A method of encoding binary messages for asynchronous transmission. Two states, A and B, are chosen. (These states could be different amplitude waves for amplitude modulation, or two different freuqencies for frequency modulation, for example.) One state is used to represent the active signal, the other state is used to represent the inactive signal. Normally the inactive signal is easier to produce, or takes less power to generate. A logic value 0 is represented by a full bit period at the inactive state, a logic value 1 is represented by a brief pulse of the active state followed by the inactive state for the rest of the bit period.
Compare to NRZ encoding and Bi-Phase encoding.