The data we are interested in consists of only two
states rather than a continuously changing signal.
This can assist in simplifying the demodulation
process. If we look at the mathematical
description of the carrier, Equation 6, modulated
with the message, Equation 7, we can see that
when the digital information, ai changes from 1 to
-1, xc(t) changes from cos(t) to –cos(t), which is
equivalent to cos(t+
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π ) or a phase inversion. This
is very similar to binary phase shift keying which
changes the phase of the carrier based on the
binary message signal.