What do the Multiplex filter or Hi-Blend functions on my tuner or receiver do?
FM broadcasts that are in stereo use the multiplexing system described in the FAQ item just above. As described there, one of the restrictions on the design of FM stereo broadcasts was that it could not affect the ability of a monophonic receiver to receive the mono component of the signal. There wasn’t quite enough room to broadcast two complete channels in the spectrum allotted to an FM channel, so the only choice left was to reduce the amount of spectrum given to the stereo signal, instead. What this does is reduces the amount of signal available for the FM Stereo encoding as compared to the mono (L+R) encoding; as a result, the tuner must have a stronger signal from an FM station to get the same amount of “quieting”, or noise reduction, in stereo as the quieting it can achieve with the same signal in mono. If you’d like to try an experiment so you can see this happening, just tune to a moderately distant FM station. If you have a hi-blend or multiplex filter function, turn them off
