Патент USA US2412964код для вставки
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.