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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 ¿if/ 70177746.25 dl1 llAl "V ls An . "W INVEN TORS Huy/7 l/.ÜEDFÜRD à Kem l?. WEA/¿77 By @QM Aug. 6, 1946. A. V. BEDFORD ET AL 2,405,280 S IGNAL CORRECTION CIRCUIT @fram/5y Aug. C, 1946. 2,405,280 A. V. BEDFORD ET AL SIGNAL CORRECTION CIRCUIT ' Filed March 29, 1945 5 Sheets-Sheet 3 IA .Y (c) ` Í i „wmf/Heime #fz ,a Te/ssieip Hyp) INVENTCRS HLM? M?EDFa/w By ¿QM Augß 5, 1945; A. v. BEDFORD ETAL ' 2,405,280 S IGNAL CORRECTION CIRCUIT Filed Mal-¿n 29, 1945 5 sheets-sheet 4 Aug. 6, 1946. l A. v. BEDFORD ET Al. 2,405,280 SIGNAL CORRECTION CIRCUIT 5 Sheets-Sheet 5 Filed March 29, 1945 INVENTORS By _ ‘ ' . , irme/vif 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.