FreeDV is a Digital Voice mode for HF radio. FreeDV allows any SSB radio
to be used for low bit rate digital voice. Speech is compressed down to
700-1600 bit/s then modulated onto a 1.25 kHz wide signal comprised of
16 QPSK carriers which is sent to the Mic input of a SSB radio. The signal
is received by an SSB radio, then demodulated and decoded by FreeDV. FreeDV
700(B) rivals SSB in it's low SNR performance. At high SNRs FreeDV 1600
sounds like FM, with no annoying analog HF radio noise.

WWW: https://freedv.org/