US2432902A - Electric selective signaling or control system - Google Patents

Description

Dec. 15, 1947. c JONES r AL 2,432,902

ELECTRIC SELECTIVE SIGNALING OR CONTROL SYSTEM Filed Aug. 25, 1945 4 Sheets-Sheet l 1D 15 2 H .i'; T3 xdki g 2 fi Diallin Key 11 V E lka-: q h i 2 DS I [L 16 IMP U:

I2 M 1 Dsclm M r 13 Y 14 l|. N Release R r. coc i 1,5 T4 i MDFIX SAX LUX i i 20 16 I H .4

j 9: A? r l 1 RY T W 1s SAY LUY l 'l MRY INVENTORS CYRIL EDWARD PALMER' JONES ROBERT RAYMOND amss ATTORNEY 16, 1947. Q JQNES ET AL 2,432,902

ELECTRIC SELECTIVE slGNALING QR CONTROL SYSTEM Filed Aug. 25, 1945 4 Sheets-$heet ,2

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SAY F PDY MRY 2 INVENTORS GYRIL EDWARD PALMER JONES I ROBERT YMON) BIRSS ATTORNEY Dec. 16, 1947. Q JONES ET AL I 2,432,902

ELECTRIC SELECTIVE SIGNALING OR CONTROL SYSTEM Filed Aug. 25, 1945 4 Sheets-Sheet 3 LE Q 89 T '6 1 v I D C A 17 W18 YA INVENTORS CYRIL EDWARD PALMER JONES ROBERT RAYMOND BIRSS ad Z ATTORNEY Dec. 16, 1947. c JONES ET AL 2,432,902

ELECTRIC SELECTIVE SIGNALING CR CONTROL SYSTEM Jr-B i i FSM (C2) r r r1 r I INVENTORS CYRIL EDWARD PALMER JONE ROBERT RAYMOND BIRSS ATTORNEY Patented Dec. 16, 1947 UNITED STATES PATENT OFFICE ELECTRIC SELECTIVE SIGNALING OR CONTROL SYSTEM Application August 25, 1943, Serial No. 499,888 In Great Britain June 26, 1942 Section 1, Publ'c Law 690, August 8, 1946, Patent expires June 26, 1962 Claims. (Cl. 177-353) The present invention relates to electric selective signalling and control systems, and is more particularly concerned with, although not limited to, automatic telephone systems.

In automatic telephone systems it is usual to convey intelligence such as digit selection signals by means of direct current or alternating current impulses, and for satisfactory operation of the signal responding equipment it has been necessary to endeavour to maintain the original wave forms of the signals. On long reactive lines in particular the problems involved are great and it is the main object of the present invention to provide means for transmitting intelligence over a circuit whereby dependence upon the envelope form or duration of the received signals for accurate interpretation is avoided.

According to the invention intelligence is adapted to be transmitted over a signalling medium by means of two currents having a particular characteristic differing quantitatively in accordance with the intelligence to be transmitted, receiving equipment being adapted to separate the respective currents and to determine the quantitative relationship of the said particular characteristic of the two currents and thereby enable the intelligence to be received.

The currents transmitted may differ in amplitude or if alternating currents in frequency or phase or if carrier currents in depth of modulation or any combination of the foregoing.

Other features of the invention relate to the detection of the two currents and to the translation of the information so received into suitable form for practical use in an electrical signalling and/or control system.

I'he invention will be better understood from the following description of alternative methods of carrying it into effect, reference being had to the accompanying drawings in which the invention is assumed to be applied to the transmission of digit selection signals over a line to automatic telephone equipment of the decimal step-by-step type.

So far as existing automatic telephone systems are concerned the invention will find particular application to dialling over lines which may include thermionic repeaters where selection is at present made by means of voice frequency alter- 2 nating current impulses. In the embodiment to be described, therefore, V. F. alternating current dialling is assumed to be employed.

Referring now to the drawings,

Figs. 1 and 2 relate to the transmitting and receiving equipment respectively at the two ends of a transmission channel in which the two currents differ in amplitude.

Figs. 3 and 4 are similar to Figs. 1 and 2 respectively and relate to equipment for dealing with currents varying relatively in frequency.

Fig. 5 relates to receiving equipment in which the currents vary in depth of modulation.

Fig. 6 indicates an arrangement for converting the signal as detected into a train of impulses, the number of which is equivalent to the signal for operating step-by-step switching apparatus.

Fig. '7 illustrates how the signal is formed prior to transmission and the reconstituted signal at the receiving end.

Referring now to Fig. 1, in which two V. F. currents are transmitted over the line, the information being carried by their voltage relationship. Thus one frequency, say X, is transmitted at a constant level, preferably a low level, during digit signalling and the other frequency, say Y, determines the significance of the digit in question. In transmission a digit is herein characterized by the number of units of difference of level between the two signalling tones, each unit being equivalent to a convenient number of decibels; for instance two and one-half decibels may prove to be a suitable value of the unit for practical purposes.

As regards the establishment of suitable send-- ing conditions, if a switchboard operator is to dial a number over the V. F. link, then the operation of her dialling key, in response to which contacts dial and dkZ are closed, will cause a constant level frequency X to be transmitted to the line I5, I6. The step-by-step switch DS is adapted to be operated by the impulse springs IMP in accordance with each digit dialled over the 3 be controlled so that attenuation of VA is progressively reduced in a step-by-step manner to a value in accordance with the digit to be transmitted. The level of the Y tone therefore increases step-by-step for each impulse of a digit number being transmitted as indicated in Fig. 7 (b) and the final relationship between the levels of the X andY' tones: is determined by the-digit concerned. After each digit has been transmitted the switch DS is restored to normal in any known manner in preparation for the dialling of the next digit; for instance, a restoring magnet RM may be energized by a special release'key in order to accomplish this.v

The detection of the intelligence-conveyed: by the signals arriving at thereceivihg. end over; the line involves, as shown in Fig. 2, the provision of selective amplifiers SAX and SAY, responsive respectively to the frequencies X'andlY and having no limiting action or imposing no volume distortion to any appreciable degree. These am-- plifiers are connected to give at separate outputs volumes of each of the two tones independentpf the other. The digit. information is still conveyed by the number of units of relative difference-oft'ransmi'ssion' level'between t'h'osetwooutputs; but the tones are nolonger-mixed together. The output from the'amplifier SAX'c'onn'ects'via rectifier MRX to logarithmic unit LUX,v andthat from amplifier SAY' connect-s via rectifier WY to-logarithlnicunit LUY, the direct current outputs from the two logarithmic units being'connected in series opposition to a load resistor YA. Asuitablelogarithmic unit is referred to in British Patent No; 438,839, but any-type'of-unit givinga D; C. output current proportional tothe logarithm of the applied input'current will be suitabl'e-toenable comparison between therelati-ve levels 'ofthe X and-Y tones to be effected on a decibel'basis. It is desirable that the charact'eri-sticsof the apparatusLUXand LUY should be substantially equal, and it will-be seenthat-the potential difference appearing across the load resistor YA- will-be related to the level difference between the two tones at any instant irrespective of their intrinsic levels provided the latter are not so low orhigh as to come outside the-range of linear performance of the lo'garithmicunits. The actual'value of the potential difference built up -step-bystep across resistor YA will be proportional to the number of units of level difference between the two tones, since, as previously mentioned, the comparison between. levels. is on a decibel basis and a unit has been hereinbefore defined as'being equivalent to a predetermined number of decibels. Assuming that a 25 db; range v of linear performance can be obtained fromthe logarithmic units, then for the selection of digit numbers l10 the value of. 2 /2 db. per unit may be therefore conveniently'used in connection with such logarithmic units. However, if. the linear range is strictly limitedto that stated,- it is evidently: desirable to counteract fortuitousrline loss variationsby the. provision of constant output control arrangements such as COC'in'. the constanttone receiving amplifier SAX and to ar rangethat the gain of-the'second amplifier SAY istherebysimilarly controlled in order that it shall have; again equal to the first at any-instant. Thus in spite: of reasonable line'loss variations the load resistor potential difference can be maintainedzproportional:to. the digit transmitted. This potential difference. will. be measured .in. voltage units; For. convenience the same reference YA is appliedto theresistor: shown in Figs. 2, 4, 5.

and 6 as these resistors all correspond as will be seen later.

With reference to the arrangement illustrated in Fig. 2 it may be pointed out that the constant output amplifier arrangement is really a refinement since the logarithmic units will correctly compensate small drifts inintrinsic overall circuitloss in the"transmissionchannel. Again if the constant output amplifier with interlocked control on the two amplifiers for the two frequencies is employed it would be reasonable in some: practical cases to discard the principle of subtraction of logarithms. For instance one frequency might-act for the purpose of setting the gain-of the-receiving amplifier only in accordance withthe signal .to. be transmitted while the other frequency.- when amplified and rectified would of itself supply the digit-significant voltage across theoutputresistor. The voltages on the bank of thes'elector shown in Fig. 6, which will be described later, in this case would not necessarily bein equal steps but might be designed to correspond both with the sending level changes employed as a code for the digits and with the known characteristic of the rectifier in the receiving:appai'atus'.. It will .b'e'understood thatthe complete receiver as; described with. relation to Fig. 2 will work better than these. cheaper schemes for it will have a greater; margin. for fortuitous variations of. overall loss;

When the frequencies to be employed'for voltage ratio'signalling are chosen fairly close togather: the receiving apparatus may conveniently bearranged as analternative to what hasalready been described to treat the two frequencies: as

carrier and sideband respectively and to demodulate them whether or not theyrwere. caused to be transmitted in that relationship at the sending end. The closer the two frequencies'areto each other the better is the chance of their not suffering differentv losses (and variations of relative loss) in transit. It will be noted that'if thetwo frequencies are generated by modulation methods the-system might be described as one in which thetwo currents vary by varying the depth of modulation,- As a further alternative upper and lower sidebands may both be used to correspond to one frequency or current and the carrier to correspond to the other current. Preferably in this case the carrier is constant in level. This scheme has'the advantage that if the frequency response. characteristic of the transmission channel is sloping in the vicinity of the signalling current frequencies the second sideband taken in conjunction with the first may result in nullifying the effect of such, slope.

It will be understood that basically two currents are, still being used in this method but by modulating one by the other new temporary frequencies appear which are not removed until demodulation-takes place. This latter arrangement leads to very convenient circuit arrangements for with a single rectification one can extract a direct current proportional to the car- I rier, then filter with a simple network and extract a direct current proportional to the modulating frequencies. These twov direct currents can be added in opposition with or without logarithmic units and with or without constant output control of the amplifier derived from say the rectified carrier or at some other convenient output point.

Fig. 5 illustrates the receiving arrangements. The current from the line L comes into an ampli- .fying, stage AS and thence through a. rectifier MR where the output of the rectified carrier is led through a suitable smoothing circuit SC to a direct current amplifier DCA. The modulating frequency is led through a low pass filter LPF. The output is again rectified by MRI and passes through a suitable smoothing circuit SCI to the direct current amplifier DCA. These two direct currents, that is the rectified carrier and the rectified low frequency, are connected to the D. C. amplifier DCA in opposition and the output is fed across the terminals of the resistor YA.

The amplifying stage AS may be controlled from either the output of the modulated carrier, the filtered component of the rectified carrier, the rectified carrier or the low frequency rectified output, as may be desired to give the best overall design. The drawing shows the alternative ways in dotted lines of how these different controls may be connected up. The object of the control is to counteract fortuitous line loss variations.

Another way of employing the depth of modulation for distinguishing between the two currents soas to give a signal-carrying medium is to have two carriers and to compare the depth to which they are severally modulated. To do this the two modulated carriers would be separated by wave filters, one of which modulated carriers would have a fixed depth of modulation and the other of which would be modulated to a varying degree by means of a variable attenuator on the lines illustrated in Fig. 1. Such variable attenuator will be placed in the modulating current feed instead of directto line.

The separated modulated carriers are separate- 1y demodulated after individual amplification controlled from one or more output circuits so that constant carrier voltages appear in the out puts. The demodulated low frequencies are fil tered and rectified before being added in opposition on resistor YA with or without logarithmic units analogous to the manner of Fig. 2.

Referring now to Fig. 3, the principal feature of which is to transmit two differing V. F, currents over the line, the information being carried by their frequency relationship. Thus one frequency, say X is transmitted at a constant level, preferably a low level, during digit signalling, and the relationship to it of the other frequency say Y determines the significance of the digit in question.

As regards the establishment of suitable sending conditions, if a switchboard operator is to dial a number over the V. F. link, then the operation of her dialling key, in response to which contacts dkl and (1102 are closed, will cause a. constant frequency X to be transmitted to the line I5, If The step-by-step switch DS is adapted to respond to each digit dialled and on advancing its wipers in response to the impulses of a train will control the lO-step variable oscillator V0 to cause currents of variable frequency to be sent to line, the effect of advancing the wipers of DS is to cause the frequency generated by the oscillator V0 to be progressively changed ina step-by-step manner to a value in accordance with the digit to be transmitted. The Y frequency generated by VO ma therefore increase or decrease as the case may be, step-by-step for each impulse of a digit number being transmitted as indicated in Fig. '7 (b), and the final relationship between the frequencies of the X and Y tones is determined by the digit concerned. After each digit has been transmitted the switch DS is restored to normal in any known manner in preparation for the dialling of the next digit; for instance, arestoring magnet RM may be energized by a special release key in order to accomplish this.

The detection of the intelligence conveyed by the signals arriving at the receiving end involves, as shown in Fig. 4, the provision of selective amplifiers SAX and SAY, SAX being responsive to the frequency X and SAY being responsive to the range of frequencies which tone Y may have. As in this case the only factor or importance is frequency some amplitude distortion can be tolerated but it is necessary to ensure that these selective amplifiers do not introduce into the circult any extraneous frequency whether by modulation, distortion or otherwise which might affeet the output to any appreciable degree. These amplifiers are connected to give in separate outputs each of the two tones independent of the other. The digit information is stillv conveyed by the relative difference of frequency between those two outputs, but the tones are no longer mixed together. The output from the amplifier -SAX connects via rectifier MRX to frequencyto-di'rect current converter FPDX, and that from amplifier SAY connects via rectifier MRY to frequency-to-direct current converted FPDY. the direct current outputs from the two frequency-todirect-current converters being connected in series opposition to a load resistor YA. A suitable frequency-to-direct-current converter is described in the publication The Review of Scientific Instruments, February 1935, in an article entitled A' direct reading frequency meter, by F. V. Hunt, but any similar converter may be used which converts a current of any frequency within a given range of frequencies into a direct current having a potential corresponding to the frequency preferably directly proportional to the frequency. It is desirable that the characteristics of the converters FPDX and FPDY should be substantially equal, and it will be seen that the potential difference appearing across the load resistor YA will be related to the frequency difference between the two tones at any instant. The actual value of the potential difference built up step-by-step across resistor YA will correspond to the frequency difference between the two tones.

Another main feature of the invention consists in translating the potential differences across YA at the receiving station into a suitable form forv practical use. Assuming that they are to be converted to make-,and-break impulses to position step-by-step switching equ pment, then this can be conveniently accomplished as shown in Fig. 6 by arranging for a uniselector FS to hunt over its bank contacts until it encounters a potential which together with the applied potential cause hunting to cease, the stepping of selector magnet FSM being derived from a local supply of step-by-step impulses which are continuously applied to lead 20 and which are connected up to magnet FSM when relays RX and RY (Fig. 2) are operated. Relays RX and RY operate in response to the reception of X and Y tones, respectively, and restore to normal in response to the cessation of these tones. The digit hunting selector FS may have its bank contacts connected as in Fig. 6 to a tapped resistor YB carrying a steady current from battery B such that the potential difference between successive contacts corresponds to the various potentials which may be established across YA in response to the transmission of the digits 1 to 0 inclusive. The switch FS may be arranged suitably to polarise a thermionic relay so that when a potential corresponding'to that existingacross YA is encounteredthe supply of step-by-step pulses to the hunting-circuit is cut off by virtue of the action of'the relay. The impulses extending to magnet FSMare also allowed to proceedover'lead 2! to switching apparatussucceeding the point of reception. In Fig. 6' the grid potential of valve V may normally be such that relay DC is operated, and assuming a potential on resistor YA is being built up step-bystep for each impulse of a transmitted digit, then when the resultant potential difference between the grid and the filament of valve V is increased as toygive a suflicient negative bias to the valve V, relay DC'will be released. The F8 switch in making one step in response to an impulse received over; lead 20 will, if no further impulses have been received by this time to indicate that the digit concerned is 1, restore the standing light negative biasand relay DC will re-operate to open the magnet driving, circuit. If the digit conoerned-is 2 or more then by the time the switch FS reaches the second contact the potential across resistor YA will have built up to a new value corresponding to the value of the digit and so switch FS' will have to continue to step until it'v balances the potential difference across resistor YA. After switch FS has registered the digit it maybe restored to normal in any known manner, for example by use of an auxiliary restoring magnet'as shown in Figs. 1 and 3.

As regards the conversion of the originating Signals to variable frequency difieren'ce form and then back again to their original form, this is shown graphically in Fig. 7 where the originating dialledimpulses of digits 3 and 2 are shown designated (a), the space between the first digit 3 and the second digit 2 being occupied by the set ting of the dial for the digit 2 and by the lost motion period provided for on the dial. In (b) is shown the step-by-step building up of the frequency of Y tone for each of the two digits concerned'. 'In (cl) and (02) is shown the step-bystep impulses released at the receiving end to the switch FS (02-) serving to indicate the eifect of a slight dlfierence in phase of the local'supply of impulses at the receiving end.

It: will be understood that the drawing of Fig. 7. will-apply to the arrangement described with reference to Figs. 1 and 2 except that in that case the ordinates for (b) will represent changes in level instead of frequency. It is clearthat (b) could apply to any characteristic whose quantitativeivalue is varied.

In the-arrangement described with relation to Figs. 3 and 4 it is the difference of frequency which determines the signal; if it is desired it may be the ratio of frequencies which determines the. signal. To do this all that is necessary is tosubstitute logarithmic units as described with reference to Fig. 2 in series with the outputs of the frequency-to-directcurrent units FPDX and FP-DY respectively. In this way the potential across YA will be the difference between two logarithms and these correspond to a ratio of the quantities concerned. The relationship between the two currents according to the invention may follow any regular or irregular law provided that there is no ambiguity due to differing signals giving-thesame result or vice versa.

It is to be understood that the signalling methods: according to the invention are not limited to-transmission over a line or radio link, for instance the signals may be set up on the early part of a gramophone record, light film or the like for therpurpose. of setting the gain, frequency respouse and speed for the subsequent reproduction. If the transmission takes place over a line or link this would apply to carrier working as well as direct signalling, and to wireless as wellas to signalling over a line or cable.

Again it is to be understood that the invention is not restricted to repeating arrangements for converting existing methods of signalling into the method according to the invention or for converting signals according to the invention into the existing methods of operating selector switches as it is contemplated that calling devices may be designed to generate signals according to the'invention and that selectors will be designed to operate direct in response to signals according to the invention, repeaters being entirely eliminated and selectors being driven smoothly and not by jerks.

We claim:

1. In a signalling system, a source of electrical current, means operated in steps responsive to received impulses for progressively changing a particular characteristic of said current in corresponding steps, the quantitative value of that characteristic of the current after a particular number of steps of said means being indicative of the number of impulses received, means for transmitting said current as changed and a reference current via the same medium of transmission, a receiver upon which both of said currents are impressed after transmission, means in said receiver for comparing said two currents and producing a voltage varying in magnitude in dependence upon the degree of difierence between said two currents in respect to said particular characteristic, and means in said receiver operated in accordance with the magnitude of said produced voltage to indicate the number of impulses received by said first means.

2. In a signallin system, a signalling circuit, means for generating an electrical signal having a particular characteristic of a certain value, means for generating a second electrical signal diifering from said first signal to a variable degree in said particular characteristic, means for transmitting said two signal over said circuit, means associated with said circuit for receiving said signals and generating two direct current voltages corresponding respectively to the received signals, the value of each voltage being commensurate with the magnitude of said particular characteristic in the corresponding received signal, a circuit to which said two voltages are applied, means for producing a third voltage, means for applying said third voltage to said circuit and increasing the value thereof in steps of a predetermined size, and means in said last circuit operated responsive to said third voltage reaching a value equal to the diiference between said first two voltages,

3. In a signalling system, means for receiving two electrical signals differing from one another to a variable degree in a particular characteristic, and for generatin two direct current voltages corresponding respectively to the received signals, the value of each voltage being commensurate with the magnitude of said particular characteristic in the corresponding received signal, a circuit to which said two voltages are applied, means for producing a third voltage, means for applying said third voltage to said circuit and increasing the value thereof in steps of a predetermined size, means for transmitting an impulse each time the value of said third voltage is increased, and means in said circuit operated responsive to. said third voltage. reaching a value 9 equal to the difierence between said first two voltages thereby to halt said impulse transmission.

4. In a signalling system, means for receiving two electrical signals differing from one another to a variable degree in a particular characteristic and for generating two direct current voltages corresponding respectively to the received signals, the'value of each voltage being commensurate with the magnitude of said particular characteristic in the corresponding received signal, a grid controlled thermionic valve, means for applying to the grid of said valve a voltage equal to the difference between said two voltages, means for adding a compensating bias to said grid in increments of a predetermined size, whereby the number of steps required to bring said thermionic valve to its critical grid potential is a measure of the degree of difference between said two signals, and means in the plate circuit of said valve operated under the control of said valve when it reaches its critical grid potential.

5. In a signalling system, means for transmitting alternating current signals of two predetermined frequencies, means operated in steps responsive to received impulses, whereby the amplitude of the signal of one frequency is progressively changed in corresponding steps, the value of the amplitude after a particular number of steps of said means being indicative of the number of impulses received, the signal of the other frequency being maintained at a constant amplitude, different degrees of difference in their am plitude being indicative of the number of impulses received, a separate receiver for each of said two frequencies, means controlled by each receiver for generating a direct current voltage whose value is commensurate with the amplitude of signals received by that receiver, a circuit upon which the two direct current voltages are impressed in opposition to one another, and means controlled variably by said circuit in accordance with the degree of difference in the value of said two voltages to indicate the number of impulses received by said second means.

6. In a. signalling system, a source of electrical current, means operated in steps responsive to received impulses for progressively changing the amplitude of said current in corresponding steps, the value of the amplitude after a particular number of steps of said means being indicative of the number of impulses received, a reference current, two alternatin current signals, means for receiving said two signals after transmission via said medium and separating them from one another, an amplifier upon whose input one of said signals is impressed, means for controlling the gain of said amplifier in accordance with the other of said signals, means for producing a voltage varying in magnitude in dependence upon the output of said amplifier, and means operated responsive to said last voltage to repeat the number of impulses received by said first means, whereby the output of said amplifier is indicative of the number of impulses received by said first means.

'7. In a signalling system, a signalling circuit, means for superimposing upon the same part of said circuit two alternating current signals differing from one another to a fixed degree in frequency, means operated in steps responsive to received impulses whereby a particular other characteristic of one of said signals is progressively changed in corresponding steps, the value of said characteristic of the current after a particular number of steps of said means being indicative of the number of impulses received, the particular characteristic of the other of said signals being kept at a constant value, tuned signal receiving means associated with said circuit for distinguishing between said two signals and generating two direct current voltages corresponding respectively thereto, the value of each voltage being commensurate with the magnitude of said particular characteristic in the corresponding received signal, means, connecting said two voltages in opposition to one another, and means operated variably in accordance with the value of the resultant voltage-when said two voltages are connected in opposition to indicate the number of impulses received by said second means.

8. In a signalling system, means for transmitting via the same medium of transmission two alternating current signals differing from one another to a variable degree in frequency, means for receiving said two signals after transmission via said medium and generating two direct current voltages corresponding respectively to the received signals, the value of each voltage being commensurate with the frequency of the corresponding received signal, means connecting said two voltages in opposition to one another, and means operated variably in accordance with the value of the resultant voltage when said two voltages are connected in opposition.

9. In a signalling system, means for generating two alternating signals differing from one another to a variable degree of frequency, means for transmitting via the same medium of transmission said two alternating current signals, means for receiving said two signals after transmission via said medium and generating two direct current voltages corresponding respectively to the received signals, the value of each voltage being commensurate with the frequency of the corresponding received signal, a circuit to which said two voltages are applied, means for producing a third voltage means for applying said third voltage to said circuit and increasing the value thereof in steps of a predetermined size, and means in said circuit operated responsive to said third voltage reaching a value equal to the difference between said first two voltages.

10. In a signalling system, means for generatign two alternating current signals differing from one another to a variable degree in frequency, means for transmitting via the same medium of transmission said two alternating current signals, means for receiving said two signals after transmission via said medium and generating two direct current voltages corresponding respectively to the received signals, the value of each voltage being commensurate with the frequency of the corresponding received signal, a circuit to which said two voltages are applied, means for producing a third voltage, means for applying said third voltage to said circuit and increasing the value thereof in steps of a predetermined size, means for transmitting an impulse each time value of said third voltage is increased, and means in said circuit operated responsive to said third voltage reaching a value equal to the difference between said first two voltages thereby to halt said impulse transmission.

CYRIL EDWARD PALMER JONES. ROBERT RAYMOND BIRSS.

(References on following pager-b REFERENCES CI-TED The'followingreferences -are of record -in the file of "this patent:

UNITED STATES PATENTS Number Name Date 1,221,262 'Wright Apr, 3, 1917 1,730,981 Murphy Oct. 8, 1929 1,849,870 Fitz Gerald Mar. 15, 1932 1,922,199 Fladd Aug. 15, 1933 1,929,216 Pfannenmuller Oct. 3, 1933 1,952,368 Gardner Mar. 27, 1934 Number Number Name Date Green May 5, 1936 Green May 5, 1936 Rothert Dec. 6, 1938 Lundstrom May 21, 1940 Mathes Sept. 30, 1941 Logan Nov. 17, 1942 Hilferty Nov. 23, 1943 -Mestas Feb. 1, 1944 FOREIGN PATENTS Country Date Great Britain June 17, 1935

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