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Keep in mind that 700 b/s is an extremely low bitrate.

I worked on Opus, which bottoms out at around 6000 b/s. A typical IP+UDP+RTP packet has 40 bytes of headers, which with 40 ms frames is 8000 b/s of overhead. Cell networks will use header compression to reduce that, but even so. 8000 b/s is almost eleven and a half 700C codec2 streams.

codec2 is just in another league. It's seriously impressive work.

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I think the old Crichton novel, Congo, mentioned using a very low bitrate codec over a digital satellite link. So maybe something already exists off the shelf that one could try over amateur radio.

AFAIK there aren't any commercial codecs with such low bit rates, as there simply isn't the demand. The minimum bit rate of GSM-AMR is 4.75kbps. Some military codecs designed for use over HF are in the same ballpark as Codec2 - STANAG 4479 operates at 800bps. There is some academic research on voice codecs below 500bps.

Fun fact: export of very low bitrate (<2400 bps) codecs has been restricted under the same munitions restrictions as crypto (And also when open source presumably subject to the same court outcomes in the US).

Gotta talk to those subs somehow, those ultra long wavelength Comms aren't exactly high bandwidth. (I'm just making this up but it feels plausible, at least a few decades ago)

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