I worked in AM radio, as an engineer and occaisionally as a DJ. AM uses lower frequencies with longer wavelengths than FM. It can travel farther over land. And while listening to the AM radio on my alarm clock, someone drove past my house in some vehicle that was jamming every AM frequency with repetitive scratchy noises. But I rememeber when the "Civil Defense" radio became "CONELRAD", set up to keep people informed in the event of a public emergency (war broke out, there was an explosion in a nuclear power plant,North Korea just nuked the moon, there's a tsunami heading for Miami, and things like that. And the two frequencies that were allocated for that were clearly marked with special emblems on any transistor radio or car radio tuning dial.
And in each locality, one AM station becomes the centerpiece of the system. When it broadcasts is buzzing electronic alert signal, it also transmits a combination of two audio tones (like the tones on a touch-tone phone) Heard as audio, this combination of frequencies sound very harsh and discordant and shrill. this particular combination of frequencies sets off alarms in OTHER radio stations who have a receiver tuned to the local "master" alert station. to the first one. This enables them to spread the necessary information quickly to the whole region.
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And in each locality, one AM station becomes the centerpiece of the system. When it broadcasts is buzzing electronic alert signal, it also transmits a combination of two audio tones (like the tones on a touch-tone phone) Heard as audio, this combination of frequencies sound very harsh and discordant and shrill. this particular combination of frequencies sets off alarms in OTHER radio stations who have a receiver tuned to the local "master" alert station. to the first one. This enables them to spread the necessary information quickly to the whole region.
It's a fairly well-designed system, and it has stood up to testing and actual https://youtube.com/shorts/vE7KlwYMVOc?feature=shareemergencies quite well since the 1950s.