https://www.johndcook.com/blog/2024/10/13/why-does-fm-sound-better-than-am/

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Why does FM sound better than AM?

Posted on 13 October 2024 by John

The original form of radio broadcast was amplitude modulation (AM).
With AM radio, the changes in the amplitude of the carrier wave
carries the signal you want to broadcast.

AM signal

Frequency modulation (FM) came later. With FM radio, changes to the
frequency of the carrier wave carry the signal.

[fm_wave]

I go into the mathematical details of AM radio here and of FM radio
here.

Pinter [1] gives a clear explanation of why the inventor of FM radio,
Edwin Howard Armstrong, correctly predicted that FM radio
transmissions would be less affected by noise.

    Armstrong reasoned that the effect of random noise is primarily
    to amplitude-modulate the carrier without consistently producing
    frequency derivations.

In other words, noise tends to be a an unwanted amplitude modulation,
not a frequency modulation.

FM radio was able to achieve levels of noise reduction that people
steeped in AM radio thought would be impossible. As J. R. Carson
eloquently but incorrectly concluded

    ... as the essential nature of the problem is more clearly
    perceived, we are unavoidably forced to the conclusion that
    static, like the poor, will always be with us.

But as Pinter observes

    The substantial reduction of noise in a FM receiver by use of a
    limiter was indeed a startling discovery, contrary to the
    behavior of AM systems, because experience with such systems had
    shown that the noise contribution to the modulation of the
    carrier could not be eliminated without partial elimination of
    the message.

Related posts

  * FM signal approximation
  * Mathematics of radio

[1] Philip F. Pinter. Modulation, Noise, and Spectral Analysis.
McGraw-Hill 1965.

Categories : Math Science
Tags : Radio
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One thought on "Why does FM sound better than AM?"

 1.
    Kyle Heironimus
    14 October 2024 at 09:04

    Great comment from Hacker News (https://news.ycombinator.com/
    item?id=41832302)

    Someone gave me an analogy some time ago that made a lot of
    sense.
    If you shine a flashlight through a tree blowing in the wind and
    vary the brightness to convey information, the signal can get
    distorted pretty easily.
    However, if you have a constant brightness source and vary the
    color, it's a lot easier to figure out what the source is trying
    to convey.

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