Frequency Modulation (FM) In frequency modulation (FM), the number of waves
that occur during one second undergoes change based on the amplitude of the modulating
signal while the amplitude and the phase of the carrier remain constant. Figure 2-21 illustrates
an FM signal and a simple modulating sine wave. The bottom portion of the figure
shows the result of modulating the FM carrier in frequency. Note how the frequency
changes proportionally, based on the change in amplitude of the input signal, which effectively
allows the receiver to reproduce the signal with the correct amplitude (or the volume
of the sound). In addition, the rate of change of the modulated signal also follows the rate
of change of the input signal, which, in turn, allows the receiver to reproduce the frequency
(pitch or tone) at the output.