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. Then 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

Here is Codec2 in an open source transceiver project, the M17 project

And the apps, "DudeStar" and "DroidStar" projects which to date, use the 3200 Codec2 mode.

Here I've modded "dudestar" to use the full Codec2 (shared) library so as to try other Codec2 modes.

My Beta source: www.unixservice.com.au/freedv