Re: [PATCH 2/3] femon: Display SNR in dB

[Stefan Richter <stefanr@s5r6.in-berlin.de>]

On Nov 25 Jean Delvare wrote:
> Hi Manu,
> 
> On Sun, 24 Nov 2013 22:51:33 +0530, Manu Abraham wrote:
> > Sorry, that I came upon this patch quite late.
> 
> No problem, better late than never! :)
> 
> > On Mon, Jun 3, 2013 at 8:51 PM, Jean Delvare <khali@linux-fr.org> wrote:
> > > SNR is supposed to be reported by the frontend drivers in dB, so print
> > > it that way for drivers which implement it properly.
> > 
> > Not all frontends do report report the SNR in dB. Well, You can say quite
> > some frontends do report it that way.
> 
> Last time I discussed this, I was told that this was the preferred way
> for frontends to report the SNR. I also referred to this document:
>   http://palosaari.fi/linux/v4l-dvb/snr_2012-05-21.txt
> I don't know now up-to-date it is by now, but back then it showed a
> significant number of frontends reporting in .1 dB already, including
> the ones I'm using right now (drx-3916k and drx-3913k.) With the
> current version of femon, "femon -H" reports it as 0%, which is quite
> useless. Thus my patch.
[...]

Hi,

I inherited this in drivers/media/firewire/firedtv-fe.c:

static int fdtv_read_snr(struct dvb_frontend *fe, u16 *snr)
{
	struct firedtv *fdtv = fe->sec_priv;
	struct firedtv_tuner_status stat;

	if (avc_tuner_status(fdtv, &stat))
		return -EINVAL;

	/* C/N[dB] = -10 * log10(snr / 65535) */
	*snr = stat.carrier_noise_ratio * 257;
	return 0;
}

As far as I understand, the comment should have been written with a "FIXME"
prefix.

I have no documentation and no personal manufacturer contact (and the
devices are EOL).  All I know from the driver source is that we do get a 16
bits wide carrier_noise_ratio.  So it appears to be something on a scale
from 0x0000 to 0xffff, and the comment makes it look like being on a linear
scale originally.

I could cross-check with a Windows based TV viewer application what signal
strength value is presented there to the user with DVB-T and DVB-S2
incarnations of FireDTV devices.  Right now I don't remember how that
application presents it (i.e. as percentage or dB or whatever...).
When I looked at that application and at kaffeine some years ago, they
displayed grossly different values.  I did not research back then whether
the Linux driver or kaffeine or both treated it wrong.

Any advice for the quoted kernel driver code?

Thanks,
-- 
Stefan Richter
-=====-===-= =-== ==--=
http://arcgraph.de/sr/