AMPLITUDE MODULATION (AM)

Amplitude modulation (AM) is the scheme in which the amplitude of the carrier signal is varied in direct proportion to the instantaneous amplitude of the baseband signal, while the carrier's frequency and phase are both held fixed. The result is a modulated waveform whose outer envelope -- the curve traced by the peaks of the rapidly oscillating carrier -- exactly reproduces the shape of the slower baseband signal, so that a listener's or viewer's original information is encoded purely in how loud (large-amplitude) or quiet (small-amplitude) the transmitted carrier becomes at each instant. AM is the scheme historically used for radio and television broadcasting. It carries several practical advantages: the transmitting and receiving circuitry needed is comparatively simple, so transmission and reception are easy; it needs a relatively small channel bandwidth (about twice the baseband signal's own bandwidth); and both of these together keep equipment cost low. Its limitations are equally real: because information is coded purely in amplitude, any amplitude-disturbing noise picked up along the way directly corrupts the received signal, giving AM a comparatively high noise level; the scheme is also relatively power-inefficient, since a large fraction o …