[linux-dvb] How to gather "Signal Quality" information from DVB drivers

Wolfgang Wegner wolfgang at leila.ping.de
Mon Jul 30 12:01:38 CEST 2007


Hi Morgan,
On Mon, Jul 30, 2007 at 12:04:51PM +0200, Morgan T?rvolt wrote:
[...]
> your real problem is not the quality part of this equation really. It
> is the fact that all drivers implement this differently. A level of
> 0x2a35 on snr could mean perfect signal on one driver, and no
> reception at all on a second driver. Some cards dont even give you
> snr, maybe even a hardware constraint. Actually this will be different
> from card to card as well (with the same frontend), given that there
> is a different sensitivity on the tuner. Getting this right is really
> a very difficult task right now, unless I am totally off here.

thank you for the excellent summary!
However, I have one thing:

the SNR value is only dependent on the demodulator you use, so even with
a different frontend, the values can be directly compared as long as you
use the same demodulator IC (and firmware, maybe...).
For most demodulators, you can give a (more or less) simple equation to
calculate the S/N (or C/N, whatever you want to call it) from the register
values, but unfortunately there are some exceptions: STV0297 DVB-C for
example.

The signal strength, in turn, is most often not even useful when comparing
devices of the same type - the tolerances are too big, you can only get
a rough idea of "any signal present" or "no signal present". (Which is,
BTW, sometimes very nice too have, too.)

> -Morgan-

Regards,
Wolfgang




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