[Lowfer] WSPR noise bandwidth, was 74.5495 QRSS 60 as usual ...

[jrusgrove at comcast.net]

I apparently missed the section in the User Guide that spelled out in detail how WSPR goes about 
determining the noise level for the SNR calculation. Please forward that information to me. Oh 
that's right ... it's not in there. Your lengthy analysis is based on your assumption of what is 
going on.

Instead of your assumption that the program noise measurement doesn't descriminate against 
"non-random noise (ie, other signals and QRM as one so often finds in the real world)" I would 
assume that it likely does since this is easily carried out when analyzing the FFT data. The program 
can easily look through the 2500 Hz bin data for areas that are quiet and make a determination of 
the 'uniform' noise level. This level can then be accurately scaled at will.

In use, under steady state daytime conditions, additional signals in the passband (unless they 
exceed 'overload' level), interference lines etc do not affect the reported SNR (unless they are in 
the desired signal bins). If you had hands on experience with the program you would know this.

There's really no sense in carrying this any further.

Jay



----- Original Message ----- 
From: "JD" <listread at lwca.org>
To: "Discussion of the Lowfer (US, European, &amp;UK) and MedFer bands" <lowfer at mailman.qth.net>
Sent: Thursday, October 10, 2013 2:01 PM
Subject: Re: [Lowfer] WSPR noise bandwidth, was 74.5495 QRSS 60 as usual ...


> Before I propose an experiment that might better illuminate the matter of SNR, let's review what I 
> have _not_ said, given all the seeming misreadings and side issues that have popped up lately. 
> This should be about simple science and engineering, after all.
>
> I have not "dissed" WSPR, only mocked what I consider its misuse of the term SNR--but even then 
> took pains to mention that the reading is still useful in its own specific way, even though it's 
> not SNR in the sense an engineer or scientist would use it.  I have only addressed Argo, WSPR, and 
> basic OPERA modes, because they all rely on FFT analysis in comparable bandwidths, differing 
> mainly in what they do with the information before displaying it. I specifically did not mention 
> WOLF because BPSK decoding is a fundamentally different process.
>
> Now, a few recent points and the experiment I mentioned:
>
>>>> Not true. The noise is analyzed in a bandwidth wider than 200 Hz - this has been well known 
>>>> since the early days of WSPR.
>
> If you re-read my post, that's what I said myself.  It's exactly what I'm complaining about.  The 
> NOISE is measured in the wider bandwidth, which is _not_ the same bandwidth analyzed and tracked 
> for SIGNAL.  That's what's wrong with calling it "SNR"...no matter how many times the user's guide 
> beats us over the head with an irrelevant bandwidth.
>
>>>> You are free to un-scale the measurements if you prefer it that way.
>
> Problem is, the number is only truly scalable where noise power is uniform over the bandwidth.  If 
> there is non-random noise (ie, other signals and QRM as one so often finds in the real world), 
> that non-random component is not scalable on a simple bandwidth ratio.  This is where the idea of 
> an arbitrary noise BW breaks down.
>
> Random or non-random, whatever stuff in the remaining 2300 Hz bandwidth that WSPR "sees" while 
> it's looking for signals is of no concern to the detection process, but it too gets counted in the 
> "noise" measurement, thus inflating the number.  Saying we can define noise BW to mean anything we 
> want is sort of an Alice in Wonderland approach to engineering.
>
> Why does it matter?  Follow this closely as a simple thought experiment, or feel free to duplicate 
> it in the real world:
>
> 1.  First find, generate, or imagine two simultaneous WSPR 2 signals of steady, nearly equal 
> levels... not overly weak and not overly strong relative to the available noise, not too close 
> together but still both within the 200 Hz detection band.  (One signal alone will demonstrate the 
> basic point, but it'll be more meaningful  to have two, because a single signal won't give an 
> "SNR" reading when it fails to decode.)
> 2.  Introduce an extraneous carrier outside the critical 200 Hz band but elsewhere within the 2500 
> Hz noise band.  Let it add maybe another 6 or 10 dB or more to the existing total alleged "noise" 
> without it ever having any chance of affecting detection.  At this point, it only further inflates 
> the reported "SNR" of the signals being detected.
> 3.  Now move that extraneous carrier right over one of those signals of interest.  The total 
> energy in the 2500 "noise" bandwidth remains exactly the same as in Step 2.  Exactly the same! 
> Remember that well.
> 4. Two minutes later, the reported "SNR" of the unaffected signal shows very nearly the same as 
> before, if propagation is stable.  But guess what--now the primary signal doesn't get decoded at 
> all.
>
> Same noise level in the overall bandwidth...exactly the same...but a completely different outcome 
> for the affected signal.
>
> Anybody still want to argue that an "SNR" based on noise outside the transmission channel is 
> interchangeable with an SNR based on noise inside the transmission channel?
>
> 73
> John
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