> Old RTTY modems, including valved ones, are not quite in this
> class although they come close

How are they different ? I've an early 80's schematic for a RTTY
transceiver which doesn't use a 555 for modulation (there's one
to set the baud rate). It has a gated twin-T oscillator for the '0'
and '1' tones and an AY3-1015-D UART to iinterface with a
Microbee computer (think Commodore 64 or VIC20)

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