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

[Johannes Stezenbach <js@convergence.de>]

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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