Amplitude modulation (AM) is a technique used in electronic communication, most commonly for transmitting information via a radio carrier wave. AM works by varying the strength of the transmitted signal in relation to the information being sent. For example, changes in signal strength may be used to specify the sounds to be reproduced by a loudspeaker, or the light intensity of television pixels. Contrast this with frequency modulation, in which the frequency is varied, and phase modulation, in which the phase is varied.

In the mid-1870s, a form of amplitude modulation—initially called “undulatory currents”—was the first method to successfully produce quality audio over telephone lines. Beginning with Reginald Fessenden’s audio demonstrations in 1906, it was also the original method used for audio radio transmissions, and remains in use today by many forms of communication—”AM” is often used to refer to the medium wave broadcast band

The AM modulation index is the measure of the amplitude variation surrounding an unmodulated carrier. As with other modulation indices, in AM this quantity (also called “modulation depth”) indicates how much the modulation varies around its “original” level. For AM, it relates to variations in carrier amplitude and is defined as:
where and are the message amplitude and carrier amplitude, respectively.

So if , carrier amplitude varies by 50% above (and below) its unmodulated level; for , it varies by 100%. To avoid distortion, modulation depth must not exceed 100 percent. Transmitter systems will usually incorporate a limiter circuit to ensure this. However, AM demodulators can be designed to detect the inversion (or 180-degree phase reversal) that occurs when modulation exceeds 100 percent; they automatically correct for this defect.[citation needed]

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