Amplitude modulation is a technique used to transmit content on a radio carrier wave. Changes, or modulations, in the transmitted signal's strength help to specify the sound or image to be reproduced on the receiving end of the signal. Amplitude modulation was first used in the early 1900s and has been in constant use since the early days of radio.
Amplitude modulation is used for broadcasting radio transmissions, ground-to-air communication, two-way radio, short-range wireless transmission, Wi-Fi and cellular telecommunications. The abbreviation "AM" is usually used to refer to radio transmissions on the medium-wave broadcast band. Although amplitude modulation is usually thought of in terms of radio, the technique can also carry video signals.
Amplitude modulation is the simplest way of modulating an electronic communications signal, and the demodulation of the signal, which converts it back into an audio frequency signal, is simple as well. An AM signal has two-thirds of its power concentrated in the carrier signal, which carries no useful information, and the rest in two mirror-image sidebands; as a result, the power usage of AM is inefficient. It also uses bandwidth inefficiently and is prone to high levels of noise because an AM receiver detects electromagnetic interference in proportions equal to its detection of the actual signal sent.