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rtl_fm -> left and right sox output?

Posted: Sun Mar 04, 2018 3:01 am
by phil
Hi all,
Briefly -
I'm running rtl_fm to PDW, using : rtl_fm -M -d 1 -g 4 -r 12.5k -f 148.68750M | play -r 12.5k -t raw -e s -b 16 -c 1 -V1 -
which is working nicely.
At the same time i am running a second dongle (same commands, different frequency) for a second pager frequency.

Is there a command that will allow me to split the output of one dongle to the left channel and one dongle to the right channel so i can run run 2 versions of PDW like i can do in windows with SDR# and its AUX VFO functions?

This is on linux mint 18.3 and pulse audio. I have read the options for sox, but all the L/R or piping options tend to relate to stereo not an analog nfm signal.

Any help appreciated

Thanks.

Re: rtl_fm -> left and right sox output?

Posted: Sun Mar 04, 2018 3:22 am
by rtlsdrblog
How are you running PDW on Linux?

Maybe it would be better to use a command line app like MultiMon-ng and this display app, if you're looking for a full Linux solution. https://www.rtl-sdr.com/pagermon-a-brow ... ltimon-ng/

Re: rtl_fm -> left and right sox output?

Posted: Sun Mar 04, 2018 4:35 am
by phil
PDW is being run through wine. I would still run into the same issue monitoring 2 pager sites regardless of which decoder i use wouldn't i?

Re: rtl_fm -> left and right sox output?

Posted: Sun Mar 04, 2018 9:45 pm
by rtlsdrblog
Multimon-NG would get the data through Linux Pipes, rather than through audio so it woudn't be an issue there.

If you want to continue with PDW, I think it should definitely be possible to split the output using sox. Using the stereo split options should work, it doesn't matter that the modulation was NFM. Once demodulated it's standard L+R audio with both LR channels having the same audio.

Another better option might be to use PulseAudio to create multiple virtual audio outputs. I've never tried that, but I found a somewhat related guide here https://www.reddit.com/r/linux/comments ... h=c5f5643c

Re: rtl_fm -> left and right sox output?

Posted: Mon Mar 05, 2018 8:39 am
by phil
Thanks, awesome info as usual. I'm going to hop right into it this week and we'll see what comes of it, will post the outcome. My RPi 3 turned up last friday so i have multimonNG running on that as well, not much chance of wine running on the pi unless i use a paid program to host it.
Incidentley, about halfway down this page: https://www.raspberrypi.org/forums/view ... 2&start=25
there is a short script for "PDW" type look for multimonNG. I've been playing with it, but its throwing up some errors that im trying to work through, but i'm no programmer.

Cheers!

Re: rtl_fm -> left and right sox output?

Posted: Thu Mar 08, 2018 2:17 am
by phil
Been faffing about with the pi for a couple of days and now have 2 dongles (using rtl_fm) feeding 2 instances of PDW via the pulse audio commands mentioned in the link you posted. Working well using between 24 and 30% cpu. The RPI 3 is an awesome little unit!

Re: rtl_fm -> left and right sox output?

Posted: Fri Mar 09, 2018 4:09 am
by rtlsdrblog
Nice! Are you able to post the commands that you used here? Might help others in the future who find this post in a search.

Re: rtl_fm -> left and right sox output?

Posted: Sat Mar 10, 2018 1:47 am
by phil
no probs:

- rtl_fm -d 0 -p 57 -g 20 -r 12.5k -f ***.****M | aplay -r 12500 -f S16_LE for each dongle and frequency

$ pacmd load-module module-combine-sink sink_name="whatever" slaves="alsa_output.platform-soc_audio.analog-stereo" for each output to PDW. I have noticed that the terminal has to stay open otherwise these will disappear from the sound controls.

Re: rtl_fm -> left and right sox output?

Posted: Fri Mar 16, 2018 11:02 pm
by phil
Quick update: I am now running the 2 frequencies (1 per dongle, i tried using 1 dongle setup to scan 2 frequencies with the squelch command like the RTL-FM scanner setup but kept hitting problems) into 1 instance of PDW and i haven't seen any decoding issues. CPU is down to 15%. I'm very happy and surprised with the outcome.