16 Chapter 1
broadcasting station and consequently a “clear chan-
nel” that Ralph Townsley utilized to modulate
20–20,000 Hz low distortion AM signals. Those of us
who had Sargent Rayment TRF tuners had AM signals
undistinguishable from FM, except during electrical
storms. Any graduating Electrical engineer who could
pass Townsley’s basic audio networks test, for a job at
WBAA, was indeed an engineer who could think for
himself or herself about audio signals.
Great audio over AM radio in
the late 1920s and early 1930s
ran from the really well-engi-
neered Atwater Kent tuned radio
frequency receiver (still the best
way to receive AM signals via
such classics as the Sargent Ray-
ment TRF tuner) to the abso-
lutely remarkable, for its time,
E.H. Scott’s Quaranta (not to be
confused with the equally
famous H.H. Scott of postwar years).
This was a 48 tube super-
heterodyne receiver built on six
chrome chassis weighing
620 lbs with five loudspeakers
(two woofers, midrange, and
high frequency units) biamped
with 50 W for the low frequen-
cies and 40 W for the high fre-
quencies. My first view of one
of these in the late 1930s
revealed that wealth could pro-
vide a cultural life.
Edwin Armstrong
(1890–1954) The Invention of Radio and Fidelity
The technical history of radio
is best realized by the inven-
tor/engineer Edwin Howard
Armstrong. Other prominent
figures were political and
other engineers were dwarfed
by comparison to Armstrong.
In the summer of 1912,
Armstrong, using the new tri-
ode vacuum tube, devised a
new regenerative circuit in
which part of the signal at the
plate was fed back to the grid to strengthen incoming
signals. In spite of his youth, Armstrong had his own
pass to the famous West Street Bell Labs because of his
regenerative circuit work. The regenerative circuit
allowed great amplification of the received signal and
also was an oscillator, if desired, making continuous
wave transmission possible. This single circuit became
not only the first radio amplifier, but also the first con-
tinuous wave transmitter that is still the heart of all
radio operations.
In 1912–1913 Armstrong
received his engineering
degree from Columbia Univer-
sity, filed for a patent, and then
returned to the university as
assistant to professor and
inventor Michael Pupin.
Dr. Pupin was a mentor to
Armstrong and a great teacher
to generations at Columbia
University.
World War I intervened and
Armstrong was commissioned as an officer in the U.S.
Army Signal Corps and sent to Paris. While there and in
the pursuit of weak enemy wireless signals, he designed
a complex eight tube receiver called the superhetero-
dyne circuit, the circuit still used in 98% of all radio and
television receivers.
In 1933 Armstrong invented and demonstrated
wide-band frequency modulation that in field tests gave
clear reception through the most violent storms and the
greatest fidelity yet witnessed. The carrier was constant
power while the frequency was modulated over the
bandpass chosen.He had built the entire FM transmitter and receiver
on breadboard circuits of Columbia University. After the
fact of physical construction, he did the mathematics.
Armstrong, in developing FM, got beyond the equa-
tions of the period which in turn laid the foundations for
information theory, which quantifies how bandwidth
can be exchanged for noise immunity.
In 1922, John R. Carson of AT&T had written an
IRE paper that discussed modulation mathematically.
He showed that FM could not reduce the station band-