Amplitude Modulation
Electromagnetic radiation or transmission of information through space
has become an indispensable part of a modern technological society.
However, transmission through space is only efficient and economical
at radio frequencies (above 20 kHz). To transmit intelligent signals—
such as for speech and music—contained in the low-frequency range
of 50 Hz to 20 kHz is expensive; it requires a huge amount of power
and large antennas. A common method of transmitting low-frequency
audio information is to transmit a high-frequency signal, called a carrier,
which is controlled in some way to correspond to the audio information.
Three characteristics (amplitude, frequency, or phase) of a carrier can
be controlled so as to allow it to carry the intelligent signal, called the
modulating signal. Here we will only consider the control of the carrier’s
amplitude. This is known as amplitude modulation.