A low bit-rate vocoder/codec allows a low bandwidth modem.

700 bits per sec. for the 700D mode. 1KHz of audio bandwidth.
Thus is useable on HF with standard SSB transceivers.
To get to "low bit-rate" one has to get smart. Forget trying to do a "waveform" thing, instead analyse what the human is doing, sound wise, voice box frequency, amplitude, filtering with throat mouth & nose, added noise. Determine a rate of data packet transmission needed to convey intelligable speach.

See: www.freedv.org

I've reworked the "freebeacon" code to be a repeater "parrot" as it records and plays back.

Runs well on a Pi clone, the Banana Pi M2 Berry and a Yaesu FT-897D transceiver.

Source now at www.unixservice.com.au/parrot/src