What is Modulation?

Modulation is the encoding of a sound signal into a separate signal, known as the carrier signal, for the purposes of data transmission. The reason for using modulation is that the carrier signal is a radio wave instead of a sound wave. Radio waves exist in a partciular frequency range of light that is too low frequency for humans to see. The benefit of using light over sound is that light travels much faster and much farther.

The concept of modulation is not specific to radio broadcasts, however. Any time there are multiple signals sent through the same medium, modulation must be used to prevent interference. For example, cable television uses a form of modulation to send multiple television channels down one wire using a modem (short for modulator-demodulator).

Analog Modulation Methods

Analog modulation is when analog data is transmitted via an analog signal. Given a range of frequencies of light at which to broadcast, how can we encode audio data into the light? If we look at the basic equation for a wave of light, we see there are three parameters.

x = A \sin(B(t - C))

In this formula,

$A$ determines the amplitude,
$B$ determine the frequency,
$C$ determine the phase,
$t$ is time,
$x$ is the "position" of a the light wave as a function of time.

There are three parameters in which we can encode the audio signal $\textcolor{royalblue}{\phi(t)}$ : $A$ , $B$ , and $C$ .

In amplitude modulation, the audio signal is encoded as $A$ . The amplitude can only be so large since higher-amplitude radio waves require more energy, so the audio signal must be scaled to fit within the amplitude limit.

x = \textcolor{royalblue}{\phi(t)} \sin(B(t - C))

In frequency modulation, the audio signal is encoded as $B$ . The frequency must be scaled to all into a narrow band, since frequencies outside the band would interfere with other radio stations.

x = A \sin(\textcolor{royalblue}{\phi(t)}(t - C))

In phase modulation, the audio signal is encoded as $C$ .

x = A \sin(B(t - \textcolor{royalblue}{\phi(t)}))

The light wave is referred to as the carrier signal since it "carries" the information about the audio.

Digital Modulation Methods

In digital modulation, the information to be transmitted is in digital form, such as binary data (1s and 0s). Digital modulation techniques involve varying certain aspects of a carrier signal (amplitude, frequency, or phase) to encode this binary data.