FM

FM, or frequency modulation, is a method of conveying information by varying the frequency of a carrier wave in response to the input signal. Unlike amplitude modulation (AM), the instantaneous frequency carries the information while the signal’s amplitude remains constant. This makes FM less susceptible to amplitude-based noise and typically enables higher fidelity and dynamic range in audio transmissions. The bandwidth required for FM depends on the maximum deviation and modulation index, and is generally larger than AM.

FM radio broadcasts use this mechanism to transmit audio over a dedicated band. In many regions, the FM broadcast band runs roughly from 87.5 to 108.0 MHz, with channel spacing commonly 200 kHz. Stereo FM uses a 19 kHz pilot tone and a 38 kHz subcarrier to encode the left and right channels. FM’s resilience to atmospheric noise and multipath, plus its ability to deliver high-quality music, helped make it a standard for radio broadcasting in the mid-20th century.

History and standardization: The practical development of FM is associated with Edwin Armstrong in the 1930s, who demonstrated and refined the technique and its advantages over AM. After patents and experimental stations, public broadcasting adopted FM in many countries, with regulatory bodies defining bands and technical standards. Over time, FM led to improvements such as stereo encoding, digital radio data services, and compatibility with RDS (Radio Data System).

Other uses: In science, fm is the symbol for femtometer, an SI unit of length equal to 10^-15 meters, used to measure nuclear and sub-nuclear distances (for example, the approximate size of a proton is about 0.8–0.9 fm). As an acronym, fm can also denote various unrelated terms in different fields, so the meaning depends on the domain.