Dear Graham, LF Group,
Remembering what I said jokingly to RA3AGC a few days ago about visually
verifying a spectrogram trace of a WSPR signal, and the discussions of
merits of WSPR vs QRSS, I thought this ought to work for diagnosis
purposes - after all, WSPR is measuring the received frequency changes and
the times at which they change, and in principle the same info is displayed
by the spectrogram.
So I spent a "hugely exciting" couple of hours studying your signal with
Spectrum Lab, and comparing it with a locally generated signal from my PC
(which everyone seems to decode OK), configured with the same beacon message
you have been transmitting. Eventually, I realised that the difference was
that your transmissions are a few seconds shorter than the locally generated
signals - in the attachment, the locally generated signals have the dark,
noise free background. This means the transmitted sequence will be a couple
of symbols out of step by the end of each transmission - I expect that could
mess up the demodulation/decoding processes quite a lot. If you have a noisy
signal, the demodulator/decoder might be using effectively just the least
noisy fraction of the signal, with less timing error over a shorter part of
the sequence.
To test this theory, I waited until about 1 minute of your signal had
appeared on the spectrogram, then unplugged the antenna, so that the
remaining minute was white noise. The result was successful decodes! I tried
this 3 times, and it worked 3 times. So your signal does not have to be
noisy in order to decode, there has to be part of it missing in order to
decode!
So that might be why the signals don't decode - whether this is a problem
with the soundcard, motherboard, or software is probably a question for
K1JT! Still, a step in the right direction...
Cheers, Jim Moritz
73 de M0BMU
G0NBD_wspr.jpg
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