Here is a sample of Mark, VK5QI, sending a FreeDV 700D signals from Adelaide, South Australia, to a Kiwi SDR at the Bay of Islands, New Zealand. It was a rather poor channel with a path length of 3200km (2000 miles). First SSB, then FreeDV 700D, then SSB again:
Here is FreeDV 700D on the waterfall of Mark’s IC7610. That little narrow signal at 7.176 MHz is 700D, note the “overweight” SSB signals to the right! This is a very bandwidth efficient mode.
Last weekend FreeDV GUI 1.3 was released, which includes the new 700D mode. I’ve been working hard for the last few months to get 700D out of the lab and onto the air. Overall, I estimate about 1000 hours were required to develop FreeDV 700D over the last 12 months.
For the last few weeks teams of beta testers dotted around the world have been running FreeDV 1.3 in the wild. FreeDV 700D is fussy about lost samples so I had to do some work with care and feeding of the sound drivers, espcially on the Windows build. Special thanks to Steve K5OKC, Richard KF5OIM, Mark VK5QI, Bill VK5DSP; Hans PA0HWB and the Dutch team; Eric GW8LJJ, Kieth GW8TRO and the UK team; Mel K0PFX, Walt K5WH and the US team, Peter VK5APR, Peter VK2TPM, Bruce K6BP, Gerhard OE3GBB, John VK5DM/VK3IC, Peter VK3RV and the Sunbury team, and my local AREG club. I apologise if I have missed anyone, all input is greatly appreciated.
Anyone who writes software should be sentenced to use it. So I’ve poked a few antennas up into the air and, conditions permitting have made 700D contacts, getting annoyed with things that don’t work, then tweaking and improving. Much to my surprise it really does handle some nasty fading, and it really does work better than SSB in many cases. Engineers aren’t used to things working, so this is a bit of an adjustment for me personally.
Here’a demo video of FreeDV 1.3 decoding a low SNR Transatlantic contact between Gerhard OE3GBB and Walt, K5WH:
You can see the fast fading on the signal. The speech quality is not great, but you get used to it after a little while and it supports conversations just fine. Remember at this stage we are targeting low SNR communications, as that has been the major challenge to date.
Here’s a screen shot of the FreeDV QSO Finder (thanks John K7VE) chat log, when the team tried SSB shortly afterwards:
FreeDV 700D also has some robustness to urban HF Noise. I’m not sure why, this still needs to be explored. Here is the off-air signal I received from Peter, VK2TPM. It’s full of nasty buzzing switching power supply noises, and is way down in the noise, but I obtained an 80% decode:
It’s hard to hear the modem signal in there!
FreeDV 700D Tips
Lots of information of FreeDV, and the latest software, at freedv.org. Here are some tips on using 700D:
- The 700 bit/s codec is’s sensitive to your microphone and the FreeDV microphone equaliser settings (Tools-Filter). Suggest you set up a local loopback to hear your own voice and tune the quality using the Tools-Filter Mic equaliser. You can play pre-recorded wave files of your own voice using Tools-Play File to Mic in or with the “voice keyer” feature.
- You need to be within +/- 60Hz for it to acquire. Once you have acquired sync it can track drift of 0.2Hz/s.
- Notes on the new features in FreeDV 1.3 User Guide.
- Look for people to talk to on the FreeDV QSO Finder (thanks John K7VE)
- Adjust the transmit drive to your radio so it’s just moving the ALC. Don’t hammer your PA! Less is more with DV. Aim for about 20W average power output on a 100W PEP radio.
- If you get stuck reach out for help on the Digital Voice mailing list (digitalvoice at googlegroups.com)
The last time a new HF voice mode was introduced was the 1950’s and it was called Single Side Band (SSB). It’s lasted so long because it works well.
So a new voice mode that competes with SSB is something rare and special. We don’t want the next HF Voice mode to be locked down by codec vendors. We want it to be open source.
I feel 700D is a turning point for FreeDV and open source digital voice. After 10 years of working on Codec 2 and FreeDV, we are now competitive with SSB on HF multipath channels at low SNRs. The 700 bits/ codec isn’t great. It’s fussy about microphones, EQ settings, and background noise. But it’s a start, and we can improve from here.
It takes some getting used to, but our growing experience has shown 700D is quite usable for conversations. Bear in mind SSB isn’t pretty at low SNRs either (see sample at the top), indeed untrained listeners struggle with SSB even at high SNRs.
Quite remarkably, the 700 bit/s codec is competitive with (and in some cases outperforms) locked down, proprietary, expensive, no you can’t look at my source or modify me, codecs like MELP and TWELP at around the same bit rate.
The FreeDV 700D waveform (the combined speech codec, FEC, modem, protocol) is competitive at low SNRs (-2dB AWGN, +2dB CCIR Poor channel), with several closed source commercial HF DV systems that we have explored.
FreeDV 700D requires about 1000 Hz of RF bandwidth, half of SSB.
Most importantly FreeDV and Codec 2 are open source. It’s freely available to not just Radio Amateurs, but emergency services, the military, humanitarian organisations, and commercial companies.
Now that we have some traction with low SNR HF fading channels, the next step is to improve the speech quality. We can further improve HF performance with experience, and I’d like to look at VHF/UHF again, and push down to 300 bit/s. The Lower SNR limit of Digital Voice is around -8dB SNR.
This is experimental radio. DV over HF is a very tough problem. Unlike other almost all other voice services (mobile phones, VHF/UHF radio), HF is still dominatted by analog SSB modulation. I’m doing much of the development by myself, so I’m taking one careful, 1000 man-hour, step at a time. Unlike other digital voice modes (I’m looking at you DStar/C4FM/DMR/P25) – we get to set the standard (especially the codec), rather than following it and being told “this is how it is”.
Here is how you can get involved:
- Support this work via Patreon or PayPal
- Refactor and maintain the FreeDV GUI source code. This will help free me to push forward the DSP code where my skills are unique. See bottom of FreeDV GUI README.
- Experienced or not, if you want to play DSP, I have some work for you too. You will learn a lot. Like Steve Did.
- Find corner cases where 700D breaks. Then help me fix it.
- Work with me to port 700D to the SM1000.
- Make freedv.org look great and maintain it.
- Help me use Deep Learning to make Codec 2 even better.
- Start a FreeDV Net.
- Set up a FreeDV beacon.
- Help me get some UHF/VHF FreeDV modes on the air. Some coding and messing with radios required.
- Help others get set up on FreeDV, 700D voice quality depends on the right microphone and equaliser settings, and noobs tend to over drive their PA.
- Create and Post Demo/instructional Videos.
Like the good people above, you have the opportunity to participate in the evolution of HF radio. This has happened once in the last 60 years. Lets get started.
If you are interested in development, please subscribe to the Codec 2 Mailing List.
Peter VK2TPM, blogs on 700D.
AREG Blog Post on FreeDV 700D
Steve Ports an OFDM modem from Octave to C. This is the sort of support I really need – thanks Steve for stepping up and helping!
Windows Installers for development versions of FreeDV.
Codec 2 700C
AMBE+2 and MELPe 600 Compared to Codec 2
Lower SNR limit of Digital Voice
700D OFDM modem README and specs
FreeDV User Manual, including new 700D features.
Bill, VK5DSP designed the LDPC code used in 700D and has helped with its care and feeding. He also encouraged me to carefully minimise the synchronisation (pilot symbol) overhead for the OFDM modem used in 700D.