From mboxrd@z Thu Jan 1 00:00:00 1970 Return-path: Received: from mail-la0-f44.google.com ([209.85.215.44]:33238 "EHLO mail-la0-f44.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1756231Ab3AHR7o (ORCPT ); Tue, 8 Jan 2013 12:59:44 -0500 Received: by mail-la0-f44.google.com with SMTP id fr10so805695lab.31 for ; Tue, 08 Jan 2013 09:59:43 -0800 (PST) Message-ID: <50EC5EAB.9050705@googlemail.com> Date: Tue, 08 Jan 2013 19:00:11 +0100 From: =?ISO-8859-1?Q?Frank_Sch=E4fer?= MIME-Version: 1.0 To: Mauro Carvalho Chehab CC: Simon Farnsworth , Linux Media Mailing List Subject: Re: [PATCH RFCv9 1/4] dvb: Add DVBv5 stats properties for Quality of Service References: <1357604750-772-1-git-send-email-mchehab@redhat.com> <1357604750-772-2-git-send-email-mchehab@redhat.com> <10526351.JB9QcZTfut@f17simon> In-Reply-To: <10526351.JB9QcZTfut@f17simon> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 8bit Sender: linux-media-owner@vger.kernel.org List-ID: 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. Just my two cents. Regards, Frank