(under subclass 279) Subject matter where the carrier is an elementary alternating current signal having for each data bit a zero crossing in the same direction at the beginning of each bit and also at the end of each bit, a constant duration, a zero crossing only once intermediate the beginning and the end of each bit, the intermediate zero crossing representing one of the two signalling conditions when occurring before the middle of the data bit and the other of the two signalling conditions when occurring after the middle of the data bit.
