Differences between AM and FM

Differences between AM and FM

Amplitude modulation (AM) and frequency modulation (FM) are two methods of impressing an information signal onto a radio carrier wave. The primary difference between them is that AM varies the amplitude of the carrier wave, while its frequency remains constant. In contrast, FM varies the frequency of the carrier wave, while its amplitude remains constant. This fundamental difference leads to significant variations in broadcast range, sound quality, and resistance to interference. American engineer Edwin Armstrong invented wide-band FM in 1933, offering a higher-fidelity alternative to AM broadcasting.

Comparison Table

| Susceptibility to Interference

Category	Amplitude Modulation (AM)	Frequency Modulation (FM)
Modulation Method	The amplitude of the carrier wave is varied in proportion to the message signal.	The frequency of the carrier wave is varied in proportion to the message signal.
Sound Quality	Lower fidelity, more susceptible to static and noise.	Higher fidelity, clearer sound, and less susceptible to noise.
Bandwidth	Narrow, typically 10 kHz per channel.	Wide, typically 200 kHz per channel.
Frequency Range	535–1705 kHz (Medium Wave).	88–108 MHz (Very High Frequency).
High. Electrical noise and atmospheric effects can distort the signal's amplitude, creating static.	Low. Receivers can ignore amplitude changes caused by noise, resulting in a clearer signal.
Transmission Range	Long distance. Lower frequency waves can travel along the Earth's surface and reflect off the ionosphere, especially at night.	Shorter, generally line-of-sight. High-frequency waves are more easily blocked by physical obstructions.
Equipment Complexity	Transmitters and receivers are relatively simple and inexpensive.	Transmitters and receivers are more complex and costly.

Venn diagram comparing Differences between AM and FM

Fidelity and interference

The most noticeable difference for a listener between AM and FM is sound quality. FM signals have a significant advantage in fidelity because they are less susceptible to interference. Many sources of radio noise, such as electrical storms and nearby machinery, produce amplitude-based interference. In AM broadcasting, this noise directly adds to the signal's amplitude, which the receiver interprets as static. FM receivers, however, are designed to detect only changes in frequency and can ignore most variations in amplitude.

This advantage is enhanced by FM's wider bandwidth allocation. An FM channel is typically allocated 200 kHz, which is 20 times greater than the narrow 10 kHz bandwidth for an AM channel. This wider bandwidth allows FM stations to transmit a greater range of audio frequencies, resulting in a richer and more detailed sound suitable for music. AM's narrower bandwidth limits its audio range, making it better suited for talk radio and news broadcasts.

= Range and spectrum usage =

Despite its lower sound quality, AM has an advantage in transmission range. AM radio waves, which are in the medium frequency (MF) band, are longer and can travel farther in two ways: by following the curvature of the Earth (groundwave) and by reflecting off the ionosphere (skywave). Skywave propagation is particularly effective at night, allowing AM signals to be received hundreds or even thousands of miles from their source.

FM signals broadcast in the very high frequency (VHF) band. These shorter waves travel in straight lines and are generally limited to the line of sight between the transmitter and receiver. They are more easily blocked by hills, buildings, and the curvature of the Earth. This limits the coverage area of a single FM station compared to an AM station, requiring a larger network of transmitters to cover the same geographic area. The different bandwidth requirements also mean that more AM stations can occupy a given portion of the radio spectrum than FM stations.