Radio Ramblings

So, the release of a new version of FDMDV reminded me of one of the things that got me interested in software radio to start with.

FDMDV takes a voice stream and puts it through a MELP codec, which gives a stream of 54 bit frames.  There are then 2 bits of text data added, and this 56 bit chunk is then split into 2 x 28 bit data frames, which are transmitted over 14 QPSK carriers, with a central BPSK carrier for tuning and framing indication.

I had been wanting to set up a compatible modulator/demodulator where it’s possible to easily change the modulation scheme (to add or remove carriers, and change from QPSK to other modulations) and voice codec used (to test the effects of codecs other than MELP.)

It’s pretty hard to get your head round how to map all of this into a GNURadio program - I’m using the GNU Radio Companion to help with this - but even so it’s a struggle.  I’ve simplified things by starting with just 4 carriers, but I’ve not managed to pass data across them yet.

If anyone has Been There Done That already, I would be really intrigued to hear how you accomplished it.  If anyone wants my work-in-progress flow graphs, then add a comment - otherwise I’ll put them up here once I’ve got something which works.