US2406019A - Pulse modulation system - Google Patents

Description

Aug; 20, 1946. E. LABIN PULSE MODULATION SYSTEM Filed April l. 1941 4 Sheets-Sheet 1 ATTORNEY Aug. 20, 1946. E. LABIN PULSE uonULATIo SYSTEM Filed April 1, 1941 4 Sheets-Sheet 2 INVENTOR .NE Y

ATTORNEY l Allg. 20, 1946. E, LABlN 2,406,019

PULSE IODLATION SYSTEM Filed April 1, 1941 4 Sheets-Sheet 3 FIGA;

lNVENTOR ATTCRN l Aug. 20, 1,946. E. LABIN PULSE MODLATION SYSTEM 4 Sheets-Sheet 4 vFiled April 1, 1941 6 M a MW w amava/.Aw

wn-calml l reared' 20 1946 amig PULSE MODULATON SYSTEM application April i, i941, serial 10,386,282

(ci. 25o-6) i2 claims. l

This invention relates to improvements in pulse modulation systems.

It is a primary objective of the invention to provide an improved pulse modulating system.

Another object is to provide more interferencefree system of radio communication.

A further object is to provide an improved system of radio transmission which may function despite deliberate attempts to jam the transmission frequency.

It is also an object to provide an improved and simplified pulse modulation reception system.

Still another object is to provide an improved system of secret radio transmission.

A further object is to provide an improved form of pulse modulation system in which a. single, constant-width impulse may suiilce for indication of intelligence.

It is also an object to provide a pulse modulation system adaptable to relatively simple receiver means.

Another object resides in provision of a pulse modulation system in which there is no phase modulation of, the impulses.

Other objects and various further features of novelty and invention will hereinafter be pointed out or will occur to those skilled in the art from a reading of the following specification in con, junction with the drawings included herewith. In said drawings Fig. l is a graphical showing of the effect of signal treatment in accordance with features of my invention;

Fig. 2 is a block diagram of a pulse transmitter according to my invention;

Fig. 3 is a block diagram of a pulse receiver incorporating features o-f my invention;

Fig. 4 is a schematic circuit diagram of a portion of the transmitter of Fig. 2; and

Fig. 5 is a graphical showing of successive changes in wave shape in portions of the circuit of Fig. 4.

Pulse modulation systems are known in which the amplitude of the pulse is always maintained constant and the duration of the pulse vmade to vary in accordance with speech or other modulating signals. It is also known that the whole pulse-so modulated need not be transmitted, but rather two pulses, each marking one determinant end of the modulated pulse. This latter method has advantages over the former, in that still greater-signal-to-noise ratios may be obtained, but has the disadvantage that a signal so transmitted requires just as much of a time-interval (i. e., from the beginning to the end of the modulated pulse) to be intelligible as in the former case. It might also be observed that neither of the above-indicated systems is free of phase modulation, as will later be clear.

In accordance with features of my improved form of pulse modulation system only one extremely short pulse is needed to identify a. signal, no matter what the degree of modulation. This improved method contemplates the time-displacement of a single pulse one side or the other about a given norm in accordance with the degree of modulation; hereinafter alternately referred to as time-modulation. Since the maximum time-displacement of the impulses is preferably small in comparison with the time-interval between pulses. a large part of the time is not used for transmission; and suitable receiving apparatus may include a sharply selective circuit tuned to the pulse frequency and blocking means energized by said selective circuit for excluding substantially all noise but the displaced impulses. Appropriate displacement responsive means may then detect out the audio` signal in a known man- I consider it preferable to a more ready understanding of my invention rst to indicate generally how I treat signals in accordance with features of this invention, and then to describe possible means for giving such treatment. Accordingly, vreference will first be made to Fig. 1 which represents graphically the general treatment of pulses in accordance with features of my modulating system.

The pulses modulated as indicated correspond to a spectrum of frequencies with bands located near the harmonics of the pulse repetition frequency. Each harmonic may be modulated in amplitude by time modulation of the pulses. If any particular harmonic of the transmitted energy be selected for detection or demodulation, the original signal may be restored by 'relatively simple means forming a, feature of this invention.

In order to avoid combined amplitude and phase modulation of harmonics and increase the percentage of amplitude modulation of harmonics, I propose to displace alternate impulses symmetrically and in opposed senses about their normal unmodulated position. In other words, the impulses are displaced to new, modulated positions which are in time successively before and after` the normal instants at which unmodulated impulses would occur. In such a modulation system the impulses may be considered as made up I, 3, kare shown schematically with respect to time and as being time-modulated in a sense opposed to that of the modulation of even impulses 2, 4, Ii.` It wil1 be clear that if to represents the maximum amplitude of time displacement for the Y impulses, one set of impulses I, 3, 5 etc. will be displaced :2te with respect tothe other (2, 4, 6 etc.) for maximum modulation, depending upon the magnitude of modulation.

As stated above, the series of pulses shown in Fig, 1 may be considered as formed of two identical series, each having a period T, which series we shall designate as P1 (for pulses I, 3, 5 and P2 (for pulses 2, 4, 6 respectively.

By appropriate choice of time origin the center of the rst pulse I of series P1 may occurat time AP: Em(- 01, and the amplitude A of pulse I may be described as- AP1=ZMAW cos mou-61) in which Ap2=2,.,.-cos mw(t02) (2) Where 62 represents the time at which the center of impulse 2 occurs with respect to the time origin.

By combining expressions (1) and (2) it will be clear that the combined eiect of both trains of impulses will be described by- If `the time origin has been appropriately selected, a simplied solution may result. Assuming, then, a time-displacement of both series P1 and P2 in opposite. directions by a time 0, We may arbitrarily describe 61 and 62 as- T 01 b- 9 Hand in which 2b arbitrarily represents the distance between the axes of modulation of the ilrst two pulses after the time origin, and from which as will be clear. After substituting Equations 4 and 5 in expression (3), we obtain Now, if we assume that the time-displacement Such an assumption will permit the following simplifications for our purposes:

cos mw0==11 and tall mw9=mw0 Wherefrom an appropriate expression for Ap becomes Ap being the sum of amplitudes at frequencies "/r each of which is amplitude modulated bythe signal 0, as will be clear.

Now, assuming the simplified case of 0 sinusoidally varying with a period a, We may write l 0=K.o. Sin (at-Ho) v(10) Upon examination of expression (9), it will at once be observed that the depth of' modulation for the mth harmonic is mao. tan mwb=K.e. tan mob It will be recalled that K is always equal to or y less than 1, and that e is much less than 1. Therefore. in order to obtain large orders of modulation Ke. `tan mwb should approximate unity, or tan mwb should equal will be clear, would give rise to the undesirable 1 effect of doubling the impulse frequency. In order, then, to avoid a doubling of the period-T,

characteristic of each seriesl of impulses, we may define b as T i b=Eto (11) in which case, for m odd,

tan mwb= =1 tan e e and cos mwb=sin e=e and, for m even,

and

-Y cos mob-:cos e=1 The general term of series (3) may now be written in two forms, depending on whether m is odd or even; thus, for m odd 2Am.e[1-K sin (at-l-qnl cos met From expressions (12) and (13) it will be noted that the presence of e in the amplitude term will make for odd harmonics of relatively small amplitude, while even harmonics, not being associated with the factor e, will he relatively large. On the other hand, odd harmonics are highly modulated, whereas there is hardly any modulation of even harmonics.

It will further be noted from these same expressions that, inasmuch as ai and 02 vary in opposite directions, there are no phase modulation terms, and that, therefore, all modulation is in amplitude. That such relative variation of 6i and 02 is responsible for'elimination of phase modulation may be seen if we consider one series of impulses as unmodulated while modulating the other. In such an assumed case, for m odd, we would obtain, instead, of expression (12) ZAWeIII-- sin (at-H] cos mw(tg) (14) Y In this case it will be noted that depth of modulation is necessarily only one half that of the preferred case of expression (l2), and that phase modulation is present. It may be observed, however, in this connection, that the phase modulation present in the case of expression (14) is not very harmful to speech transmission; but, of course, for utmost fidelity purposes, it would be considered undesirable.

Returning again to the preferred system in which both series of impulses are modulated in opposite directions, let us consider reception and detection of signal impulses. Since each odd harmonic, as will be clear from expression (12), is purely amplitude-modulated, a circuit tuned to some such odd harmonic of l/T would permit the amplitude modulated term to be removed and thus result in detection of the signal. Further utilizing received energy in accordance with features of the invention, any Veven harmonic term, which as we have seen from expression (13) may be very large, may be employed, after suitable shaping. to obtain a generally squareshaped blocking signal for blocking out reception of energy for substantially the interval .between pulses. By making the even harmonic circuit as selective as possible, it will be clear that parasitic disturbances may be reduced to a minimum, and that substantially only the signals themselves will be admitted to the detection circuits.

A further desirable feature of my invention may be seen in its use as animproved secrecy system. Since the steady component existing in the signal is modulated but very slightly, ordinary detection methods will only be able to detect a small unintelligible disturbance, and common circuit and other noise will probably be large enough compared with such very slight modulation that the signal may be completely unintel-y ligible, as will be clear.

This brings us to a consideration of the signalto-noise ratio and related factors. If we assume that the receiver is blocked continuously except during the pulse and modulation interval 2me-d, d rheling the length of the pulse, the probability of interference for equal amplitudes of desired and interfering signals may be expressed assuming an interfering signal substantially uniform with respect to time. Of courseffor best reception, free as possible from interference.

.should be made small as possible; and in the case of It will be seen that, for any given value YofV e, p is inversely proportional to m, and that, therefore the higher the harmonic chosen for detection, the greater will be freedom from interference. Since e may be selected as desired, it will also be appreciated that to should be as small as possible, both in order to obtain less probability of interference and to see that modulation (see expression (7)) follows a substantially linear4 rather than a tangential law. In review then, as high a harmonic as possible should so be chosen that, when considered in view of the smallest possible to and d, the term e is not so large that linear modulation is impaired. In any case, the final value chosen will be a compromise between the frequency band available and freedom from interference.

In the drawings I show a preferred possible circuit for obtaining the above-indicated desired results in accordance withfeatures of the invention. In Fig. 2, which shows a transmitter in block diagram form, impulse energy may be the transmitter diagram.

Fig. 4 shows a possible circuit for converter 2| schematically and in some detail. In a preferred form the circuit is designed to set up the two series of pulses l, 3, 5 and 2, d, 6 so that, when speech or other signals are applied, time-modulation of one set of pulses will be opposed to time-modulation of the other. The circuit of Fig. 4 may more readily be understood when viewed jointly with the wave forms of Fig. 5. First, we shall consider the case of no modulation, tracing wave treatment throughout the circuit of Fig. 4, as depicted, in an exaggerated `manner for purposes of clarity, by waves a is. so that the impulses later to be superimposed occur at instants of time corresponding to positive and negative maxima of the input voltage. A circuit for eiecting the desired phase displacement may. for example, include series capacitance means Ca and shunt resistance means Re, as shown.` 'I'he input sinusoidal voltage, so shifted in phase may then be substantially squared on by well-known limiter means including an overexcited amplifier tube V1 and appropriate circuit elements. Output energy from tube V1 may be taken from across a potentiometer 25 and is preferably divided for application to tubes V2 and V3 in push-pull. The outputs of tubes V2 and Va may be connected in parallel as shown so that a unidirectional output signal may be obtained. For purposes hereinafter to be indicated the t'ap of potentiometer 25 is so adjusted that the push-pull control potentials applied to tubes V2 and V3 are unbalanced; that is, one is substantiallygreater than the other. As a result of the limiting eiiect, adjustment of `potentiometer 25, and the parallel output connection ofthe tubes V2 and V3, an output wave form somewhat conforming to Fig'. `5b is obtained.

At this Ystage f the sinusoidal input signal treatment impulse energy (shown in Fig. c to be substantially in quadrature with the sinusoi'clal voltage, that is, characterized by impulses` occurring as the sinusoidal input passes through zero) may be superimposed upon the wave form of Fig. 5b by appropriate grid control means 26, 21 shown associated with tubes^V2 and Va', respectively. The resultant wave so obtained will be of the general form shown in Fig. 5d as will be clear.

In accordance with features of my invention, I next apply the Wave of Fig. 5d to a multivibrator circuit of known form including tubes V4 and V5. These multivibrator tubes are preferably so excited that an impulse, say 21 (Fig. 5d) of a certain magnitude will energize one of the tubes V4 and V5, and a drop below a certain exciting potential, say 28 (Fig. 5e), will cause operationof the other multivibrator tube. The circuit of these tubes (V4 and V5) preferably includes appropriate time-constant decay circuit means for deriving an output from the multivibrator of the general form shown in Fig. 5e. The multivibratorcrcuit including tubes V4 and V5 is one capable of controlling the instant of transition from one condition to another, and operates spontaneously without external control to return to the rst condition. The multi-vibrator action is attained by virtue of a conventional forward coupling from tube V4 to tube V5 over a resistance capacitance arrangement R7, C1, R1, together with a back coupling from the output of tube V5 to the input of the tube V4 by virtue of a common cathode resistor R. The forward coupling through R7, C1, R1 is like an ordinary interstage coupling except that the values of condenser C1 and resistance R1'are such that condenser Ci may be charged to a substantial value in a few microseconds.-

In considering operation of the multi-vibrator circuit, assume as an initial condition an instant or so before occurrence of impulse 21 (Fig. 5d), when tube V4 is carrying relatively high plate current; thusmaking its plate far less positive than the potential of plate supply. This reduction in the plate potential of tube V4 has the effect of applying a'negative potential on the control grid of tube V5 after a delay, instituted by condenser C1 and resistor R1, suflicient to allow the negative potential to be substantially attenuated and permit tube V5 to draw grid current. Thisy is the condition of tubes V4 and Vs when pulse 21 appears. Application of pulse 21 to tube V4 momentarily drives thatidischarge device below cut-oi, thus permittingv a sharp increase in plate voltage. This relatively positive potential on the plateis transmitted through condenser C, to render tube V5 conducting. Thereupon, because' of the conducting state of tube Vt, a resultant drop in cathode resistor R shifts cathodes of both multi-vibrator tubes positively. This shift has the effect of making the control grid of tube V4 so negative with respect to the cathode that the cut-off condition set up by the momentary impulse is maintained a little longer. v

The cut-oi state of tube V4 will in every instance be maintained for a period of time depending upon the magnitude of negative impulse applied. For example, pulse 21 being more negative than pulse 30 will retain tube V4 in the cutoi state longer than pulse 30 will. This will be clearly seen from the fact that condenser C1 is variously charged depending upon the magnitude of the positive pulse output of tube V4 when the latters conductivity is first cut off. This positive potential in the form of a charge -on condenser Ci will continue to be applied to the control grid of tube V5 (thus keeping tube Va conducting) until the leakage effect of resistance Riso reduces the magnitude of the charge that the output current of tube V5 is insuiiicient to maintain the drop in cathode resistor R necessary to keep tube V4 below cut-oil. When the drop in resistor R gets that low, then, tube V4 will again conduct and tube Vs will be non-conducting. This is the point at which the operation of the multi-vibrator was assumed to commence, and so a complete cycle of multi-vibrator action has been described.

It has just been pointed out that a longer time will elapse before decay to a predetermined trip level 28 for larger applied pulse magnitudes than for lesser applied pulse magnitudes. An attempt has been made Ato show this phenomenon in the curve of Fig. 5e. It will be obserbed that pulse 21 and pulse 38 of Fig. .5d represent respectively pulses one of greater magnitude than the other. Pulse 21, being relatively larger, causes a relatively large output voltage 29, with the result that decay from this voltage to the level 28 takes a relatively long time t'. Pulse 30, being xsomewhat smaller in magnitude than 21, consequently causes a lesser output voltage 3l, with the result that decay from yvoltage 3| to level 28 takes a correspondingly smaller time interval t than t in the case of pulse 21.

It will now be clear that the respective widths t and t" each recur regularly with a periodicity T. In accordance with features of my inven tion I employ the relative position of the instant at which the decaying voltages reach level 28, that is, when the drop across cathode resistor R due to conduction of tube V5 decreases to such a magnitude that the control grid of tube V4 is no longer effectively biased beyond cut-oil?, to determine the two series of impulses I, 3, 5 and 2, 4, 6 of period T.

To obtain the two series of pulses from the curve of Fig. 5e I prefer to employ a Well-known resistance-capacitance derivative circuit, which mayinclude capacitance C2, resistance R2 and an over-biased tube Vs, in the output of the multi-vibrator. Output from the derivative circuit, as obtained across R3 may be of the form of Fig. 5f, as will be clear. It will further be clear that peaks 32, 33, 34, etc. so obtained represent the desired. appropriately displaced two series of impulses. Since the other peaks 35, 36, 31, etc. obtained from the derivative circuit are all regularly spaced with period T/2 andare independent of the magnitudes of pulses 2l, 3d, they are of no use to a transmitter in accordance with my invention, and may therefore be suppressed by well-known means (not shown), as I have indicated schematically in Fig. g. The unmoduiated signal of Fig. 5g is now fully prepared for its transmission by any desired means.

The case of unmodulated pulses has first been taken for discussion because it is obviously simpler than when the impulses are modulated. Figs. 5h through 5l illustrate this latter more complicated case for an assumed sinusoidal modulating potential, shown in Fg. 5h. It will be clear that the voltage of Fig. 5h may represent speech or other intelligence to be transmitted..

In a preferred form this speech may be applied to modulate the pulse signals at the push-pull rectider stage V2, V3 of the converter of Fig. 4. According to this embodiment, balanced, that is equal, speech signals are applied by means of an input transformer 33 to appropriate control grids of rectiers Ve and Va.

The edect of such application of speech signais may be to vary the curve of Fig. 5d as shown in Fig. 5i, that is, to make pulses corresponding to 2l and 3d of either greater-or lesser relative magnitude in accordance with the modulation. Thus. pulse 39, being at an unmodulated point, is unmodulated, and hence of the same magnitude as pulse 217. However, pulse 40, being displaced by the rising portion of the modulating potential, is of lesser magnitude than pulse 3i). Recalling the discussion in connection with Fig. 5e, it will be seen that pulse 39 will cause a multivibrator output signal of duration t'; Whereas pulse dd, being of lesser magnitude than pulse 3d, will cause a multivibrator output signal of still shorter duration (tm) than t", as will be clear.

The next succeeding impulse il of series 39, el, d2, which may correspond to series i, 3, 5 of Fig. l, has been so modulated that its peak is of considerably greater magnitude than those of 2t or 39. Consequently, voltage decay from the multivibrator output voltage corresponding to pulse di may be of proportionally greater duration tm' than t', for pulses 2l and 39. Considering this phenomenon as to its effect on the displaced peaks of Fig. 5k, it will be apparent that, as the modulating signal is, say, increasing, the pulses (3W di) of one series (39, di, d2)

are time-modulated or displaced in one direction (i. e.. retarded in time) from their 'unmodulated relative positions, and the pulses (say tii) of the other series (fili, 43, M) are time-modulated or displaced in the opposite sense (i. e., advanced in time) from their unmodulated relative positions. When prepared for transmission treatment, the modulated pulses, corresponding to a modulating signal of Fig. 5h, may then resemble the curve of Fig. 5e, as will be clear.

In the circuit of Fig. 4 it will be appreciated that many adjustments may be made to vary the relative displacement of the two series of pulses, width of pulses, modulation depth, etc. etc. For instance, by changing the value of resistor R,- or varying the bias on tube V4, the width of pulse in Figs. 5e or 5i may be controlled,in that amplication, thus controlled, willoccasion diiferent pulse widths t or t", etc. Thus, an increase of resistance R, or increase in the negative bias of tube V4, will displace all pulses by increaslngthe retardation or decay time. Conversely; a decrease in R, or decrease in bias, will result in an opposite displacement eect. Relative displacement of the two series of pulses, one with respect to the other, may be varied by adjustment of the ground tap on potentiometer 25, which, as we have seen, controls the relative amplitudes of adjacent lobes of the curve of Fie. 5b. v

While a form of converter circuit 'shown has been described in particular detail, it is to be understood, of course, that such description is merely by way of example to illustrate how the desired form of impulse treatment may be effected.

A possible and preferred form of receiver for detecting impulses, transmitted as outlined above, is shown schematically in Fig. 3. Impulse energy received by antenna t5 may rst be treated in a customary manner in radio and intermediate frequency amplier stages,f shown generally as a block B6. Thereupon it may be detectedl and some disturbing effects removed by suitable limiter and detector means dl. At this point, in accordance with a feature of my invention I provide means, responsive to the output of detector t?, for blocking out reception of any signals substantially for the period between successive impulses. Such blocking means preferably includes a sharp selective circuit 68, tuned twice to the pulse frequency (i. e., of period T) and a square-wave generator t9 controlled thereby. By suitable adjustment means (not shown) the square-wave output of generatoril may be made to produce a blocking signal occurring only when no impulses are due for reception. 'Ihis blocking signal when applied to the earlier stages 46 or l of the receiver may effectively cut out substantially any and all parasitic disturbances which would otherwise interfere with reception of the impulses.

A relatively simple blocking signal of uniformly recurring duration may be derived by well-known means utilizing, say, the second harmonic of the impulse period T. Such a signal could, for example, be obtained by half-wave rectification of the second harmonic and adjusting generator 49 to give as large a constant-width blocking signal as possible without interfering with impulse reception. Suitable oparatus for obtaining a signal of the indicated nature having a desired l `width has been disclosed in the copending application of H. G. Busignies, Ser. No. 380,186, filed February 24, 1941, and entitled "Radioelectric impulse systems." Such a blocking signal is shown schematically by the dash-dot-dot line 50 in Fig. l, and is designated as a simple blocking signal. It will be observed, however, that such a blocking signal is not effective completely to block out reception between pulses in that one series of pulses, as we have seen, is other than exactly in phase opposition to the other series of impulses, and, furthermore, modulation of one series is opposite in sense to that of the other. What is needed, therefore. for much greater blocking efciency, is some sort of complex signal which will block reception for both the short interval of time between, say, pulses i A possible arrangement for this odd propor` tioning of blocking signals could include apparatus similar to that outlined above in connection with the simple blocking signal (for the one particular odd harmonic.

. y 11 shorter intervals) and an additional superimposed blocking signal suitably phased with respect to the first-mentioned blocking signal in accordance with teachings in the above mentioned H. G. Busignies application and recurring with a period T for substantially blocking out the longer intervals. It will be clear that known wave-shaping methods may bel employed for utilizing preferably the second harmonic to obtain the additional superimposed, as well as the firstindicated blocking'signal. The complex blocking signal so obtained has been designated as such in Fig. 1 and is shown as a dotted line.

After the above-indicated treatment o f received signals, it will be seen that output tothe rest of the receiver vial line 52 will include only the pulses themselves with a very slight amount of adjacent parasitic noise, which may completely have been eliminated in the limiter circuit 41. It will be recalled from the above discussion of theory involved that any odd harmonic in line 52 will include amplitude-modulated pulses; see expression (12). Thusfall that now is needed is an appropriate band-pass filter 53 for selecting the desired odd harmonic, the passband being, of course, wide enough to accommodate the modulating frequencies either side of the harmonic selected. Thereafter, simple detector 54, audio amplier 55, and speaker means 56 may restore the original input signals with theoretically perfect ildelty. as will be appreciated.

It might here be observed that theoretically any amplitude-modulation receiving set ought to be able to listen-in on pulse-modulated sig. nals` in accordance with features of this invention. Practically, however, it seems unlikely that such will be possible. for it would be necessary to have a set including an extremely stable and critical local oscillator for receiving only Furthermore, an ordinary set would not be provided with my novel blocking means, so that the average signal-tonoise ratio might, and would in all probability, be so great'as not torpermit reception at all. Thus, it will be seen that transmission and appropriate reception in accordance with my invention may even take place without suspicion by anyone that any such communication is taking place. N

In accordance with ,still another feature of the invention, still better signal-to-noise ratios may be obtained. I propose to utilize the interval between blocking impulses (in the receiver), which interval is relatively short compared to the time between pulses, momentarily to increase the gain,

say, of amplifier 55. Although the means for so improving reception has not been shown, it will be appreciated that it may include suitable wave-shaping means for applying a momentary abnormally high voltage to the anode or, say, an

accelerating electrode in an amplifier tube of circuit 55. While sustained application of such a voltage might ordinarily harm this tube, a momentary application thereof conceivably will not. At the same time, it will be clear that amplification and efliciency of the tube may be materially increased for this, the desired and useful instant of time.

It will be seen that I have provided a relatively simple radio transmission apparatus of greatly improved efllciency and having many other desirable features. While the invention has been described in particular detail and preferred forms illustrated, it is, of course, to be selecting frequencies in the neighborhood of an odd harmonic of the' impulse frequency, whereby it is possible to demodulate by ordinary ampli,

tude detection of an odd harmonic of the impulse frequency.

2. A radio communication system including a transmitter and receiver, said transmitter including impulse generator means for generating two series of impulses of the same impulse frequency, and means for time-modulating one of said series of impulses with respect to th'e other of said series, said receiver including filter means passing substantially only a band of frequencies immediately about an odd harmonic of the impulse frequency of one of said series, and amplitude-detection means.

3. A receiver for an impulse-modulation communication system, said receiver'including lter means passing a band of frequencies including an oddv harmonic of the frequency of the received impulses, amplitude-detecting means, 'and blocking means responsive to an even harmonic of said frequency of the received impulses, said blocking means including a generator responsive to said even harmonic for supplying energy to cut out reception of signals during substantially the period between received impulses.

4. In a receiver for receiving and detecting time-modulated impulsev energy, blocking means for suppressing reception during the-interval between impulses of the received impulse energy, said blocking means including a sharp selective circuit attuned to an even harmonic of the frequency of said received impulses, wave-shaping means for generating a generally square-shaped signal to occur substantially during the'interval between said impulses, and means responsive to the output of said last-mentioned means for suppressing effective operation of said receiver in accordance with said generally square-shaped signal.

5. An impulse time-modulation transmitter according to claim 11, in which the impulses generated by said impulse generator are relatively short with respect to the interval between successive impulses, and in which, for the same degree of modulation, time retardation of impulses of one of said series is of substantially the same magnitude as the time advance of impulses of the other of said series.

6. In an impulse time-modulating transmitter, an impulse generator for supplying a series of regularly spaced impulses, circuit means synchronously related to said impulses for supplying a full-wave rectified alternating wave of a frequency one-half the recurrence frequency of said impulses and having oddv maxima greater than even maxima, means for superposing said impulses and said rectified alternating wave, multivibrator means responsive to such superposed 13 multivibrator means for deriving impulse peaks corresponding to alternative action of said multivibrator means.

7. A transmitter according to claim 6, in which additional circuit means responsive to modulating energy is provided for modulating the magnitude of successive maxima of said Wave.

munication system, said receiver including filter means passing a band of frequencies including a harmonic of the frequency of the received impulses, amplitude-detection means, and blocking means responsive to a harmonic of said frequency of the received impulses, said blocking means including a generator responsive to said last-mentioned harmonic for supplying energy to cut out reception of signals during substantially the period between received impulses.

l0. A receiver for detecting time modulated impulse energy, odd impulses of said energy recurring with a given periodicity, said receiver comprising filter1 means passing substantially only a band of frequencies immediately about an odd harmonic of said periodicity and amplitude detection means.

11. In an impulse time-modulation transmitter, an impulse generator for generating a rst series of regularly spaced impulses and another series of regularly spaced impulses of the same frequency of recurrence as said first mentioned series, modulating means for time-modulating impulses of said first mentioned series of im# pulses in one sense and for time-modulating impulses of said other series in an opposed sense, and means displacing impulses of said rst series in time with respect to those of said other series, said displacing means being eiective to displace said iirst and said other series respectively to an extent that an impulse of said first series always precedes an impulse of said other series with an interval of time greater than that by which an impulse of said rst series follows animpulse of said other series, whereby succeeding impulses alternately determine an interval greater than one-half the periodic recurrence of impulses of one of said series and a succeeding interval less than said one-half.

EMILE LABIN.

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