We’ve just released FreeDV v0.98 GUI software, which includes the new FreeDV 700 mode. This new mode has poorer speech quality than FreeDV 1600 but is far more robust, close to SSB on low SNR fading HF channels. Mel Whitten and the test team have made contacts over 1000 km using just 1 Watt!
You can download the Windows version of FreeDV 0.98 here.
To build it you need the latest codec2-dev and fdmdv2-dev from SVN, follow the Quickstart 1 instructions in fdmdv-dev/README.txt. I’ve been cross compiling for Windows on my Ubuntu Linux machine which is a time saver for me. Thanks Richard Shaw for your help with the cmake build system.
Mel and the team have been testing the software for the past few weeks and we’ve removed most of the small UI bugs. Thanks guys! I’m working on some further improvements to the robustness which I will release in a few weeks. Once we are happy with the FreeDV 700 mode, it will be ported to the SM1000. If you have time, and gcc/embedded experience I’d love to have some help with this!
It’s sounds pretty bad at 700 bit/s but so does SSB at 0dB SNR. The new modem uses a pilot symbol assisted coherent PSK modem (FreeDV 1600 uses a differential PSK modem). The new modem also has diversity; the 7 x 75 symb/s QPSK carriers are copied to form a total of 14 half power carriers. Overall this gives us significantly lower operating point SNR than FreeDV 1600 for fading channels. However the bandwidth is a little wider (800 – 2400 Hz), lets see how that goes through real radios.
Simulations indicate it has readability 4/5 at 0dB SNR on CCIR poor (fast) fading channels. It also has a PAPR of 7dB so if your PA can handle it you can hammer out 5dB more power than FreeDV 1600 (be careful).
For those of you who are integrating FreeDV into your own applications the FreeDV API now contains the 700 bit/s mode and freedv_tx and freedv_rx have been updated to demo it. The API interface has changed, we now have variables for the number of modem and speech samples which change with the mode. The coherent PSK modem has the very strange sample rate of 7500 Hz which at this stage the user (that’s you) has to deal with (libresample is your friend).
The 700 bit/s codec (actually 650 bit/s plus 2 data bits/frame) band limits the input speech between 600 and 2200 Hz to reduce the amount of information we need to encode. This might be something we can tweak, however Mel and the team have shown we can communicate OK using this mode. Here are some samples at 1300 (the codec rate used in FreeDV 1600) and 700 bit/s with no errors for comparison.
Lots more to talk about. I’ll blog some more when I pause and take a breath.