AM and FM
Whether in analog or digital format, information has to be converted into radio waves before it can be sent. This process is known as modulation. It involves altering a radio wave of the desired frequency, called the carrier wave, so that a receiver will be able to extract useful information. There are two main types, familiar to all radio listeners: AM (amplitude modulation) and FM (frequency modulation).
The simplest way to transmit a signal is to superimpose it over the carrier wave at the transmitter. As illustrated in Figure 2.5, this results in waves with amplitude (height) that varies in proportion to the information signal.
AM is unusual in modern communications systems because it uses spectrum inefficiently ...