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, and added noise. Then determine a rate of data packet transmission needed to convey intelligable speach.

See: www.freedv.org


============ My HF Repeater project ===========
I've reworked the "freebeacon" code from the above website, to be a repeater "parrot",
as it records and plays back. As a reverse beacon, it was 99% there.

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

FreeDV Ver 1.9.x with Reporting ON.
To trigger "parrot" having noticed my current frequency on QSO.freedv.org
put "0" (zeros) on the end of your callsign to be in position 7 or 8 eg. K1AA => K1AA000 since few if any callsigns end in a zero.
Only modes 700D or 700E until more work is done on the LPCNet modes
to allow them (2020x) to run on a low power ARM platform. March 2023

2024, the Pi 5B has the power to run the 2020x modes.
LATE 2024, a new mode RADE or RADAE blows SSB away with a Machine Learned algorithm.
But (now) does not send a callsign or any flag useful for, say a repeater.
The lookup code table is huge, gigabytes. (no problem today, 2024)

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

And on Github, by contributor AlanVK2ZIW

Here is Codec2 in an open source transceiver project, the M17 project
https://m17project.org/

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