Technology
Frequency Modulation
Quick fact
FM was invented by Edwin Armstrong in 1933 to overcome the static and interference problems of AM radio. His system used wideband FM, which traded bandwidth for improved signal quality.
Why this is interesting
You've probably noticed that FM radio stations sound clearer than AM stations, especially during a thunderstorm. Why does changing the frequency rather than the amplitude make such a difference?
Read the full explanation
Understanding Frequency Modulation
Imagine a steady sound wave, like a constant beep. In radio, such a wave is called a carrier wave. To send information—like music or speech—we need to modify that wave in a way that can be reversed later. Frequency modulation does this by slightly speeding up or slowing down the carrier wave's cycles according to the incoming audio signal. When the audio gets louder, the carrier frequency increases; when it gets quieter, the frequency decreases. The receiver detects these frequency changes and converts them back to sound. Because natural and man-made noise mostly affects the amplitude (height) of waves, FM's reliance on frequency makes it far more robust against interference.
A deeper explanation
At its core, frequency modulation encodes information in the instantaneous frequency of a carrier wave. The modulating signal (e.g., an audio waveform) controls the amount by which the carrier frequency shifts from its center value—this shift is called the deviation. For example, in commercial FM radio, a ±75 kHz deviation is used for a 100 MHz carrier. The receiver uses a discriminator or a phase-locked loop to convert frequency variations back into voltage fluctuations, reconstructing the original signal. FM's noise immunity comes from the fact that most electrical noise adds amplitude changes, not frequency changes; by limiting (clipping) the amplitude variations at the receiver, the noise is largely removed. However, FM requires more bandwidth than AM, which explains why FM stations are spaced farther apart on the dial. This trade-off of bandwidth for robustness is why FM is preferred for high-fidelity music broadcasting and for communications in environments with high interference, such as mobile radios.