David’s FreeDV Update – April 2024

This month I took a vacation, so less work that usual on FreeDV. I traveled by road from my home in Adelaide to Western Australia (WA), reaching the South-Western tip of Australia, about 3000km away. South Western WA is a lovely part of the country, and the trip included the adventure of crossing the 1200km Nullarbor plane (translated: no trees).

But back to the business of HF digital voice. Given the encouraging results from our initial Radio Autoencoder (RADAE) over the air (OTA) tests, we have expanded our program of testing to include Hams from different countries. It takes a lot of work to develop a new speech communication system , so it’s important to validate the design as early as possible. Much smarter to do this in the current simulation form, rather than put in ten times the work on a real time implementation, release it, and find out it falls over in a common use case. This is experience talking – we’ve learned many lessons after a decade of FreeDV development.

So we are testing the prototype RADAE design using crowd sourcing. I have approached several Hams for help in testing RADAE signals over their local radio channels and in different languages. They provide me with a 10s speech sample in their language, I send them back a file of RADAE samples that they can transmit over the air. The received RADAE signal is recorded off air by them (e.g. using a KiwiSDR), we then decode and evaluate. This is all done in non real time using stored files being emailed back and forth.

In particular I would like to thank Kanda JH0PCF, Yuichi JH0VEQ, and Simon DJ2LS for your help. Some take aways:

1. RADAE works well in Japanese and German, as well as English.
   It handles Near Vertical Incidence Skywave (NVIS) channels quite well. This is typical of local (several hundred km) HF communication in countries like Japan and the UK. The signal goes straight up and straight down.
2. However on the samples tested so far, RADAE is falling over with long distance (e.g. Japan to Australia) communications, so some more work required there.
3. The speech quality is competitive with SSB at high and low SNRs, however we need a better way to measure the actual SNR of off air signals to “calibrate” the results. So I need to hit the math on that one to develop a suitable algorithm.

Some comments from the test team:

I listened to the voice that was replied. RADAE has no problem with Japanese demodulation. In fact, I feel that RADAE’s demodulated audio is easier to hear than SSB. I thought the sound quality was close to the 2020 mode implemented in the current version. It’s a strange feeling to be able to experience FM mode quality audio with SSB. (Kanda, JH0PCF)

It feels, if we can hear SSB clearly, then RADAE also works, but as soon as we are coming closer to the edge of ability of hearing SSB, also RADAE struggles. But the radae output is really nice, much better voice output compared to SSB. Using your last example, I first thought, you’ve send me the default audio example (ie source sample), so yes, its really nice. (Simon, DJ2LS)

Here is a example from Kanda at low SNR

Kanda JH0PCF, 20W, received by Tokyo KiwiSDR (RADAE)

The two signals are transmitted one after the other, so get (more or less) the same channel conditions. The spectrogram below shows (left to right) the sine wave tone, SSB, then RADAE. Note about half the RADAE signal is wiped out at the start, but it seems to sound OK.

Here’s a medium SNR (8dB-ish) sample from Simon, DJ2LS, over a 1100km path at about 10W peak (1-2W RMS):

Yuichi performed a couple of novel experiments that I hadn’t thought of. Here is RADAE compared to existing FreeDV modes, transmitted a few seconds apart, so over more or less the same channel at roughly the same power. SNRs are around 8dB.

Unfortunately 2020B didn’t sync on this channel/SNR. I feel the 700E sample could possibly be improved (e.g. levels, microphone, filtering), but it does illustrate the problems with current FreeDV modes – the speech quality borders on the lower level required for communication and it can be a struggle to get consistent results. This is something we have recognized and are attempting to address with our ARDC funded R&D program.

As a fairer comparison, here is the sample used for RADAE testing passed through a FreeDV 700E simulation with no channel errors:

Project Plan Pivot

Given the encouraging results with RADAE, we’ve pivoted our ARDC project plan to focus on RADAE, and have paused development of Codec 2 and FreeDV modes. RADAE appears to be our strongest candidate for satisfying the top three goals we set for ourselves when applying for the ARDC grant:

1. Improve speech quality to a level comparable to commercial codecs.
2. Develop a “rag chew” FreeDV mode with subjective speech quality comparable to SSB at high SNRs.
3. Improve low SNR operation such that FreeDV is superior to SSB over poor HF channels.

We are on track to meet (and indeed exceed) the first two goals, but I think the final goal has yet to be demonstrated (e.g. SSB and the current incarnation of RADAE fall over at roughly the same SNR). There are a few bugs and many practical issues to work through before we have a real world version of RADAE that anyone can use. Plus there will be a few “gotchas” we haven’t thought of yet. Plenty for me to do in the coming months!

I’ve also been working on HF data modem software with Simon from the FreeDATA project. In next months report we hope to present a new FreeDATA release incorporating this work, resulting in a significant boost in FreeDATA performance.