A great lecture night at the Manly Warringah Radio Society, with the Queen looking on with a knowing smile (I bet she understands I/Q demodulators).
Andrew Gilbett, VK2XAG, gave a presentation on his dissertation on using software defined radio for a repeater system including how this might be scaled up to a "repeater hotel" where a single SDR might implement multiple repeaters on different frequencies.
Andrew talked through convolution, gausian minimum shift keying (GMSK), which is used in GSM and DStar, and how all this is easily done in software.
For me, the hi-light was seeing GNU Radio in action using GNU Radio Companion (grc). Although GNU Radio can be used with a Linux PC's audio I/O, Andrew brought along an Ettus USRP (Universal Software Radio Peripheral) for the demonstration.
For the demo an antenna was attached to the transmit output connector, the receiver was simply the un-terminated socket, Andrew then used a VHF transceiver to talk and whistle while we watched the FFT and waveform on the live displays in GNU Radio Companion.
I rushed home to my Ubuntu 11.04 machine, installed GNU Radio, but it doesn't currently have GRC in there so I grabbed the source from the git repository and built using instructions here and here. (There are quite a few dependancies to be satisfied before grc will build).
Thanks to the organisers and thanks to Andrew for an excellent presentation.