The United States was fretting over the possibility of a sneak attack by those pesky Soviets in 1951, so President Harry Truman announced the launch of CONELRAD, or the Control of Electromagnetic Radiation system of emergency notification. One major fear was that the USSR might hone in on American radio signals and use them as beacons for their atomic missiles. Under CONELRAD, all U.S. radio stations would cease broadcasting after an alert from the White House. Listeners were then supposed to tune in to either 640 or 1240 on their AM dials for further information and instruction. (Radios manufactured during this time had Civil Defense logos marked on those dial positions.)

By 1963, the Soviet Union had switched to ballistic missiles, and zeroing in on radio signals was no longer necessary since long-distance attacks were now possible using other techniques. CONELRAD was retired and replaced by the Emergency Broadcast System. The EBS was a relay system: an official alert from Washington first went to primary stations, then to secondary stations, and so on. The primary station sent the Alert Tone, made up of sine waves of 853 and 960 Hz, and then stations across the U.S. automatically switched over to broadcast the emergency information being sent from the source. By FCC law, all radio and television stations were required to perform a random “Weekly Transmission Test of the Attention Signal and Test Script” (“This is only a test. Had this been a real emergency…”).

The EBS was replaced in 1997 by the Emergency Alert Service, which allows broadcast stations, satellite radio, cable systems, DBS systems, participating satellite companies, and other services to receive emergency information automatically, even if their facilities are unattended.