An Introduction to FM MPX

The MPX stereo signal consists three parts.
  1. The first is a normal audio signal made up of the Sum of the left and right channels. This is the signal you hear on a Mono radio and is the same as switching the Stereo/Mono Switch on an amplifier to "Mono'.
  2. In addition a difference signal (Left - Right) is generated and then used to modulate a 38 Khz subcarrier using Double sideband suppressed carrier (DSBSC) modulation. This is an AM modulation of the subcarrier.
  3. To keep the receiver decoder locked into the 38Khz subcarrier a 19 Khz pilot tone (EXACTLY 1/2 of 38 Khz) is transmitted at well. The relative percentage of modulation put into the pilot is 10%.

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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