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Патент USA US2405280

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Aug. 6, 19%.
2,405,28@
A. V. BEDFORD ET AL
SIGNAL CORRECTION CIRCUIT
Filed March 29, 1945
5 Sheets-Sheet 1
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A. V. BEDFORD ET AL
2,405,280
S IGNAL CORRECTION CIRCUIT
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A. V. BEDFORD ET AL
SIGNAL CORRECTION CIRCUIT '
Filed March 29, 1945
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2,405,280
S IGNAL CORRECTION CIRCUIT
Filed Mal-¿n 29, 1945
5 sheets-sheet 4
Aug. 6, 1946.
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A. v. BEDFORD ET Al.
2,405,280
SIGNAL CORRECTION CIRCUIT
5 Sheets-Sheet 5
Filed March 29, 1945
INVENTORS
By
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2,405,28
Patented Aug. 6, 1946
NT OFFICE
2,405,280
SEGNALI CGR-RECTION CIRCUIT
Alda V. Bedford, Princeton, and Karl R. Wendt,
Hightstovvn, N. J., assignors to Radio Corpora
tion of America, a corporation of Delaware
Application lillarch 29, 1945, Serial No. 585,526
l1 Claims. (Cl. 17g-_1.5)
l
2
The present invention relates to wave trans
mission systems and more particularly to an irn
proved method of and means for improving the
axis may be employed to control a D.-C. inser
tion circuit, for correcting the product signal SK
to cross its A.-C. axis at the same instant.
The
low-frequency fidelity of received communication
split-channel filter system is utilized to remove
ing signal.
improved secret telecommunication system in
cluding a novel D.-C. insertion circuit combined
signals by means of an improved D.-C. insertion Ul spurious high~frequency signal components which
are introduced by the D.-C. insertion network.
circuit in, a synchronized communication system..
Among the objects of the invention are to pro
The invention, by way of example, will be de
vide an improved method of and means for im
scribed hereinafter as an improvement in a se
proving the low-frequency fidelity of received
cret telecommunication system of the general
type described in the copending U.
applica» 10 communication signals in a synchronized com
munication system. Another object of the inven
tion of Alda V. Bedford, Serial No. 36,630, filed
tion is to provide an improved method of and
May 20, 194C/l. Said copending application dis
means for inserting D.-C. and low-frequency sig
closes a system wherein, for example, a speech
nal components which have been lost during
signal comprising a complex wave S is modiñed
transmission of a communication signal. A fur
by means of a coding signal comprising a comm
ther object of the invention is to provide an im
plex wave K in a manner whereby the instantane
. proved D.~C. insertion circuit in combination
ous ordinates of the resulting coded signals are
with a synchronized secret telecommunication
the product SK of the corresponding instantane
system. An additional object is to provide an
ous ordinates of the speech signal and the cod"
The resulting unintelligible coded
signals are transmitted by any conventional
means to receiver wherein thev coded signals are
combined with decoding signals generated in the
receiver and having instantaneous ordinates cor
responding to the reciprocals of the correspond
ing instantaneous ordinates of the coding signal
component of the transmitted signalY The final
signals. therefore, are derived from the product
of the transmitted signal SK and the decoding
signal
with a split-channel filter network. Another ob
ject is to provide an improved secret telecom
munication system including a receiver having,
in combination, a novel D.-C. insertion network
synchronized with the telecommunication signals
and a unique split-channel ûlter system for im
proving the low-frequency fidelity of received sig
hals,
The invention will be described in greater de
30
tail by reference to the accompanying drawings
of which Figure l is a block schematic diagram
showing the basic elements of a complete tele
communication system including the instant in
The coding and decoding signal generators at the
vention, Figure 2 is a partially schematic, more
transmitter and receiver, respectively, are syn
detailed, block diagram of a complete telecom
chronized by a unique system, wherein synchro
munication system including the instant inven-nizing pulse signals, each. comprising a first sig
tion, Figures 3 and 4 are groups of graphs which
nal pulse immediately followed by a second sig
are explanatory of the operational characteris
nal pulse of opposite polarity, are superimposed
upon the coded signals SK at predetermined in 40 tics of the circuit of Figure 2, Figure 5 is a sche
matic circuit diagram of a signal multiplier net
tervals. At the receiver the reversal in polarity
work forming one of the components of the cir
between the two synchronizing pulses is employed
cuits of Figures i and 2, Figure 6 is a schematic
to synchronize the decoding wave generator.
circuit diagram of a signal reciprocal network
The instant invention comprises an improved
forming another component of the circuits of
method of and means for improving the low
Figures 1 and 2, Figure 7 is a schematic circuit
frequency ñdelity of received coded signals by
diagram of the D.-C. insertion and split-channel
means of a D.-C. insertion network combined
filter networks comprising the novel circuit ele
with a novel split/«channel filter system disclosed
ments of the instant system, and Figure 8 is a
in the copending application of Alda V. Bedford.
group of graphs explaining the operation of the
Serial No. 583,343, filed March 17, 1945. In the
circuit of Figure 7. Similar reference characters
instant system, the coded signal is corrected for
are applied to similar elements throughout the
spurl-ous D.~C. and low-frequency components by
drawings.
deriving control signals corresponding to the in
stants when the coding signa] component K
BAsIc SYSTEM
crosses its A.-C_ axis. Since the coded signals
Coding transmitter
have instantaneous ordinates which are the prod
Referring to Figure l, the circuit to be de
uct of the corresponding instantaneous ordinates
scribed includes transmit-receive switches 49, 5l
of the communication signal S and >the coding
and el which control the circuit for transmis»
signal K, the control signals corresponding to the
times when the coding signal K crosses its A.-C. so sion of coded signals when the switches are in
2,405,280
3
4
the positions T1,T2, and T3, and alternately con
received decoded signal is designated S' because
trol the system for receiving and decoding such
of inherent distortion in transmission and recep
signals when the switches are in the positions
tion.
D1, D2 and D3, respectively. In the transmitting
condition, a complex coding wave generated by
scribed heretofore and the Voperational charac
teristics thereof will be described in greater de
Vtail by reference to subsequent figures of the
the code wave generator 2 and code wave syn
chronizing network 2’ is applied to one input cir
‘
cuit of a wave multiplier circuit 59. Speech sig
to a second input circuit of the wave multiplier 10
stantaneous ordinates which are the product of
the instantaneous ordinates of the speech signal
drawings.
COMPLETE SYSTEM
nals, derived from a microphone 55, are applied
59 whereby complex coded. signals SK, having in
The details of the various circuits de
Coding wave generator
Referring to Figure 2, the coding wave gener
ator employed for both transmitting and receiv
S and decoding signal K are applied to a signal
ing coded speech signals comprises a conven
circuit 63 are limited in a limiter circuit ‘IS and
E. L. C. White on December 16, 1941. It should
be understood that pulses of either polarity may
mixer circuit 63. Synchronizing signals, each 15 tional free-running multivibrator circuit I which
generates pulses at a rate, forl example, of one
comprising a pair of pulses of opposite polarity
hundred pulses per second. A typical multivi
occurring at regular spaced time intervals, are
brator of this type, the frequency of which may
superimposed upon the coded signals SK in the
be controlled by recurrent applied control pulses,
signal mixer circuit 63. Peak values of the mixed
coded and synchronizing signals from the mixer 20 is described in U. S. Patent 2,266,526, granted to
are applied to modulate the conventional radio
transmitter 8l connected to an antenna 83.
BASIC SYSTEM
Decoding receiver
When the circuit is employed for the reception
be applied in any known manner to key the mul
tivibrator, and that similarly output pulses of
either polarity may be derived therefrom. The
25 generated pulses are applied to the input of a
conventional delay network 2 comprising a plu- '
rality of series inductors 3, 5, 6, 9, II and a plu
rality of shunt capacitors 4, S, 8, Ill, I2, I4. The
tional receiver 85 which detects the coded signals 30 remote terminals of the resultant pulse delay
network 2 are terminated by a resistor I 3 match
SK and the synchronizing pulses from the signal
carrier. The coded signals and synchronizing l ing the surge impedance of the network. It
should be understood that the delay network 2
pulses thence are applied to a novel D.-C. clamp
of coded signals, the signals received on the
receiving antenna 84 are applied to a conven
may include a relatively large number of filter
circuit IBO, comprising a D.-C. insertion net
work and a split-channel ñlter system which 35 sections as indicated by the dash lines inter
will be described in detail hereinafter. The D.-C.
clamp circuit I0!) is controlled by square Wave
pulses derived from the coding wave generator
connecting the iilter sections l', 8 and 9, Ii), and
that equalizers and booster amplifiers may be in
serted in the delay network at desired points to
maintain pulse amplitude relations at optimum
2 through limiter circuits of a reciprocal wave
values.
network IûI. The square wave control pulses 40
applied to the D.-C. clamp circuit Iliii are char
acteristic in time of the intersections of the cod
ing `wave K with its A.-C. axis. The D.-C. clamp
. circuit corrects D.-C. and low-frequency spurious
components of the received signal at each in
stant that the coding wave K crosses its A.-C.
axis, whereby the received coded and synchroniz
ing signals are corrected in amplitude at closely
spaced time intervals.
Y
Pulses applied by the multivibrator I to the
input of the delay network 2 provide similar
pulses at the junction of each of the succeed
ing series inductors 3, 5, 6, 9, II wherein each
succeeding pulse is delayed a predetermined
amount with respect to pulses occurring at other
prior network terminals. A complex coding wave
thus may be obtained in response to each pulse
applied to the delay network by combining in
The corrected signals derived from the D.-C. 50 either polarity differently delayed pulses derived
from a plurality of such predetermined points
clamp circuit Iillì4 are applied to a differentiating
circuit 8'! which selects the synchronizing pulses
along the delay network.
Separate isolating resistors l5, Il, I9, 2|, 23, 25
and controls the code wave synchronizing net
each have one terminal connected to different
work 2' for synchronously pulsing the code wave
generator 2. The corrected signals also are con 55 points along the delay network, and have their
remaining terminals connected to separate mov
nected to a synchronizing pulse blanking circuit
able contacts of a plurality of singleepole double
93 which removes the synchronizing pulses from
throw switches 21, 2Q, 3i, 33, 35, 31. The cor
the coded signals and applied the blanked, cor
responding i’ixed contacts of the several switches
rected SK signal to the wave multiplier circuit
59. The coding wave K derived from the code 60 are connected together to provide two lines 35i, «1 I,
which are terminated through resistors 43, 45,
wave generator 2 is applied to the reciprocal cir
respectively, to ground. The remaining terminal
cuit IßI which converts the instantaneous sig
of the line 39 is connected through a coupling
nal ordinates to a value
l
K
The reciprocal signals
l
K
alsoare applied to the wave multiplier 59 where
by the product of signals
sKXIlí or s'
resistor 41 to one iixed contact T1 of a first
single-pole, double-throw
65 “transmit-receive”
switch 49. The remaining terminal of the sec
ond line 4I is connected through a polarity-re
versing ampliñer 5I and a second coupling re
sistor 53 to said ñrst fixed contact T1 of the first
“transmit-receive” switch 49. Thus each of the
70 100 pulses per second, dervied from the multivi
brator I and applied to the input of the delay
network 2, provides a plurality of pulses of either
polarity occurring at predetermined intervals
during each one-hundredth second period, as de
is derived and applied to a reproducer ID3. The 75 termined by the points of connection to the delay
2,405,280
5
6
network and the arrangement of the switches 21,
Z9, 3l, 33, 35, 3l. Therefore, a very complex
ative square wave pulse c is applied through a fifth
“transmit-receive” switch T3 to a second input
circuit of the ñrst mixer circuit 63, and is applied
through a sixth “transmit-receive” switch T5 to
key a third multivibrator ‘il which generates a
positive square ‘wave pulse indicated by the graph
d of Figure 3. It will be understood that the posi
tive square wave pulse d will be initiated at the
termination of the negative square wave pulse c
coding wave may be applied to the first iixed con
tact T1 of the first “transmit-receive” switch @97,
merely by selecting the desired arrangement of
the pulse selecting switches. It should be under
stood that the total delay provided by the pulse
delay network should be at least slightly less than
the pulse period of the multivibrator l in order
that only one pulse may be traveling along the
delay network at any predetermined instant.
In the typical secret telecommunication system
of the general type described in applicant’s1 co
pending applicati-on identiiîed he etofore, the
coding signal generator includes a delay network
having 80 sections ?and a plurality of seamen `al
switches which may be preset to any desired code
and selectively actuated by a clock mechanism to
in a manner well known in the multivibrator art.
The positive square wave pulse d is applied to a
third input circuit of the mixer circuit 93 where
by the coded signal SK, the negative square wave
pulse c and the positive square wave pulse d are
combine-:l to provide a communication signal in
cludin-g-the coded/wave SK andY the synchronizing
signal comprising a negative sanare wave pulse
immediately followed by a positive square wave
By means of simple “transmit-receive” switches
pulse. It should be understood that, if desired.
the synchronizing signal may comprise a positive
pulse followed by a negative pulse since multi
vibrators may be keyed by, and can provide,
pulses of either polarity, providing proper con
the coding signal either is combined with the
nections thereto are provided in a manner known
speech signal for transmitting a coded wave, or ‘l
in the art. rï‘he combined coded signal and syn
chronizing signal derived from the mixer 63 will
have a wave form, for example, of the type illus
change the code continuously or at predetermined
desired intervals. Identical coding signal genera
tors are employed
both> the tra.. mitter and re
ceiver in such a secret telecommunication system.
reciprocal values` of the coding signal are derived
from a reciprocal circuit responsive to the coding
signal generator and are combined with the re
ceived coded signal to decode said received. sig
nal. Much oi the decoding apparatus including
the generator for the code signal is identical to
the coding apparatus. Hence, by means oi' the
simple “transmit-receive” switches, the various
trated in graph f of Figure 3, including the pulses
I, I, shown in dash lines.
A pulse derived from the third multivibrator
"il also is applied to key the ñrst multivi rat-or
l to generate a positive square wave pulse e, illus
trated in Figure 3, which is applied to the input
l’ the delay network â to initiate a succeeding
elements of the apparatus may be employed at
different times for dual purposes, in a single unit 35 pulse which will be progressively delayed along
for either transmitting or receiving the coded
the delay network. Since the ñrst multivibrator
signals.
Coding transmitter
i is keyed by the pulse from the third multi
vibrator ll immediately preceding the time for
Referring to Figure 2, the system may be em 40 the generation of a normal pulse by said nrst
multivibrator, it will be seen that the coding wave
ployed as a coding transmitter by switching the
generator will be self-running, and will be main
movable contacts of each of
single-pole` dou
tained at a substantially constant treuren , 7
ble-throw “transmit-receive” switches included
since the pulse rate therethrough will be sub-etan
therein to engage the liked contacts T1, T2, T3, T4,
tially dependent upon the time
of the suc
T5, Te corresponding to the “transmit” condition.
cessive pulses applied to the delay network 2. if
Signals derived, for example, from a microphone
for any reason the first multivibrator i is not
55, which may be fed through a speech amplifier,
properly keyed by the third multivibrator fr". the
not shown, are applied through a second “trans
îirst multivibrator will merely generate a pulse
mit-receive” switch El to one input cir
of a
e’ which will be applied to the delay network 2
wave multiplier 5€, which will be described '
at a slightly later interval. The slightly delayed
ence to Figure 5 of tl A.,
pulse upon reaching the seventy-ninth tap of the
delay network therefore will key the second andgenerator described heretofore, are
third multivibrators in the manner described
through the *first switch lid., to a `second input cir
cuit of said wave multiplier
whereby coded 55 heretofore and provide a new set of synchronizing
pulses which will actuate the ñrst multivibrator
signals SK having instantaneous ordinates corre
l in 'synchro-nism thereafter.
sponding to the products of the corresponding 1n
The coded signals SK combined with the synstantaneous ordinates of the speech signal S and
chronic-ing pulses c and d are applied to second
the coding signal K are applied through a third
limiter 'iii whereby the high amplitude portions l
“transmit-receive” switch tl to one input circuit
of the synchronizing signal are clipped to a maxi
of a first mixer circuit 63, which may comprise
mum level 1I indicated by the dash lines in grap-h
any conventional network wherein applied sig
f of Figure 3. The thus limited combined coded
nals are combined algebraically.
Transmitter synchronizing pulse operator
Regularly recurrent pulses indicated by the
graph a of Figure 3 are derived, for example, from
the seventy-ninth tap on the delay network 2
and applied to a conventional thermionic tube
amplitude limiter circuit
which clips the wave
a at the level P to derive ii‘idividual li1 ited pulsa
represented by graph b of Figure
T le limited
pulses b are applied through a fourth “t“ansmit
receive” switch *it to key a second muli vibrator
'H to derive a negative, substantially 'square-wave
pulse illustrated by graph c of Figure
The neg
and synchronizing signals are applied as a com
munication signal to a conventional radio trans
mitter 3l which includes a transmitting an
tenna 83.
Coding signal rocciosi“
In order to convert the circuit thus described
to operate as a coded signal receiver, the mov
able contacts of each of the “transmit-receive”
switches
"i3 and
are switched to
the corresponding fixed contacts D1, D2, Da, D4,
D5, D5, corresponding to thA “re eive” condition.
The combined coded signal and synchronizing
2,405,280
7
8
signals transmitted from the transmitter 8| are
vide in its output circuit a short somewhat tri
“smeared” and phase-shifted somewhat due to
non-linearity in transmission to resemble the
solid portion a: of the graph f of Figure 3, and
angular pulse, illustrated by graph m of Figure 4.
The triangular pulse m is applied through’the
sixth “transmit-receive” switch 15 to key the
third multivibrator ll to provide a positive pulse
represented by graph n of Figure 4 which is ap
plied to key th'e first multivibrator l as described
heretofore with respect to the pulse d in the
transmitting network. It should be understood
as received by means of a conventional radio re
ceiver 85 are applied, through the equalizer net
work 86 and D.-C. clamp circuit lei), described
hereinafter, to a conventional wave differentiat
ing network 81 which may be of any type well
known in the art. For example, a wave may be 10 that, if desired for extremely precise synchronism,
the pulse m may be changed from triangular to
differentiated by applying it to a network com
square Wave shape by clipping at a low-level and
prising a small series capacitor and a shunt re
then by amplifying the clipped lower portion of
sistor. The transmitted signal f of Figure 3 after
the pulse in a manner known in the art. The
being differentiated at the receiver resembles the
graph g of Figure 4 wherein a relatively large 15 pulse n therefore causes th'e first multivibrator vI
to generate a positive pulse-o which iseappliedY to
-pulse Q occurs at an instant correspondingto the
reversal in polarity between the received syn
the delay network 2 in the same manner as de
scribed heretofore with respect to the positiv ,
chronizing negative and positive pulses and
wherein low-frequency components are substan
pulse o of the transmitting network.
=
As explained heretofore with respect to the op
tially removed from the pulse Q. It should be 20
eration of the multivibrator circuits in the “trans
understood that instead of differentiating the re
ceived signal, it may be treated in any other
mitting” condition, if the circuit falls out of syn
known manner to derive a control pulse in re
chronism, the various multivibrators will provide
sponse to the reversal in polarity of the negative Y
pulses at somewhat increased time intervals un
and positive synchronizing pulses.
25 til such time as a synchronizing pulse occurs at a
proper instant to pull all of the multivibrators
The receiver first multivibrator l being free
back into synchronism. Since pulses are derived
running, as described heretofore, th‘e delay net
from the delay network 2 at intervals of the order
work 2 will provide recurrent pulses at its sev
enty-eighth tap which will be limited by means
of .01 second, it is apparent that the various cir
of a third limiter 8S to provide limited pulses rep 30 cuits will fall into synchronism in a relatively
resented by the graph h, of Figure 4. The thus
short time which seldom will exceed one full
limited pulses h are applied `to key a fourth multi
second.
vibrator 9| which generates a relatively long
Due to phase distortion in the transmission or
blanking pulse illustrated in graph' i of Figure 4.
radio circuits interconnecting th‘e transmitter and
The long blanking pulse iis applied to a blanking 35 receiver units, it is possible that the effective
time of occurrence of the received synchronizing
circuit 93 which blanks out portions of the re
ceived signal, as will be explained in greater de
pulses will vary in different receivers with respect
to the received coded speech. To correct for such
tail hereinafter.
Receiver synchronizing circuits
variations, the circuit constants of the third mul
40 tivibrator Tl may, in any known manner, be al
Similarly, each of the recurrent pulses derived
from the eightieth tap of the delay network 2 are
applied to a fourth limiter 95 which clips th'e
upper portion of the applied pulse as explained
heretofore with respect to pulse b, to provide a
tered in the receiving condition so that the width
of the pulse n may be varied to provide keying of
the first multivibrator l at the precise desired
instant. The manner of varying .the circuit con
stants of multivibrators to provide pulses of de
short pulse illustrated by graph 7' of Figure 4,
The limited pulsey’ is applied through the fourth
“transmit-receive” switch 69 to key the second
multivibrator ‘Il to provide a relatively long posi
Signal decoding system
tive square wave pulse lc.
sired polarity and duration in response to prede
termined applied keying pulses is well known in
the art.
It will be noted that 50
The received signals derived from the radio
receiver 85 are applied, through the phase equal
izer network 86 and the D.-C. clamp circuit |00,
generated by the second multivibrator 'il when
to the input of the blanking circuit 93 which in
said multivibrator is employed in the transmitting
circuit. The different pulse polarity and dura 55 terrupts the received coded signals during the
occurrences of the recurrent blanking pulses i,
tion may be accomplished in any well known
whereby the transmitted positive and negative
manner by changes provided in the multivibrator
synchronizing pulses may be removed from the
circuit constants and the connections thereto,
received coded signal. This condition obtains
when the multivibrator is switched from th'e
60 when the coding signal generator of the receiver
“transmitting” to the “receiving” condition.
is in synchronism with the transmitter coding
The positive square wave pulse 1c derived from
signal generator, since the fourth multivibrator
the second multivibrator l! is applied through
9! is responsive to pulses derived from the sev
the fifth “transmit-receive” switch 'i3 to a second
enty-eighth tap on the delay network 2. Blank
mixer circuit 91, to which also is applied the dif
the positive pulse 7c is of relatively longer duration
than the negative pulse c previously described as
ferentiated wave y derived from the differentiat
65 ing circuits are Well known in the art.
ing circuit 8l. The thus mixed signals illustrat
ed by graph l of Figure 4 include a pulse peak Z’
which corresponds in time to the occurrence of
the large positive pulse Q 0f the differentiated
received wave g. As explained heretofore, the 70
pulse Q corresponds to the reversal in polarity
of the received synchronizing negative and posi
tive pulses. The wave l derived from the second
mixer circuit 97 is applied to a ñfth limiter 99
which' clips the mixed signal at a level z to pro 75
They may
comprise, for example, a push-pull amplifier for
the signal, arranged so that the blanking pulses
i are superimposed on the grid-cathode circuits
so that both tubes are simultaneously driven to
cut-off during the blanking period. The thus
blanked received signals comprise the transmit
ted signal components SK which are applied
through the second “transmit-receive” switch 51
to one of the input circuits of the wave multi
plier 59.
2,405,280
9
10
Similarly, the coding signals K generated by
rality of small copper oxide rectiñers V1, V2, Vs,
the receiver coding generator are applied to the
input circuit of a reciprocal circuit IGI, which
will be described in detail hereinafter by refer
ence to Figure 6 of the drawings. Signals de
VA, known commercially as “Varistors” Be
cause of the particular variable resistance char
acteristics of the “Varistor,” the current there
through is substantially proportional to the
rived from the reciprocal circuit il!! will have in
stantaneous ordinates corresponding to the re
ciprocal values of the instantaneous ordinates of
the synchronized coding wave K generated in
the receiver. rEhe reciprocal wave
square of the applied voltage over a reasonable
l
K
is applied through the first “transmit-receive”
switch ¿i9 to a second input circuit of the multi
plier 59. Since the wave multiplier 53 Provides
output signals which have instantaneous ordi
nates corresponding to the product of the instan
taneous ordinates of the waves
:le
range of applied voltage of a single polarity.
The multiplier network 5S is shown as including
a ñrst triode thermionic tubo à! i having its grid
electrode connected to the movable contact of
the first “transmit-receive" switch 49, whereby
signals characteristic of either the coding wave
K or the reciprocal thereof
l
K
may be appiied to the tube grid cathode circuit.
A second thermionic tube H3 has its grid elec
trode connected to the movable Contact of the
second “transmit-receive” switch 51, whereby
either the speech signals S or the blanked, re
ceived signals SK may be applied to the tube
grid-cathode circuit. The operation of the cir
and SK applied thereto, the output signals ap
cuit will be explained hereinafter with the
plied through the third “transmit-receive”
switch SI to a reproducer E93 will be substan- -’-' switches 49 and 57 in the “transmitting” posi
tion whereby the signals K and S, respectively,
tially characteristic of the original speech modu
are applied to the grid-cathode circuits of the
lation signals S. The signals applied to the re
tubes IH and H3. Push-pull output signals are
producer H53 have been indicated as S’ since
derived from each of the tubes by means of con
some phase distortion is inherent in the various
circuits described and especially in many radio " nections to the corresponding tube 'anode and
cathode circuits as indicated in the drawings.
transmission circuits. It should be understood
In order that the desired sum voltages be ob
that the signals S’ derived from the third “trans
tained, the signals S and K are applied to a net
mit-receive” switch 6l may be applied to actuate
any other desired type of utilization apparatus, n work of resistors in the following manner; Sig
nals S andK respectively traverse resistors Rs
not shown,
and R7 to provide a signal proportional to (S-i-K)
Signal 'mult/¿plier
at point (S-i-K); the signals S and -K respec
Figure 5 shows a 'typical wave multiplier circuit
tively traverse resistors R5 and R12 to provide sig
forming a portion of both the coding wave trans
nal (1S-K); the signals -S and -K respec
mitter and receiver circuits described heretofore ^ tively traverse resistors R10 and R11 to provide
with reference to Figures 1 and 2 cf the draw
signal (-S-K) ; and the signals -S and K
ings. This multiplier circuit is described and
traverse respectively resistors R9 and Ra to pro
claimed in the copending U. S. application of
sived signal (-S-l-K). Thus, at each of the four
designated junction points, a sum of voltage is
Alda V. Bedford, Serial No. 517,967, ñled Janu
ary l2, 1944, and assigned to the same assignee 45 obtained as indicated in the circuit diagram.
as the instant application. The circuit utilizes
The network also includes resistors R12 and R15
the property of well known electrical devices
leading respectively from points (S~--K) and
which provide an instantaneous output voltage
(-S-l-K) to ground, and resistors R11 and R15
which is proportional to the square of the instan
leading respectively from points (S-l-K) and
taneous input voltage over a reasonable voltage 50 (-S-K) to the positive terminal of the source
range in a single polarity. Such circuits or de
of bias voltage which is applied through a volt
vices will be referred to as “squaring circuits,”
age-reducing resistor R13. An SOOO-ohm resist
and will be designated as “V” where referred to
ance has been found satisfactory for the resistors
hereinafter.
R13, R14, R15, and R16, while 100,000-0hm resist
l'n the preferred form of the multiplying cir 55 ance has been selected as the value of resistors
cuit, the waves S and K, to be multiplied, are
R5, R6, R7, Re, Re, Rio, R11 and R12.
added together with four dilîerent polarity com
The sum voltages at the four points of the net
binations and “squared” in four different signal
work are applied with bias voltage A and -A to
channels. Then the four “squared” signals are
the four Varistors V1, V2, V3, and V1 respectively,
added together with suitable polarities to obtain
all of which control the current through the
the product SK in the output circuit of the mul
common load resistor R17 to provide thereacross
the product output voltage SK. The output
tiplier network, as will be l lustrated by the fol
lowing equations:
across R17 is proportional to the sum of all the
(1)
voltages which would have been generated if each
It will be understood that the term A in the 7" Varistor had supplied current to a separate re
sistor, as indicated by the foregoing squaring
above equations is the D.-C. bias added to the
equations. It is to be noted that the Varistors
A.-C. waves to cause all of the signal amplitude
V2 and V4 are connected with opposite polarities
variations to have> the same polarity with respect
from the Varistors V1 and V3, so that the D.-C.
to the squaring devices.
The squaring circuit illustrated employs a plu 75 bias voltage must be different, By reference, re
11
Y
,
spectively, to the third and fourth equations it
will be seen that the values (-S-i-K-A) and
(S-K-Al are each preceded by another minus
sign and included in brackets before squaring to
indicate properly mathematically the effect of the Ul
reversed
connection on these two Varistors.
These ñve equations show that, ideally, only the
.
12
amplitude, for example, of the order of G0 volts.
The ampliñed K wave then is applied through a
blocking capacitor |5| and a resistor |52 to a cop
per oxide rectifier unit |53 which functions as a
non-linear resistor having the property of de
creasing in resistance as the applied Voltage in
creases. The resistor |52 is of high enough re
sistance so that the driving source for the non
desired voltage SK is produced across the output
resistor R17.
For compensating for small dissimilarities in
the Varistors and other circuit elements, it has
been found desirable to provide variable res'mtors
R1 and R3 connected as voltage dividers in the
linear resistance unit |53 is of high impedance
whereby there is only a slight variation in the
current flow through the unit |53. The unit | 53
may consist of a pair of copper oxide rectiñers
|53a and |53b connected to conduct current in
opposite directions.
anode circuits of the tubes ||| and H3, respec
tively for adjusting the relative amplitudes of
that only the side band frequencies are produced,
~while the input frequencies and the harmonics
rI'he voltage appearing across the non-linear
unit |53 is the voltage wave H, which is the wave
K having a flattened wave form. This voltage,
which is ampliñed by a cathode biased vacuum
tube |54, appears across an anode resistor |56
and a portion of the anode resistor LiL-|51’ of
a second amplifier tube |58.
The rectangular wave I is produced, in this par
4thereof are suppressed.
ticular example, by applying the output of the
The output signals SK derived from across the
output resistor R17 are applied to the movable
contact of the third “transmit-receive” switch 6|,
the reproducer |63 or to the ñrst mixel` 63, de
pending upon the desired operation of the cir
tube |56 through a blocking capacitor |59 and a
high impedance resistor |5|to a pair of diodes |62
and |53, which are connected to conduct in oppo
site directions. Resistors |54 and |56, of com
paratively low resistance, are connected in series
with the diodes |52 and |53, respectively. A bias
cuitsof Figures 1 and 2.
ing voltage drop for opposing current flow
»_S and -K_
AWhile in the foregoing the term “multiplying
circuiv ” has been used to deñne the circuit, it will
be seen that the circuit actually is a sort of modu
lator which is completely balanced in the sense
whereby they may be selectively applied to either
,
through diode |63 is produced across the resistor
Signal reciprocal circuit
§55 by connecting a source of voltage (not shown)
thereacross,
a resistor |51 being in series with
,The reciprocal circuit |û| shown in Figure 6 of
the voltage source. The diodes |52 and |63 clip
the drawings is described and claimed in the co
pending application of Carl A. Meneley, Serial No. 35 the applied wave H symmetrically about its A.-C.
axis, because a voltage which causes current iiow
484,304,_iiled IApril 23, 1943, and assignedto the
through the diode |52 and resistor |64 is built up
same lassignee as the instant application. In this
across the capacitor |59 by the positive cycle
circuit instantaneous reciprocal values of an ap
pulses ñow‘lng through the diode |63. Thus, the
plied coding wave K are obtained by means of an
electrical network in which the wave K is clipped 40 diodes |62 and |63 become conducting on alter
natecycles when the signal voltage exceeds the
on both its positive cycle and on its negative cycle
to produce a substantially rectangular wave, and
D.-C. voltage drop across the resistors |64 and
|65, respectively, The resulting rectangular wave
I is amplified and reversed in polarity by the tube
' in- which the wave K and the -rectangular wave
are added together with one of them reversed in
polarity, preferably after the peaks of the posi
|58.
-tive and negative cycles of the wave K have been
“squashed” or ñattened somewhat. 4The circuit
includes no appreciable capacitive or inductive
reactances (the blocking capacitors in the circuit
in the portion of the anode resistor WiL-|51’ that
The wave I and the flattened wave H add
is common to the tubes |54 and |58 to produce
the desired reciprocal wave 1 /K shown in graph J.
If the wave H is flattened correctly, and if the
50 Waves H and I are added with the correct rela
presenting negligible impedance) and, therefore,
provides the reciprocal of substantially any ap
tive amplitudes, the resulting signal will be sub
plied signal waveform regardless of its frequency
stantially a true reciprocal of the wave K.
components.
only substantial departure from a true reciprocal
signal Will be where the wave K crosses the A.-C.
axis. Here the reciprocal value is infinity where
Referring to Figure 6, the graph G represents
a typical coding wave K which is applied to the
input terminals |2| of the circuit. The graph J
represents the reciprocal Wave
The
as the maximum amplitude of the wave l /K nec
essarily has a finite limit. The waves H and I
may be mixed with the correct relative ampli
tudes by adjusting a variable tap |1| on the an
60
ode resistor |51-|5'|’. The correct shaping of
which is the sum of the flattened coding wave K,
the flattened wave H may be obtained by select
1
K
represented by the graph H, of reversed polarity,
ing a non-linear resistor unit |53 having a suit
and of the rectangular wave shown in graph I.
. -able voltage-resistance characteristic and by ad
The squashed or flattened wave H may be ob
"justing the value of the variable resistor |52.
tained by passing the wave G through a circuit 65
>As stated heretofore, the above-described re
that changes its resistance `with a change in `ap
plied voltage The rectangular Wave I maybe
produced kby clipping the positive and negative
4
` ciprocal circuit is purely resistive so that its op
eration is independent of frequency. The in
stantaneous
voltage output of the circuit is a1
cycles of the flattened wave H at the voltage levels
U and L respectively, for example, close to the 70 ways substantially the reciprocal of the instan
taneous applied voltage. It follows that if the
A.-C. axis of the signal, and then by amplifying
reciprocal circuit is adjusted to produce the re
the clipped signal.
ciprocal of an applied signal having one waveform,
The wave K applied tothe input terminals |2|
the circuit will then always produce the recip
may, ifA desired, be amplified by meansof an
-'amplifier tube |23 to provide a peak-toépeak 75 rocal of an applied signal regardlessof its wave
2,405,280
i4
form. There are various ways of determining
when the circuit has been adjusted to give sub
stantially a true reciprocal. One way is to con
neet the reciprocal circuit into the signalling sys
tems of Figs. 1 or 2 and, while transmitting speech
or music, adjust the resistor |52 and the variable
tap |`|| at the receiver until the speech or music
p derived from the terminals |75 of the recip
rocal circuit IGI (see Fig. 6) are applied to a
phase inverter tube Ill to derive the inverted
wave q, shown in the graph q of Fig. 8. (Wave
p is another View of the wave I of Fig. 6.) 'I‘he
opposite polarity square waves p and q are effec
tively limited, and then added through a pair of
diodes |19 and IBI to derive the triangular wave
shown in the graph r of Fig. 8. The triangular
It should be understood that oppositely-con
nected diodes may be substituted for the copper l10 wave 1' is differentiated by means of a small series
capacitor |83 and a shunt resistor |85 to derive
oxide rectifiers |530 and |5317, described hereto..
the pulsed wave shown in the graph s of Fig. 8.
fore. When properly biased, the two diodes
The synchronizing signal blanking pulses
should be operated along the lower knee of their
shown in graph i of Fig. 3 or in graph t of Fig.
operating characteristic and in the proper region
to shape the wave K in the desired manner to 15 8 are applied to the grid of a blanking tube |81
to blank out all signals occurring during a short
provide the iiattened Wave H.
time interval immediately preceding, following
It will be understood that the device not lim
Vand including the synchronizing pulses. VThe
ited to the particular circuits illustrated since
blanked pulse signals s are applied to limiter tubes
the waves H and I may be derived from the wave
K in various other ways, and since the two waves 20 |89 and I9! which clip the pulses of the signals
s at the levels |93 and |95, whereby pulses co1'
may be combined by means of a variety of other
responding to the graph u are applied in opposite
circuits.
polaríties to a pair of oppositely-connected di
The square wave signal I also is applied through
odes |91 and |99. The pulses u and _u com
a suitable coupling capacitor l'lfâ to output ter
mence shortly before the instant at which the
minals |15, which are connected to the D.-C.
coding wave K crosses its A.-C. axis, and they
clamp circuit |09.
end at precisely the instant at which the wave
D.-C. clamp circuit
K reaches a zero value, as indicated by the two
vertical dot-dash lines in Fig. 8. Since the di
Referring to Figures 7 and 8, the received coded
odes are connected in opposition and are pulsed
Wave SK including the synchronizing pulses is
has a minimum of distortion.
a product of the speech wave S and the code wave
K, with the synchronizing pulses superimposed
thereon at regularly recurring intervals.
Since
the coded wave SK is a product and includes a
multiplication factor comprising the coding wave
K, the coded wave SK should have zero ordinates
at the same times that the coding wave K has
zero ordinates. If the low-frequency components
of the coded wave SK are attenuated or shifted
in phase, those portions of the wave SK occur
ring at times corresponding to zero ordinates of
the coding wave K will be displaced from their
zero values.
The D.-C. clamp circuit l iid, comprising a com
ponent of the receiver network, operates to dis
place the distorted received coded Wave SK up
ward or downward as required to have zero ordi
by the pulses u and -u, the common connection
of the cathode of the diode |91 and the anode
of the diode |99 is effectively brought to ground
potential at the termination of each of said
pulses, as indicated by the hypothetical switch
2li l , shown in dash lines.
The received coded and synchronizing signals
derived from the receiver 8d, are applied through
a coupling capacitor 293 to the grid of a iirst filter
amplifier 2 E. Similarly the signals from the re
ceiver £5 ar e applied through a second coupling
capacitor 29 l to the grid of a second ñlter ampli
ner
Due to the D.-C. setting action of the
pulsed diodes |91 and |99 connected to the grid of
nrst filter ampliñer 2&5, the received commu
nication signal, shown in graph w as including
spurious low-frequency components, as indicated
by the broken line 2li, is corrected as shown in
nate values at each instant when the locally
graph .r so that its ordinates are zero at each oc
generated coding wave K has zero ordinate val '50 currence oí one of the control pulses shown in
ues. This tends to resto-re the low-frequency
graphs u and o. Graph c is similar to graph u
components of the communication signal includ
with the exception that it is compressed to show
ing its D.-C. components.
By the same action,
thev D.-C. clamp circuit also tends to remove any
spurious low-frequency components, such as come
from the power supply, surges from switching
more control pulses.
Graphs o, w, a: and y are
drawn to the same time scale.
rEhe abrupt D.-C. setting action of the circuit
thus described tends to provide spurious high
in the power supplies, etc. Another possible
frequency components in the corrected communi
source of low-frequency surges is high pulses of
cation signal, at the grid of the first filter ampli
radio noise, which may cause a momentary flow
fier
although spurious low-frequency com
of grid current in some amplifier tube of the sys 60 ponente have :been effectively removed. By
tern. Such grid current iiow would charge the
means of the unique split-channel complemen
associated coupling capacitor, and the charges
tary filter system described in copending applica
would leak oii‘ relatively slowly through the as
tion Serial No, 583,343, filed March 17, 1945, men
sociated grid leak. A spurious signal comprising
tioned heretofore, the signal is further corrected
Gb
relatively long duration, low-frequency surges
to provide the fully compensated signal shown in
would be generated by these narrow noise pulses.
the graph y which is applied to the terminals of
Since the low-frequency noise surges are con
the synchronizing blanking circuit 93 and the
verted to objectionable spurious frequencies by
diiîerentiator 3l of the circuits of Figs. 1 and 2.
rl‘he series capacitor 297 and shunt resistor 2| l
multiplication with the decoding wave l/K, the
in the grid circuit of the second ñlter ampliiier
use of the D.-C. clamp circuit effectively de
299 provide a high-pass filter therefor, whereby
creases the eiTect of this type of interference.
Also by improving the overall low-frequency fidel
ity, the clamp circuit improves the quality of
the transmitted speech.
substantially only the high-frequency compo
nents w’ of the received signal w are applied to
the succeeding circuits 8l and S3. Also because
In the circuit of Fig. 7, the square coding Wave 75 of the low-pass filter provided by the series re
2,405,280
16
sistor 2|3 and shunt capacitor 2l5 in the'cath
ode circuit of the first filter amplifier 205, sub
stantially only the complementary low-frequency
components œ' of the corrected received signal are
applied to ' the succeeding circuits » 8l and 93.
complex signal having a signal amplitude multi
plication factor component and a spurious am
plitude component, a circuit for removing said
spurious signal component comprising means for
generating a signal which is substantially a rep
lica of said signal multiplication component,
This eifect is obtained by selecting the parame
ters of the filters 261, 2H and 2l3, 2li to be ex
actly complementary in that each have 70 percent
response and 45° phase shift at some selected
mean frequency.
Thus, the invention disclosed comprises a novel
secret telecommunication system including a
means for converting said replica signal to sub
stantially square waveform, means responsive to
said square waveform signal component forde
riving control potentials, a network responsive to
said complex signal and means for applying said
control potentials to said network to remove said
D.-C. clamp circuit for improving the low-fre
spurious signal component of said complex
signal.
quency ñdelity of received signals in a synchro
6. In Ya communication system employing a
ized communication system. The D.-C'. clamp 15
complex signal having a signal amplitude multi
circuit is controlled by the coding wave to dis
plication factor component and a spurious am
place the received signal to its A.-C. axis at each
plitude component, Ya circuit for removing said
instant that the coding signal, which is a multi
spurious signal component comprising means for
plication factor thereof, crosses its A.-C‘. axis.
We claim as our invention:
20 generating a signal which is substantially a rep
lica of said signal multiplication component,
l. In a system employing a complex signal hav
means for converting said replica signal to sub
ing a signal amplitude multiplication factor com
stantially square waveform, means including sig
ponent and a spurious amplitude component, a
nal differentiating means responsive to said
circuit for removing said spurious signal compo
nent comprising means for deriving said multipli 25 square waveform signal component for deriving
cation component signal, a switching device,
Y control potentials, a network including a capac
itor responsive to said complex signal, a selective
ly bi-directional network forV discharging said
capacitor, and means for applying said control
lectively actuating said switching device, and
potenti-als to selectively actuate said bi-direc
30
means responsive to said actuation of said switch
.tional network to remove said spurious signal
ing device and operable upon said complex signal
component of said complex signal.
for removing said spurious component of said
'7. In a communication system including a
complex signal.
complex signal having a signal amplitude mul
means responsive to reversals in polarity of said
derived multiplication component signal for se
2. In a communication system employing a
complex signal having a signal amplitude multi
plication factor component and a spurious ampli
tude component, a circuit for removing said spu
rious signal component comprising means for
35 tiplication factor component and a spurious am
plitude component, a circuit for removing said
spurious signal component comprising means for
generating a signal which is substantially a rep
lica of said signal multiplication component,v
generating a signal which is substantially a repli 40 limiting means for converting said replica signal
ca of said signal multiplication component, means
to substantially square waveformJ means includ
responsive to said replica multiplication signal
ing signal differentiating means responsive to re
component for deriving control potentials, and
versals in polarity in said square Waveform sig
means for combining said complex signal and said
nal component for deriving control pulses, a net
control potentials to remove said spurious signal 45 work including a capacitor responsive to said
component of said complex signal.
complex signal, a selectively 'bi-directional net
3. In a communication system employing a
work for discharging said capacitor, and means
complex signal having a signal amplitude mul
for applying said control pulses to said bi-direc-_
tiplication factor component and a spurious am
tional network to selectively provide discharging
plitude component, a circuit for removing said 50 paths for charges on said capacitor through said
spurious signal component comprising means for
bi-directional network to remove said spurious
generating a signal which is substantially a repli
signal component of said complex signal.A
ca of said signal multiplication component, means
` 8. Apparatus of the type described in claim 2
including signal differentiating means responsive
characterized in that said control signal intro
to said replica multiplication signal component 55 duces distortion in said combined signals, said
for deriving control potentials, and means for
apparatus including separate complementary
combining said complex signal and said control
filter means for said complex signal and said
potentials to remove said spurious' signal com
, combined signals and means including said filter
ponent of said complex signal.
means for substantially removing said distortion
4. In a communication system employing a 60 from said combined signals.
'
_ _
complex signal having a signal amplitude mul
9.,Apparatus of the type described in claim 7
tiplication factor component and a spurious am
characterized in that said selective discharging
plitude component, a circuit for removing said
of said capacitor introduces distortion in said
spurious signal component comprising means for
complex signal with said spurious signal . re
generating a signal which is substantially a rep 65 moved, said apparatus including complementary
lica of said signal multiplication component,
ñlter means respectively for saidV original com
means for limiting said replica signal to con
plex signal and said complexrsignal with said
vert said signal Á‘to substantially square wave
spurious component removed, and means includ
form, means including signal differentiating
ing said filter means for substantially removing
means responsive to said limited signal compo 70 said distortion from said iiltered signals.
nent for deriving control potentials, and means
10. V’I’he method of removing a spurious signal
for combining said complex signal and said con
component from a complex signal which also in
trol potentials to remove said spurious signal
cludes an amplitude multiplication factor com
component of said complex signal.
5. In a communication system employing ' a
ponent comprising the steps of deriving said mul
tiplication component signal, deriving control
2,405,280
17
18
potentials in response to reversals in polarity of
terized in that said amplitude changing of said
said derived multiplication component signal, and
complex signal introduces additional distortion
selectively changing said complex signal ampli-
of said complex signal, and including the step
tude to a predetermined value in response to said
of filtering out said additional distortion there
control potentials to remove said spurious signal 6 from.
component from said complex signal,
11. The method described in claim 10 charac-
ALDA V. BEDFORD.
KARL R. WENDT.
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