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Patented Oct. 29, 1946
-
UNITED
2,410,27
TES
ATENT OFFIC
2,410,276
SECRET COIVIIWUNICATION SYSTEM
Stanley D. Eilenberger, Kenosha, Wis., assignor,
by mesne assignments, of sixty-?ve percent to
S. E. Steen, Kenosha, Wis., seventeen and one
half percent to L.
G. Voorhees, Macedonia,
Ohio, and seventeen and one-half percent to
Howard W. Taft, Balboa Heights, 0. Z.
Application December 17, 1941, Serial No. 423,384
16 Claims. (01. 250-6)
This invention relates to a method of secret
communication, principally for military use,
where a false or masking signal is provided where
in the false signal in itself is a complete, intelligi
ble signal which may be designed to mislead
unauthorized persons, which method has consid
- erable value over so-called masking signals con
sisting of noise modulation or other spurious fre
quencies, which in themselves provide evidence
2
otherwise shown in this speci?cation, and ref
erence should be made to this co-pending appli
cation for this additional detail.
Referring more particularly to Fig. 1, A—l rep
resents the source of the true signal which may
well be a microphone, or tone modulated tele
graph signals, and B—l an ampli?er for this true
signal input. A--2 represents a source of intel
ligible false signal and B—-2 an ampli?er for this
that communication of a secret nature is being 10.
false signal. The output of ampli?er 13-! is
carried out.
connected to the input of reactance modulator
The purposes of this invention are:
unit C, which is so designed as to frequency mod
To, provide a method of secret communication
ulate controlled oscillator D throughout a range
in which the communication shall be disguised
in such a manner that reception shall be di?icult 15 of 20 C. P. S. above and 20 C. P. S. below a mean
frequency of 100 kc. The output of controlled
for any except the authorized parties.
oscillator D beats with the output of ?xed oscil
To provide a method of secret communication
lator
E, which is adjusted to a ?xed frequency
employing a true signal lying in the ultra-sonic
of
125
kc. This provides a beat-note having a
frequency range and a false intelligible signal
mean frequency of 25 kc, and this beat note is
lying in the sonic frequency range.
20 frequency modulated between the limits of
To provide a method of secret communication
25 kc¢20 C. P. S., by the true signal originat
employing two signals, one of which is the true
ing in microphone A-l. If desired, this 25 kc.
signal, the other being designated as a false sig
subcarrier might be obtained directly, without
nal, the true signal being so constituted as to be
receivable only with special receiving equipment 25 the use of beat frequency oscillators, by adjust
ing oscillator D to oscillate at 25 kc. In either
case, as shown in detail in my above quoted co
tional types of equipment.
pending
application, the resultant carrier will be
To provide a means of secret communication
modulated
at an index less than unity, whereby
in which two diiferent signals are transmitted’
simultaneously without any increase in carrier 30 the modulation index is de?ned as the ratio of
frequency deviation from the mean carrier fre
or modulation power beyond that which would
and the false signal being receivable on conven
be required to radiate only one signal.
This invention will be best understood from a
quency over the signal frequency.
Unit BP is a band-pass ?lter designed to pass
this frequency modulated carrier, together with
consideration of the following detailed descrip
its associated sidebands. The output of ?lter
tion, in view of the accompanying drawing form
ing a part of the speci?cation; nevertheless, it 35 BP is connected to input terminal I of switching
unit X, while the output of ampli?er unit B—2
is to be understood that the invention is not con
is connected to input terminal 3 of switching
?ned to the disclosure, being susceptible of such
uni-t X.
~
changes and modi?cations as de?ne no material
Switching unit X is a two pole switch, with
departure from the salient features of the inven 40
two input terminals I and 3 and two output ter
tion as expressed in the appended claims.
minals 2 and 4; this switching unit is arranged
In the drawing:
that contact must always be between terminals
Figure 1 represents one form of a transmitter
I and 2, or between terminals 3 and 4. Figure 1
for use with this system.
Figure 2_ represents a receiver for use with the 45 shows this switch with contacts closed between
input terminal I and output terminal 2, the dotted
transmitter of Fig. 1.
line from input terminal 3 to output terminal ll
Figure 3 shows detail of unit X in the trans
mitter of Figure 1.
For additional detail regarding this system, ref
erence should also be made to my co-pending 50
application, Serial No. 422,915, ?led December 13,
1941, entitled “Signalling by sub-modulation.”
All letter designations in the drawing represent
units identical with like lettered units in the co
representing the alternate position of the switch.
With switch X in position l—2 the output of
band-pass ?lter BP is connected-to the input of
modulation ampli?er H.
This switch is designed to change positions at
a very rapid rate, such as 50,000 times per second.
This could be done mechanically by mounting
pending application referred to above, unless 55 the switching blade on a high speed motor shaft
at the center terminal 5 and while such switches
2,410,276
3
are practical, and have been used, particularly
with a mean resonant frequency of 100 cycles and
in commutator form, a mechanical switch moving
at such a high speed is not completely satisfac
tory. For this reason, an electronic switch is
recommended-for this switching unit
which
accomplishes the same results without moving
parts. One simple form of such an electronic
switch is illustrated by Fig. 3, and will be de
scribed in detail later in this speci?cation.
The balance of the system is arranged for con
4
Unit R is a. frequency discriminator designed
further designed to provide e?icient recovery of
the modulating signal on the 40 C. P. S. frequency
swing. Unit S2 represents a high pass ?lter with
a low frequency cut-01f at 120 cycles, designed
to remove residual subcarrier frequencies. Unit
T represents the audio frequency ampli?er, and
U the reproducer. The reception of such a modu
10 lated carrier wave, which has been amplitude
ventional amplitude modulation at any desired
carrier frequency, where unit Mist-he modulator,’
L the power supply, I the carrier-frequency am
modulated by a frequency modulated subcarrier,
is fullyv describedin my co-pending application
referred to above. .
A full study of, this co-pending application Will
pli?er, E2 the carrier-frequency, oscillator and K‘ - reveal'that numerous additional methods of dis
the antenna. All of these units are conventional
guising the true signal are possible and it should
except that modulation ampli?er H and modu
be further. understood that any existing type of
lator unit M should be arrangedto carry out. am
disguising a signal, such as frequency inversion,
plitude modulation at an ultra-sonic frequency,
might be used in lieu of the system shown here.
since switching unit X is being moved from the
If such alternate method» were employed the ap
contact position l-—2 to the contact position 3-4 20 paratus used'to disguise the signal would bacon
atthe rate, of approximately 50,090 timesper sec
nected between the output terminal of ampli?er
ond; With this switch in the position 3——fi the
B1.- and' the input» to switch terminal I. A re
output of false signal ampli?er B—-2 is connected
quirement of such alternate method is that the
directlyv to the inputv of modulation ampli?er H,
switching operation and the converted true sig
andis consequently radiated as a conventional 25 nal, as supplied to the» input of modulation ampli
amplitude-modulated signal, which may be re
?er H, be at on ultra-sonic frequency.
ceived on any conventional type of receiver.
Figure 3 shows adetailed arrangement of» a
Summarizing the operation of the transmitter
simpli?ed electronic switch-for use as switching
of vFig. l, the true signal is radiated as amplitude
unitzX'inthe transmitter of Figure 1. Termi
modulation of the carrier by a frequency-modu 30 nals No, l and 3 represent the input-terminals
lated subcarrier having- a mean frequency of 25
and terminals No. 2‘ and'ritrepresent theoutput
kc; and-a-total frequency swing of 40 C. P. S.; the
terminals, these terminalsbeing numbered iden
false signal is radiated-as conventional amplitude
tically' with the arrangement shown in Fig. 1.
modulation. Only one of these signals is present
inthe modulator and carrier units at one time,
and therefore the modulator and carrier units
neednot operate at increased power, even though
two/separate signals are being radiated.
Thevacuum tubes-9| and- Hli might be commer
cial type 6L7 or any other triple grid type ar
ranged as an ampli?er; 9 isthea-mpli?er for in
put‘ terminal! and if}: is the ampli?er for input
terminal 3. rf‘liese tubes-are arranged to alter
Referring to Fig-.- 2, there is- shown one form.
nately amplify andblock- atthe frequency of the
of 'a receiver for use in recovering the true signal 40
pulses supplied from‘ square-wave generators
radiated by the transmitter of Fig. 1. In Fig. 2, K
represents the’ receiving antenna, N the radio fre
quency ampli?er, O the convertor, P the inter
mediate frequency. ampli?er, and .Q the amplitude
demodulator‘; all of these units are conventional,
except, in the example given, the acceptance band
of units N, O, P, Q must be at. least wide enough
topassv the received‘ carrier and its associated
sidebands, including the switching frequency,
SW-—i and SW--2, the frequency and synchro
nization of which are- controlled by unit PG,
which is a pulse generatoror oscillator, operat
~ ingat-the desired switchingifrequency, which'in
this. case is- 50,000 times per- second. Blocking
pulses are supplied to auxiliary control-grids l3‘
and l'itso that the signal inputs received at pri
rn-a'ry'1Contr01- grids H and» i2‘ are alternately
which, in the example given,is 50110.; the carrier 50 ampli?ed, and the signal voltages developed
across resistors‘ l'l- applied through coupling con
and its associated sidebands alsooccupy 50 kc, if
densers E8 to ioutputiterminalsez and ll. Conden
both sidebandsare transmitted. If desired, either
ser - i9 ‘is arrangedto by-pass any inter-coupling
or lower sideband may be removed by
between stages. Terminal 2?} represents the com
commonly used methods. The original
modulated subcarrier, having a mean 55 mon ground connection-and terminal 21 a source
ofhigh voltage of approximately 250 volts. Cath
of 25- kc., is present at the output of
ode, primary control grid and screen grid cir
demodulator Q, also the original signal frequen
cuits are» conventionally- arranged. Electronic
cies impressed as the false signal at microphone
switching- devices similar toy that illustrated by
A-—2 at the transmitter of Fig. 1; this combined
output of demodulator Q is passed through high 60 Figure 3 are in common-‘use and commercial units
the upper
any of the
frequency
frequency
pass ?lter S-l , which has, for example, a low fre
quency cut-off at 5000 cycles, which eliminates
the so-called false signal, and passes the 25 kc.
subcarrier on to ampli?er-mixer unit F. Here it
beats with a constant frequency from ?xed oscil
lator E, which has a frequency of 24.9 kc., so that
the resultant beat note at the output of, mixer
unit F is a frequency modulated audio frequency
subcarrier having a mean frequency of 100 cycles.
may be obtainedoperating onthis principle.
While the foregoing description servesto il
lustratethe manner in which the objects'of my
invention may be carried-out, nevertheless, it is
understood that I do not desirev myinvention to
be limited thereby, but only bythe-scopeofthe
following claims.
I claim:
1.- The method of secret communication, which
comprises the steps of p-roducinggan ultra-sonic
70
This frequency modulated’ 100 cycle subcarrier
carrier wave, frequency modulating said ultra
is impressed on ?lter unit G, which is- a parallel
sonic'carrier'by‘a true signal containing the in
resonant ?lter with a resonant frequency’ of 108
telligence to be transmitted'toan authorized-re
cycles, identical in function and purpose to ‘filter
ceiver at a modulation'index smaller than unity,
G described in detail in my above quoted co-pend
producing a signal containing any arbitrary in_
ing application.
76
telligénce, and alternately amplitude modulating
modulated high frequency carrier wav‘e,.l,y?lteringv
a'high frequency carrier wave by‘the frequency
the demodulated wave through a high pass ?lter
modulated ultra-sonic carrier wave and by the
to attenuate all frequency components contained
in. the false :signal and to. obtain the‘ frequency
false signal containing any arbitrary intelligence
at an'ultra-sonic rate of alternation;
'2. The method of secret communication, com
prising the steps of producing a ?rst high fre
quency carrier Wave, frequency modulating. said
?rst'high frequency carrier wave by a true signal
containing the intelligence to be transmitted to 10
an" authorized receiver at a modulation index
modulated ultra-sonic carrier, beating theirecov-'
ered ultra-sonic carrier with a constant frequen-.
cy Wave‘ to produce an "audio frequency frequencyv
modulated subcarrier, and demodulating theatre-'
quency modulated subcarrier to recover the true
signal;
'
.1
'
I
_-
.
'
>
9. In, a transmitting circuit" a source of-true
signal; .a source of false signal, a high frequency
smaller than unity, beating said frequency mod
ulated high frequency carrier with a constant fre
oscillator frequency modulated at an index. less
quency wave to produce a frequency modulated
than unity i" by the true signal, a high‘ speed
ultra-sonic carrier, producing a false signal con 15 change~over switching circuit, the input sides of
taining any arbitrary intelligence, alternately
which being connected to receive both true and
amplitude modulating a second high frequency
false
signals, and a high frequency oscillator am
carrier by both the frequency modulated ultra
plitude modulated by the output of the high speed
sonic carrier and the false signal at an ultra
switchingcircuit.
sonic rate of alternation.
20
10. In a transmitting circuit a source of true
3. The method of receiving a true signal con
signal, a source of false signal, a source of ultra
taining the intelligence transmitted to an author
sonic oscillations and means for frequency mod
ized receiver by the method described in claim 1,
ulating said ultra-sonic oscillation at a modula
comprising the steps of demodulating the ampli
tion index less than unity by the true signal, a
tude modulated high-frequency carrier wave, ?l
source of high frequency oscillations and means
tering the demodulated wave to attenuate all
for alternately causing the false signal and the
the frequency components contained in the false
‘ frequency modulated ultra-sonic oscillations to
signal and to obtain the modulated ultra-sonic
modulate the high frequency oscillations at an
wave, demodulating the ultra-sonic Wave to re
cover the true signal containing the intelligence 30 ultra-sonic rate of alternation,
11. The method of receiving a signal contain
intended for reception.
ing the intelligence transmitted to an authorized
4. Themethod of secret communication, Which
receiver by the method described in claim 2, com
comprises producing a true signal and an intelli
gible false signal of sonic frequency, converting
the true signal to an ultra-sonic signal frequency
modulated at an index less than unity, combin
ing the sonic signal and the ultra-sonic signal,
transmitting thecombined signals to a receiving
point, eliminating the sonic signal, and recon
prising the steps of demodulating the amplitude
modulated high frequency carrier, ?ltering the
demodulated Wave to attenuate all the frequency
components contained in the false signal and to
obtain the frequency modulated ultra-sonic car
rier, and demodulating said frequency modulated
carrier to recover the true signal.
verting the ultra-sonic signal into the true signal. 40 ultra-sonic
12. The method of receiving a signal contain
5. The method of secret communication, which
comprises producing an intelligible true signal
and an intelligible false signal, converting the
true signal into an ultra-sonic signal frequency
modulated at an index less than unity, alternate
ly transmitting the ultra-sonic signal and the
false signal at an ultra-sonic rate of alternation
to a receiving point, eliminating the false signal
and reconverting the ultra-sonic signal to the
true signal.
-
6. A system of secret communication, compris
ing a source of intelligible true signals, a source
of intelligible false signals, means for converting
the true signals into ultra-sonic unintelligible
ing the intelligence transmitted to an authorized
receiver by the method described in claim 2, com
prising the steps of demodulating the amplitude
modulated high frequency carrier Wave, ?ltering
the demodulated Wave through a high pass ?lter
to attenuate all frequency components contained
in the false signal and to obtain the frequency
modulated ultra-sonic carrier, beating the ‘re
covered ultra-sonic carrier with a constant fre
quency Wave to produce an audio frequency fre
quency modulated subcarrier, and demodulating
the frequency modulated subcarrier to recover
the true signal.
_
.
13. The method of secret communication which
signals frequency-modulated , at an index less 55
comprises producing a true signal and an intelli
than unity, means for alternately transmitting
the intelligible false signals and the ultra-sonic
unintelligible signals to a receiving point, means
for eliminating the false signals, and means for
reconverting the ultra-sonic unintelligible signals
to true signals.
7. The method of receiving a, signal containing
gible false signal of sonic frequency, converting
the true signal toan ultra-sonic signal frequency
modulated at an index less than unity, combining
the sonic signal and the ultra-sonic signal, modu
60 lating a carrier Wave by the combined signals, .
the intelligence transmitted to an authorized re
transmitting the carrier Wave to a receiving point,
then demodulating the carrier Wave to recover
ceiver by the method described in claim 1, com
prising the steps of demodulating the amplitude
the combined signals, eliminating the sonic sig
nal, and reconverting the ultra-sonic signal into
modulated high frequency carrier, ?ltering the
the true signal.
demodulated wave to attenuate all the frequency
components contained in the false signal and to
obtain the frequency modulated ultra-sonic car
_
14. The method of secret communication which
comprises producing an intelligible true signal
and an intelligible false signal, converting the
rier, and demodulating said frequency modulated 70 true signal into an ultra-sonic signal frequency
ultra-sonic carrier to recover the true signal.
modula'ted at an index less than unity, alter
8. The method of receiving a signal containing
nately modulating a carrier wave by the ultra-.
the intelligence transmitted to an authorized re
sonic signal and the false signal at an ultra-sonic
ceiver by the method described in claim 1, com
rate,
transmitting the carrier Wave to recover the
prising the steps of demodulating the amplitude
ultra-sonic signal and the false signal, eliminat
2,410,276
7
ing the false signal and reconverting the ultra
sonic signal to the true signal.
15. ,A secret communication system comprising
a source of true signals, a source of intelligible
false signals, means for converting the true sig
8
a ‘source- of true signals, a source of ‘intelligible
false signals, ‘means for converting the true sig
nal to an ultra-sonic ‘signal frequency-modulated
at an index less than unity, means for producing
a carrier wave, switching means ‘for alternately
nal 'tora'n ultra-sonic signal frequency-modulated
causing the false signal and the ultra-sonic signal
signal.
signal.
to modulate the carrier wave for transmission to
atan index less than unity, switching means for
a receiving point, means at the receiving point
alternately ‘introducing the false signal and the
for demodulating the carrier Wave to recover ‘the
ultra-sonic ‘signal ‘into a channel for transmis
sion to a receiving point, means at the receiving 10 false signal and the ultra-sonic signal, means for
eliminating the false signal, and means for re—
point for eliminating the false signal, and means
converting the ultra-sonic signal to the true
for ‘reconverting the ultra-sonic signal to the true
16. .A secret communication system comprising
STANLEY D. EILENBERGER.
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