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. Just my two cents. Regards, Frank -- 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
