Frequency Modulation

Frequency Modulation (FM), system of radio transmission in which the carrier wave is modulated so that its frequency varies with the audio signal being transmitted. The first workable system for FM radio communication was described by the American inventor Edwin H. Armstrong in 1936. Digital FM broadcasting was authorized by the Federal Communications Commission in 2002.

Frequency modulation has several advantages over the system of amplitude modulation (AM) used in the alternate form of radio broadcasting. The most important of these advantages is that an FM system has greater freedom from interference and static. Various electrical disturbances, such as those caused by thunderstorms and automobile ignition systems, create amplitude modulated radio signals that are received as noise by AM receivers. A well-designed FM receiver is not sensitive to such disturbances when it is tuned to an FM signal of sufficient strength. Also, the signal-to-noise ratio in an FM system is much higher than that of an AM system. Finally, FM broadcasting stations can be operated in the very-high-frequency bands at which AM interference is frequently severe; commercial FM radio stations are assigned frequencies between 88 and 108 Mhz. The range of transmission on these bands was limited so that stations operating on the same frequency could be located within a few hundred miles of one another without mutual interference.

These features, coupled with the comparatively low cost of equipment for an FM broadcasting station, resulted in rapid growth in the years following World War II (1939-1945). Within three years after the close of the war, 600 licensed FM stations were broadcasting in the United States. By the early 21st century there were thousands of FM stations.

Because of crowding in the AM broadcast band and the inability of standard AM receivers to eliminate noise, the tonal fidelity of standard stations was purposely limited. FM does not have these drawbacks and therefore can be used to transmit musical programs that reproduce the original performance with a degree of fidelity that cannot be reached on standard AM bands. In 1961 the Federal Communications Commission authorized FM stereophonic broadcasting. Thereafter, the FM band drew increasing numbers of listeners to popular as well as classical music, and commercial FM stations began to draw higher audience ratings than AM stations.

In 2002 the Federal Communications Commission authorized a form of digital broadcasting called HD radio. The radio signal is compressed and broken into small data packets for digital transmission. The digital data stream is then decoded by a digital receiver. Digital FM broadcasting eliminates interference and static, and provides sound quality described as equivalent to a compact disc. In turn, digital AM broadcasting permits sound quality equal to standard FM. Radio stations can broadcast a standard analog FM signal and a digital FM signal combined on the same frequency, along with additional digital FM channels as an option called multicasting. A listener needs a digital receiver to hear the HD radio signals, however.