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

[Georg Acher <acher@in.tum.de>]

On Sat, Oct 18, 2008 at 06:42:08AM +0100, Darron Broad wrote:
<cx24116> 
> The trouble there is that the scaling for the cx24116 already works
> from an end-user perspective. The value derived in the code is
> a possible maximum of 160 from the chip. REELBOX decided on 176
> which may be more accurate.

The reelbox code was just a heuristic approach to scale the value so that
less than 30-40% is where the trouble starts... I've more or less matched it
to femon's colors. There was no intention to indicate dBs, as end users
don't understand dBs anyway ;-)

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...

The signal strength in 9e/9d is the value of the AGC voltage. Register da
seems to contain the estimated power level (-25 to -70dBm), but there's no
further information about that (step size etc). I guess the firmware derives
it from the AGC settings.
-- 
         Georg Acher, acher@in.tum.de         
         http://www.lrr.in.tum.de/~acher
         "Oh no, not again !" The bowl of petunias          

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