Frequency modulation
In telecommunications and signal processing, frequency modulation transmits information over a carrier wave by varying the frequency. This technique is different from amplitude modulation which varies the amplitude, but keeps the frequency constant. This kind of modulation is used in broadcasting and other radio work.
In the context of broadcasting, Frequency modulation is often shortened to FM. When transmitting analogue sound, the sound quality of FM signals is better than that of amplitude modulation (AM) signals. However, FM signals do not travel as far as AM beause they use higher frequencies that do not bounce off the Kennelly–Heaviside layer.
Many radio stations send out both kinds of signals. AM may be used for talk shows, and FM for music. FM broadcasting usually includes a difference signal, which can cause two different speakers at home to create different sounds. This creates stereo sound.