Here's information on the "750 Hz tone" received at this QTH. I am
listing K8R's frequency as reported on my screen. Please explain. The
tone frequency reported matched the left edge of the modulated waveforms
on the waterfall when I could see tones.*
July 11 RX 21091.0 830 Hz successful QSO audio "loud", tones obvious
July 14 RX 14091.0 763 Hz successful QSO audio "loud", tones obvious
July 23 RX 18095.0 795 Hz no QSO yet* Might be propagation is
unfavorable.
SO: what is the frequency tolerance of 750 Hz tone for successful decoding?
*FYI several times on the bands I have copied K8R when there were no
visible signals from anyone in the waterfall, and including no visible
"750 Hz" tone. To me that means Superfox modulation provides excellent
results.
Equipment: Windows 10 Dell laptop, TS590SG rig, various antennas.
--Glenn, AF8C
On 7/23/2024 12:16 PM, Steven Franke via wsjt-devel wrote:
On Jul 23, 2024, at 10:41 AM, Black Michael via wsjt-devel
<wsjt-devel@lists.sourceforge.net> wrote:
Was looking at this example SuperFox transmission using Audacity
https://www.dropbox.com/scl/fi/stohwy3veidevmwr4j517/240605_181330.wav?rlkey=85h1c9riskq1mmbqzex9hpov3&dl=1
<https://www.dropbox.com/scl/fi/stohwy3veidevmwr4j517/240605_181330.wav?rlkey=85h1c9riskq1mmbqzex9hpov3&dl=1>
It looks like there is amplitude modulation going on which would seem
to be undesirable.
For example, analyzing this one section there is a 1697Hz signal. But
if the amplitude drops the SNR will drop too limiting the decoding
ability.
Is amplitude modulation part of the Q-ary polar code?
The SuperFox audio waveform produced by WSJT-X is a constant-envelope
Gaussian frequency shift keyed (GFSK) signal with bandwidth-time product
BT=8. The GFSK waveform includes 24 sync symbols at audio tone
frequency 750 Hz and 127 polar code symbols at tone frequencies 750 Hz +
n*11.71875 Hz, where n is in the range 1-128.
If the transmitter’s upper-sideband audio-to-RF frequency response is
flat over the bandwidth of the SuperFox signal (750 Hz - 2262 Hz audio)
then the transmitted signal will also have constant envelope. What you
are seeing in the example that you posted is the result of
frequency-selective fading, i.e. the propagation channel’s frequency
response is not flat over the 1512 Hz bandwidth of the SuperFox signal.
Most signals will exhibit this effect so some extent.
Steve K9AN
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