M17 mod/demod

[sp5wwp]

How about adding some support for the M17 mode?

[abraxasneo]

I am a big fan of the M17 and such a receiver and transmitter implemented in your sdrangel would be something great. Because nobody has made a transmitter so far. There is only a receiver/decoder plugin for SDR++.
M17 is an amateur alternative to commercial products like D-STAR, C4FN or DMR. Work is underway to implement this codec in the most popular transceivers such as the TYT MD-UV380.
With your sdrangel, users could build a simple M17 transceiver and use it even for communications via the qo100 geostationary satellite. I personally managed to build a simple transceiver from the command line on a raspberry pi and using the right syntax and the rpitx program, which transcodes the speech from the connected microphone in real time and transmits on a specific frequency. Based on this, I also sent a transmitter suspended under a stratospheric balloon, which transmitted the voice signal and in the digital part transmitted the height of the balloon to Earth. I am sure this is just the beginning of the possibilities of this M17 project.

[f4exb]

Could be interesting indeed.

[f4exb]

https://github.com/mobilinkd/m17-cxx-demod

Test with sox /home/f4exb/f4exb_call.wav -t raw -r 8k -c 1 -b 16 -L -e signed - | apps/m17-mod -S F4EXB | apps/m17-demod -l -d | play -q -b 16 -r 8000 -c1 -t s16 -

Formatting modulation file to be injected in NFM modulator: sox /home/f4exb/f4exb_call.wav -t raw -r 8k -c 1 -b 16 -L -e signed - | apps/m17-mod -S F4EXB -D F5SFU | sox -r 48k -c 1 -b 16 -L -e signed -t raw - -t raw -r 48k -c 1 -b 32 -L -e float m17-sdrangel.raw

Then if I am not mistaken the NFM modulator parameters should be like this to produce a test signal:

[f4exb]

NFM modulator needs more options to disable high pass filtering and maybe pre-emphasis. Note: done in 76ed92c985b50a47d5c38a41466f09a16b48e85f

[f4exb]

Verify the input file to NFM modulator: sox -t raw -r 48k -c 1 -b 32 -L -e float /opt/build/sdrangel/build/m17-sdrangel.raw -r 48k -c 1 -b 16 -L -e signed -t raw - | apps/m17-demod -l -d | play -q -b 16 -r 8000 -c1 -t s16 -

[sp5wwp]

This is how M17 RF spectrum should look like:

[f4exb]

What is your FM deviation? From the picture the base of the spectrum occupies roughly a third of the span which makes it ~16.7 kHz at -60 dB. It looks like it is obtained with a deviation much larger than the specified +/- 2.4 kHz. If I use +/-2.4 kHz I obtain something more narrow and spiky:

[sp5wwp]

2.4 kHz. What's the baseband sample you use to get this?

Formatting modulation file to be injected in NFM modulator: sox /home/f4exb/f4exb_call.wav -t raw -r 8k -c 1 -b 16 -L -e signed - | apps/m17-mod -S F4EXB -D F5SFU | sox -r 48k -c 1 -b 16 -L -e signed -t raw - -t raw -r 48k -c 1 -b 32 -L -e float m17-sdrangel.raw

This one? Can you share the resulting .raw file?

[f4exb]

It is here now: https://transfert.free.fr/q2t84g

Internally in SDRangel it is played at a sample rate of 48 kS/s and interpolated before being sent to HackRF at 3.072 MS/s.

Note that I am using the m17-mod of https://github.com/mobilinkd/m17-cxx-demod so you should be able to play the same way with it. One thing I noticed is that the coefficients of the rrc filter are not those I obtain for aplha=0.5 Ts=1/4800 and Fs=48000 which should correspond to recommendations. However it does not seem to make a big difference.

[sp5wwp]

This is what I get with GNU Radio for your baseband file:

The baseband generated by cxx-demod tools isn't an IQ stream, but rather an instantaneous frequency shift.

[f4exb]

Due to too many issues the M17 plugins will be removed from the default build.