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

Wed Oct 9 23:46:05 EDT 2013

JD

>There could be a lot of people in that boat if they figure a purported SNR number is meaningful 
>when the stated noise bandwidth is not related in any way to that needed for communication.

The WSPR 3.0 Users Guide states quite clearly:

"Signal levels reported for each WSPR decode are the measured ratio of signal power to average noise 
power, scaled to a reference bandwidth of 2500 Hz."

One should always review the User Guide when using a new mode.

Joe's choice of reference bandwidth is consistent for modes FSK441, JT6M, JT65B,C, JT2, JT4A, B, C, 
D, E, F, G, JT64A, WSPR2, WSPR15 and is clearly spelled out in all of the Users Guides. The common 
bandwidth reference allows comparison of the theoretical SNR limits. Makes perfect sense to me ...

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: Wednesday, October 09, 2013 9:31 PM
Subject: Re: [Lowfer] WSPR noise bandwidth, was 74.5495 QRSS 60 as usual ...

>>>> I sincerely doubt Joe Taylor would attempt to 'phony-balony' anyone ... if you're unfamiliar 
>>>> with his work you might try 'google-ing'
> Joe Taylor Princeton university.
>
> I've long been aware of Joe Taylor's background, and I don't casually argue with any Nobel 
> Laureate's choice of words in most matters.  But...
>
>>>> The only way anyone could be fooled is due to their lack of understanding the concept of 
>>>> signal, noise and bandwidth ... and how they are related to each other.
>
> There could be a lot of people in that boat if they figure a purported SNR number is meaningful 
> when the stated noise bandwidth is not related in any way to that needed for communication.  This 
> and OPERA are the only communication tools I've ever used where we just pick an arbitrary noise 
> bandwidth, totally unrelated to the signal as transmitted, and call the result SNR.  In 40+ years 
> of measuring S/N ratio in all kinds of transmission systems for a living, I never encountered such 
> a thing before. (Note, please, that I'm only quibbling over this specific term; more on that in 
> the third paragraph below.)  You don't simply pick a number and everybody rallies around it.  If 
> you're going to compare Signal to Noise and call it a Ratio, then they really need to be 
> apples-to-apples measurements.
>
> In ATSC digital television, for instance, I can't pretend my station is viewable at a level 26 dB 
> below noise by choosing to average the noise over the entire VHF band.  If I don't deliver a 
> signal to your receiver that is an honest 15 dB above all noise sources _within the same 6 MHz 
> channel I'm using_ then you simply won't see a picture...the bit error rate goes through the roof 
> and we'll fall off the infamous "digital cliff."  Likewise, neither Argo nor WSPR nor OPERA will 
> detect anything unless the signal at the instant of measurement is some few dB above the noise in 
> the few-millihertz-wide FFT bin(s) where it finds itself.  They all have that in common.  Viewed 
> at the specific place and time where actual detection of signal takes place, there must always be 
> more signal than noise if you expect valid output.  Always.  What you do with that signal 
> afterward... all the processing and decision making needed to extract information from it... is 
> where the differences are, and the necessary bandwidth for communication (a function, in part, of 
> the expected throughput) becomes inextricably entangled in that process.
>
> That's why a legitimate SIGNAL to noise ratio must relate both measurements to the signal's own 
> bandwidth.  Garry mentions the Shannon theorem, the very heart of what I'm saying.  The 
> signal-to-noise expression in Shannon's equation certainly does not assume some arbitrary 
> bandwidth for noise!!  It explicitly specifies the same bandwidth for noise as the width of 
> transmission channel.  When you talk "signal-to-noise," either to a practicing engineer or to an 
> information theorist, that's the condition they always expect to be true.
>
> Now, if you're simply comparing detection thresholds, then sure!  That's a very different matter. 
> Saying WSPR can pull a signal out of X dB of random noise in a 2500 Hz bandwidth, compared to Y dB 
> for OPERA in the same bandwidth, is a perfectly legitimate comparison.  (Assuming the data 
> throughputs are also similar, of course.)  Arbitrary bandwidth is entirely fine there, provided it 
> is mutually understood.  I'm absolutely and completely not arguing against that.
>
> But detection threshold is _not_ the same thing as SNR from a communication engineering 
> standpoint.
>
> It really should be called something else if one wants to be accurate and truthful.  That's the 
> point I'm trying to get across.
>
> 73
> John
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