Digital modulation method for standard broadcast FM subcarrier

摘要

A digital transmission system using standard FM broadcast stations employs an SCA subcarrier which is amplitude modulated with a second subcarrier modulated to have different phase orientations representative of digital data. Bits are separated from a digital data stream in groups and are differentially encoded into one of at least four phase orientations of the second subcarrier. The amplitude modulated SCA subcarrier is combined with the stereo signal output of a stereo multiplexer to form the modulating signal for the station FM modulator. The receiver includes a differential phase detector for reconstruction of the differentially encoded digital data. A system which additionally includes means for modulating the SCA subcarrier with a composite signal having an audio signal in addition to the differentially encoded second subcarrier is disclosed.