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From Citizendium, the Citizens' Compendium

Frequency modulation (FM) is a technique for imposing information onto a electromagnetic signal of constant frequency -- the "carrier wave" -- by changing the frequency in proportion to the instantaneous strength of the information signal to be transmitted via FM.

For audio broadcasting, FM is much preferred to amplitude modulation due to its much greater ability to reject noise. Since noise is random, it can reinforce the instanteous amplitude of an AM signal, but a random noise spike has very little effect on FM.