This is 100% nonsense. Phase noise exists too, not just amplitude noise.
The answer is actually rather simple. AM stations are limited to 10KHz band width. FM gets 200KHz. More bandwidth allows representing a higher fidelity signal…
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The answer is actually rather simple. AM stations are limited to 10KHz band width. FM gets 200KHz. More bandwidth allows representing a higher fidelity signal…
If we look only at the audio bandwidth, AM stations are limited to 5 kHz of audio spectrum. The 10 kHz figure comes from the fact that AM is double sideband modulation (as opposed to single sideband as used in ham radio and other radio services). So the broadcast signal uses twice the bandwidth of the audio.
FM stations have 15 kHz of audio bandwidth, three times that of AM. They are able to do this because they transmit at a much higher frequency.
The 200 kHz figure includes other things like stereo (two channels of audio), subcarriers for RDS data and such, and the "Carson bandwidth rule" that 'basementcat' mentioned.
I am surprised that the article overlooked this simple and obvious explanation.