Amplitude modulation (AM) changes a radio carrier’s amplitude to carry information; frequency modulation (FM) changes its frequency. Both are ways of encoding a message onto a radio-frequency wave for transmission. Their bandwidth is not determined by the labels AM or FM alone: it depends on the particular signal and its parameters.
What modulation does
A carrier is a radio-frequency wave used to carry a message. Modulation changes a property of that carrier according to the information being sent. The Federal Communications Commission (FCC) training material describes modulation as combining information, or intelligence, with a radio carrier.
For a simple mental picture, imagine a repeating wave. Its amplitude is the size, or height, of its swings; its frequency is how often those swings repeat over time. Modulation encodes information by varying one of these properties in a controlled way.
How AM and FM encode information
Amplitude modulation (AM)
In AM, the carrier’s amplitude changes in response to the information signal. The carrier’s frequency remains the basis of the transmission, while its changing amplitude represents the message.
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Frequency modulation (FM)
In FM, the carrier’s frequency changes in response to the information signal. The FCC glossary defines FM as “A method of modulation where the amplitude remains constant and the frequency of the carrier wave is varied according to the modulating wave.” This definition describes broadcast FM; it is published by the Federal Communications Commission.
What bandwidth means—and why it varies
Bandwidth is the portion of the radio spectrum occupied by a particular emission. It is a property of the signal being transmitted, not a fixed value that follows automatically from calling it AM or FM. The signal type and its modulation parameters matter. The ITU-R Recommendation SM.1708 also says a receiver’s bandwidth should be wide enough to receive the signal, including the essential parts of its modulation spectrum.
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For examples, the recommendation’s 2005 table lists these minimum bandwidths for specific signal types:
| Signal type | Example minimum bandwidth in ITU-R SM.1708 |
|---|---|
| AM double-sideband signals | 9 or 10 kHz |
| AM single-sideband signals | 2.4 kHz |
| FM broadcast signals | 170 kHz or greater |
These are the recommendation’s examples for the named signal types, not universal channel-width requirements or a general comparison of AM and FM sound quality. In particular, the AM figures refer to different AM signal types, while the FM figure refers to broadcast FM.
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Where broadcast FM fits
As a geographic example, the FCC glossary gives the U.S. FM broadcast band as 88–108 MHz. That figure describes the United States; it should not be assumed to apply in other countries or used by itself as current operational guidance. The glossary result does not state a publication date.
Broadcast examples make AM and FM familiar, but the underlying distinction is about what property of a carrier changes—not a universal ranking of range or sound quality. Reception and audio depend on circumstances such as propagation, interference, the receiver, and the service.
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Listening to the difference
A portable AM/FM radio can tune broadcast stations and make the terms concrete as listening formats. It is a listening aid, not an instrument that displays the modulated waveform or measures frequency deviation or bandwidth. The ITU’s 2023 discussion of broadcast radio notes that people expect small battery-powered receivers to provide broadcasts during emergencies; that general observation is not a recommendation or a test of any particular radio.
Quick Recap
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