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Re: [Discuss-gnuradio] PSK31 (from audio to radio)

From: pen tester
Subject: Re: [Discuss-gnuradio] PSK31 (from audio to radio)
Date: Sat, 10 Sep 2016 11:40:23 +1000

   Still playing around and I have a question about the value which should go in the Polyphase Resampler on the tx side please. Any advice on the AM rx too would be helpful :-)



[1] https://github.com/tkuester/gr-psk31/ (examples/psk31_tx.grc)
[2] https://github.com/tkuester/gr-psk31/ (examples/psk31_rx.grc)
[3] https://www.csun.edu/~skatz/katzpage/sdr_project/sdr/grc_tutorial3.pdf 

On 6 September 2016 at 05:49, pen tester <address@hidden> wrote:
   Yes, your graphs are really useful :-) Thank you for responding in such detail! 

   Here is what I am up to:

Transmit side:
  • I made your changes to your transmit graph [1]
  • All the changes were fine but I was confused about the use of the Polyphase Resampler as I could not get the values to work without underflows when executing i.e. "UUUU..."
  • So instead (this is probably wrong) I tried using a Rational Resampler with an Interpolation of 1e6 and a Decimation of 48e3.
  • Updated version: http://pasteboard.co/hmyGOGTE4.png
  • Either way, the tx graph looks healthier - just need some advice on the resampler perhaps?

Receive side:
  • I am using the start of this receive graph [2] as it was using radio rx already 
  • and just modifying the demod portion to be more like your rx graph 
  • Updated version: http://pasteboard.co/hmzrMBAIi.png
  • I also need advice from anyone who could point in the right direction on the AM rx side please?

[1] https://github.com/tkuester/gr-psk31/ (examples/psk31_tx.grc)
[2] https://github.com/argilo/sdr-examples (tutorial/6_psk.grc)
[3] https://github.com/tkuester/gr-psk31/ (examples/psk31_rx.grc)

On 4 September 2016 at 12:33, Tim K <address@hidden> wrote:
Hey hey!

Glad to see someone using this. (And that it's apparently still working! Goodness... I need to clean this up...)

I'm giving advice freehand right now, as I don't have my radios with me to test this. There are definitely more elegant ways for this solution, this is just the quickest I could think of without reverse engineering what I wrote forever ago.

Since this is AM, I think you'll only need the Real port on your "Float to Complex" block. You can attach the "Null Source" to the Imaginary port. Do a quick check to make sure the real signal's not exceeding +/- 1... you might want to attenuate it to +/- 0.7 for now, someone else should comment.

As far as the transmit side goes, I'd leave everything up to the "Multiply" block untouched. Consider that your "audio input" to the radio. After that...
 - Multiply Const (set to 0.7 for safety)
 - Polyphase Resampler (float/float) set to (1e6 / 48e3) - This gets your sample rate up to radio speeds
 - Float to Complex (in the Real port, Null Source to the Imag port) - This makes your signal "complex AM"
 - Send the Complex stream into your Osmocom Sink (set to 1e6 sps) - And transmit!

If I remember correctly, some radios don't like working at non-integer sample rates. (The hackRF may be ok though...)

Play gently with your TX gain until you can see it on another radio. (IIRC the HackRF is simplex... perhaps an RTL?) Use AM to demodulate.

I've been meaning to do a write up on this for a while. If I ever get around to it, I'll make sure to CC you.

- Tim

On Fri, Sep 2, 2016 at 11:55 PM, pen tester <address@hidden> wrote:
I am trying to get a simple working example of PSk31 working (transmitting with a hackrf and receiving with a rtlsdr dongle). Here is what I have so far.

Working PSK31 tx and rx with a sound card
Transmit: https://github.com/tkuester/gr-psk31/ (examples/psk31_tx.grc)
Receive: https://github.com/tkuester/gr-psk31/ (examples/psk31_rx.grc)

These work great for sending a PSK31 message over audio card and receiving with the audio card.

I just slightly modified them to take input from the "Audio Source" block instead of a "Wav File Source" to use the sound card directly:

Turning up the audio a little and tuning to 2003 approx on the rx graph results in reception of the sent message "Hello World!"

Reference version for using hackrf instead of sound card
Next, I wanted to modify these to audio versions to work over Radio using a HackRF and RTLSDR dongle.

These graphs seems very useful but I couldn't get it working end to end for some reason.

Receive:  https://github.com/argilo/sdr-examples (tutorial/6_psk.grc)

I made some changes to the freq to be in range for RTLSDR .e.g changed freq from 441e6 in ref graph, changed to 841e6. 

My merged attempt
The attempt I am currently trying to get working is based on the audio soundcard flies for the transmit:

Transmit: Using  audio tx (psk31_tx_attempt2): where I just replaced the audio sink with the Osmocom sink (not sure what else is needed here for transmission over hackrf)

For the receive, I just left radio rx part in and replaced the decoder parts with decoding logic from https://github.com/tkuester/gr-psk31/ (examples/psk31_rx.grc). I made other minor changes such as freq and sample rates.

My question
So in summary, just looking for a little guidance on my merged attempt to get psk31 out over a hackrf and received using a RTLSDR. Any advice for a learner would be great :-)

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