- Canadian physicist
who worked in the USA. He patented the modulation of radio waves (transmission
of a signal using a carrier wave), an essential technique for voice
transmission. At the time of his death, he held 500 patents.
Early radio communications relied on telegraphy by using bursts of single-frequency
signals in Morse code. In 1900 Fessenden devised a method of making
audio-frequency speech (or music) signals modulate the amplitude of
a transmitted radio-frequency carrier wave - the basis of AM radio broadcasting.
Fessenden's other major invention was the heterodyne effect. In this,
the received radio wave is combined with a wave of frequency slightly
different to that of the carrier wave. The resulting intermediate frequency
wave is easier to amplify before being demodulated to generate the original
sound wave.
Fessenden was born in East Bolton, Québec, and educated at Bishop's
University, Lennoxville, Québec. He went to the USA to work for
inventors Thomas Edison and, 1890-92, George Westinghouse. Fessenden
became professor of electrical engineering at Purdue University, Lafayette,
and then at the Western University of Pennsylvania (now the University
of Pittsburgh). It was there that Fessenden began major work on the
problems of radio communication.
In 1902 Fessenden organized the building of a 50-kHz alternator for
radiotelephony by the General Electric Company.
This was followed by his building a transmitting station at Brant Rock,
Massachussetts. On Christmas Eve 1906, the first amplitude-modulated
radio message was broadcast.
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