[linux-dvb] Re: Needs to get the SNR value in dB, with dvbtune

["Samuel Torton" <samuel@ncslab.com>]

Johannes,

Thank you very much for your information.
I don't know if your answer is very satisfying for my needs, but at
least, that is very clear !

I've just read on this mailing-list archives (Apr-2002) that the SNR
results should be divided by 1.000.000 (1E-6) in order to be read in dB
(?). One of my collegues did an experimentation between a field measurer
(MCP3000) and the SNR value on the card output... and I noticed that the
comparison between both results are <<<APPROXIMATIVELY>>> "1.000.000"...

I will wait for Holger's comments, if he has some more info or patches.

Thanks again

Samuel

> -----Original Message-----
> From: linux-dvb-bounce@linuxtv.org
[mailto:linux-dvb-bounce@linuxtv.org]
> On Behalf Of Johannes Stezenbach
> Sent: lundi 2 juin 2003 13:21
> To: Samuel Torton
> Cc: linux-dvb@linuxtv.org
> Subject: [linux-dvb] Re: Needs to get the SNR value in dB, with
dvbtune
> 
> Samuel Torton wrote:
> >
> > In order to get the SNR value, I used the following lines in the
> > "tune.c" source code:
> >       ------------------------------------------
> >       strength=0;
> >       ioctl(fd_frontend,FE_READ_SNR,&strength);
> >       fprintf(stderr,"SNR: %d\n",strength);
> >       ------------------------------------------
> >
> > My questions are :
> > - What's the unit of the value FE_READ_SNR to be displayed ?
> >   Usually, a big number is displayed, but I want to display this
value
> >   in dB, so is there any math formulae to apply, in order to get
> >   this value in dB ?
> >   If yes, what is it ? (*)
> > - Same questions for FE_READ_BER, FE_READ_SIGNAL_STRENGTH
> 
> The values SNR and signal strength are whatever the frontend hardware
> gives us, possibly stretched into a range of 0..0xffff.
> For the BER, I don't even know...
> 
> > (*) For example, if I install this DVB card on a Windows box, with
> > WinTV software tools on Windows, I can see the SNR level in a color
bar,
> > and the value expressed in dB, such as in the example provided in
this
> > image: http://www.hauppauge.fr/sup/cap/nexus2.jpg
> 
> We don't know how to convert the values from the hardware into dB,
> because a) we don't have enough information about the hardware,
> b) we don't have time to do it by experimentation.
> 
> Patches are probably welcome, check with Holger.
> 
> I also doubt that the values that WinTV displays are calibrated, they
> might be off by some considerable amount. After all the DVB cards are
> not made for signal measurements.
> 
> 
> Johannes
> 
> 
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