability of local impulse interference this method of optimising the system can take weeks. Therein lies the nature of the tinkerer's curse - if the tinkerer is trying to get rid of the maddening once per half hour sort of picture freezes in DTT he is damned to go round in circles unless he changes something and evaluates it over many hours, taking careful records. The curse is that there are just not enough hours in the day to do this right without measuring equipment, and even if you had the time it's a boring job and theres a lot more interesting things to do in the world..

Basically either your DTT rig works flawlessly, or it doesn't. If it doesn't, you need to borrow a DTT  meter to get some science into what you do. 

Bear in mind the tinkerer's curse when you read on a message board that "the booster amp solved all my problems". Maybe it did. Maybe it didn't. Maybe their  neighbour turned using their mobile off or put it in a different room, while they charged off to Argos to get their booster. Maybe they shifted their leads a tiny bit away from some interference when connecting in the booster amp. With a meter they would be able to tell which of those it was and by how much the signal had improved - the instant feedback we had on any analogue TV picture is now the variation of the meter's bit error rate bargraph. There still tends to be a slight lag so even with a meter you have to pan the Freeview aerial sloooowly. 

So you need a hefty pinch of salt and some realism when reading message boards and drawing conclusions. If someone said I raised my aerial height by a metre and I got much less freezing I'd be inclined to believe them. If someone said I changed by 15 year old brown coax cable for CT100 and I got an improvement I'd also be inclined to believe them. If someone said I just got this super amplified set-top aerial and it made things so much better I would wonder what else changed - or indeed whether realignment, moving cables about didn't have just as much an effect.

Selecting and siting your TV aerial

Aerial system components

Why DTT is different from an analogue install

 

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  1. Using Channel State Information to characterise DVB-T reception, J Lago-Fernandez, BBC
    (available on http://www.broadcastpapers.com/tvtran/IBCBBCCSIDVB-T.pdf ) He actually recommends using another measure, which I have never seen on an installer grade meter, and CBER as the next best, with graphs and an explanation why.

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