How FM Turns Sound into Frequency Changes
A microphone converts sound into a changing electrical signal: a quiet note produces small voltage changes, while a louder one produces larger ones. A radio transmitter combines that signal with a steady, high-frequency radio wave called a carrier. Instead of changing the carrier’s height, as amplitude modulation does, the transmitter shifts its frequency slightly above and below its resting value. The audio signal controls those shifts moment by moment.
The result is not a wave whose frequency simply rises for loud sounds. Its instantaneous frequency follows the full audio signal: the direction and speed of the changes carry the sound’s changing pitch and shape, while the size of the frequency swing reflects the signal’s strength.
How the Receiver Gets the Sound Back
A receiver first selects the desired station, then uses a circuit called a frequency discriminator to turn the carrier’s frequency shifts back into voltage changes. Those changes reproduce the original audio signal, which an amplifier and speaker turn into sound.
Because ordinary interference often affects a radio wave’s height more than its frequency, FM receivers can limit unwanted height fluctuations before decoding the signal. That helps make FM less vulnerable to the crackle caused by some forms of static. It does not make FM immune to interference: a weak or obstructed signal can still become noisy or drop out.
The Tradeoff: Cleaner Sound Takes More Space
FM’s larger frequency swings can represent audio with good fidelity, but they require more room on the radio dial. That is why broadcast FM stations occupy a broad range of frequencies rather than being packed as tightly as AM stations. FM trades spectrum space for a signal that can better resist many kinds of noise—a useful bargain for high-quality local broadcasting, but one that limits how many stations fit in a given band.
