I was playing with HDSDR+ExtIO and Ettus USRP1 to track a BPSK signal. In the end I successfully heard the signal.

But I would like to see the data, I mean the raw data (HEX values). For that I tried fldigi in which I just selected BPSP-31 under PSK (in the Op Mode menu).

I've used a cable to forward audio from speaker to mic and I saw something in the waterfall but cannot get any data.

Which things should I check? (Any lower level tutorial?)

I know the source operates at 1200bps (symbol rate) but I'm not aware of line coding, preamble (, et cetera) at this moment.

Also, i successfully experimented to decode some public BPSK sounds:

  • Got the message "CQ CQ CQ de EA2BAJ EA2BAJ EA2BAJ PSE K" for this.

  • Got the message "The quick brown fox jumps over the lazy dog 1234567890" for this.

  • 2
    $\begingroup$ Every digital signal has many parameters besides the type of modulation (BPSK) — symbol rate, line coding, preamble, et cetera. Please edit your question to include all the information you have about the signal you are trying to receive. You tagged this question “satellites”, so including the satellite you're trying to receive from, if that's what you're doing, would also be good. $\endgroup$
    – Kevin Reid AG6YO
    Oct 26, 2015 at 14:02

2 Answers 2


So as I understand you, the Ettus USRP1 is a software-defined receiver, and HDSDR+ExtIO is software that you are using to control the receiver. So the audio comes from HDSDR running on the computer, and the audio cable loops right back to the computer for fldigi to decode? Interesting! Do you have a hardware attenuator in the audio circuit? If not you're probably way, way overdriving the mic input; your problem could be distortion. The speaker output is a few tenths of a volt perhaps, but the microphone input is expecting millivolts. Check out the attenuator circuit in the picture in this web page, and any PSK31 tutorial such as this one for advice on adjusting the signal level. There are some great PSK31 tutorials on YouTube also.

If I were you I'd set things up to receive PSK31 signals successfully first. Once you do that, then hopefully it shouldn't be too hard to tweak fldigi or some other program to get the raw hex stream you're after. Have fun!


I did something similar a year or so ago. Eventually I ended up creating a GnuradioCompanion block in python that would take the output after being demodulated and converted it to a binary stream.


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