Multi-frequency modulation modem. 375bps on a line with 2kHz bandwidth. Not sure if good.
General principles, not exact components. Not quite finished, it would probably be better with different filters, impeding rather than sinking out of band signals, but that would mean re-testing and I couldn't be bothered.
To save space, inverters are used for analog amps and mixer, and the Schmitt triggers are made from AND gates. The switching FETs provide input resistance for the mixer. The mixer in the sender is also effectively the pre-amp in the receiver here.
