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.
Frequency modulation (FM) came later. With FM radio, changes to the frequency of the carrier wave carry the signal.
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 effected 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
[1] Philip F. Pinter. Modulation, Noise, and Spectral Analysis. McGraw-Hill 1965.
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