Tibor Bece and George Karan are collaborating with me on the baseband FM (BBFM) project. Tibor and George are veterans of the land mobile radio (LMR) industry, having worked together for many years and helped develop commercial VHF and UHF radio hardware with over 2 million units manufactured. They are pretty excited about the Radio Autoencoder work and what it could mean for LMR.
George has managed to build the RADE V1 stack, and run the ctests on a variety of embedded platforms, including AM625 – this is a high end embedded processor with enough power to run RADE (including the FARGAN stack); and a Librem 5 phone!
Tibor has been interfacing the BBFM ML stack to a COTs LMR radio, using a modified conventional digital voice frame structure to carry the “analog” BBFM symbols. Unlike my passband demo, this implementation has direct access to the FM modulator and discriminator so it’s a “DC coupled” arrangement – closer to what a real world, commercial implementation would look like.
Like me, Tibor was initially thinking the speech quality and low SNR performance of this technology was in the “too good to be true” category. However he has now performed controlled experiments on his (very well equipped) RF work bench, as was quite surprised to be getting high quality speech at RX signals levels down to -125dBm, several dB lower than analog FM or digital LMR systems like P25 would allow. At this low RF level the cut off is due to framing of the RADE symbols (not BBFM), as he never dreamed it would be necessary to operate at such a low SNR.
Tibor writes:
The 11dB SINAD point (around -121dBm) is where the squelch would normally fail to open, and a P25 frame would start dropping out. The RADE decoder munches through this with great ease, there is some barely perceptible degradation.
All I can say – WOW!
Here are samples (over the same radios) of analog FM and BBFM at various RF input levels from Tibor’s workbench: