For the last 2 months I have been working with Dave Witten KD0EAG, coding a GUI application called FreeDV. It combines Codec 2 and the FDMDV modem into single, user friendly application that runs on Linux and Windows. It enables anyone with a SSB radio start using digital voice.
It works really well. FreeDV uses just 1100 Hz of bandwidth, much less that the 2400 Hz required for an analog SSB signal. Compared to SSB it provides a “noise free” audio experience, and continues to work during fades and multipath at quite low SNRs. Mel Whitten has experimented with many Digital Voice systems over the years. This practical experience has led to the current design – a fast sync, no FEC, low latency system that gives a “SSB” type feel for operators.
Here is a video showing FreeDV in action, with analog SSB for comparison:
It’s been a long time since I did any GUI programming and I found it a nice change from the command line signal processing work that I usually do. The programing problems I had to solve didn’t involve maths or complex signal processing algorithms. However bringing FreeDV to life has it’s own special problems, for example spending hours messing with wxWidgets “sizers” to get a check box positioned just right! It was also much larger than the usual program I work on, so there was a certain complexity navigating large files and keeping several balls in the air at once.
I have also really enjoyed working with a nice team of guys, including Dave Witten, Mel Whitten and Bruce Perens. Also involved were a wonderful group of alpha testers and kind people helping us document, support, and improve FreeDV. One example is this fantastic FreeDV Getting Started video produced by Tony, K2MO.
I also feel a sense of importance in our work – FreeDV is the only open source digital voice system for Amateur Radio. It’s an opportunity to prevent Ham Radio (and digital voice over radio in general) being “locked down” to proprietary codecs.
Over the next few months we will gradually improve FreeDV. In particular I would like work on improvements to the low SNR performance. In the medium term I am interested in other applications for narrowband digital voice over radio, such as telephony in the developing world. Ham Radio is an ideal test bed for refining the algorithms and experimenting with integration of the various buildlng blocks.