The most difficult radio riddles

On Dec. 6, 1923, the first presidential address was broadcast on the radio. President Calvin Coolidge delivered what is now known as the State of the Union address to a joint session of Congress. Mr. Coolidge was known as a quiet man, but he was the first president to use the medium of radio to his advantage.

In telecommunications and signal processing, frequency modulation (FM) is the encoding of information in a carrier wave by varying the instantaneous frequency of the wave.

The SOS signal was first adopted by the German government in 1905, and became the worldwide standard in 1908. There is no meaning behind SOS letters, they were chosen as easy to recognize.