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>
>
>
>
>
>
>
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