It's a sham that faster digital modes haven't really caught on. I've read about 56k packet in the past. Most radios today support 9600 baud packet, if you can find the discontinued equipment that support those speeds. What does this have to do with digital voice?
Codec2 can squeeze voice into 2550bit/s AMBE can do voice in 2400bit/s and MELP can do it in 2000bit/s. D-Star does a continuous bitstream at 4800baud with 2400 for AMBE voice and 1200 for FEC(Forward Error Correction) (And the voice frames are transmitted blank if you're doing data only, FEC doesn't cover data and you really only get 751bit/s data, I'm not a fan of the protocol)
If someone built a system with Codec2, some FEC and a packetized modem at 9600 baud (or, better 19.2k for the overhead). I'm betting we could have a single-channel digital voice digipeater. I think you'd have to be careful not to use 50% of the channel on each side just because of overhead from headers and the like. It'd be like 802.11, I tx some time, you tx some time, I tx some time, etc. 1 frequency digital repeater. No duplexer cans.
In fact, if the data speed was fast enough we could probably do multiple logical channels with simultaneous users on the same frequency.
More to come.