Science Questions

Why does bad weather alter digital signal?

Tue, 18th Dec 2012

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Roy asked:

Digital television signals. We’ve got a nice number of channels to choose from, okay, that's all very nice, but the signal qualities and the strength of those signals are absolutely hopeless. Whenever we get unsettled weather conditions like high winds or wind, rain, and even frost, we get picture break up and we also get channel losses where we when you look at the signal percentage, it’s zero.


Chris -   John, what do you think about Roy’s issue with weather conditions and sort of deterioration in his signal?

John -  Right, well that was specifically about television, but yes it’s back to this digital “shelf factor”.  What is this?  In the analogue world, the signal gets better and better as you increase the signal strength, until you get an absolutely noise free signal.

In the digital world, you get perfect signal, until all of a sudden, your decoder says, the signal is a bit weak, I can’t make any sense of it.  So there are three states in digital television.  You've either got perfect picture which may be with quite a small signal; then you get a very marginal signal, which means that the picture breaks up, it goes all blocky, pixelates, and the sound chirps and burbles at you; or you’ll get nothing at all.  It’s just these three states and that's the problem.

So, what it says to me is you're starting off with a fairly marginal signal to start with.  You could do with a stronger signal by whatever means, so that when you do get adverse weather conditions and winds blowing your aerial around and what not, then you've got a bit more margin to play with.


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