A carrier is generated at the transmitter. A carrier signal is a single-frequency signal that is 1. used to carry the intelligence (data). There are three characteristics of a sinusoidal carrier signal that can be changed over time: amplitude, phase, and frequency. (Note, however, that phase and frequency are just different ways to view or measure the same signal change.) The carrier is modulated with the information to be transmitted. Modulation is the process 2. of overlaying a signal that has (some encoded) intelligence over an appropriate underlying carrier, thereby enabling the long-distance transport and reception of the intelligent signal. Any reliably detectable change in signal characteristics can carry information. When the underlying signal is modulated, it is able to carry a message over a distance; in general, the intelligent signal would otherwise be unfit for direct baseband transmission.*

At the receiver, the signal modifications or changes are detected and demodulated at this 3. point; the modulating signal (intelligence or data) is recovered.