Interested in new open source digital radio protocols? Check out FreeDV.
FreeDV is a GUI application for Windows and Linux (MacOS and BSD are in testing) that allows any SSB radio to be used for low bit rate digital voice.
Speech is compressed down to 1400 bit/s then modulated onto a 1100 Hz wide QPSK signal which is sent to the Mic input of a SSB radio. On receive, the signal is received by the SSB radio, then demodulated and decoded by FreeDV.
FreeDV was built by an international team of Radio Amateurs working together on coding, design, user interface and testing. FreeDV is open source software, released under the GNU Public License version 2.1. The FDMDV modem and Codec 2 Speech codec used in FreeDV are also open source.
In addition to the FreeDV software you also need an interface device which connects between your PC soundcard and the speaker and microphone on your amateur radio transceiver to provide isloation between the devices. FreeDV may also be used with a shortwave receiver for listening only. In this case you may be able to get away with a direct connection between your shortwave receiver’s headphone jack and your PC’s microphone or line-in jack, though this does not provide protection for your PC soundcard.
All audio processing is done by the PC, with I/O via the PC speakers and microphone. Most FreeDV ops are centered around 14.236 MHz. FreeDV is distributed as open source software and is available for Windows and Linux from FreeDV.