in reply to: software physical layer posted by Claudio Weicman on 18 April 2003 at 17:15:51.
Possibly for the transmitter you want to use DDS i.e run a 100us interrupt, compute the phase angle or the 1200Hz or 2200Hz signal being transmitted (by adding the phase increment for 100us, and look up the sine of the angle in a look up table and transmit that value through a dac. A anti alias filter about 4 poles at 4kHz should be placed after the dac. I suspect the receiver will be more difficult to do in a micro, though at 1200 baud Bell 202 you can use a PLL for demodulation