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) ------------------------------------------------------------------------------ Transform Data into Opportunity. Accelerate data analysis in your applications with Intel Data Analytics Acceleration Library. Click to learn more. http://makebettercode.com/inteldaal-eval _______________________________________________ Freetel-codec2 mailing list Freetel-codec2@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/freetel-codec2
