[mythtv-users] Off Topic - DVB-T, Muxes and reception...

[Michael Cowell]

If the muxes are broadcast at different frequencies (which they would 
be), then it is quite possible, even probable, for the SNR (signal to 
noise ratio) to be different for each.

The channel between you and the transmitter (which includes the trees) 
would have a different response for each carrier frequency. The 
amplitude / phase response could be very different for different 
carriers. If something between you and the transmitter was, say, 
resonant at a particular frequency, that would cause a lot of noise on 
that particular frequency, but little on others.

A booster cannot change the signal to nosie ratio - only boost the 
amplitude of both. However, that may solve the problem. It all depends 
on the amplitude that your tuner needs to be able to effectively 
demodulate the data signal from the carrier. If all the other 
multiplexed streams are received fine, then you should make sure that 
the booster you picks leaves them alone.

Now, unless you buy a good one you're probably going to *decrease* the 
SNR for most muxes. So if they are already somewhat prone to error - I'd 
be wary about adding more hardware between you and the signal.

I'm not sure how fir trees affect signals at the 100s of MHz range - so 
I cannot comment on whether they are the likely cause of your 
interference, or whether the problems you are having are indicative of 
the channel in your areas, as a whole. Maybe someone in you area could 
provide good advice on what they had to do to get a good signal.

Hope that was some help,

Michael

Jules Gosnell wrote:

> a quick question...
>
> I stuck up an DVB-T aerial on my garden office and with the aid of a 
> compass pointed it towards the Guildford transmitter, 3 or 4 miles 
> away. I have some fir trees between me and the transmitter.
>
> MythTV receives all but one mux without problems, but one is 
> problematic to tune into (lock keeps breaking) and channels associated 
> with tend to be the first to break up....
>
> If all muxes are being transmitted from the same transmitter, why 
> should one bundle of channels feel like it is coming from the moon ? 
> Are muxes simply s/w abstractions or do they correspond somehow to 
> physical divisions in the h/w - i.e. is my problem mux maybe being 
> pumped out at a lower power than all the others etc...
>
> Would a booster be likely to solve the problem, or do I just live with 
> it - they are tall fir trees and not mine :-)
>
> cheers,
>
> Jules
>