A look at Emley Moor’s output, on a rainy day in April 2010
This little investigation was done in a quick and casual manner on a cold and miserable afternoon when I didn’t feel like doing anything ‘sensible’. It isn’t particularly scientific, but I think it illustrates a few points about the pre-switchover situation, where digital and analogue TV run side by side. It was provoked by the appearance of the temporary HD mux on Emley Moor. For a while this was 9dB below the adjacent mux, which would have been pretty useless, but after a few days it settled down at a level roughly the same as mux D, which makes sense because both are the same ERP.
In the past we have been accustomed to mux D being the troublesome one; now we have two troublesome ones! The main analyser screenshot shows the full spectrum used by Emley Moor.
You can see how the 4kW muxes are significantly lower than the rest, which are 10kW. The analogue video carriers are all about 870kW, which comes out at about 20dB above the muxes. You can’t see the ones on channels 41 and 51 very well because I carelessly left markers over them. This screenshot was taken with the analyser storing the maximum levels for about a minute, and this does tend to exaggerate analogue carriers by about 8dB.
The screenshot with channels 39, 40, and 41 shows that there is only a tiny gap between the muxes on channels 39 and 40. The ch40 mux is offset downwards by 167kHz to move it away from the not-quite-perfectly-suppressed lower sideband of the ch41 analogue video carrier, but there is no offset on ch39, so the two muxes are almost touching. Let’s hope the receivers can cope.
As you can see most of the muxes are about 20dB below the average level of the analogue signals. That’s how it should be because digital TV will work with a signal to noise ratio that’s about 20dB lower than analogue needs. So, although the signal strength is much lower, the coverage is about the same.
This little bit of nerdy fun was inspired by Justin Smith, who has dedicated himself to the investigation of aerial characteristics, and has brought it to a fine art. It’s heroic work that takes all his spare time, yet his wife still stands by him! Justin has done what the aerial manufacturers don’t want anyone to do: he has compared aerials objectively and has stripped away what he calls the ‘marketing bollocks’.
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