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Measuring signal quality -
The Poor Man's DTT test meter

In 1883, Lord Kelvin summarised the problem you have with DTT.

If you can not measure it, you can not improve it

Far seeing old boy, knowing about the problems we were going to have with digital TV huh? In the case of DTT, where there is a single parameter, bit error rate, that determines the quality of your viewing experience, Kelvin is right on - there is a lot to be said for measuring it. In the days of analogue TV, signal strength was a good measure of quality. Most analogue installer meters focused on measuring signal strength.

With DTT, where the system is working horribly close to the noise floor with only about 10dB of signal loss in reserve, rather than the 40dB[2] of an analogue system, signal quality is a better metric that signal strength. A good installer meter measures ideally[1 the bit error rate before the Viterbi decoder. The Viterbi decoder ekes somewhat more performance by adjusting the data thresholds based on the past history of the signal, and the signal then passes to the forward error correction which catches more. Which is a good thing when you are watching the programme, but it sharpens the digital cliff between good operation and no operation. That isn't helpful if you are trying to optimise signal quality, where you want a more gradual change of increasing error rate with worsening signal. 

You've thought long and hard about it but you don't want to stump up the £500 for an installer meter? So the first thing people reach for is the signal strength and quality indicator on their set top box. You can now measure signal quality, right?

Wrong. You can measure something, but you aren't sure what 'it' is. Signal strength is measured in dBuV and the bit error rate should be cited as a dimensionless ratio, measured before the Viterbi decoder. Manufacturers don't want  their set-top to get the reputation of being low sensitivity and demanding of the aerial system so they tend to bias things to say you have enough signal even if you haven't really. It is like the signal meters on FM tuners, which were always at 10 even though the sound was like a bag of hissing cats. You need science in this subject.

There are two ways to do this job. The best way is to have something that measures the BER pre-viterbi such as some Nebula DigiTV boxes - here is a guide to how you use that. The second way is to gauge your noise margin with attenuators, which gives you a reasonable estimate if the only degradation of your signal is steady noise due to a lack of signal or suboptimal aerial position, but does not give you a terribly accurate indication if you suffer impulse interference.

Get yourself some UHF attenuators. If you're having problems a 3 and a 6 should be fine. I needed a 3 and a 6 and a 12. In selecting them start with a 3, then a 6, then a 12 if necessary. That way you can cover attenuation steps as follows

attenuation how
3 3
6 6
9 3+6
12 12
15 12+3
18 12+6
21 12+6+9

Now insert attenuation into the aerial feed before any booster amplifiers and find out where you lose signal

In my case

channel hold lost
BBC1 12 12+3
Ch5 12 12+3
Sky Trav 12 12+3
QVC 12 12+3
The Hits 12 12+3
E4 6 6+3
E4+1 12 12+3
various shopping 12 12+3
BBC news 12 12+3
ITV news 6 6+3
Sky Spts news 12 12+3
Sky news 12 12+3
ITV 6 6+3
ITV2 6 6+3
ITV3 6 6+3
Ch4 6 6+3

you sort of  see a pattern emerging. Anything on the ITV mux - any ITV channel, channel 4 and e4 are 6dB closer to being lost than anything else. At the time of writing, clearly E4+1 must have been on another mux since it can take an extra 6dB hit before being lost, like all the other non ITV channels.

So how do you use this to optimise your aerial? You iterate, slowly, through the positioning, such that you can insert the highest amount of attenuation before losing the picture. If you can't insert at least a 6dB attenuator without losing the picture than your installation is marginal and you should expect blockiness under certain conditions.

This only helps you optimise you aerial for maximum signal strength from the wanted signal You aren't going to have great success if you suffer from a lot of interference - to track that down you need better test kit. But hey, for few quid it ain't bad. It is objective, and permits comparison between different set-top boxes on the same aerial, and indeed different aerial installations on the same set-top.

Selecting and siting your TV aerial

Aerial system components

 

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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.
  2. the 40 dB representing the variation between 70dBuV nominal spec and about 30dBuV which gives you a picture like this which is probably common in student flats...
  3. Lord Kelvin delivered himself of some other pithy epithets, such as "To measure is to know" and "In physical science the first essential step in the direction of learning any subject is to find principles of numerical reckoning and practicable methods for measuring some quality connected with it. I often say that when you can measure what you are speaking about, and express it in numbers, you know something about it; but when you cannot measure it, when you cannot express it in numbers, your knowledge is of a meagre and unsatisfactory kind; it may be the beginning of knowledge, but you have scarcely in your thoughts advanced to the state of Science, whatever the matter may be." One of his finest, though nothing to do with measurement, is "The true measure of a man is what he would do if he knew he would never be caught."

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Text and photographs RM 2005 unless otherwise credited