How does my tuner or receiver decode stereo broadcasts?
The basic idea of encoding more than one channel into the essentially mono signal that FM broadcasts were originally conceived with was to create a mono signal that would work for receivers that didn’t have stereo capability. This choice was made so that older mono-only receivers would not become obsolete – useless – when stations began to broadcast in stereo. With the compatible mono signal in place, the broadcasters would transmit other information that could not be sensed by mono receivers, but could by stereo receivers. This new information would be used by additional circuitry in an FM stereo tuner to create a stereo signal from the mono signal. The problem with creating a mono-compatible broadcast is that this information has to be added in such a way as to not be audible to the person listening to the original mono signal – it can’t make the mono signal distort, lose volume or become more difficult to receive. That was really tough to do if we wanted equal quality for the stereo
