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15.10 Multiplex
We have so far discussed methods of modulating a carrier by a single
modulating signal. But in stereo radio and colour television
broadcasting, a second modulating signal is needed (as explained in
later chapters) and it would be very convenient if the same carrier
could be used to carry both, provided that the signals could be
recovered without mutual interference at the receiver.
This can be achieved, and the same method is used in both radio
and television. The second signal is used to amplitude-modulate a
second carrier, known as a subcarrier, which is added to the first
signal, the frequency of the subcarrier being