[mythtv-users] LNB configuration for second DVB-S2

Simon Hobson linux at thehobsons.co.uk
Fri Aug 12 10:20:58 UTC 2011

Josu Lazkano wrote:
>Hello forum, I just bought an other DVB-S2 device and I have problem
>with LNB frecuency.
>I have a dish pointed to Astra19.2E and with one DVB-S2 device I have
>no problem, I just configure it as Universal LNB and it works great.
>But for the second device, I add this on the cable:
>I connect the two outputs to both devices and now I have no signal on
>both devices. I must configure the LNB with other frecuencies?

You have a big problem doing that.

The LNB actually does part of the tuning - it selects one of four 
combinations of polarisation (horizontal or vertical) and band (low 
or high). Thus at any time you will only ever get about 1/4 of the 
available channels down the cable. The tuner sends signals up the 
cable that both power the LNB, and instruct it which of the four sets 
of signals to select.

This is VERY different to terrestrial or cable systems where the 
entire set of channels comes down the wire at the same time - and you 
just pick out the one you want.

It is possible to split the signal - provided you have enough of it 
to start with - but you will have to do two things :

1) You will need to disable or block the signals from one of the 
tuners - otherwise you could have one sending a control signal for 
one group, and the other sending a different signal. Even if they 
select the same group of channels to be sent down the wire, the 
signals can interfere such that the LNB doesn't get anything it 

2) You will need to configure the system so that it knows that it can 
only tune channels on tuner 2 that are in the group selected on tuner 
1. I don't know if Myth can handle this, and if it does, how to 
configure it.

The other problem you may have is that a passive splitter loses a lot 
of signal - each output gets less than half the power fed into the 
splitter. Unless you have an excess of signal, you may not have 
enough left for the tuners to work (reliably, or possibly at all).

So in summary, no matter how difficult, you really want to get a twin 
output from the LNB. I'm guessing you have an old Sky minidish - you 
may find the following links useful :
This guy has a LOT of information on his site.

Or just stump up and get a whole new dish fitted.

Simon Hobson

Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.

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