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: > > <snip> > >> + <itemizedlist mark='bullet'> > >> + > >> <listitem><para><constant>FE_SCALE_NOT_AVAILABLE</constant> - If it is not > >> possible to collect a given parameter (could be a transitory or permanent > >> condition)</para></listitem> > >> + > >> <listitem><para><constant>FE_SCALE_DECIBEL</constant> - parameter is a > >> signed value, measured in 0.1 dB</para></listitem> > >> + > >> <listitem><para><constant>FE_SCALE_RELATIVE</constant> - parameter is a > >> unsigned value, where 0 means 0% and 65535 means 100%.</para></listitem> > >> + > >> <listitem><para><constant>FE_SCALE_COUNTER</constant> - parameter is a > >> unsigned value that counts the occurrence of an event, like bit error, > >> block error, or lapsed time.</para></listitem> > >> + </itemizedlist> > > <snip> > >> + <section id="DTV-QOS-SIGNAL-STRENGTH"> > >> + <title><constant>DTV_QOS_SIGNAL_STRENGTH</constant></title> > >> + <para>Indicates the signal strength level at the analog part of > >> the tuner.</para> > >> + </section> > > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-media" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
