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EDN Europe's Editor Graham Prophet posts a selection of comments and insights prompted by the many items of industry news and rumour that cross the editorial desk or are gathered on his frequent travels to interviews, press conferences and events around Europe - and further afield - and somehow never find their way to the magazine or the web site, recovering some of the information otherwise lost in the noise level...

Monday, October 11, 2010

Can’t handle that.

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One more illustration that you can’t get something for nothing; Right now, we have a large high-pressure (anti-cyclone) weather system in residence over the UK and Northern France. Just the right conditions for anomalous RF propagation effects (such as, but not limited to, sporadic-E) when well-defined and stable thermal inversion layers form in the atmosphere.
When we had analogue television, this would typically result in multipath reception or co-channel reception of signals that would normally be well out of range or over-the-horizon. Ghosted pictures, TV-set AFC captured by adjacent-channel signals – all that sort of thing.
But now we have digital compressed channel multiplexes. A signal-processing miracle; all that content squeezed into the bandwidth once occupied by just a few analogue channels. And everything is well, because OFDM is resistant to multipath signals, right? Well… not all that much. Not if the decoders in my sets are anything to go by. One in a set-top-box, one in an integrated set, one in a DVD recorder. Unanimous in their response; a bit of a half-hearted attempt to stay locked to the MPEG stream, much fragmentation of the picture into MPEG blocks, followed by a descent into a sulk with report of (depending on decoder) “bad signal” or “no signal”. No ghosted or slightly disturbed picture; just – nothing at all.
That’s progress, I suppose.
To be fair, transmitted power is scheduled to be stepped up when the analogue service is turned off and the channel multiplexes re-aligned. It is reportedly 7dB down on where it will eventually be; but all the same, I’m supposed to getting the benefits of “Digital” technology, in the service area of a transmitter covering a capital city.
I expect the HF and VHF amateur radio enthusiasts have been having fun, though.

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