Re: Weird WSPR decode anomaly - Probably a FEC problem with wspr decoding software


Hans Summers
 

Hi Ron, Tony

That is indeed a mystery. 

Ron, you are correct about 6-char grid squares in the extended WSPR transmission format - but the QCX is not capable of transmitting this extended format. It can only handle the simple normal WSPR transmissions. 

Tony, it cannot be a problem with Forward Error Correction because this relates to the encoding of the transmission itself, on the QCX. Forward Error Correction is, somewhat loosely, the term applied to the encoding of data with additional bits via the used encoding protocol, that allow the decoding station to correct errors in the transmission. The QCX cannot be transmitting or encoding your friend's callsign, since you have replaced it with your own. 

Furthermore it cannot be a problem with the QCX somehow remembering the encoding of your friend's callsign, since the encoded sequence of 162 symbols in a WSPR transmission is not stored in EEPROM, it is always calculated dynamically and exists in volatile RAM only. Therefore it is not persisted through power cycles. It is hard to see therefore, how this mystery could be caused by any problem in the QCX firmware itself. 

Some digital modes augment their decoding logic with a probability-based fuzzy match, looking for probable (but not algorithmically certain) matches between the decoded data, and a database of existing callsigns - which could be callsigns the decoding station has heard before, or in some cases an internet-based list of callsigns. This is a somewhat controversial practice, as far as I know it is not done in the WSJT-X software for WSPR at all; I know JT65 and Opera are two modes that use this controversial technique but I have not heard of it being applied to WSPR, though I could be wrong. 

All of which makes it a bit of a mystery... I do not see how this phenomenon could ever be caused by the QCX firmware itself, though equally well I don't see that it could be caused by WSJT-X either... unless there is some feature of WSJT-X I don't know of, or maybe there are other more aggressive WSPR decoder software packages out there which DO apply the controversial probability match. 

73 Hans G0UPL
http://qrp-labs.com

On Fri, Feb 26, 2021 at 12:53 PM Ron Carr <rcarr@...> wrote:
The receiving software keeps a database that it uses to match complex calls and 6 character grid squares to the correct call.  And the matching algorithm is not perfect as I believe it uses a hash or similar scheme.  I could see something like this happening if you or he used a call like  K1URC/3   or a grid like FN54fk.

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