Modulation is a process in which a modulator changes some attribute of a higher frequency carrier signal proportional to a lower frequency message signal. If the carrier is represented by the equation
Figure 1. Carrier Signal Equation
a change in the message signal will produce a corresponding change in either the amplitude, frequency, or phase of the carrier. A transmitter can then send this carrier signal through the communication medium more efficiently than the message signal alone. Finally, a receiver will demodulate the signal, recovering the original message.
In Amplitude Modulation (AM), pictured below, the amplitude of the carrier sinusoid changes based on the amplitude of the message.
Figure 2. Amplitude Modulation
The message signal (red) rides on top of the carrier as the amplitudes of both vary with time. The frequency of the carrier, however, is much higher than the frequency of the message. This carrier frequency is the center of the 'channel,' or frequency allocation of this RF signal. Frequency allocations vary depending on the medium of transmission. For broadcast transmissions, where signals are sent through the air, the government regulates frequency allocation. If the RF signal is transmitted over wire, such as in cable television, there is more freedom in the choice of carrier.
In addition to amplitude modulation, frequency modulation varies the frequency of the carrier sinusoid based on the amplitude of the message signal. Similarly, phase modulation changes the phase of the carrier in response to a change in amplitude of the message.