FM multiplex stereo decoder

How it works

The decoder accepts the multiplex output of an FM receiver’s discriminator and uses its stereo information to reconstruct left and right channels. The indicator lights when the decoder recognizes the stereo condition.

A normal mono audio output may already have removed the pilot and stereo subcarrier information. Connecting this circuit to such an output cannot recreate stereo that is no longer present.

On the bench

Use the receiver’s appropriate multiplex test point, with the correct level and coupling. Check channel separation with a known stereo signal rather than relying on the indicator alone.

Resources & provenance

Rewritten from the original circuits/rfr/hg65q0p_rfr012.php page. Component values and historical performance claims have not been independently bench-tested. The circuit figures have had their watermarks removed.