What is Amplitude modulation?
[[File:Amfm2.gif|thumb|right|A sound wave, an '''AM''' wave, and an '''FM''' wave compared]] '''Amplitude Modulation''', usually shortened to '''AM''', is a simple way to send a [[radio]] [[signal]]. The signal can travel long distances, and appear in faraway places, because of the earth's [[Kennelly-Heaviside layer|ionosphere]]. [[Amplitude]] is the [[height]] of a [[wave]]. A [[wiktionary:carrier wave|carrier wave]] is [[wiktionary:modulate|modulated]] (changed) by adding [[audio]]. The combined signal is [[wiktionary:transmit|transmitted]] over a [[frequency]]. AM signals can be received by a [[crystal]]-powered [[crystal radio|radio set]], that needs no [[battery|batteries]] or electrical [[wiktionary:power source|power source]]. Many early radios were homemade, and used [[wiktionary:earphone|earphones]]. AM signals are mostly sent out on [[wiktionary:medium wave|medium wave]] and [[wiktionary:shortwave|shortwave]] [[frequency|frequencies]]. They do not carry [[high fidelity]] sound, but the sound is good enough to hear most things. [[Analog signal]]s can fade in and out, or have [[:wikt:interference|interference]] from [[wiktionary:power line|power lines]] and [[power station]]s. [[Sunspot]] activity can also weaken signals. In the early years of [[Commerce|commercial]] [[broadcasting]] in the [[1920s]], medium wave AM (from 540 to 1600 [[wiktionary:kilocycle|kilocycles]]) was the only kind of radio widely used. [[Radio program]]<nowiki/>s were much like today's [[Television programme|television program]]<nowiki/>s, including [[news]], [[sports]], [[drama]], [[music]], [[comedy]], and other [[entertainment]]. Later [[Frequency modulation|Frequency Modulation]] (FM) radio was developed. It had better [[wiktionary:fidelity|fidelity]], but the signals could not travel as far, so AM was still important. After [[television]] became common in the [[1950s]], AM became a major [[:wikt:format|format]] for popular music. As FM became common in the [[1970s]], AM stations featured more news, and [[talk show]]s. [[