A periodic modulated (AM) radio signal has the form
$$y=(A+B \sin 2 \pi f t) \sin 2 \pi f_{c}\left(t-\frac{x}{v}\right)$$
The factor \(\sin 2 \pi f_{c}(t-x / v)\) is called the carrier wave; it has a
very high frequency (called radio frequency; \(f_{c}\) is of the order of
\(10^{6}\) cycles per second). The amplitude of the carrier wave is \((A+B \sin 2
\pi f t)\). This amplitude varies with time-hence the term "amplitude
modulation" - with the much smaller frequency of the sound being transmitted
(called audio frequency; \(f\) is of the order of \(10^{2}\) cycles per second).
In order to see the general appearance of such a wave, use the following
simple but unrealistic data to sketch a graph of \(y\) as a function of \(t\) for
\(x=0\) over two periods of the amplitude function: \(A=3, B=1, f=1, f_{c}=20 .\)
Using trigonometric formulas, show that \(y\) can be written as a sum of three
waves of frequencies \(f_{c}\) \(f_{c}+f,\) and \(f_{c}-f ;\) the first of these is
the carrier wave and the other two are called side bands.