CONELRAD had a simple system for alerting the public and other "downstream" stations, consisting of a sequence of shutting the station off for five seconds, returning to the air for five seconds, again shutting down for five seconds, and then transmitting a tone for 15 seconds. Key stations would be alerted directly. All other broadcast stations would monitor a designated station in their area.
In the event of an emergency, all United States TV and FM stations were required to stop broadcasting. Upon alert, most AM stations shut down. The stations that stayed on the air would transmit on either 640 or 1240 kHz. They would transmit for several minutes, and then go off the air and another station would take over on the same frequency in a "round robin" chain. This was to confuse enemy aircraft who might be navigating using Radio Direction Finding. By law, radio sets manufactured between 1953 and 1963 had these frequencies marked by the triangle-in-circle ("CD Mark") symbol of Civil Defense.
False Alert - 1970s.