Re: [PATCH RFCv9 1/4] dvb: Add DVBv5 stats properties for Quality of Service

["Frank Schäfer" <fschaefer.oss@googlemail.com>]

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