These are systems for alerting the public to emergency situations.
Conelrad (CONtrol of ELectronic RADiation) was set up in 1951 to provide warnings to the public during the Cold War. Upon alert, most stations would go off the air. Each remaining station was to move to either 640 or 1240 kHz, and alternate with other such local stations, supposedly so no enemy Direction Finding equipment could lock onto locations in the US. Or course, most stations were not really that far apart, in air miles, so it was not a very useful system. Actual activations were apparently very few. (IF you have a story about a Conelrad activation, please share it.)
Conalert II - a professional Broadcast Receiver
Model DS-9660B Conelrad Receiver serial 3127
Even Ham operators were required to shut down in a Conelrad alert. One alarm, the Heathkit Conelrad alarm unit could be used to automatically turn off the transmitter of a monitoring station.
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Phil Alexander recalls: Radios of the late 50's and early 60's had the "CD" triangle in a circle emblem at 640 and 1240, and late one night I spent 2 or 3 hours turning a DA station on 1550 into a non DA on 1240 because about 9 PM EST one Saturday night in October '62, the White House seized all the wires and sent a message that all ConelraD authorized stations should be prepared to begin ConelraD operations at, or at any time after, 10 AM the following morning. I tested on 1240 during the experimental period, marked all the strap points and adjustments, put the station back on 1550, left a note for the morning guy to call me if an alert came through and went home (5 minutes away) and went to bed about 3 AM. The sun came up, the Russian ships on the way to Cuba turned around, and the 1240 crystal was never used again.
To replace Conelrad, the EBS (Emergency Broadcast System) was put into place in 1963. Originally as outlined, stations were to test weekly. They were supposed to set off "carrier detect" receivers by the following sequence (As Conelrad receivers operated on the "loss of carrier" principle, they were still used for that purpose with the EBS program.):
Ball/Miratel EBS receiver
Among other unintended consequences, during lightning storms dozens of false "alarms" were often initiated. And there were many stories of stations going off, and suddenly founding out through the "EBS Stress Test" that they couldn't get the transmitter to come back on!
Worse yet, due to a slipup (supposedly in NORAD headquarters) on February 20, 1971 at 9:33 AM EST, during a regular weekly test period, a false alert was sent out. The story was that the "wrong tape" was put in the teletype machine. Many stations, including some key primary stations, either ignored the alert or found that their staff didn't know what to do.
One account of interest about that strange day had pictures of the teletype paper, found here. Audio is here from WOWO. The report in Wikipedia even has a name for the operator.
The bureaucrats, after a year or so of talking and "testing" decided to "upgrade" the system to provide "better and more accurate handling" of alert receptions. In addition to other procedural changes, the alert signal was changed to a dual-tone to reduce "false alerts." EBS, Version 2 was put into operation in about 1976.
TFT 760 Two Tone EBS Encoder/Decoder
Gorman-Redlich EBS monitor
A web page with more information on the EBS system, including samples of the scripts and authentication lists, can be found at http://www.akdart.com/ebs.html
In November 1994, the EAS (Emergency Alert System) was approved by the FCC, with operations to begin January 1, 1997. Using digital signaling, the EAS was to permit sending more than an alert, actual information could be sent, printed out, and rebroadcast on command.
TFT Model 911 EAS Encoder/Decoder
EAS - CAP
What might be considerd EAS V 2.0 had a deadline of June 30, 2012, when all stations were required to have a receiver that could accept messages from the national IPAWS OPEN CAP server at the FEMA.