US 2381847 A
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Aug. 7, 1945.
E. H. ULLRxcH y 2,381,847
SYSTEM OE' COMMUNICATION BY MEANS OF ELECTRICAL WAVES Filed Nov. 8, 1941 2 Sheets-Sheet l FIGA.
MPL/ 705i INVENTOR.
Aug. 7,- 1945. E. H. uLLRlcH 2,381,847
SYSTEM oF COMMUNICATION BY MEANS QF ELEc'rEIcf-aJ wAvEs Filed Nov. 8, 1941 2 sheets-sheet 2 P1455 F7L TEE a FIGA@ INVENTOR. 50W/mp i ya /e/cw Patented Aug. 7, 1945 SYSTEM oFcoMMUNIcArIoN BY MEANS OF ELECTRICAL WAVES Edwardv Hill assignor to Ullrich, London W. C.
International Standard Electric 2, England,
Corporation, New York, N. Y.
Application November s, 194i, serial No. 418,281
Am France April 4, 1940 '7 Claims. (Qi. 179-15) The present invention relates to a systemof communication by means of electrical waves and more particularly, though not exclusively, to electrical wave communication system in which the waves' are modulated by impulses whether thev transmission is over metallic connections, dielectric guides, or by radio.
One ot the objects of thg invention is the provision of means for increasing for a given band width, the number of communications that can be transmitted simultaneously.
Another object of the invention is` the provision of. means forpermitting satisfactory 4reception of a transmission modulated by impulses and independently of parasites or undesired signals which may exist at the same wave length of transmission.
A communication system which embodies the features of the present invention comprises one or more transmitters, the outputs of which are modulated by impulses and which cover one another partially or entirely, means being provided at the receiver for discriminating between the ,ceiver, or toseparate a desired communication from another communication or from natural different emissions either in accordance with the cess of that of the threshold. In order for the difference in the duration of the impulses. or in accordance with the difference in the constant amplitudes of the impulses, these two factors varying from one transmission to another.
The invention will be explained in detail -with reference to the drawings in which:
Fig. 1 represents two signals having the same energy. but different durations, because the first signal has double amplitude but the second signal quadruple duration;
Fig. 2arepresents a series of square impulses which are repeated at equal timeintervals;
Fig. 2b represents the variations of the amplitudein function of the frequency for developing in the Fourier series two diilerent impulses;
Fig. 3 represents three series of impulses corvresponding to three diierent simultaneous communications; i
Fig. 4 represents schematically the embodiment of the invention in a receiver for the simultaneous reception of two different communications the series of impulses of which correspond to two of the series of Fig. 3; and
Fig. 4a shows a modication of -the circuiil of Fig. 4 to the left of the dotted line L -a.
The'invention will be described as applied to a system in which the emission is modulated by impulses in which the band width being larger,
it is particularly advantageous to sixperirripose` several communications on the-carrier at the transmission and to :separate them in the reor artificial parasitic disturbanecs.
The invention may be particularly applied tov a system of transmitting a certain number of communications from one and the same point over metallic links or radio channels or to transmissions from a network of radio transmission stations, having a plurality of transmitters at different points.
In the known transmission systems employing modulation by impulses, the receivers use a threshold'which in the absence of signals suppresses only the peaks of the parasitics or undesired signals so that nothing will be received. When the signalsarrive, then the intensities of the desired and undesired signals may be added or subtracted. In the first case, the level will be above the threshold no matter what the intensity of the desiredvsignals may be. In the second case, the peak of the level exceeds the threshold only if the intensity of the desired signal is greater than the intensity of the undesired signal by a potential which is just in expeak to rise above the threshold even when the undesired signals have a maximum negative value, it is necessary that the amplitude of the desiredl signal 'exceed the sum of the potential of the negative peak and of the threshold. Since. generally speaking, the negative peak of undesired signals is equal to the positive peak which, in turn, is equal to the threshold, it follows that the desired signal must at least be greater than twice the peak of undesired signals (i. e., greater by 6 db.) whereby, if `we assume that the edges of the signalling yimpulses are absolutely` vertical, then the effect'of undesired signals will be zero. However, if the intensity of the signals isnot quite twice the peak of undesired signals, then the peak of the desired signal will coincide only rarely with the negative peak of undesired signals.' Under these conditions, the message or the communication will in practice be only slightly mutilated.
Under certain special conditions the signal may be appreciably weaker than twice the VApeak of undesired signals as will be explained below.
In accordance withone embodiment of theinvention, the means provided for distinguishing between two emissions (e. g. having the same strength) coming from the same point are based on the difference in the duration of impulses. Asl a matter of fact, itis possible to distinguish the short last case between signals having diilerent duration in var# `ious ways, two oi' which will now be described.
' the present case I. For a given total energy, the
peak is inversely proportional to the square root of the duration. 'I'he relationship between the peaks in Fig'. l will therefore be 2.
By considering an impulse 3 having a duration t which is repeated at intervals T, Fig. 2a, if F'is the frequency having a period T and f the frequency having period t, if the series of impulses of Fig. 2a are expressed in the Fourier series, we will obtain frequencies of the form-nF `where n will have successively all the integral values from 1 to infinity. 'I'he curve in solid lines l of Fig. 2b represents approximately the manner in which the amplitude (the ordinate) of frequency nF (the abscissa) varies in function of n. `It will be seen that although the' amplitude has substantially a constant value until nF becomes equal to approximately f/2, the value of the amplitude will have peaks around frequencies 43]/2, ]/2, etc. and depressions around frequencies f, 2f, 3f, etc.
If we deal with a single impulse which is not repeated, then T becomes infinite and the diagram of Fig. 2b continuous. If an impulse (which may be a direct current impulse on a wire or a series of periods .of high frequency on a wire or by radio and the amplitude of which is like that 'of the impulse illustrated in Fig. 2a) has a square form at the vmoment of transmission, its form in the receiver will depend on the passing band of the receiver. Roughly speaking, one may admit that in order to obtain satisfactory reproduction at the receiver, a passing band of the order of f is required. If the bandV which is necessary for suitably receiving the long impulse 2 of Fig. l is f, then the `band which is necessary for receiving impulse I willbe 4f. However, in the the amplitudes ofthe frequencies of the Fourier series will be substantially one-half that' of the frequencies corresponding to the long impulses 2. `The amplitudes will follow the dotted curve 5 in Fig. 2b. It will be seen,.there fore, that if the two impulses are received in a receiver, the passing bandf'fof whichis 4f, then the peak of the short impulse I- will be approximately twice as strong as the peak of the long impulse 2. If, on the other hand, the passing band of the receiver is f, then the amplitude of the long impulse 2 will be almost twice that of the short impulse I. From this standpoint, the object of the invention is to separate in the receiver the received signals having d iil'erent durations of emission by suitably selecting the passing band width so as to increase the relationship of the peaks of two impulseshaving diil'erent duration.
If, in order to preserve in the receiver the more abrupt edges of which is wider than f the long impulse, and the short impulse, then the results will be quantitatively but not qualitatively modiiled.
It must be noted that by suitably selecting the band width, one may at will either separa the impulses of di'erent Aduration by increasing the relationship between their peaks or diminish this relationshi i. e. favour a mixed reception. i
A receiver arranged in accordance with this aspect ofthe invention will, therefore, have such theimpulses, a band is passedin the assumed case for I, which is wider than 4f for different duration,
'signals of two messages from assisi? a passing band that it will establish the desired relationship between the peaks of impulses of and means for reproducing these -impulses comprising threshold arrangements which permit the separate reproduction of the diierent impulses. Such arrangements are well known in themselves and will, therefore not be described in greater detail.
The main advantage of the invention from this standpoint is the possibility of providing a minimum band width whereby parasitics caused by other stations will be reduced to a minimum. However, it may be desirable in accordance with another aspect of the invention to provide multiple receivers having a different arrangement. Such receivers may be usedl nomatter what may be the relative duration of the impulses in the different messages as long as the amplitude which is supposed to be constant for each series of impulses does not have the same intensity as the impulses corresponding to another message.
Before describing in detail the invention from this view-point, I wish to note that generally speaking the duration of an impulse is short with respect to the time elapsing between the impulses of a given communication. It happens, therefore, only rarely that two impulses of two different messages are received at the same instant.
Bearing this in mind, Fig. 3 represents three series of impulses, 6, 1, 8, corresponding to three different messages.- After amplification, these impulses may in accordance with the present invention actuate respectively, three receivers which for convenience will be The threshold of receiver A has the value 9 of Fig. 3, of receiver B. the value is III, and of receiver C, the value is II. The impulses received by the different receivers correspond in the case of receiver A, the impulses 6, in the case of receiver B, to impulses 6 1 and in the case of receiver C to 6 1 8.
Receiver A picks up, therefore, the message transmitted by the impulses 8 without disturbance by the other messages. I'he impulses thus received by A are amplified and re-introduced in the opposite sense either before `or after detection into the receiver B in a manner to render the latter inoperative during the impulses. Unless an impulse 6 coincides with an impulse 1, in the output ofthe receiver B, we will obtain aseries of impulses which4 are not disturbed by the messages I and l.
In the same manner, the two receivers A and B may be arranged to render the receiver C inoperative during the impulses B and 1. In this manner, one will obtain only the impulses 8 in the output of receiver C. Obviously, the same arrangement may be applied to a larger number of messages at a point in the reception. In each case, the impulses in the individual receiversfwill actuate known receivers which will transform the series of impulses into intelligible messages.
A schematic representation of this embodiment of the invention is shown in Fig. 4 for only two messages in order to simplify the drawings.
In this figure, an antenna I2 collects the two transmitters I2A and I 2B, which are arranged to, transmit signals corresponding, for instance, to the impulses 6 and 1 respectively, and after amplification in any suitable amplifier I3, these are transmitted to two receivers schematically repre sented at Il and I5 in the form o1 a usual electronic discharge tube circuit, although obviously termed A, B, C.
these receivers may be of any other type. Each receiver Il or II has its output connected' to an amplier II tector Il or I! and the output of the detector` or I1 and these, in turn, to a deafter amplieation at or 2| is transmitted to an impulse demodulator 22 or 23, i. e. to an Aarrangement for retransforming the series of impulses into an intelligible message which can be heard in the telephone receivers 2l and 25.
The impulses of the receiver Il must be reintroduced into the receiver I5. If these impulses are reintroduced before detection, then their .intensity and phase may be regulated. This regulator, which may be of any well-known type is indicated at26. Alternatively the impulses in the upper receiving path may be introduced into the lower receiving path after the detectors inwhich case the phase. regulator 26 is notrequired.`v f 'v Itshould be understood that 'the word amplier as above used includes, if desired, frequency modulation amplifiers also.
An impulse 6`wi1l from time to time coincide with an impulse 1 and will blot it out in the receiver I5, whereby a distortion will be produced in the channel of receiver I5. If the duration of the impulses is three microseconds and the frequency of theimpulses 10,000 per second (impulses spaced by 100 microseconds) there will be partial blotting out once in 33, i. e. 300 times per second. However, this is not important and the only thing that counts is complete blotting out. Complete blotting out occurs when the two impulses coincide, the time difference being less than one period of the high.
frequency, i. e. at 6 meter wave 1,50 microsecond.
' At an impulse frequency of 10,000, this will occur less than once in 5,000, i. e. twice every second.
If We have n channels, then the nth will be subjected to blotting out n times more often and the quality' of its circuit will be reduced by that much.
Until now we'have considered only channiis in which the duration of the impulses was the same. If this duration is different in' each ease, then there never will be complete blotting out if the diiference in the duration of the impagses is at least equal to one period of the hign irequency, i. e. in the example given to 1/50 of a microsecond in 3 microseconds, and if the rst channel has the shortest duration. If the duration of the impulses 1 is longer than the duration of the impulses 6, then the blotting out of the receiver I5 by the impulses 6 might produce fa void in the middle of each impulse 1 and thus two instead of one impulse 1 will be obtained.
In order to avoid this diiliculty, in accordance with the present invention, during an interval of time which is slightly longer than the duration of the impulses 6 which are introduced -in the receiver I5, the arrangement for transforming the output of I5 into intelligible messages is rendered inoperative.
Fig. 4a illustrates how the difference between the amplitude peaks of two impulse modulated transmissions, the impulses of which are of different duration, may bey increased by inserting passlters and 26 ahead of the limiters Il, I5, the pass bands of these filters being selected as previously explained.
In accordance with another feature of the invention, .carrier frequency waves may be used which are slightly different for the different messages, although the band width in the repartial blotting out ceiver is wide enoughto receive more than one message. If the band is wider than is necessary for the impulses of, e. g. two-messages, just to attain their peaks in a time equal to the duration of the impulses which are assumed to be square, then according to the present invention, the relative change in phase between these two channels is utilized. As above stated, generally a difference in intensity of the order of six decibels is needed between two signals in order to separate them. This condition is imposed by the fact that the weaker signal may be in phase opposition with the stronger signal. By admitting several high-frequency oscillations by im` pulses, the lmessage which it -is desired to suppress must beof the same phase as the signal so that such a difference of six decibels is, necessary.
`Considering now `the case of 4an impulse of three microseconds at a frequency 10 megacycles, there will be 30 oscillations per impulse. If the undesired signal to which we shall refer as the disturbance signal has a phase change of 180, it`
turbance signal, and that the signal must gener-l ally surpass this threshold evenl in the absence of a disturbance, the signal must exceed the disturbance simply by a value. which depends upon the duration of the impulses, on the number of impulses per second, and on the difference in the relative energy carried by the impulses, but which may be substantially lower than 6 decibels,
It should be noted that the arrangements above described are particularly applicable to radiochannel transmission in which one message is on purpose disturbed. By means of the invention it is possible to separate from the communication a disturbance unless the duration. frequency, in-l tensity, and phase of the disturbance relative to the impulses are such that the two signals cannot be separated any more. However, it is very dlii-A cult to obtain all these characteristics whereby it may be said that a system of communication arranged in accordance with the present invention cannot in practice be disturbed by disturbances produced on purpose. If, however, unintentional )l `disturbances were produced, then the duration of the impulses could easily be varied, e. g. in accordance with a code predetermined between transmitting and receiving stations.
In the case of a network of radio transmitting- -stations, the fading will generally be different for the different stations. By network, I wish to include also the above mentioned case of a desired transmitting and a disturbing station.
Although the invention has been explained as applied to waves modulated by impulses, it need not be limited tosuch systems, but may be applied to. frequency-modulation systems also. In fact, although the image frequencies are not the same in the output of the limiters, it is possible to place in opposition the waves of undesired staf lied in many ways without departing from the spirit thereof.
What is claimed is:
1. A receiving arrangement for an electric wave A signalling system comprising means for simultaneously deriving from two signal transmitting channels respectively two signal wave trains of substantially constant but different amplitudes, ilrst and second receiving paths each including a limiting device, and means located in the ilrst path receiving said `signal wave train of greater amplitude for injecting in opposition the resultant of said latter wave train into the second path receiving additionally said signal wave train of lesser amplitude, whereby in said second-path the effect of said signal wave train of greater amplitude is substantially cancelled out.
2. A receiving arrangement for an electric wave signalling system' comprising means for simultaneously deriving from` n signal transmitting channels respectively, n signal wave trains o! substantially constant but different amplitudes, n receiving paths each including a limiting device the several limiting devices each being adjusted to a diil'erent threshold point, and means including said limiting devices for discriminating between said signal wave trains in said receiving paths, said discriminating means'also including means for injecting in opposition a tlrst signal from one path receiving said :first signal alone, into another path receiving said signal and a second signal, whereby in said last mentioned path the eiect of said nrst signal is substantially cancelled out.
3. A receiving arrangement for an electric wave signalling system comprising means for simultaneously deriving iromtwo transmitting signal channels respectivelytwo signal wave trains of substantially constant but diilerent amplitudes, first and second receiving paths each including a limiting device, the limiting device in said ilrst path being adjusted to have such a threshold that it passes the peaks of both said wave trains and the limiting device in the second path being adjusted to have such a threshold that it passes the peaks of one wave trainonly, and means controlled by the wave train in said second path for excluding the same wave train from the output of said rstpath.
4. A receiving arrangement for an Ielectric wave signalling system comprising means for simultaneously deriving from n transmitting signal channels respectively n signal wave trains of substantially constant but different amplitudes, n receiving paths each including a limiting device n the several limiting devices each being adjusted to train,
a diilerent threshold point so that a firstreceiv i118 path is receptive to one Signal channel wave a second receiving path is receptive to two Signal channel wave trains, a third recel is receptive to three signal channel wave trains and so on, an output circuit for each receiving path. and means coupled between each said receiving path which is receptive to at least two signal channel wave trains and at least one of the other receiving paths for excluding all but one ot said signal channel wave trains from the output circuits of the receiving paths which are re- :xeive to more than one signal channel wave 5. A receiving arrangement according to claim 4 for simultaneously and selectively receiving a plurality of impulse modulated signal wave transmissions distinguished by different constant im. pulse amplitudes. wherein said means coupled between each 4said receiving path which is receptive to at least two signal channel wave trains and at least one of the other receiving paths includes means for introducing into each said rstmentioned receiving path wave trains which are of equalintensity and opposite phase to the wave trains which arerrequired to be excluded from such path.
8. A receiving arrangement according to claim 4 for simultaneously and selectively receiving a, plurality of impulse modulated signal wave transmissions each distinguished by a. different duration of impulses constituting the signals, wherein a passillter is included in each said receiving path at a point preceding the limiting device in the respective path, the pass bands of said illtcrs being selected so as to increase the dinerence between the peaks of the impulses constituting the several signal wave transmissions.
7. An electric wave signaling system comprising means for transmitting two signals from which two signal wave trains of substantially constant but dilerent amplitudes may be respectively derived, means to receive said signals and 'l to derive said two signal wave trains therefrom.
first and second receiving paths each including a limited device, and means located in the first path receiving said signal wave train of great amplitude for injecting in opposition the resultant of said latter wave train into the second path receiving additionally said signal wave train of lesser amplitude. whereby in said second path the eilect of -said signal wave train or greater amplitude is substantially cancelled out.
. EDWARD HILL ULLRICH.