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

[Darron Broad <darron@kewl.org>]

In message <20081019215225.GA11678@braindead1.acher>, Georg Acher wrote:

hiya.

>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.

Okay, thanks.

>> 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...

Thank you for this information Georg. I am now going to leave this
case closed mostly because I have looked at too many graphs, mistrusted
registers, failed to see the wood for the trees and finally you give
me absolute evidence to trust what were simple observations as
being partially true at least.

Thanks again

darron

--

 // /
{:)==={ Darron Broad <darron@kewl.org>
 \\ \ 


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