Just out of interest, can anyone explain what exactly it is they need to do in preparation for DSO, and why so much cabling needs to be done? I know a lot of main transmitter sites have also undergone a lot of work. But with them already broadcasting DTT
, I would have presumed all the relevant feeds and connections are already there, and it would just be a case of switching off the analogue transmitters and turning up the power of the DTT transmissions and remapping the frequencies. Evidently that's not the case, so what else is it they're doing?
The aerials and feeders (the large co-ax cables that connect the transmitters at the bottom of the mast to the aerials at the top) are being replaced. There is a difference in terms of average power level between analogue and digital modulation. An analogue video signal is only at full power when the video signal is totally white, which happens rarely even when there's 4 going up the same feeder/aerial. A digital signal is many thousands of different 'carriers' all combined together so are at peak power all the time, at main sites there could be 6 of these in one feeder/aerial.
Therefore it's running at a higher combined power for longer and so the feeders etc need to be replaced as they can't (and weren't designed to) cope. If you saw what happened at Oxford, that's the sort of thing that happens when you shove up more power than it can handle! Also the infrastructure at most sites will be getting old so it's a good opportunity to replace everything
The sites are also getting their aerials replaced for more modern ones designed for digital, these are taking the place of the old analogue 'cylinders' at the top of the mast. They're also getting full reserve aerials further down the mast, something that wasn't standard in the past (reserve aerials varied in quality from site to site and often couldn't transmit to the full power of the mains). This is what has been causing some masts to be replaced or extended. Either the weight of the new aerial systems was too much (in the case of Caldbeck this was because it had extra ones pointing towards Scotland) or the assembly at the top of the mast wasn't long enough.
It doesn't really matter if the site had DTT in the first place, the original DTT system is being totally replaced. The transmitters it uses are a fraction of the power and the aerials are usually further down the mast than the analogue ones. Also it wasn't fully redundant... basically it was a quick add-on to each site