Although still needing some work on the audio / microphone area, after several years of development the FreeDV digital Voice mode for HF has now reached a stage where it out-performs SSB in low signal situations. Up to three FreeDV conversations can take place in the bandwidth needed by ONE SSB conversation.
Required to run FreeDV is a either a computer running Windows, iOS or Linux (including a Raspberry Pi version) or alternatively there is a speaker microphone with built in FreeDV coder/decoder that can be purchased. In both cases the generated digital voice goes to a standard SSB transceiver. PC-rig Interfacing uses the same hardware as used for digital data modes.
Flex and the $500 RS918 (built MCHF kit clone) and R928 (X5105 clone) Chinese HF 10/15W SDR Transceivers support FreeDV in their soft / firmware.
Example of SSB, FreeDV and then SSB - 2000 mile transmission on a noisy path from Australia to New Zealand along with a spectrum scope picture - http://www.rowetel.com/?p=6103