Re: Some questions about DSO
- From: Mark Carver <mark.carver@xxxxxxxxxxxxxxx>
- Date: Mon, 06 Apr 2009 07:45:52 +0100
Java Jive wrote:
Being doing some background work on Digital Switch Over aka Analogue
Switch Off, but need some clarification on some particular points, if
anyone can help out:
1) In Ofcom's info ...
... those transmitters that have been switched over, for example
Selkirk, are still shown as broadcasting analogue. Presumably this is
just old data? I take it that none are actually still broadcasting in
No, Selkirk, nor none of its relay stations are broadcasting in analogue.
It's Ofcom being totally useless again. Even the Beeb with their revamped engineering info website still have this page:-
2) I have figures for the transmitter ERP's, but am not sure how to
relate this to range. I know that the signal decreases by an inverse
square law, but am not sure how to relate a given ERP to the
sensitivity of an 'average' receiver, analogue or digital, connected
to an 'average' aerial.
ERPs quoted for analogue TV transmissions are peak sync. In System I the video signal is inverted for transmission. Therefore the sync bottoms become the radiated signal peak. 'Average' ERP at any given moment over the course of a field will be defined by the video signal's APL (Average Picture Level) inverted. In other words average ERP levels will be lower. For an all grey flat picture, average ERP will be about 50% of the peak value quoted.
COFDM signals as used for DTT take on the characteristic of Gaussian noise, when viewed on a spectrum analyser they appear as a 8 MHz wide 'lump of grass'. AIUI ERP quoted relates to the average level of this lump.
So, to compare the quoted ERP for DTT, you have to do so against average ERP for analogue. However DTT receivers can resolve a useful signal at lower levels than analogue (until of course they drop over the digital cliff).
To provide the same size service area, and equivalent penetration, from a given transmitter the average DTT ERP has to be about 7dB lower than the peak sync ERP for analogue. Or doing the maths, 7dB is a 5:1 power ratio, therefore Crystal Palace analogue ERP is 1000 kW, its post DSO DTT ERP will be 200 kW.
Most sites after DSO will have ERP levels of -7dB (rel to analogue) for the PSB muxes. Some will run at -10dB for the COM muxes, AIUI because of frequency planning constraints.
Please replace invalid and invalid with gmx and net to reply.
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