FreeDV is a program for digital voice transmission over the air with a smaller bandwidth than SSB. This is achieved by compression to a transmission rate of 1400 bits/s and QPSK modulation with a width of 1100Hz. This data signal is fed from the sound card to the microphone input. During reception, the audio signal from the radio station is fed to the sound card and the program creates an intelligible sound from it. This configuration is therefore similar to other digital modes.
You can compare classic and digital transmission in the demo video. You can download FreeDV at http://freedv.org. FreeDV and the necessary codec are free. You can consult or share information about digital voice transmission in the discussion group http://groups.google.com/group/digitalvoice