I have done this with an AVR microcontroller and it turned out to be very easy. Just program one output to generate a 1 KHz carrier and another to modulate the amplitude using a resistive divider. A simple LPF will knock the edges off the carrier and not affect the timing accuracy too much. Receive is easy too if you have a micro with a comparator which can tell the difference between the two amplitude levels. I would extract the carrier first and feed it to another input of the micro to provide a phase reference and then do the decoding all in firmware. Morris > > > i would think, given that the audio carrier is 1kHz-ish, that almost > any > of the small microcontrollers would work, using a single bit in/out > with > some RC signal conditioning. Maybe a bit of a challenge for the > receive.. you'd need two inputs with different resistors. >