In Amplitude modulation, the amplitude of a high-frequency carrier wave is modified in accordance with the strength of a low-frequency audio or video modulating signal. When the amplitude of the modulating wave increases, the amplitude of the modulated carrier also increases and vice-versa.
The envelope of the modulated wave takes the form depending on the amplitude and frequency of modulating signal. Instantaneous amplitudes of audio signal and carrier wave are:
va(t) = vao sin ωat
vc(t) = vco sin ωct
where wa and wc are the angular frequencies and vao and vco denote the amplitudes of audio and carrier waves, respectively. In amplitude modulation the modulating (audio) signal is superimposed on the carrier wave, so that the amplitude of the resultant modulated wave can be expressed as
A(t) = vco + va(t) = vco + vao sin ωat
The instantaneous amplitude of the modulated wave is determined by the amplitude and frequency of the analog audio signal. The ratio vao/vco gives a measure of the extent to which carrier amplitude is varied by the analog modulating signal and is known as amplitude modulation index.