How is an FM radio signal encoded with sound information?

In frequency modulation (FM), sound information is encoded by varying the frequency of a carrier wave in direct accordance with the amplitude of the incoming audio signal.123

Unlike amplitude modulation, where the carrier's strength fluctuates, FM keeps the amplitude of the carrier wave constant. The information is instead embedded by shifting the carrier's frequency—the number of cycles per second—up or down based on the intensity of the sound. This variation is known as frequency deviation. Because noise and interference typically affect the amplitude of a radio wave rather than its frequency, FM signals remain significantly more stable and clear than AM signals.23

For stereophonic broadcasting, the system transmits two distinct sets of information: the sum of the left and right audio channels (L + R) and the difference between them (L − R). A receiver then processes these signals to recover the individual left and right audio outputs. This process is essential for high-fidelity sound reproduction.2