Ten bells, and not seven trumpets, announced the apocalypse on February 20, 1971. It was 10:33AM, and teletypes in every single radio and TV station across the country rang those bells to announce an incoming message that nobody had received before.
It was a message that announced the end of the world, sent by the North American Aerospace Defense Command—NORAD.
It was the teletype you are seeing above. It told everyone that any broadcast had to be interrupted and emergency protocols had to be activated—a non-descriptive way of saying that, from that moment, the United States of America was at war with the Soviet Union.