Hello Andy, 

>As the WSJT standard for S/N is to express it in a 2.5kHz 
>bandwidth, simply dividing the previously measured S value by 
>this N (for 2.5kH input)   will give the final S/N value.   If 
>DSP prefiltering has been included in the software, the noise 
>will have been measured over just this pre-filtered bandwidth 
>and the lower resulting N is then scaled up to what it would 
>be in 2.5kHz.  Noise power is proportional to bandwidth, so a 
>250Hz prefilter means the measured total noise is now 10dB 
>lower and this correction has ot be added on

*If* there is a DSP pre-filtering in the SW and the noise measured in this
BW is scaled up to 2.5kHz which includes the appropriate correction
then a hardware filter (or digital filter in a SDR like perseus) narrower than 
2.5kHz in the RX itself 
*ahead* of the WSPR-DSP would not affect/falsify the calculated S/N as long as 
this filter
is not narrower than the DSP prefilter of the WSPR SW.
I have the impression that in the 'old' WSPR SW there is no DSP prefiltering in 
the SW but in WSJT-X
there is.

73
Clemens
DL4RAJ 


>-----Original Message-----
>From: [email protected] 
>[mailto:[email protected]] On Behalf Of Andy Talbot
>Sent: Thursday, October 13, 2016 2:17 PM
>To: [email protected]
>Subject: Re: Re[2]: LF: Running 2 WSPR transmissions, how to 
>stop them TX'ing at the same time??
>
>No 41dB is NOT a real S/N figure and is due solely to your 
>artificially restricting the noise bandwidth gon gin !    If 
>you use a receiver bandwidth any narrower than the full SSB 
>bandwidth then the S/N reporting will be wrong.
>
>With just one exception (*) there is no benefit at all to be 
>gained from using a narrow receiver filter.  It does 
>absolutely nothing to improve signal reception and just makes 
>your noise measurement wrong.  The reasons are as follows :
>
>All filtering of the signal is done in the DSP software, where 
>for WSPR it is bandpass filtered to a bank of 1.46Hz wide 
>filters, called bins (as in dustbin) - this being the tone 
>spacing and the speed of a WSPR signal.    There may be some 
>incidental pre-filtering on the way to getting down to this 
>narrow bandwidth, but it is not relevant to the process.  ONLY 
>the final 1.46Hz bandwidth is the one that matters.   
>
>Measuring the signal level  (the S part of the S/N) is 
>straightforward.   Your four tones fall in four of the 1.46Hz 
>wide adjacent parallel filter banks corresponding to each of 
>the four tones transmitted.   The sum of the power in the 
>tones is then the signal in the four adjacent bins  added 
>together and  is just a relative number.
>
>But to get S/N we now have to make a noise measurement and 
>here is where it gets difficult.  There are many ways of  
>estimating the noise and one of the simplest is to take 
>everything going into the soundcard / software and assume that 
>as it is so wide, a few weak signals present  won't affect the 
>average too much; ie.  that it is all noise.   
>An assumption has to be made about the input now, and since 
>SSB radios are used, the assumption is made that the noise is 
>bandpass filtered at this point to 2.5kHz width - this being 
>the bandwidth of a typical SSB filter.    By measuring the 
>signal over this full bandwidth a value for N in 2.5kHz can be 
>obtained.
>
>As the WSJT standard for S/N is to express it in a 2.5kHz 
>bandwidth, simply dividing the previously measured S value by 
>this N (for 2.5kH input)   will give the final S/N value.   If 
>DSP prefiltering has been included in the software, the noise 
>will have been measured over just this pre-filtered bandwidth 
>and the lower resulting N is then scaled up to what it would 
>be in 2.5kHz.  Noise power is proportional to bandwidth, so a 
>250Hz prefilter means the measured total noise is now 10dB 
>lower and this correction has ot be added on
>
>Simples !   Except it is all too easy to go very wrong.
>
>First of all, if the input noise is externally filtered (say 
>by a narrow CW filter) its total power will be lower by the 
>proportion of this BW to 2.5kHz.  So the value of N will be 
>less.    S stays the same (it is filtered in software to 4 * 
>1.46Hz)  so same S, lower N  results in a higher S/N  when the 
>input is bandpass  filtered.
>
>This is clearly why a certain F station consistently reports 
>ridiculously high values of S/N for everyone.
>
>Next is what happen when strong signals come up inside the SSB 
>filter width but outside the WSPR band.   These can get lumped 
>into the overall measurement and give an artificially high 
>value of N.   They are some way away from the signal of 
>interest so don't actually affect the decoding process, but to 
>lead to a falsely low value for S/N.     This is probably 
>happening for stations who consistently report lower than 
>expected S/N for many stations.
>
>There are many ways that more accurate estimates of noise can 
>be obtained, using statistics and probability distributions to 
>separate out genuine noise from signals.   Others just by 
>looking for the presence of signals and adding everything else 
>up that doesn't fit that criteria of 'signal'.   All are used 
>in the electronic Warfare and Communications Intercept 
>business to automatically detect and then classify signals.  
>But it is a complicated process just to obtain an incidental 
>measurement value for information only, that does nothing to 
>help the actual decoding process.
>
>(*) The one exception, where a narrow input filter will help 
>is if a very strong local signal appears in the SSB filter 
>passband that is sufficient to depress the AGC.    This can 
>degrade the wanted signal.   However, if the receiver is 
>properly designed, the S/N of the wanted signal in the now 
>depressed audio should still be the same.   A soundcard has 
>typically 80dB dynamic range or more, so unless the unwanted 
>signal  is say  60 to 70dB above the WSPR signal of interest, 
>it should still cope.  Although the N measurement will be 
>wrong for the reasons detailed above
>
>For a description of how signals can be detected in noise, 
>take a look at the  RadCom  Data columns for  April and June 
>2008.  There's also a description on page 47  my book 
>"Command,  Computers, Microcontrollers and DSP for the Radio 
>Amateur".   RSGB still have a few copies - I saw it on the 
>stand at the Convention!  (Although several sections in there 
>are embarrassingly out of date by now).
>
>Andy  G4JNT
>
>
>
>
>
>On 13 October 2016 at 10:36, Chris Wilson <[email protected]> wrote:
>
>
>       Hello Andy,
>       
>       Thursday, October 13, 2016
>       
>       Thanks for the detailed explanation Andy. Good copy 
>thoughout the
>       night on your WSPR2 and 15 signals. Early this morning I greatly
>       reduced the filter width to just encompass the needed 
>section of the
>       band to hear your WSPR2 signal and the level reported 
>from WSPR went
>       mad high. I read a paper you wrote on S/N levels in 
>WSPR and filter
>       width but it rather went over my head. I assume +41 SNR is not a
>       "real" figure when filter width is reduced right down?
>       
>       
>       Best regards,
>        Chris  2E0ILY                          
>mailto:[email protected]
>       
>        My part time LF grabber is at 
>http://www.chriswilson.tv/grabber.html 
><http://www.chriswilson.tv/grabber.html> 
>       
>       
>       > Correct.   I have a PIC that reads timing data from a 
>GPS and at
>       > the right timing uses a set of pre-stored WSPR 
>symbols to calculate
>       > the frequencies for the tones and sends the resulting 
>data to an
>       > AD9852 DDS.  Different symbol sets (for the two messages) and
>       > frequency data is stored for each of the two modes.
>       
>       <SNIP>
>       
>       
>       
>
>
>