[linux-dvb] [Proposal] Meaningful reporting of SNR

Rusty Scott rustys at ieee.org
Wed Apr 12 18:15:55 CEST 2006


On Wed, 2006-04-12 at 16:57 +0200, Robert Schlabbach wrote:
> From: "Rusty Scott" <rustys at ieee.org>
> > 1)  That it be encouraged to report SNR in dB whenever possible.
> 
> I'm afraid most hardware is incapable of producing such measurements.
> Instead, I'd propose scaling all signal measurements to _percentages_.
Currently done.  Causes confusion.  Percentage is meaningless.

Why penalize the cards that are capable?  If SNR cannot be reported by a
card then it shouldn't be throwing out a number that isn't
representative of SNR.  If it is representative of SNR in some form, the
manufacturers I've dealt with provide the formula to compute SNR in dB.
SNR as a percentage is a useless number (Percentage of what? Largest
possible value a card can produce?).  Given the number of questions
concerning "What should I be seeing for SNR from XXX?" where the reply
is it should be "big", there are a number of people looking for a
meaningful, consistent, method to provide this information.  For ATSC
and QAM there is an SNR point where errors begin to degrade the quality
of what you see; the same applies to every transmission coding scheme.

> > 3)  For cards that don't provide signal strength directly (read from a
> > register), the strength be calculated from the SNR by mapping 30dB SNR
> > to full scale signal.
> 
> That'd boil down to the same thing, except that instead of scaling 0..100
> you'd scale to 0..3000. You could also go for permille and scale to
> 0..1000.

Current method: All numbers are meaningless because all references are
based on the card.  Signal is x% of what the card can report.  Is that
good or bad? Look at the output and decide if it's good or bad for you.

Proposed method: SNR has meaning.  A user can tell if their signal is on
the edge or not; if an amplifier might help them; etc.  Signal level as
a percentage of 30dB SNR now makes signal level card independent.

This does not "boil down to the same thing".  If cards are incapable of
producing meaningful numbers, they shouldn't be throwing out "feel good"
numbers.

Rusty





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