The father of FM radio was born on this day in 1890. In the 1920s, radio broadcasting used only amplitude modulation (AM), in which a signal is transmitted by variations in the amplitude of a radio wave. Such signals were susceptible to the crackle of static from storms and electrical disturbances. In 1933, electronics engineer Edwin Armstrong introduced radio broadcasting by frequency modulation (FM), in which the signal is transmitted by variations in the frequency of the radio wave over a wide waveband. This method, which now dominates radio, television, microwave, and satellite transmissions, is unaffected by static, although it is limited to line-of-sight distances. Armstrong is also known for his invention of the superheterodyne receiving circuit, which was used during World War I to detect enemy aircraft by the radio waves given off by the ignition systems of their engines.
[Source: Roy Porter, Ed. The Biographical Dictionary of Scientists, Second Edition. Oxford University Press. 1994.]