DISTINGUISHING BETWEEN VARIOUS FM BROADCAST MULTIPLEX TRANSMISSIONS

摘要

A 38kHz flag subcarrier accompanying an FM multiplex transmission identifies whether such multiplex transmission includes two channel or surround stereophonic information. Apparatus to provide such a flag subcarrier comprises, for example, a source of direct potential to unbalance a then unused balanced quadrature subchannel modulator within the transmitter during two channel transmission. Receiver apparatus to detect the transmission of such a flag subcarrier comprises a threshold detector coupled by a subsonic low pass filter to the corresponding synchronous subchannel detector. Indicating and/or automatic switching apparatus responsive to the detector output is provided.