At the receiver the L+R and L-R signals are added for the Left channel ((L+R)+(L-R)=2L) and subtracted for the right channel ((L+R)-(L-R)=2R).
This is the 'Classic' method. An easier to build method simply switches between the left and right channels at a 38 Khz rate. The mathematics of this produce a signal the same as the classic method but there is less likelyhood of differences between channels sneaking in.
Pre-empahssis (giving High frequencies a boost) is applied to both channels before the Multiplexing process. A corresponding de-emphasis is applied at the receiver. The idea of this is to lift the high frequency signal some so that in the receiver you can reduce the high frequency ALONG WITH some of the hiss from the receiver. In Australia we use a 50 uS time constant network for pre-emphasis, in USA they use 75uS.
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