Modulation is defined as the process by which some characteristics of a carrier signal is varied in accordance with a modulating signal. The base band signal is referred to as the modulating signal and the output of the modulation process is called as the modulation signal.
Amplitude modulation is defined as the process in which is the amplitude of the carrier wave is varied about a means values linearly with the base band signal. The envelope of the modulating wave has the same shape as the base band signal provided the following two requirements are satisfied
01. The carrier frequency fc must be much greater then the highest frequency components fm of the message signal m (t) i.e. fc >> fm
02. The modulation index must be less than unity. if the modulation index is greater than unity, the carrier wave becomes over modulated.