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@xxxxxxxxxxxx> 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/ -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
