This month I conducted a successful test of the Baseband FM (BBFM) waveform, over a short UHF radio link on my bench. This demonstrates high quality, 8000 Hz audio bandwidth speech being transmitted over the air (OTA) using commodity FM UHF radios and machine learning. It’s early days, but the speech quality already appears competitive with analog FM and any VHF/UHF digital voice system I am aware of.
Here is a sample of the “ideal” BBFM audio ( a perfect channel), and the audio through the UHF radio link. The initial word “G” is missing due a sync issue that will be cleaned up soon.
The experimental system was a Yaesu FT-817 with a Rigblaster USB sound interface as the transmitter into a dummy load, and a Yaesu VX3 handheld with a USB dongle sound card as the receiver. I used the Python “passband” modem developed last month so the signal could be sent over the regular 300-3000 Hz audio bandwidth that commodity FM radios provide (i.e. no DC coupling to the FM modulator or special mods).
To test the modem I can send BPSK symbols instead of ML symbols – in this case I could see a bit of distortion on the scatter diagram. However when I plug the ML symbols back in the audio sounds just fine, indicating the system is quite robust to noise as expected. It’s early days so I haven’t set the deviation carefully or fine tuned the system, but this is a fine start.
C Port of Core ML
The next chunk of work from November was a C port of the Python core encoder and decoder at the heart of the RADE system. Fortunately, this is very close to RDOVAE that is now part of Opus, so much of the Opus ML code could be re-used, with the main change being a new set of weights. The C port resulted in a significant drop in CPU load, in fact it’s now hard to measure on my laptop.
Profiling suggests the remaining receiver Python DSP now dominates the CPU load. However I am reluctant to port this to C as (a) it’s complicated so this would take months and (b) I have some improvements planned for RADE V2 which, if successful, will make much of this DSP unnecessary.
End of Over Text
Unlike earlier FreeDV modes RADE V1 does not, at present, have a way of sending small amounts of text over the channel (along side the voice). This is particularly useful for “spotting” RADE signals, e.g. on FreeDV reporter and PSK reporter. We have plans for a text system in RADE V2. but this is several months away. As an interim solution for RADE V1, we are building up a text system that uses the currently empty “End of Over” frame to send digital data. Turns out we have room for 180 bits available there. So every time an over ends, a chunk of text can be sent by the system. I have developed the modem DSP side of this, and it seems to work OK on simulated fading channels at moderate SNRs (e.g. 6dB on fading channels).
Conference Paper
Finally, I have been working on a conference paper on the HF RADE system. This is new technology for HF speech communications, and combines several disparate technologies, e.g. machine learning, speech coding, OFDM, and HF radio. So I am putting in some effort to document and publish the work in a paper, hopefully at a conference (TBD) in 2025.