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 of an analogue system, signal quality is a better metric that signal strength. A good installer meter measures ideally 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
Now insert attenuation into the aerial feed before any booster amplifiers and find out where you lose signal
In my case
|Sky Spts news||12||12+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