August 1935 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
A momentous development that changed the field of radio communications
warranted merely a half-page announcement in 1935 when frequency
modulation inventor Edwin Armstrong had his article published in
Radio-Craft magazine. Spread spectrum modulation / demodulation
would be the next big advance. I cannot think of any fundamentally
new communications technology since that time, other than maybe
the still-in-the-laboratory method of quantum entanglement ('spooky
action at a distance,' per Einstein) information transfer. This
is an good example of where having access to the original material
from nearly a century ago is priceless for gaining first-hand insight.
"Frequency" vs. "Amplitude" Modulation
The new "frequency" modulation system just announced by "the"
Armstrong of regeneration, super-regeneration and superheterodyne
fame, offers amazing advantages to (1) television, and (2) high-fidelity
broadcasting - especially, on 5 meters and less.
Edwin H. Armstrong
A new and revolutionary system of radio transmission
which wipes out the effects of static, tube noises, and fading,
has been invented by Major Edwin H. Armstrong (as announced last
month on page 6) and is described briefly below. -Editor.
Modulating and multiplexing equipment at the Empire State transmitter.
The principle is carried out by the use of a discarded method
of modulation known as frequency modulation. Although this method
of modulation has been known for over 20 years, the hitherto unsurmounted
difficulties due to distortion and other troubles in both transmitter
and receiver have caused its abandonment by all who worked with
The original demonstrations of the system were made at Columbia
University in the beginning of 1934, where, it was explained to
some of the leading engineers of the country. As a result of these
demonstrations, the short-wave transmitter on the Empire State building
was placed at my disposal by the National Broadcasting Co. about
a year ago.
Tests have been continuous but so secret that our work passed
unnoticed, except by amateurs who being unequipped with proper receiving
equipment, frequently advised the engineer in charge of the station
to find out what was "wrong" with his transmitter!
Part of the receiving station at Westhampton, L.I. (Amateurs
unequipped with suitable receivers reported, "Fix your apparatus
- modulation is poor"!)
Although the power used at the Empire State transmitter was under
2 kw., at no time during the last year either at Westhampton or
at Haddonfield (where the receivers were located) were the programs
interrupted by either static or fading. On the other hand, in the
summer time it was frequently impossible to listen to either of
the 50 kw. stations WJZ and WEAF on account of the static.
In the winter time the selective fading frequently interrupted
the programs almost as badly as static in the summer time. Neither
occurred with the new system.
The amount of noise reduction which can be obtained depends on
the strength of the noise. One of the worst disturbances encountered
on short wavelengths are the noises due to the motion of the electrons
in the circuits and in the tubes of the radio receiving set itself.
(See "The Limit of Amplification," in the preceding July issue.
-Ed.) On the New York-Haddonfield circuit the energy of this disturbance
is reduced to one one-thousandth part without losing any of the
signal strength. As the strength of the disturbance increases, the
ratio of the improvement becomes less.
The effect is something like that of the "tin hat" worn overseas
in the AEF - practically perfect against fragments up to a certain
size, but not effective against a six-inch shell.
The practical utility of the system will be principally on the
ultra-short and microwave signaling systems, as the bands of frequency
or width of the channel required is greater than on normal broadcast
For example, the band width at present used on the Empire State-Haddonfield
set-up is about 150,000 cycles. This would not be a practical band
width to use on present-day broadcast channels, but it is quite
feasible on the 40,000,000 cycle wave used at the Empire State.
The range of modulation frequencies which can be transmitted from
the best transmission systems today does not extend beyond 8,000
cycles, and only frequencies up to about 5,000 cycles can be effectively
used without encountering interference from adjacent channels.
On account of the extremely short wavelengths it has been possible
to transmit all modulation frequencies from thirty to 16,000 cycles,
and to receive them with what engineers call a flat characteristic.
The theory on which the problem was solved flies directly in
the face of all previous mathematical deductions. The old theory
of the way to shut out static assumed that the best that could be
done was to narrow the band of the selective systems at the receiver
as much as possible without shutting out the signal. By narrowing
the band down to a width just sufficient to admit the signal, it
was believed that under these conditions the signal-to-static ratio
would be the best.
Where the signals and disturbances are of the same order of magnitude,
I find the exact opposite to be true. With proper methods of transmission
and reception, the wider the band, the better will be the signal-to-noise
(This completes all the information that can be obtained at this
time from Major Armstrong or Columbia University where the Major
is a Professor of electrical engineering. However, Radio-Craft has
in preparation some very interesting data concerning modulation
in its various forms - especially frequency modulation which is
the basis of the Armstrong system. This information will appear
in a forthcoming issue. -Ed.
Posted February 8, 2016