The SM1000 Manual which tells you how to upgrade to the new firmware
I’d also like to thanks Steve (K5OKC) for his fine work on the OFDM modem, Danilo (DB4PLE), and Richard (KF5OIM) for their work on the build system, Github/Travis integration and testing; and Walter (K5WH) and George (AC6RB) for helping us sort out the Windows flashing procedure.
The software engineering side of Codec 2 has come a long way in 2019 – we now have about 50 automated tests and a very nice build system for the x86 and embedded stm32 ports. About half the tests run on a stm32 Discovery card and ensure the stm32 port keeps running as we push the x86 side of the project forward.
The SM1000 hardware was developed by myself and Rick Barnich KA8BMA a few years ago. It is being manufactured, tested and shipped by our good friend Edwin at Dragino in Shenzhen, China.
The FreeDV 700D port includes some sophisticated signal processing:
The Codec 2 700C speech codec.
The OFDM modem developed by Steve and I that has been optimised for HF radio.
State of the art LDPC forward error correction software designed by Bill, VK5DSP.
Largely through Don’s effort that is all running on a tiny 168MHz micro-controller in just 192k of RAM. With identical results to the x86 version.
I’ve really enjoyed working with the team on this project, in particular through GitHub. I like GitHub more that Git – which I still get into trouble with occasionally.
The stm32 side of the Codec 2 project is much improved with a shiny new build system, up to date documentation, and we have the automated tests to ensure we keep it that way. Thanks guys!
Here’s my SM1000 on the bench during development. I’ve hooked up a serial port to print debug messages, and am using laptops to feed it with FreeDV signals:
Brad (AC0ZJ), Richard (KF5OIM) and I have been putting the pieces required for the new FreeDV 2020 mode, which uses LPCNet Neural Net speech synthesis technology developed by Jean-Marc Valin. The goal of this mode is 8kHz audio bandwidth in just 1600 Hz of RF bandwidth. FreeDV 2020 is designed for HF channels where SSB an “armchair copy” – SNRs of better than 10dB and slow fading.
FreeDV 2020 uses the fine OFDM modem ported to C by Steve (K5OKC) for the FreeDV 700D mode. Steve and I have modified this modem so it can operate at the higher bit rate required for FreeDV 2020. In fact, the modem can now be configured on the command line for any bandwidth and bit rate that you like, and even adapt the wonderful LDPC FEC codes developed by Bill (VK5DSP) to suit.
Brad is working on the integration of the FreeDV 2020 mode into the FreeDV GUI program. It’s going well, and he has made 1200 mile transmissions across the US to a SDR using the Linux version. Brad has also done some work on making FreeDV GUI deal with USB sound cards that come and go in different order.
Mark, VK5QI has just made a 3200km FreeDV transmission from Adelaide, South Australia to a KiwiSDR in the Bay of Islands, New Zealand. He decoded it with the partially working OSX version (we do most of our development on Ubuntu Linux).
I’m surprised as I didn’t think it would work so well over such long paths! There’s a bit of acoustic echo from Mark’s shack but you can get an idea of the speech quality compared to SSB. Thanks Mark!
For the adventurous, the freedv-gui source code 2020 development branch is here). We are currently performing on air tests with the Linux version, and Brad is working on the Windows build.
My local radio club, the Amateur Radio Experimenters Group (AREG), have organised a special FreeDV QSO Party Weekend from April 27th 0300z to April 28th 0300z 2019. This is a great chance to try out FreeDV, work Australia using open source HF digital voice, and even talk to me!
All the details including frequencies, times and the point scoring system over on the AREG site.
Is it possible to move data over HF radio using very simple, low cost hardware and clever SDR software? In the last few posts (here and here) I’ve been constructing and testing building blocks for a simple HF data terminal. This post describes a few more, a 3-8 MHz Band Pass Filter (BPF) and 1W Power Amplifier (PA).
Band Pass Filter
The RTL-SDR samples at 28.8 MHz, capturing a broad chunk of spectrum. In direct mode we just sample the Q-channel, so any energy above 14.4 MHz will be aliased into our passband; e.g. both 21 and 7 MHz will appear as a 7 MHz sampled signal.
In the previous post we determined the ADC overloads at -30dBm, so we want to remove any strong signals above or near that level. One source of strong signals is broadcast band AM radio between 500 to 1600 kHz.
The use case is “100 mile” data links so I’d like the receiver to work on the 80M (3.5 MHz) as well as 40M (7.1 MHz) bands, which sets the BPF passband at 3 to 8 MHz. I hooked up my spec-an to a 40M antenna and could see AM broadcast signals peaking at -40dBm, so I set a BPF specification of > 20dB attenuation at 1.5 MHz to keep the sum of all those signals well away from the -30dBm limit. At the high frequency end I specified at > 30dB attenuation at 21 MHz, to reduce any energy aliased down to 7 MHz.
I designed a cascaded High Pass Low Pass/Filter using some tables from my ancient (but still excellent) copy of “RF Circuit Design”, by Chris Bowick. The Octave rtl_sdr script does the calculations for me. A spreadsheet would work well too.
I simulated the BPF using LTSpice, fixed a few bugs, and tweaked it for real world component values. Here is the circuit and frequency response on log and linear scales:
I soldered up the BPF Manhattan style using commercial axial 1uH inductors and ceramic capacitors, then tested it using the spec-an and tracking generator (note linear scale):
The table at the bottom shows the measured attenuation at some important frequencies. The attenuation is a bit low at 21 MHz, perhaps due to the finite Q of the real world inductors. Quite a good match to the LTSpice simulation and close enough for my experiments. The little step at around 10 MHz is a tracking generator artefact.
The next plot shows the effect of the BPF when my spec-an is connected to my 40M dipole (0 to 10MHz span). Yellow is the received signal without the filter, purple with the filter.
The big spike around 0 Hz is an artefact on the spec-an. The filter is doing a good job of nailing the AM broadcast band energy. You can see a peak around 7.4 MHz where the dipole is resonant. Actually this is a bit of a surprise to me, as I want it resonant around 7.2MHz, I better check that out! At 7.2-ish the insertion loss (difference between the purple and yellow) is a few dB as per the tracking generator plot above. It’s more like 6dB at 7.4 MHz (the dipole peak), not quite sure why. An insertion loss of 3dB at 7.2 MHz is OK for my application.
A few weeks ago I hooked the rpitx to my 40M dipole and managed to demodulate the 11mW signal a few km away (over an urban channel) using a mag loop and my FT-817. I decided to build a small 1W PA to make the system usable over “100 mile” HF channels. The actual power is not that critical, as we can trade power off against bit rate. For example if a given HF channel supports 100 bit/s at 1W, we then know we can get 1000 bit/s at 10W.
Even low bit rates can be very useful if you have no other communication. A text message or Tweet, allowing for some overhead, averages about 1000 bits. So at 1000 bit/s you can send 1 txt per second, 3600 an hour, or 86,000/day. That’s very useful communication if you are in a disaster situation and want to tell family you are alive. Or perhaps live in a remote area with no other communication. Of course HF channels come and go, so the actual throughput will be less than that.
I explored the junk box and found a partially constructed Beach 40. I isolated the driver and PA stage and poked it with my signal generator. Turns out it had a bit too much gain (the rpitx has plenty of drive) so I ended up with this simple PA circuit:
The only spurious output I can see is the 2nd harmonic is at -44 dBC, meeting ACMA specs:
The low pass filter at the output has a 3dB point at about 10 MHz which is a little high. It could be brought down a little to increase stop-band attenuation and reduce the 2nd harmonic further. I haven’t done anything about impedance matching the input, as it hits 1W (30dBm) output with 14dBm drive from the rpitx. The 1 inch square heatsink is quite warm after 10 minutes but I can still hold it. It’s not very efficient, 2.9W DC input power for 1W out, however 16dB power gain is quite good for a PA. Anyhoo, it’s a fine starting point for my experiments, we can optimise the PA later if necessary.
OK, so I have most of the building blocks I need for some over the air HF data experiments. There was a bit of engineering involved in building the BPF and PA, but the designs are very simple and can be constructed for a few $ or even from road kill (recycled) components. We now have a very low cost HF data radio, running high performance modems, connected to a Linux computer and Wifi.
Next I will put some software together to estimate data throughput, set the system up with real antennas, and gather some experimental results over real world HF channels.
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.
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”.
Mark, VK5QI has just performed a SSB versus FreeDV 700D comparison between his home in Adelaide and the Manly Warringah Radio Society WebSDR SDR in Sydney, about 1200km away. The band was 40m, and the channel very poor, with some slow fading. Mark used SVN revision 3581, built himself on Ubuntu, with an interleaver setting (Tools-Options menu) of 1 frame. Transmit power for SSB and FreeDV 700D was about the same.
I’m still finishing off FreeDV 700D integration and tuning the mode – but this is a very encouraging start. Thanks Mark!