FM Multiplex

When the FCC investigated methods of broadcasting in stereo on the FM band on the late 1950’s one of the conditions was that the stereo signal should be compatible with mono receivers. ( This condition was also adopted with colour television ensuring the colour transmission was compatible with monochrome receivers. ) Therefore the Left and Right audio was added together and modulated onto the carrier. This mono audio (L + R) occupied the area from 0 – 15KHZ. The laft and right audio is deliberately band limited to 15KHz to ensure that the various components of the FM Multiplex remain separate. The clever people that developed FM stereo then had to work out how to break the Mono audio back to Left and Right. What they did was add the Left and Right audio again out of phase creating L – R. This Left – Right audio was also modulated onto the carrier as DSB suppressed carrier at 38KHz. In order for the receiver to accurately demodulate the L – R , a pilot tone of 19KHz was also transmitted. This pilot tone made up 10% of the modulation level and it’s presence lights the stereo indicator on the receiver.

To recap, we transmit a L + R and and L – R

In the receiver we demodulate the two signals, back to L + R (mono) and L – R. We then add them together in and out op phase.

(L + R) + (L – R) = L +L + R – R = 2L (in phase)

(L + R) – (L – R) = L – L + R – – R = 2R (out of phase)