[linux-dvb] [RFC] SNR units in tuners

Georg Acher acher at in.tum.de
Sun Oct 19 23:52:26 CEST 2008


On Sun, Oct 19, 2008 at 09:43:53PM +0100, Darron Broad wrote:
 
> >The docs for the 24116 say that the snr is measured in 0.1dB steps. The
> >absolute range of registers a3:d5 is 0 to 300, so full scale is 30dB. I
> >doubt we will see the 30dB in a real-world setup...
> 
> Okay, so we know the step size of 0.1 per bit and that's measured
> within a range of 0 to 300 but that doesn't actually say what it's
> value is? Ie, is 50=5dB or something else?

I guess so.
 
> All the graphs I see for QPSK and 8PSK in use in the real-world
> suggest the theoretical limit of esn0 is a lot less than that available
> range. I don't know what is the accepted error rate to set this limit.
> Perhaps someone who has authority on this subject can chime in?
> 
> On the cx24116 testing observed that a register max of 160 from QPSK
> gave good approximation to that given by regular sat-kit sitting
> around 100%. If that really means 16dB then it doesn't look right
> compared to the graphs I see, what's wrong here?

I've looked at the IF output of the frontend (it should be always a CX24118)
with spectrum analyzer. The input was from Astra 19.2 with a 80cm dish. The
slope top above the noise floor indicates also a SNR of 16, maybe 18dB, but
not more. It's really that low...

-- 
         Georg Acher, acher at in.tum.de         
         http://www.lrr.in.tum.de/~acher
         "Oh no, not again !" The bowl of petunias          



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