US 4,817,151 AGranted

Selective decoder for compatible FM stereophonic system utilizing companding of difference signal

AssigneeBroadcast Technology Partners

InventorsMurray L. Bod, Renville H. McMann, Emil L. Torick

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Abstract

A decoder for use in an FM stereo broadcasting system in which the usual difference signal S and a compressed version S' of the difference signal are transmitted, automatically selects on a continuous basis for de-matrixing with the received sum signal M either an attenuated signal S' or the usual difference signal S based on the protocol that the attenuated signal S' is always selected unless the usual difference signal S is higher in level than a predetermined threshold potential related to the signal level at the knee of the expansion characteristic. The decoder eliminates the need for the relatively expensive analog variable gain amplifier used in prior art decoders and is less sensitive to time constant errors and errors due to multipath distortions.

Claims14

Dependent claims

For use in a compatible stereo transmission system, a receiver for receiving an FM multiplex signal derived from left and right audio stereo signals and including at least a sum signal M, a stereo difference signal S amplitude-modulated on a first subcarrier, a compressed version S' of said stereo difference signal amplitude-modulated on a second subcarrier, said signal S' being said signal S compressed in accordance with a compression characteristic which has a knee at a selected input signal level below which said compression characteristic has a compression slope of unity and a fixed gain difference relative to the unity gain slope of said difference signal S and above which said compression characteristic has a compression slope substantially of infinity, said receiver comprising:

means for receiving said FM multiplex signal and deriving therefrom said sum signal M, said difference signal S and said compressed difference signal S',
means for controllably attenuating said compressed difference signal S' responsive to the measured level of said difference signal S and producing an output attenuated compressed difference signal S',
de-matrixing means for normally producing output left and right audio signals obtained from combining said sum signal M with said output attenuated compressed difference signal S', and
means including level detector means for continuously measuring the level of said difference signal S and said compressed difference signal S' and responsively when the level of the difference signal S differs from the level of the compressed difference signal S' by a predetermined amount for causing said de-matrixing means to produce output left and right signals obtained from combining said sum signal M with said difference signal S.

1 more element

A receiver for receiving a compatible FM multiplex signal derived from left and right stereo signals and including a sum signal M, a stereo difference signal S amplitude-modulated on a first subcarrier, and a compressed version S' of said stereo difference signal amplitude-modulated on a second subcarrier of the same frequency as said first sub-carrier and in quadrature therewith, said signal S' being said signal S compressed in accordance with a compression characteristic having a knee at a selected input signal level below which said compression characteristic has a compression slope of unity and a fixed gain difference relative to the unity gain slope of said difference signal S and above which said compression characteristic has a compression slope of substantially infinity, said receiver comprising:

means for receiving said FM multiplex signal and deriving therefrom said sum signal M, said difference signal S, and said compressed difference signal S',
means for controllably attenuating said compressed difference signal S' responsive to the measured level of said difference signal S and producing an output attenuated compressed difference signal S',
de-matrixing means for normally producing output left and right audio signals obtained from combining said sum signal M with said output attenuated compressed difference signal S',
means including level detector means connected to receive an input signal having a level related to the level of said difference signal S and for comparing the level of said input signal with a reference potential and in response to detection that the level of said input signal differs from said reference potential by a predetermined amount causing said de-matrixing means to produce output left and right audio signals obtained from combining said sum signal with said difference signal S.

1 more element

Dependencies are inferred from the claim text.