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The Zenith Quadraphonic FM Broadcast Systems (NQRC designations, 2A, 3B) are capable of broadcasting four discrete audio signals and are compatible with existing monophonic and biphonic FM services as well as with the Subsidiary Communication Authorization (SCA) service presently in practice. Two additional subchannels are broadcast in addition to the monophonic channel and the biphonic subchannel to accommodate the additional audio signals. The first of these additional subchannels uses double sideband suppressed carrier (DSBSC) AM modulation and is broadcast in quadrature with the existing biphonic subchannel at 38 kHz. System 2A, utilizing upper single sideband suppressed carrier (SSBSC) AM, broadcasts the second additional subchannel at 76 kHz while system 3B, utilizing vestigial sideband suppressed carrier (VSBSC) AM, broadcasts the second additional subchannel at 90.25 kHz. Both systems provide the necessary spectrum for the existing SCA subchannel centered nominally around 67 kHz. Standard pre-emphasis is employed to broadcast the four discrete audio signals in the band from 50 Hz to 15 kHz. Both systems broadcast an additional pilot subcarrier (at 76 kHz for system 2A and at 90.25 kHz for system 3B) to provide automatic decoder operation for monophonic, biphonic or quadraphonic signals. Sufficient phase linearity is maintained in the baseband spectrum (50 Hz to 91 kHz for system 2A, 50 Hz to 93 kHz for system 3B) to assure adequate separation between the audio outputs.