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Патент USA US2412964

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Dec. 24, 1946.
P. K. cHATTl-:RJEA ETÍ AL
2,412,954 ‘
SEGRECY COMMUNICATION SYSTEM
Filed Aug. 28, 1945l
1576.3.
Inventors
Patented Dec. 24, 1946
-
UNITED STTS
,insel
TENT @FFI
2,412,964
.
SECRECY COMMUNICATION SYSTEM
E'rafulla Kumar Chatterjea and Leslie Wilfred
Houghton, London, England, assignors to
Standard Telephones and Cables Limited,
London, England, a British company
Application August 28, 1943, Serial No. 500,416
l
In Great Britain October 23, 1942
8 Claims. ( Cl. Z50-17)
The present invention relates to an electrical
transmission system for secret communication in
which the secret intelligence is masked by the
transmission of other signals, which may or may
not convey intelligence, in such a way that the
existence of the secret communication is hidden.
2
a train of short regularly repeated pulses, means
to modulate the amplitude of the pulses in ac
cordance with the mixed wave and means for
adding the masking wave to the modulated pulses
with such amplitude, sense, and phase'that the
proportion of the modulation of the pulses which
was due to the masking wave is removed.
The invention will be described with reference
'I‘he invention is a modification of the pulse
transmission system described and claimed in the
specification of our copending application No.
to the accompanying drawing in which:
468,573 ñled December 10, 1943, and is for the 10
Figure l shows a `block schematic diagram of
purpose of avoiding certain practical diñìculties
agtransmitting arrangement according to the in
which have been experienced in meeting the re
vention;
~
quirements of the system.
Figures 2 and 3 show explanatory waveform
In this system, the signal wave actually trans
diagrams; and
mitted (which may, for example, be used to mod 15 , Figure 4 shows a, schematic circuit diagram of
ulate a radio transmitter or carrier frequency
a pulse modulator according to the invention.
system according to known principles) comprises
By reference to the specification describing the
a mixture of a masking wave and a series of reg
original invention it Will be seen that in the
ularly repeated short duration pulses which are
transmitter a train of equally spaced short pulses
amplitude modulated by the secret signal. The 20 is amplitude modulated by the secret signal. At
signals conveyed by the masking wave may be
the same time the amplitude of a suitable mask
received in any ordinary manner, and the pres
ing signal wave is reduced periodically to zero for
ence of the pulses -conveying the secret informa
successive intervals each equal to the duration
tion is not suspected. These pulses can only be
of the pulses, and the amplitude modulated pulses
rendered evident by the use of the special receiv
are inserted in these intervals, so that the pulses
ing arrangements described in the speciñcation
in the composite wave cannot be perceived by
referred to.
.
ordinary methods. The success of this operation
It is found, however, that although the mixing
depends'on the >accurate fitting of the modulated
arrangements at the transmitting end produce
pulses in the spaces cut out of the masking sig
a combined wave in which the pulses are theoreti- f nal Wave, otherwise pulses which may be easily
cally obscured in the manner explained, in prac
detected will be produced. This has been found
tice, owing to the imperfections of the circuits
difiicult to arrange and according to the present
and apparatus used, it is difficult to prevent the
invention the composite wave is produced by iirst
appearance of pulses which will betray the pres
adding together the masking signal and the secret
ence of the secret signal. The present invention
signal waves and using the product to modulate
is concerned with an improved arrangement at » the amplitude of the pulse train. The masking
the transmitting end which produces the same
signal wave is then mixed with the modulated
type of mixed signal wave without the liability
pulse train in such a sense that the masking
to generate pulses which betray the secret signal.
components of the pulse amplitude modulation
The receiving arrangements may be the same as 40 are balanced out, leaving only the modulation due
those described in the above mentioned specifica
to the secret signal, while in the intervals be
tion.
tween the pulses the masking signal wave is pres
According to the invention there is provided a
ent normally as before. This avoids the neces
secrecy communication system in which the ín
sity of fitting the pulses into the periods cut out
telligence to be communicated is transmitted as
of the wave by a separate operation.
a, composite electrical wave comprising a masking
Figure 1 shows a block schematic circuit diae
Wave periodically interrupted for short equal time
gram of a transmitting arrangement according
intervals for the insertion therein of the individ
to the present invention. An oscillator l supplies
ual pulses of a pulse train amplitude modulated
a frequency fp to a pulse generator 2 from which
in accordance with the intelligence wave, the
areobtained short pulses having a duration small
pulses having the same period and duration as
compared with the intervals between the pulses,
the interruptions; provided with a transmitting
the frequency of repetition being fp. The oscil
‘arrangement comprising means for adding to
lator l also supplies the frequency fp to a source
gather the masking wave and the intelligence
3 of a masking signal which may be an amplifier
wave to produce a mixed Wave, means to generate 55 to which is supplied a signal at MS, o_r it may be,`
2,412,964
3
d
ing signal interrupted bythe pulses whose tips
pulses will obviously be positive. The Voutput -at
terminals I5 and> I6 will accordingly be of .the
with unidirectional pulses (for example positive
a device in which any suitable signal is generated.
pulses) from the pulse generator 2 (Fig. 1)
The secret signal which is to be masked is sup
through the terminals II and I2. The resistances
plied at SS to an amplifier 5 and thence to a de
R1 and R2 being earthed for the signals through
vice 6 where it is added to the masking signal
the corresponding by-pass condensers C, equal
wave which already contains the frequency fp
negative pulses will be obtained 'from the anodes
which is added -ior demodulation purposes at the
oi V1 and V2; and also equal positive pulses Will
receiving end, as described in the previously men
be obtained from the resistances R3 and R4. The
tioned speciiication. For convenience, the out
resistances should be so chosen that the positive
put of the device 3 will be called the masking
signal, in what follows. _The mixed signal wave 10 pulses obtained across R4 are equal in amplitude
to the negative pulses obtained across R1.
from S is supplied at b to a pulse modulator ¿i for
The control grids of the valves V3 and V4 are
the purpose of amplitude modulating the pulses
connected respectively to R1 and R4 as shown so
generated in the device 2, which are supplied to
that one of them is energized positively and one
the modulator fi at a.. rI‘he output c is connected
negatively. As a result the drop across the com
to an adding device 'I, which is also supplied with
mon anode rload resistance R5 remains constant,
the masking signal wave from 3, The output of
and there will be no output at the terminals I5
'i is then fed to a radio-transmitter -or other
and I6.
translating device represented by the block 3.
Owing to the phase reversal in the valves, V3
The pulses obtained at c are amplitude modu
lated by a composite wave comprising a mixture 20 will produce a negative pulse at the output ter
minals l5 and iii, which is exactly neutralized by
of the masking signal and the secret signal waves.
the positive pulse produced by Vi, and there will,
Figure 2 shows a series of such pulses, the dotted
herefore, be no output pulses. However, when
outline representing the above mentioned com
the mixed signal wave from the device 5 (Fig. 1)
posite Wave. The masking signal wave is added
is applied to the terminals I3 and I4, the anode
to the modulated pulses in the device ‘l with such
potential of V2 will be modulated. When the
an amplitude, sense, and phase that the propor
instantaneous amplitude of the signal wave ap
tion of the amplitude modulation which was due
plied to terminals i3 and I4 is positive, the posi
to the masking signal is neutralised, so that only
tive pulses applied to Va will be increased inam
the modulation due to the secret signal remains.
plitude and will, therefore, preponderate over the
This is indicated in Figure 3, in which the outline
negative pulses applied to V4, and accordingly
of thefull-line curve represents the masking sig
negative output pulses will be obtained at ter
nal wave which when added to the pulses of Fig
minals I5 and i6 having amplitude equal to the
ure 2 causes their tips to follow the dotted line
difference between the amplitudes of the applied
curve of Figure 3, which is the curve corre
pulses, and proportional to the amplitude of the
sponding to the secret signal. 'The wave ob
applied signal wave. When the instantaneous
tained at the output d of the device 'I is the full
amplitude of the latter is negative the >output
line curve of Figure 3, which comprises the mask
denne the secret signal. This is the same type of
wave as is provided at the output of the adding
circuit l in Figure 7 of the speciñcation pre
viously referred to.
All the numbered elements in Figure 1 may be
constructed in any well known Way; but a de
tailed schematic circuit diagram of a preferred L*
form of the pulse modulator 4 is shown in Figure
4. This circuit comprises four thermionic am
plifying valves of any suitable type (shown as
triodes for clearness), designated V1, V2, V3 and
V4. Besides the special arrangements according '
to the invention which will be fully described, the
circuit also includes conventional operating ar
rangementsfor the valves well known in the art;
and of these A indicates an anode decoupling re
sistance, G a grid leak resistance, Q a cathode
bias resistance, C a by-pass condenser and K a
blocking condenser, these all having appropriate
values.
type shown in Figure 2.
The valves V1 and V2 ,should bebiased beyond
the cut-0E, and this will prevent them from be
ing operated during ythe intervals 4between .the
pulses. `The anode circuits are decoupled by
means of the condensers .C in order to yprevent
the modulating signal wave from aiîecting the
anode voltage of V1.
`
`It will be evident that positive output pulses
could be obtained by applying >the modulating
signal wave to V1 instead of to V2, or by supply
ing an eXtra reversing stage of ampliñcation (not
shown), or by supplying negative pulses from the
pulse generator 2 (Figure 1) instead of »positive
pulses.
What is claimed is:
1. A secrecy communication system in which
the intelligence to be communicated is transmit
B represents a suitable resistance for
ted as a composite `electrical wave comprising a
providing cathode bias from the anode potential
source I-IT-i- Which is connected to terminal I8.
The input terminals for the pulses to be modu
lated are I II and I2, those for the modulating sig
nal are i3 and Hi, and the output terminals for
the modulated pulses are i5 and I6, These pairs
masking wave periodically interrupted for short
equaltime intervals for the insertion therein of
the individual pulses of a pulse train amplitude
modulated in accordance with the intelligence
wave, the pulses having the same period anddu
ration as the interruptions; provided with »a
transmitting arrangement comprising means for
adding together the masking wave and the intel
of terminals are respectively marked a, b and c to
correspond with the markings applied to the
pulse modulator «i in Figure 41.
The valves V1 and V2 should be as closely alike
as possible, and form one pair, while V3 and V4
should also be alike and form another pair; but
they need not be the same as V1 and V2. The
valves V1 and V2 are respectively provided with
equal anode load resistances R1 and Rz and equal
cathode load resistances R3 and R4. The control
grids of these two valves are supplied in parallel
ligence wave to produce a mixed wave, lmeans to
generate a train of short regularly repeated
pulses, means to modulate the amplitude of the
pulses in accordance with the mixed wave .and
means for adding the masking wave tothe modu
lated pulses with such amplitude, sense, and
phase that the proportion of the modulation of
the pulses which was clue .to the .masking wave Yis
removed.
'
2,412,964
6
2. A system according to claim 1 comprising a'
pulse generator controlled by an oscillator for
modulating means comprises a first pair of simi
lar thermionic valves, means to apply the regu
producing the regularly repeated pulses, the
larly repeated impulses simultaneously to the
masking wave being adapted to contain a sinu
soidal wave component derived from the oscilla
tor and having a frequency equal to the pulse
repetition frequency.
3. A system according to claim l in which the
modulating means comprises a first pair of sim
ilar thermionic valves and means to apply the
regularly repeated pulses simultaneously to the
control grids of said two valves and means for
5
applying said masking signal wave to the anode
of one of said valves.
7. A system according to claim 1 in which the
modulating means comprises a first pair of simi
lar thermionic valves, means to apply the regu
larly repeated pulses simultaneously to the con
trol grids of said two valves, a first load resist
control grids of the two valves, each valve being
ance connected in series with the anode of one
of said valves, a second load resistance connect
posed that pulses of equal amplitudes but of op
ed in series with the cathode of the second said
posite signs are produced across the two load re 15 valve, a second pair of similar thermionic valves
sistances, respectively.
having their anodes connected through a com
4. A system according to claim l in which the
mon resistance to the source of anode potential
modulating means comprises a ñrst pair of simi
and their control grids connected respectively to
provided with an output load resistance so dis
lar thermionic valves, means to apply the regu
larly repeated pulses simultaneously to the con
trol grids of said two valves, a ñrst load resist
the two load resistances and an output circuit for
20 obtaining modulated output pulses from the
anodes oi the said second pair of valves.
3. A system according to claim l in which the
modulating means comprises a ñrst pair of simi
series with the cathode of the other valve.
lar thermionic valves, means for biasing said
5. A system according to claim 1 in which the 25 valves beyond the cut-off point, means to apply
modulating means comprises a first pair of sim
the regularly repeated pulses simultaneously to
ilar thermionic valves, means to apply the regu
the control grids of said two Valves, a ñrst load
larly repeated pulses simultaneously to the con
resistance connected in series with the anode of
trol grids of said valves, a first load resistance
one of said valves, a second load resistance con
connected in series with the anode of one of said 30 nected in series with the cathode of the other of
valves, a second load resistance connected in se
said valves, and a second pair of similar thermi
ries with the cathode of the other of said valves
onic valves having their anodes connected
and a second pair of similar thermionic valves
through a common resistance to the source of
having their anodes connected through a com
anode potential and their control grids connect
mon resistance to the source of anode potential 35 ed respectively to said first and second load re
and their control grids connected respectively to
sistances.
said ñrst and second load resistances.
PRAFULLA KUMAR CHATTERJEA.
ance connected in series with the anode of one
valve and a second load resistance connected in
6. A system according to claim 1 in which the
LESLIE WILF‘RED HOUGHTON.
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