When I first became interested in Ham Radio as a 12 year old in the late 70′s my grandfather bought me the 1979 ARRL handbook. Quite an honor to one day be contributing back to this fine hobby that was my start in a communications and electronics career.
That version of the handbook even had a chapter on “Narrow Band Voice Modulation” – an esoteric analog technique to compress speech by removing chunks of audio bandwidth. Who would have thought that 30 years later I ‘d be contributing in the same area…….
The Codec 2 project is moving along nicely. Recently I have been working on integration of the FDMDV modem with Codec 2, and have written a GUI program called fl_fdmdv to help me debug the combined system. Here is a screen shot (click for large version):
This looks really cool as the graphics update in real time, a static image doesn’t really do it justice. Displaying parameters in real time has helped me spot a few bugs, which I missed with the static plots I get from the Octave simulations. I have used fl_fdmdv to send the Codec2/FDMDV signal over an audio cable between two laptops. It’s really exciting to see the bits being modulated onto the waveform on the GUI while listening to audio flowing over the system! Next step is to replace the audio cable with SSB radios and do some over the air testing.
I am also working on a high quality version of Codec 2 at about 4000 bit/s. The target is speech quality similar to CELP type algorithms such as g.729/Speex/Opus that run at 8000 bits/s. The main application is VOIP, but it might also be useful for a “FM quality” mode for VHF digital radio. The key to high quality is quantising and transmitting the phase of the sine waves used by Codec 2 to model speech. The challenges are working with phase (modulo 2-pi hurts my head) and the time varying number of sine waves and hence phases that must be transmitted. The technique I am currently working on is “sparse vector quantisation of phases”. Tough work but I am slowly making progress. This work is being generously supported by a company who wishes to remain anonymous – but I wanted to thank them anyway!