Re: Drift


Graham, VE3GTC
 

Good morning Alan (and all),

Agreed with all you said.

This discussion has gotten me wondering just how the WSPR does determine the reported figure for DRIFT and SNR. There are documents with the details, I guess I will just have to do some reading to get a better understanding.

Perhaps we need to stop making this so complicated and just accept that for our simple amateur use that whatever is reported as SNR or DRIFT is OK and is acceptable and has more value as a relative figure of merit  within +/- X of the reported value rather than a absolute and precise value.

VAC's add another layer of uncertainty. Yes, you can adjust sample rates in and out but I don't know enough about how it's done to feel comfortable that any loss in the conversion will have no impact.

I use a couple of Lexicon Alpha USB sound cards. These have a max sample rate of 48k. However, if I just plug and go on a Windows machine I find that when I look at the properties of these devices under recording devices that Windows has defaulted them to a 44,100 sample rate.  Even though the WSPR/WSJT-X programs state that a requirement for the program is a sound card having a 48k sample rate, they work just fine with my Alpha USB sound card.

I have for some time been running an experiment comparing my 40m off center fed dipole (apex just over 7 meters) and my short unterminated beverage on 40m.  One is connected to my Yaesu FT-950 and the other to my FT-817. I spent some time ensuring each radio was set up so as to be as nearly the same as the other. Each radio feeds an Alpha USB sound card, one to a Windows 10 machine and the other to a Raspberry Pi. WSJT-X v1.7.0 is used on both the Windows 10 and Raspberry Pi where the setting for each program are the same.

I don't know if you are familiar with KB9AMG's WSPR statistics web page. This is the link to the one showing 40m

http://mardie4.100webspace.net/wspr/top_wspr_spotters_40m.html

listed are callsigns and counts relative to the placing for that specific band. I am using both of my callsigns ve3gtc and ve3ghm, stats for each here:

http://mardie4.100webspace.net/wspr/sptr.pl/2,281676

http://mardie4.100webspace.net/wspr/sptr.pl/2,281369

Those last two links may not work from day to day. I don't know if they are more or less static based on callsign or dynamically created every day.

In any case, you can see that the counts for 40m for these two calls various from day to day, sometimes a little, sometimes a lot. You too have noted this phenomenon in some of your comparisons.

More interesting is that for the same date-time, one will report stations that the other does not and vice versa, not always but sometimes. And there does not seem to be a preference for one over the other to be providing decodes that the other is not.

Are these differences the result of propagation and antenna characteristics (most likely) or the result of differences in processing (Windows PC vs Linux Raspberry pi) and other hardware (FT-950 vs FT-817) (not as likely unless the software and hardware are grossly and differently configured). Spot checking of wspr spots has shown that when the same TX is received at the same time by both that DRIFT is never different but SNR is sometimes the same, sometimes different where sometimes the dipole has provided the highest SNR and other times not.

I have read through G3ZIL's report but need to spend some more time with it; I am not yet quite sure what is being reported.

Cheers, Graham ve3gtc

On 2017-11-24 10:54, Alan G4ZFQ wrote:

 A quick
search will turn up many. Here a few to get started:
Graham,

OK:-)

But as you say only these specialised programs have compensation. I think this is either for when the card crystal is used as a frequency counter reference or in order to allow for differences between the soundcard and the computer system clocks when this can affect the program output.

WSPR will be looking the 4 tones which might themselves be varying due to propagation and/or TX/RX drift. It is not looking for precise frequencies, just tones separated by approximately the WSPR standard.
SpecLab is a complex program, maybe there is a way to compensate for sample rate and send the corrected audio to WSPR.
Or, how about using VAC? The in/out sample rate can be compensated, would that work?

As I said, I'm amazed at the computer power used for WSPR analysis. In my opinion there is so much variation in propagation and probably in the way WSPR finally decides to present a low SNR that low SNRs will not be reproducible. (I think this is basically what G3ZIL says?)
Certainly it seems propagation affect things, several times I have compared two RX systems, one clearly wins over one day, the other one wins the next day.  Or, are these just random variations?
Even two seemingly identical RX systems will have small differences which could affect weak signal decoding.

73 Alan G4ZFQ

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