> 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
>
> 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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