Three-Channel FM Stereo Multiplex System for Compatible Broadcasting

A system for transmitting and receiving three independent stereophonic channels by FM multiplex is described. This system has the advantage of being compatible with monophonic and two-channel stereophonic FM receivers without any significant loss in their output signal-to-noise ratios (less than 1 dB) and without having to utilize any part of the SCA band. Compatibility of the three-channel stereophonic receiver with a one-or two-channel broadcast is achieved by the use of an additional pilot signal that does not cause any increase in the instantaneous frequency deviation of the FM carrier. The additional noise in the output of the three-channel receiver appears to be less than 6 dB above the noise that is now present in two-channel receivers.