[mythtv-users] DVB technical issues about scan

[Simon Hobson]

James Linder <jam at tigger.ws> wrote:

> Long Answer:
> I’m in perth - flat ground for 50 km, minor escapement then more flat ground.
> In Aus all stations use the same frequency for every site.
> The receiver *must* do magic to ensure packets are from the correct site *not* (maybe it can’t) that the best packet is used (or a good vs bad packet)
> 
> By scanning and renumbering conflicts it is clear that some are good (same channel) and some are bad. The good is always good, the bad are never good.
> 
> The conflicts must come from ‘over the hills towns’ (say 100 Km away) and quality is not-good to aweful.

It sounds a lot like they gone for the worst of all worlds then.

If it were an SFN then you wouldn't see any duplicates - there would just be one set of channels as the bitstream from every tx would be the same. The fact that you see multiple versions means that the txs are sending different data streams - and in that case, no broadcast engineer in their right mind would put (near) adjacent txs on the same frequency, that's like chapter one of broadcast engineering for dummies !

AIUI, with DVB-T there is data in the stream which identifies the transmitter (or at least, region) and it may be worth looking carefully at the data in the database to see if there is anything at all (some tx/region/area identifier ?) you could use to clean up the bad channels you are picking up.

Otherwise, there are some other tricks that may be worth trying.
First off, see if there's some information available on where the txs are, and then try and work out which you are getting a signal from. In some cases, I've read that you can improve the situation by deliberately mis-aiming your aerial :
Say the unwanted tx is 60˚ off from your wanted tx. Instead of pointing the aerial directly at your wanted tx, point it 30˚ off - it'll significantly reduce the wanted signal, but it should completely eliminate the unwanted signal which will now be hitting the aerial at 90˚ which should be a null in it's sensitivity.
Alternatively, you might try an attenuator and see if you can drop the unwanted signal below the threshold of the tuner. I was going to suggest only doing this when channel scanning, but I think it would need to be permanent if the muxes are on the same frequency.

I think these latter two (or combination) are the way to go. With two txs on the same frequency, I'd have thought that's always going to cause major problems for the tuner - in fact I can't really see how it'd work at all really. So eliminating the unwanted tx by dropping it's signal strength low enough that it stops being a problem has to be the way to go.

Is there no official body/trade group/whatever you can go to for help and advice on this ? If it's as you describe, I can't see how it can't be a big problem for lots of people.