Digital modes with SDR

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Posted by w0ep on June 14, 2018 at 7:53 am

[today’s run: 3.5 miles]

Yesterday I hooked up my SDR receiver to this computer and played with it a little bit.

I decided to see how hard it would be to get a digital-modes program running and see if I could decode some of that activity.  There are a lot of digital modulation modes being used on the ham bands and no easy way to know what signal is using what mode other than experience.

I already had CubicSDR running.  I found applications fldigi and wsjt-x  and installed them.

Around 7.075 there was a group of signals that sound like a bunch of people playing tennis with penny whistles: they take turns of about 15 seconds length and there can be one or a few notes on each round.  I played around with fldigi for quite a while trying to find a modulation mode that would decode this traffic.  Eventually I had to download a newer version of wsjt-x and, sure enough, it was the new “FT8” mode.  And I was able to decode it and see what was happening.

Conversations in FT8 are somewhat “canned”.  There are just a few types of messages and the goal of the thing is to make a bare contact, not to have conversation.  But the draw of it is that there are a lot of DX (far away) stations on the air and the mode works in very low signal conditions.  So it is good for making contacts with far away stations.

Here is a screenshot.  The bottom part is CubicSDR showing the band-wide waterfall at the bottom, then the smaller local waterfall above that.  To the top right is the decoding screen showing the messages being decoded by WSJT-X  and to the top left is the “checker board” waterfall display of WSJT-X.  It shows green lines separating the 15 second intervals and yellow/orange/red squares of the signals depending on signal strength.

One of the keys of this and some other weak signal modulation modes is that they require an accurate system clock on your computer in order to coordinate the transmissions.  So there is an out-of-band time-domain coordination going on.

(note to self:  remember to run ‘pavucontrol’ command, under the “recording” tab, set the WSJT-X source to be “monitor of built-in audio analog stereo”)

Filed under Ham Radio, Operating
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