A while back I blogged about how to listen to DMR using a SDR. I tried to do the same with D-Star, but was having problems. I ended up reaching out to the author. He just released a fix, so if you download / install from now on, you shouldn't run into the head banging that I was.
I am not going to lie, it doesn't sound the best. There was no information for coder's to work off of, so what we have is rather crude, but intelligible.
If you are looking to help improve it, start here. This is from the folks from the community that brought us what we have today.
There was also this tip from user "groovy"
If anybody wants to continue the research / work, I suggest you look at the osmocom GMR code that Sylvain Munaut worked on. Those phones use a similar codec - I believe with longer frames for the satellite latency. Initially he used the mbelib code, enhanced it for things like tone support, but he later rewrote the synthesis code completely. See OsmocomGMR for his presentations and source code.
Unless other developers take this on, DVSI releases specs, or you switch to a hardware-based decoder like the thumbDV, I don't see the dstar voice quality improving in the short term.
I'd love to see further work on OP25. More so on the transmit part (hooked to analog radio, perhaps using the MMDVM hardware/Arduino. Anyone care to join forces with Max?
Perhaps an update to Johnathan Naylor's (2009) GUI Linux client that would decode and generate GMSK using a soundcard and interface to a radio with a 9600 Baud packet connector. ..
Wouldn't mind a way to hook op25 to Allstar either :-)