[mythtv-users] How to limit 2xDVB-S (+DiSEqC) to 1 LNB at a time?

Simon Hobson linux at thehobsons.co.uk
Sun Apr 26 17:58:20 UTC 2009


David Lister wrote:

>Is there a way which would allow e.g. 2-4 sats for 2 tuners with one
>cable? :) I'm still new to this...

No.

The reason is that unlike terrestrial where you just feed a load of 
signal down a cable and the tuner picks out what it wants, with sat, 
the LNB does some of the conversion. In a basic single sat system, 
signals sent up the cable will typically select high or low band, and 
horizontal or vertical polarisation - thus selecting one of four sets 
of signals. Since only one of the four sets of signals can be sent 
down the cable at once, that puts limitations on what signals can be 
tuned if you split the signal and run multiple tuners off it - ie the 
separate tuners can only access signals that are in the same band and 
on the same polarisation.

To expand on that a bit.

High and low band refer to the frequency range selected. The LNB, to 
put it in laymans terms, 'pre tunes' part of the frequency band used 
for satellite transmissions - and it converts the selected band down 
to a lower frequency that is more easily handled by passing it down a 
bit of caox cable and into a tuner. By selecting one of two frequency 
ranges for conversion in the LNB, a tuner with a relatively modest 
tuning band can tune into twice as many channels.

Horizontal and vertical refer to polarisation. A signal can be 
polarised, and a receiving antenna set at the wrong attitude cannot 
pick up the signal. The best analogy I can think of is that if two 
people stand with a piece of string between them, and one waves the 
string up and down to set up a wave along it - if you stand to one 
side you will see the string moving up and down, but if you look from 
above, you will just see it as a straight line. Similarly, if the 
string is waves side to side, then from the side you won't see it 
move, but you will from above. Conveniently, this allows the same set 
of frequencies to be used twice over - with different polarisations.

This Wikipedia article goes into it a bit more.
http://en.wikipedia.org/wiki/Low-noise_block_converter

-- 
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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