Adrian Musceac
9 years ago
Hi all,
Motivated by David's work with VHF modems, I have started to
investigate various Tx-Rx chains that might work with legacy narrow FM
radios. My goal is to develop a completely portable system composed of
a development board running open-source software paired with an FM
transceiver. The Linux board could be any ARM board capable of running
a supported Linux distribution and providing a decent interface to a
7-9 inch LCD screen with touch controller. My setup is fully described
in a Youtube video I mentioned before.
At the moment I have all the hardware components available and have
run the first tests on the air. The software was first based on
Kristoff's Codec2-GMSK modem and a Qt user interface designed by me.
Although it had worked fine, it required a radio with a data port,
which meant portable ops were restricted to a very narrow list of
handheld radios.
Since then, I thought of using the huge power of the Gnuradio
ecosystem to design my own modems that could go through the audio
passband of a handheld. I have learned many things during this
process, especially that I can very quickly prototype and test a modem
without going too deep in the maths, which has never been my strong
point.
My findings are somewhat surprising: I can use any of the modems over
a narrow FM channel, but there are a few which perform well enough for
my purposes.
Considering a RF chain composed of one laptop running Gnuradio, one
Yaesu handheld, one Yaesu mobile transceiver and one Linux board with
soundcard ports also running Gnuradio, here is the performance of my
modems in descending order:
1. DBPSK (~10 dB S/N)
2. DQPSK ( ~12 dB S/N)
3. GMSK (~14 dB S/N)
4. 4FSK (~15 dB S/N)
6. AFSK (~16 dB S/N)
I am especially fond of the GMSK modem which gives me 2 kbit/sec with
a reasonable SNR.
Since I have full control over the modems, I can select the signal
width, samples per second and tone frequencies in such a way that they
provide the maximum bitrate for a bandwidth limited to 300 Hz - 3 kHz.
Now that I have some real results, I am thinking of starting to move
the modems to C++ space and integrate them with the Qt GUI application
destined to run on the Linux board. This application will be based on
qradiolink v0.2 and will have support for VOIP via the 2.4 GHz WiFi on
board, enabling the station to be quickly transformed into a portable
repeater.
You can find my test modems on Github if you want to play with them:
https://github.com/kantooon/gnuradio_audio_modems
Obviously David's modem will probably be more robust and performant,
however I am providing a huge selection of GRC graphs here :)
Cheers,
Adrian (YO8RZZ)
Motivated by David's work with VHF modems, I have started to
investigate various Tx-Rx chains that might work with legacy narrow FM
radios. My goal is to develop a completely portable system composed of
a development board running open-source software paired with an FM
transceiver. The Linux board could be any ARM board capable of running
a supported Linux distribution and providing a decent interface to a
7-9 inch LCD screen with touch controller. My setup is fully described
in a Youtube video I mentioned before.
At the moment I have all the hardware components available and have
run the first tests on the air. The software was first based on
Kristoff's Codec2-GMSK modem and a Qt user interface designed by me.
Although it had worked fine, it required a radio with a data port,
which meant portable ops were restricted to a very narrow list of
handheld radios.
Since then, I thought of using the huge power of the Gnuradio
ecosystem to design my own modems that could go through the audio
passband of a handheld. I have learned many things during this
process, especially that I can very quickly prototype and test a modem
without going too deep in the maths, which has never been my strong
point.
My findings are somewhat surprising: I can use any of the modems over
a narrow FM channel, but there are a few which perform well enough for
my purposes.
Considering a RF chain composed of one laptop running Gnuradio, one
Yaesu handheld, one Yaesu mobile transceiver and one Linux board with
soundcard ports also running Gnuradio, here is the performance of my
modems in descending order:
1. DBPSK (~10 dB S/N)
2. DQPSK ( ~12 dB S/N)
3. GMSK (~14 dB S/N)
4. 4FSK (~15 dB S/N)
6. AFSK (~16 dB S/N)
I am especially fond of the GMSK modem which gives me 2 kbit/sec with
a reasonable SNR.
Since I have full control over the modems, I can select the signal
width, samples per second and tone frequencies in such a way that they
provide the maximum bitrate for a bandwidth limited to 300 Hz - 3 kHz.
Now that I have some real results, I am thinking of starting to move
the modems to C++ space and integrate them with the Qt GUI application
destined to run on the Linux board. This application will be based on
qradiolink v0.2 and will have support for VOIP via the 2.4 GHz WiFi on
board, enabling the station to be quickly transformed into a portable
repeater.
You can find my test modems on Github if you want to play with them:
https://github.com/kantooon/gnuradio_audio_modems
Obviously David's modem will probably be more robust and performant,
however I am providing a huge selection of GRC graphs here :)
Cheers,
Adrian (YO8RZZ)