[mythtvnz] Re: Conflicting voltage / signal on the same coax

[Sam Hadley-Jones]

Hey guys,

This is still an issue even if both dvb-s receivers are requesting the
same polarity. If the 22kHz signal is sent from both devices to the LNB,
the signals may cause the LNB to misbehave (due to aliased low
frequencies or harmonics). The tone sources are also fairly low
impedance meaning that they may be damaged. In electronics you learn
quickly that two low impedance (in other words, more powerful) sources
don't like talking to each other. The problem would be a whole lot worse
with 13/18V switching. Even a difference of a few millivolts is enough
to cause high circulating currents on a short length of conductor.

Passive splitters should have a high pass filter comprising usually of a
high value resistor and a low value capacitor. The HPF is designed to
block the 13/18VDC and 22kHz tone but pass the higher frequencies (in
our case, 950-2150MHz). The idea is that you have one receiver
(designated as the one which can control the LNB) connected straight
through to the LNB while all other receivers are connected to the same
coax via HPFs.

If in doubt, open up your splitter and make sure the receiver sources
are seperated by at least one filter. Other than that you should be
fine.

Here's a thought, can you get lightening arrestors for satellite coax?
I've sunk a lot of money into my mythbox and don't really want to see it
go up in smoke. I don't know what the earthing arrangement is on a
set-top box but a dvb-s card in a computer has the outer conductor of
the coax tied hard to earth. This presents an attractive path for a
lightening strike!

Sam.

> On 7/29/07, Simon Third <simon@simtec.co.nz> wrote:
> 
> 
> Steve, I figured the HVR-3000 card would drive the DC power and clash with
> the Sky box.  Maybe this is not he case?
> 
> 
> It can be an issue if one device wants horizontal and one wants vertical -
> horizontal will be the polarity chosen. That's what happens now with Sky on
> V and Freeview on H (though it seems to me that it's still better have
> Freeview with the potential to screw up a simultaneous Sky recording than to
> not have Freeview). But once Sky switch to H on Tuesday morning both will be
> the same anyway so it would only be an issue if you were using some vertical
> polarity DVB channel ( i.e. non-Freeview). AFAIK it shouldn't be a problem
> to have two devices both driving the voltage that selects horizontal through
> a multi-power-pass splitter. I was wondering if you'd heard something
> different.
> 
> Steve 
> 
> My electronics background tells me this is not a good idea.  If you tried it
> and the smoke didn't come out, then lucky you.
> I would not recommend this to the list.  Someone is bound to have a
> configuration one day that generates smoke!
>  
> S3