Apparatus for distinguishing between various fm broadcast multiplex transmissions

Claim 1

In an FM radio receiver, apparatus for distinguishing between reception of two channel and surround stereophonic sound information comprising: means for providing a composite signal including, during two channel stereophonic reception, an audio sum signal component, a continuous wave pilot signal component at a predetermined frequency, a stereophonic audio difference signal component impressed as a first pair of amplitude modulation sidebands on a first suppressed subcarrier, the subcarrier being at the second harmonic frequency of said pilot signal and in predetermined time relationship therewith, and an unmodulated subcarrier at a frequency equal to said suppressed subcarrier and in quadrature phase relationship therewith, said composite signal further including, during surround stereophonic reception, a surround stereophonic audio difference signal component impressed as a second pair of amplitude modulation sidebands on a second suppressed subcarrier, the second subcarrier being at the same frequency and phase as said unmodulated subcarrier and being substituted for said unmodulated subcarrier, synchronous detection means, coupled to said composite signal providing means, responsive during two channel reception to said unmodulated subcarrier and responsive during surround stereophonic reception to said sidebands associated with said second suppressed subcarrier for providing, respectively, output signals including an indication of two channel reception and output signals including said surround stereophonic audio difference signal component, sub-audio bandpass filter means coupled to said synchronous detection means for separating said indication of two channel reception from said output signals, and means coupled to said filter and responsive to said separated indications for producing an output signal during reception of two channel information.