Receiver for compatible FM stereophonic system utilizing companding of difference signal

Claim 1

A receiver for receiving an FM multiplex signal derived from left and right audio stereo signals and including a sum signal, a stereo difference signal S amplitude-modulated on a first sub-carrier, a compressed version S' of said stereo difference signal compressed according to a compression characteristic approximating the loudness response of the human ear such that signals having frequencies in a predetermined lower portion of the audio frequency spectrum are not compressed, said compressed difference signal S' being amplitude-modulated on a second sub-carrier, and a pilot signal, said receiver comprising: means for deriving said sum signal, said difference signal S and said compressed difference signal S'; expander means including a variable gain element having input, output and control terminals, means for applying said compressed difference signal S' to the input terminal of said variable gain element, and control signal generating means for generating and applying a control signal to the control terminal of said variable gain element for controlling the gain thereof, said control signal generating means including means for dynamically comparing a first signal proportional to said difference signal S against a second signal which is proportional to a third signal produced by summing the signal appearing at the output terminal of said variable gain element and a signal comprising said predetermined lower portion of said difference signal S, and means for dematrixing said sum signal and said third signal and producing said left and right audio stereo signals.