In order for information to be transmitted via radio waves, the information must be somehow embedded into the radio waves. The earliest developed method for embedding information is known as amplitude modulation. Amplitude modulation is a way of modifying a carrier signal with an audio signal so that the audio signal can be transmitted to a receiver. It is used today in two-way radios as well as in radio broadcasting. The AM in AM radio stands for amplitude modulation.
Amplitude is the amount of energy in a sound wave. Loud sounds are made by high amplitude waves while quieter sounds are made by lower amplitude waves. Frequency is the number of waves that pass a fixed point during a given amount of time. During amplitude modulation the frequency of the carrier signal remains constant but its amplitude changes to fit the amplitude of the audio signal. In order to prevent distortion the amplitude of the carrier should be greater than that of the audio signal.
The device used to produce amplitude modulation is an non-liner device known as a modulator. It is called a non-liner device because it has two inputs, the carrier and the audio signal, but only one output, the modulated signal. This output is transmitted to a receiver, where it must then be split once again into its component parts in a process known as demodulating.
Demodulation removes the audio signal from the carrier signal. Without this step it would be impossible to hear the audio signal through the interference created by the carrier signal. There are several types of demodulators, but all serve to strip away the carrier signal so that the audio signal can be heard.
The main advantage of amplitude modulation is that it is an inexpensive way to transmit radio signals because it requires only a few easily acquired parts. Yet in many cases, it has been replaced with frequency modulation (FM) and other forms of transmission. The reason for this is two-fold: it is inefficient and it is subject to high levels of background noise.
Amplitude modulation is inefficient in its use of both power and bandwidth. A large percentage of the necessary power is used up by the carrier signal, which is then stripped away. It also uses nearly twice as much bandwidth as other methods of transmission. One practical consequence of this is that fewer radio stations are able to operate in a given area. Amplitude modulation is also sensitive to background noise which can result in a poor audio quality.