Seventy-five
years ago this month, on 26 December 1933,
Edwin Howard Armstrong received four patents
for frequency modulation (FM) inventions.
Our present FM radio owes more to Armstrong
than to anyone else: he developed much of
the technology himself, and he worked for
two decades to establish FM broadcasting in
the United States.
Edwin Howard
Armstrong (1890-1954) PHOTO: IEEE
History Center
Edwin Howard
Armstrong was born in New York City on 18
December 1890, and studied electrical
engineering at Columbia University under the
celebrated Michael J. Pupin. Radio was
Armstrong's passion, and about a year before
he graduated in 1913, he devised a circuit
that revolutionized the radio art. Using a
triode as an amplifier, he fed back part of
the output to the input, and thereby
obtained much greater amplification.
Armstrong made a further discovery with this
circuit: just when maximum amplification was
obtained, the signal changed suddenly to a
hissing or a whistling. He realized this
meant that the circuit was generating its
own oscillations, and thus that the triode
could be used as a frequency generator. The
first of these discoveries — of a powerful
amplifier — vastly increased the sensitivity
of radio receivers, while the second — of an
oscillator — led to the use of the electron
tube in transmitters and also in receivers
for an added function, heterodyne reception.
In 1901,
Reginald Fessenden had introduced to radio
the heterodyne principle: if two tones of
frequencies A and B are combined, one may
hear a tone with frequency A minus B.
Armstrong used this principle in devising
what came to be called the superheterodyne
receiver. The essential idea is to convert
the high-frequency received signal to one of
intermediate frequency by heterodyning it
with an oscillation generated in the
receiver, then amplifying that
intermediate-frequency signal before
subjecting it to the detection and
amplification usual in receivers. RCA
marketed the superheterodyne beginning in
1924, and soon licensed the invention to
other manufacturers. It became — and remains
today — the standard type of radio receiver.
In the early
1920s, Armstrong turned his attention to
what seemed to him, and to many other radio
engineers, as the greatest problem, namely,
the elimination of static. He wrote, "This
is a terrific problem. It is the only one I
ever encountered that, approached from any
direction, always seems to be a stone wall."
Armstrong eventually found a solution in
frequency modulation, which is a different
way of impressing an audio signal on a
radio-frequency carrier wave. In the usual
technique, known as amplitude modulation
(AM), the amplitude of the carrier wave is
regulated by the amplitude of the audio
signal. With frequency modulation, the audio
signal alters instead the frequency of the
carrier, shifting it down or up to mirror
the changes in amplitude of the audio wave.
He soon found it necessary to use a much
broader bandwidth than AM stations used
(today an FM radio channel occupies 200 kHz,
twenty times the bandwidth of an AM
channel), but doing so gave not only
relative freedom from static but also much
higher sound-fidelity than AM radio offered.
With the four
patents for his FM techniques that he
obtained in 1933, Armstrong set about
gaining the support of RCA for his new
system. RCA engineers were impressed, but
the sales and legal departments saw FM as a
threat to RCA's corporate position. David
Sarnoff, the head of RCA, had already
decided to promote television vigorously and
believed the company did not have the
resources to develop a new radio medium at
the same time. Moreover, in the economically
distressed 1930s, better sound quality was
regarded as a luxury, so there was not
thought to be a large market for products
offering it.
Armstrong did
gain some support from General Electric and
Zenith, but it was largely on his own that
he carried out the development and field
testing of a practical broadcasting system.
He gradually gained the interest of
engineers, broadcasters, and radio
listeners, and in 1939 FM broadcasts were
coming from twenty or so experimental
stations. These stations could not,
according to FCC rules, sell advertising or
derive income in any other way from
broadcasting, but finally, in 1940, the FCC
decided to authorize commercial FM
broadcasting, allocating the region of the
spectrum from 42 MHz to 50 MHz to forty FM
channels. In October of that year, it granted
permits for 15 stations. Zenith and other
manufacturers marketed FM receivers, and by
the end of 1941, nearly 400,000 sets had been
sold.
U.S. entry into
the war brought a halt both to the granting
of licenses for FM stations and to the
production of FM receivers. After the war, FM
broadcasting was dealt a severe blow when
the FCC made one of its most unpopular
decisions, moving the FM spectrum allocation
(to the range from 88 to 108 MHz) and thus
making obsolete the 400,000 receivers, as
well as the transmitters of dozens of
broadcasters. This allocation, however,
allowed for two and a half times as many
channels, and the FM industry slowly
recovered, though it did not enjoy rapid
growth until the late 1950s. In the late
1970s, FM broadcasting surpassed AM in share
of the radio audience, and by the end of the
century, its share had grown to three times
that of AM broadcasting.
Among the many
honors Armstrong received were the Edison
Medal of the American Institute of
Electrical Engineers (AIEE) and the Medal of
Honor of the Institute of Radio Engineers
(IRE). These were the highest honors of
those two predecessor organizations of the
IEEE. Sadly, the last years of Armstrong's life
were darkened by continual patent
litigation, and he committed suicide on 31
January 1954.
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