If you grew up in the United States during the Cold War era, you are probably old enough to remember the US Civil Defense (CD) frequencies marked on AM (medium wave) radios. A "CD symbol" with simple white or red triangle highlighted the frequencies 640 kHz and 1240 kHz. These designated frequencies aided listeners in tuning to CONELRAD stations.
|Car radio with CD markers
|Portable AM radio with symbols
The purpose of the test was to allow the US Federal Communications Commission and broadcasters to verify that EBS tone transmitters and decoders were functioning properly. In addition to the weekly test, test activations of the entire system were conducted periodically for many years. These tests showed that about 80% of broadcast outlets nationwide would carry emergency programming within a period of five minutes when the system was activated. Over the years the message and procedure changed, but essentially the content remained.
Today, you will find no radios with this symbol. It is the relic of a dreadful past, hopefully gone forever.