MF-AM stereo broadcasting: the choice of modulation

Abstract

Several AM stereo transmitting systems have been applied to local radio-broadcasting networks on the MF (medium-frequency) band in the US. These sysems represent a certain compromise between two mutually contrary requirements: (i) that the broadcast spectrum be at the MF carrier within the determined boundaries (the channel occupancy criterion) and (ii) that the transmitted signal be compatible with MF-AM monaural receivers (the compatibility criterion). It is proved that a transmission system that perfectly satisfies both requirements does not exist. A modulation method called modified compatible quadrature amplitude modulation (MC-QAM) is proposed. The system has simpler circuitry and less distortion, and occupies, under many circumstances, a narrower frequency bandwidth. With slight modifications, existing receivers for C-QAM could be used for MC-QAM.