Some argue when the advent of radio truly began, but early in 1906 was the advent of AM radio. FM stations began broadcasting in the late 1930s and early 1940s. At the beginning of World War II, broadcast radio's resources were diverted to the war effort, rendering several FM radios worthless.
AM, or amplitude modulation, is a radio band that broadcasts from 535 to 1705 kilohertz, with stations being possible at every 10 kilohertz. FM, or frequency modulated, radio is higher on the radio spectrum, being broadcast from 88 to 108 megahertz. FM stations are possible at every 200 kilohertz.
AM radio had a 30-year head start and proved its superiority for almost a century. FM radio has had two major listener and station boosts: After the inventor of FM radio died, his FM radio idea caught on, and in the early to mid 1990s, FM radio surpassed its AM cousin in listeners and stations.
AM and FM radio behave and transmit differently. AM radios transmit based on power and the ionosphere interferences with these radio waves. The ionosphere bounces AM's signal back to the ground, potentially being picked up in other cities, states or countries. FM radio is more line of sight, with antennas high in the air so as to widen the broadcast range. This allows for more stations because one station will not necessarily interfere with another station.
AM radio is not primarily talk radio. Because FM radio allows for a greater range of frequencies, music will tend to sound better on FM radio because music covers a greater range of frequencies than the human voice.