[mythtv-users] OSD Signal Strength

[Joe Votour]

Quoting noelcollins at optusnet.com.au:

> Does anyone know how or what the OSD signal % means?
>
> I am using the current release of Mythbuntu (7.10) with a Leadtek   
> DTV2000H card receiving Free to Air DVB-T ( all VHF freq ) in   
> Sydney, Australia
>
> Last night I was playing around with the antenna system which has a   
> masthead amp and got the following readings
>
> 1) Lead direct to wall ***Signal 50% - S/N 4.8 db - BE 0 - Lock OK
> 2) 10db attenuation ***Signal 55% - S/N 4.8db - BE 0 Lock OK
> 3) 20bd attenuation *** Signal 60% - S/N 4.8db - BE 0 Lock OK
> 4) 2 way splitter *** Signal 75% - S/N 4.8db - BE 0 Lock OK
> 5) 3 way splitter *** Signal 100% - S/N 4.8db - BE 0 No Lock
> 6) 4 way splitter/Amp +12db *** Signal 35% - S/N 4.8db - BE 0 Lock OK
> 7) 2 piggy backed splitter/amps ***Signal 20% - S/N 4.8db - BE 1000 No Lock
>
> From this I am getting the idea that the % is inverse to the actual   
> signal strength and that below 20% I am actually overloading the card.

Disclaimer: I'm a QAM/DVB-C guy, I don't know much about DVB-T.

The numbers come from from the Video4Linux driver, and there are some  
drivers that are known to give incorrect numbers (the driver for the  
DViCO Fusion Lite back when I used it).  A 4.8DB Signal-to-Noise ratio  
is horrendous, and would not actually give a usable signal (at least  
not in a DVB-C/QAM world).

It's been over a year since I've done any V4L work (not driver  
development, just a simple QAM capture program), and I found that the  
API for fetching the signal level at that time *suggested* a return  
value from 0-65535 for signal strength (0 being 0%, 65535 being 100%),  
but not all drivers necessarily honor that.

I would say that direct to the wall is probably your best signal, 20DB  
is some serious attenuation.   However, you may very well have to do  
things by trial and error.

If you really need to know the signal levels, then you'll either need  
a different DVB-T card (where the numbers are implemented correctly),  
or use another OS/piece of hardware that gets it right.

-- Joe


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