The use of amplitude modulation (AM) for early radio broadcasts made them subject to interference. Among the inventors who worked toward remedying this situation, Edwin H. Armstrong stands out as one of the developers of frequency modulation (FM). He filed four patents for frequency modulation between 1930 and 1933 for the purpose of creating a new kind of broadcasting system. In 1934, with the assistance of RCA, Armstrong conducted FM tests from the top of the Empire State Building in New York. In 1935 he publicly demonstrated his discovery to radio engineers in New York. RCA nevertheless felt there was too much at stake in the development of FM and dropped all support. Armstrong then went to work for competitors Zenith and General Electric. World War II interrupted further development of FM-band radio, although it was abundantly used by the military. The beginnings of FM...