You may find the MELP Vocoder and history interesting: https://melp.org/ https://melp.org/ (MIL-STD-3005) MELP targets 2.4kbps, and there are later examples f

You may find the MELP Vocoder and history interesting: https://melp.org/ https://melp.org/ (MIL-STD-3005) MELP targets 2.4kbps, and there are later examples for even lower bitrates (e.g. 1.2kbps, 600bps) One thing to keep in mind is that a lot of these codecs or technologies are designed to operate at very low bit rates because the signal carrier is required to be capable of operation in "contested environments" where jamming and/or other environmental effects are present (and throughput potential is the tradeoff for assurance).