The first radio broadcasts made at the beginning of the twentieth century were based on the principles of amplitude modulation, or AM. Radio waves were broadcast at a specific frequency, and the receiver would detect amplitude variations before decoding the signal. Although poor in sound quality—primarily a deliberate technical limitation to deal with the sheer volume of AM radio stations—and prone to interference, it remained the dominant commercial mode for broadcast until the 1970s.
In 1933, Edwin H. Armstrong (1890-1954) patented an alternative method for making radio broadcasts using frequency modulation—FM. The basic principle has the carrier wave modulated so that its frequency varies with the audio signal being transmitted. The main benefit of FM over AM is that it enables broadcasts to be received with a minimum of interference: assorted atmospheric conditions, such as thunderstorms, or surrounding electrical activity, such as car ignitions, can themselves create AM signals thus interfering with AM broadcasts but FM is not prone to these problems.
Armstrong presented his paper, "A Method of Reducing Disturbances in Radio Signaling by a System of Frequency Modulation," to the Institute of Radio Engineers in November 1935, and two years later the first FM radio station, W1ZOJ, began broadcasting. Until the advent of digital radio in the late 1990s, FM was the only practical means for making high-fidelity, stereo broadcasts. FM remains the most commonly used broadcast method worldwide and is still used in analog television broadcasting.
