Frequency Modulation

A radio wave has a fixed frequency, the number of times the wave vibrates per second. It also has a definite amplitude (size). In frequency modulation, the frequency of the transmitting radio wave is made higher or lower to correspond with the vibrations of the sound to be sent. But the amplitude of the wave is not varied. In contrast, amplitude modulation keeps the frequency of the transmitting wave constant. But it changes the wave's amplitude in accordance with the vibrations of the sound signal being transmitted.

FM has some advantages over AM. It is relatively free of static from thunderstorms and of other types of interference that affect AM broadcasts. FM also provides a more faithful reproduction of music and speech.

One of the main uses of frequency modulation is FM radio broadcasting. The transmission of stereophonic programs ranks as an important development in this area. In FM stereo broadcasting, sound signals from two microphones or from both channels (transmission paths) of a stereo record are sent on the same radio wave. Transmitting a program by this method is called multiplexing. For the best results, a listener needs a special receiver that can "decode" the sounds from the two channels and send them through two separate speakers.

A commercial FM station can transmit special programs of uninterrupted music in addition to its regular or stereo broadcasts. Such programs provide pleasant background music for offices, restaurants, and stores.

Frequency modulation also has other uses. For example, television stations transmit the audio portion of their programs by this method. Telephone companies also use FM in microwave radio relaying, a system designed to send long-distance phone calls.

Edwin H. Armstrong, an American electrical engineer, invented frequency modulation in 1933. FM became widely used in the 1940's.

Contributor: Richard W. Henry, Ph.D., Prof. of Physics, Bucknell Univ.

See also Armstrong, Edwin Howard; Radio; Stereophonic Sound System.

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