From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from claranet-outbound-smtp05.uk.clara.net ([195.8.89.38]:36034 "EHLO claranet-outbound-smtp05.uk.clara.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753114Ab3AHXSR convert rfc822-to-8bit (ORCPT ); Tue, 8 Jan 2013 18:18:17 -0500 From: Simon Farnsworth To: Frank =?ISO-8859-1?Q?Sch=E4fer?= Cc: Mauro Carvalho Chehab , Linux Media Mailing List Subject: Re: [PATCH RFCv9 1/4] dvb: Add DVBv5 stats properties for Quality of Service Date: Tue, 08 Jan 2013 23:18:01 +0000 Message-ID: <1718385.5pOCXcV7mc@f17simon> In-Reply-To: <50EC5EAB.9050705@googlemail.com> References: <1357604750-772-1-git-send-email-mchehab@redhat.com> <10526351.JB9QcZTfut@f17simon> <50EC5EAB.9050705@googlemail.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8BIT Content-Type: text/plain; charset="iso-8859-1" Sender: linux-media-owner@vger.kernel.org List-ID: On Tuesday 8 January 2013 19:00:11 Frank Schäfer wrote: > Am 08.01.2013 12:45, schrieb Simon Farnsworth: > > On Monday 7 January 2013 22:25:47 Mauro Carvalho Chehab wrote: > > > >> + > >> + FE_SCALE_NOT_AVAILABLE - If it is not possible to collect a given parameter (could be a transitory or permanent condition) > >> + FE_SCALE_DECIBEL - parameter is a signed value, measured in 0.1 dB > >> + FE_SCALE_RELATIVE - parameter is a unsigned value, where 0 means 0% and 65535 means 100%. > >> + FE_SCALE_COUNTER - parameter is a unsigned value that counts the occurrence of an event, like bit error, block error, or lapsed time. > >> + > > > >> +
> >> + <constant>DTV_QOS_SIGNAL_STRENGTH</constant> > >> + Indicates the signal strength level at the analog part of the tuner. > >> +
> > Signal strength is traditionally an absolute field strength; there's no way in > > this API for me to provide my reference point, so two different front ends > > could represent the same signal strength as "0 dB" (where the reference point > > is one microwatt), "-30 dB" (where the reference point is one milliwatt), or > > "17 dB" (using a reference point of 1 millivolt on a 50 ohm impedance). > > > > Could you choose a reference point for signal strength, and specify that if > > you're using FE_SCALE_DECIBEL, you're referenced against that point? > > > > My preference would be to reference against 1 microwatt, as (on the DVB-T and > > ATSC cards I use) that leads to the signal measure being 0 dBµW if you've got > > perfect signal, negative number if your signal is weak, and positive numbers > > if your signal is strong. However, referenced against 1 milliwatt also works > > well for me, as the conversion is trivial. > > Yeah, that's one of the most popular mistakes in the technical world. > Decibel is a relative unit. X dB says nothing about the absolute value > without a reference value. > Hence these reference values must be specified in the document. > Otherwise the reported signal strengths are meaningless / not comparable. > > It might be worth to take a look at what the wireles network people have > done. > IIRC, they had the same discussion about signal strength reporting a > (longer) while ago. > The wireless folk use dBm (reference point 1 milliwatt), as that's the reference point used in the 802.11 standard. Perhaps we need an extra FE_SCALE constant; FE_SCALE_DECIBEL has no reference point (so suitable for carrier to noise etc, or for when the reference point is unknown), and FE_SCALE_DECIBEL_MILLIWATT for when the reference point is 1mW, so that frontends report in dBm? Note that if the frontend internally uses a different reference point, the conversion is always going to be adding or subtracting a constant. -- Simon Farnsworth Software Engineer ONELAN Ltd http://www.onelan.com