US1678163A - Modulation - Google Patents

Description

July 24, 1928.

1,678,163 E. PETERSON MODULATION Filed Dec. 29, 1923 2 Sheets-Sheet l f/g/ ffy;

Ec 2 @A y /m/e/f/ar:

' [age/7e fe/effo/r I July 24, 928, I 1,678,163

' E. PETERSON MODULATION Filed Deo. 29, 192s 2 sheets-sheet 2 smEBANn /m/efifor: [ae/7e Pe/erJo/y W Y Any Patented July i24,l 1928 UNITED EUGENE PETERSON, or NEW YORK, N. Y., AssmNoR To 'WESTERN ELEoTRIc comm, INCORPORATED, or 'NEW YORK, N. `ir., n ooRPoRA'rioN or Naw YoRx.

' .iaoii-u.i:a'1irc JN. Application led December 29, 1923. Serial No. 683,801.

This invention relates to modulation, and components or side bandSfare distinguished particularly tovmodulation of higher order from each other by the number of times the than the second, or other niethodsof moducarrier and modulating frequencies, takenlation producing side bands of similar fr e-` together, occur. Thisv distinction 1s made quency characteristics and its application in use of in defining the order` ofgmodulation 60 carrier wave communication systems. which produces these componen s. For ex- Modulation, as used herein, relates bnQadample, second order modulation produces' ly to that class of phenomena which acsldebands of theI usual kind, -1n which the" companies or, in fact, is produced by distor- Carller and modulating frequencles each ootion of electric waves, and especially to llIS 0I1ce,th1rd order.of modulation results 165,. ,phenomena which occur when two waves, one 1n Side bands in which the iirst even the so-called carrier wave and the other the momo 0f either the carrier or modulating modulating wave, are jointly impressed ona frequency OCCIIIS, elle. f distorting instrumentality, Anobgect of the in vention is to provide l5 It is well known that modulation of this mod ulalng-means Which-are simpler, mOre 70"' kind yields side frequencies made up of variiC1eI1t, rugged, `and less expensive, than ous combinations involving thefrequencies mOdllla-tlllg Ineens heretofore. invented.` of the impressed waves. When one of the AIlQlJheI ObJeC? 1S the ICtlCal lltillzatlll waves, either the modulating'or carrier wave 0f thll'd, and hlglll 0I' er, mdulfltlon, 0r v 2" consists ofaband of frequencies the side fre 0f modulation side bands of similar fre- 'Ni quencies likewise assume the form of bands. 'quenCX- Charactellstls- The number, amplitud@ and frequency A stlll further object is toproduce a s u characteristics of these side bands is a vunc- Pressed CaI`1`191, S1g11&1mofhlkttedwave, Wit tion of the kind and extent of the distortion out the US@ Ofi balllcd CHQUIS .g5 as measured, for example by the equation Another obJect isto effect o dd-orderinodu- 80 of the characteristic curve which expresses 135mm 't0 the Slbstantl-QXCIUS-l-Ol 0fv ,even the relation between the amplitude of the Order n lodulalom input and output quantities. 'A 513111. further 0bJeCt;1$ -'t0 Pr0V1de 1,1m- In the present systems. use is made of the proved means f Ol' emratlng thildlderslde i square characteristic of this curve, that'is, bands o f relative y greatamplitude.' 85. the characteristic which makes the equation `Add-11-0na1 Oblecs 0f tlle'lllvelltlOn are: I ofthe curve approach the form i/=c2, in t9 aChlVBhQmJmC multlplex 009111111111125- which a: and ,g/ are respectively the 'ampli- UCP, 15mg a' .Slngl `S0UFC9 0f Carmel. WaVeSf tudes of the input and output quantities. Pllme 0 1' Seconflal'yff 11 0f the Channels; Other terms having other powers of w may, t0 PlOVlfe @Il @IDPI'PVB Sgsfem 0f WO-Way 90. to some extent/,be present. Those in other CQmmPlllCatlOIl h YlDg d1 eljellf C alllfld" even powers of will contribute to the effect quencle and a Single Cal''le. SUICBifthGH secured by the square term as will be ex- P fodllctloll )f OddhPmOmQS for u Seas Carplained later. On account of this square nel' ffeqenclesiand t0 PIPV1C-1@ im. Improved .40 characteristic, modulation yields side bandA System 0f Sgecret pnlmulllcatlo 95 components having frequencies equal respec- The Val'lOllS means .Suggested for pIO- tively to the sum and difference of the 'f re: ducing pronounced' higher Aorder modulaquencies of the modulating and carrier tion side bands each involves means'for prowaves. .These side. bands with or Withoi1`t\d iici'ng', andl operating over, a'characteristic some of the original -carrier constitutev a, A cnrvekwhich has one or more pronounced 190 modulated carrier wave from which the bends. Ini-the particular case -in .which it y I mOdlllatlPg C OIHPQHGH may he reproduced is desired that odd order modulation side f at 'a receiving station by a similar distortion. bands should be obtained to the-exclusion 0f The terms besides the square term, if the even order side bands, the .curve should 5o present, in addition to contributing to the have -two symmetrical bends ,in relatively 105 effect 0f the sql1al`e elm', @S abOVe, 111:0- opposite directions, that is the curve-should duce other side bands in.` which lharinonlcs rhave, 0 positesymmetry about each of the of the carrier or modulating' frequencies, or axes o? coordinates Vthrough a oint on theY both, insteadof the fundamental frequencies, curve and the operation s ould occur occur. These various combination frequency equally over -thej curve in both directions u about the point. If the operation does not occur about the point of symmetry or ,if the curve lacks symmetry of this kind, even order side bands will be present. Simil larly, even order side bands can be obtained,

tothe exclusion of odd order side bands by operating over a curve which has opposite symmetry with respect to only oney of the axes, as, for example, a parabola. These two forms of symmetry are often, and more simply, designated respectively symmetry about a point and symmetr about a line In general, for either case, t e acuteness of the bends indicates proportionall the order ltothe substantial exclusion of even order side bands. -The lmodulating and carrier currents are used to variably magnetize the core by impressing them on a common coil, or on different coils, mounted on thecore. On accountof `its unusual magnetic qualities the alloy known as permalloy has been found to be ideal for the purpose. This alloy is described in a paper by Arnold and Elmen in the Journal of the Franklin Institute for May, 1923.

lAl similar effect can be obtained, using electron dischargerdevices by inserting a high .impedance in series with the grid of a conventional three-electrode discharge modulator. The impedance may be a conductive element of the usual kind or a'halfwave rectifier. An abrupt bend in the normal space current grid voltage characteristic curve occurs at the point corresponding to the instant when .an increasing im ressed alternating'current voltage passes t rough zero value, on accountv of the rrelatively large voltage vrdrop that occurs in the series impedance at and subsequent to this linstant of time. According to the criterion. mentioned above, pronounced even order modulation effects can be obtainedfrom altube having this characteristic. By operating beyond the foot of the normal characteristic,

i a second abrupt bend in the opposite .direcfrom a modulator having this c aracteristic. Somewhat similar effects canv be obtained" tion can be made to occur at this point. Pronounced odd Vorder modulation eifects can be secured, as has been ointed out,

by the use of an'arrangement consistingof be secured by the use of oneisuch combination.

The symmetry obtained by using a push.

be utilized to produce odd order modulation Y side bands to the exclusion'of even order side bands, oi vice versa, with accompanying suppression of the impressed carrier orV certain of its derivatives. The use of this general arrangement or another arrangement including a vsingle tube circuit having the requisite symmetry of characteristic with an out ut filter, makes possible theproduction ci) higher order side bands with complete suppression yof. the components which could otherwise be used inv combination'with the received side bands at a distant station, b even or odd order deinodulation, to repro uce the signal.' In particular a single tube or a magnetic core modulator when larranged to have syinmetr i about a point suppresses, for example, t e double carrier. Since the .fundamental carrier may easily be separated by a filter from the resultant third orderv side bands the use of a modulator of this type can accomplish a result similar to that accomplished by -a vbalanced arrangement of tubes in second order modulation with carrier suppression.'

Systems using higher order modulation side bands of similar frequency characteristics may use the same order of demodula-A tion l,if the original carrier is transmitted' or,

'locally vintroduced at the receiver. If a and demodulation may be different.

If a lower third order side band vor a side band of `similar frequency character-y istics produced by other orders of modulaor other methods of modulation producing tion and the im ressed carrier are transmitted, and the ouble frequency carrieris suppressed, and if the frequency of the carrier is chosen close to the upper limit of the impressed signal band, secret telephone communieation is possible since an interceptor provided only with second order demodula-l tion circuits will hear inverted speech. 'A receiver provided with a third order demodulator which suppresses the even order side.'

bands would be able to reproduce speech to the exclusion of theiinverted speech. l

Harmonic multiplex communication is made possible by using one carrier sourceI and different orders of modulation for the -f Levanta 3 The invention both as to its theoretical principles and its practical embodiments will be better understood by reference to the following detailed description, together with the accompanying drawing, in which,

Fig. l is a graphical diagram illustrating certain theoretical principles of the invention. i v

Fig. 2 discloses a circuit for obtaining' odd order modulation with complete carrier suppression.

Fig. 3 discloses-a circuit for obtaining pronounced odd order modulation side bands with elimination of even order side bands and even power carrier harmonics, or conversely for obtaining pronounced even order modulation side bands with elimination of odd order side bands and odd powerl harmonics.

Figs. 4, 5, 6 and 6" are graphical diagrams illustrating certain theoreticalprinciples of the operation of the systems of Figs. 3 and 7,.

Fig. 7 discloses a modulator which may be used alternatively to that of-F ig. 3, to acF complish substantially the same result.

Fig. 8 discloses a typical one-'Way carrier current system employing higher order modulation and also one form of magnetic core modulator.

Fig. 9 illustrates graphically certain thei `oret-ical principles involved in the operation embodied .and to properly appraise its ad-l lvantages as applied therein, the following' of the modulator of F ign-8.

Figs. 10, 11, 12, 13 and 14 disclose other orins of magnetic core modulator, and ligs.A 15 and 16 disclose systems alternative to that of F ig. '8 for utilizing the principle of modulation of higher-order than the second or of methods of modulation producing side bands of similar frequency characteristics.

In order to aid in an understanding of the general nature of theinvention and to elucidate the characteristic ualities ofthe various specific systems'in w ich it maybe mathematical analysis of modulation, especially higher order modulation, is given.

If currents of different frequencies p and g are fed into 'a distorting (modulating) device', there results a complex currentv in which the frequencies have the general form mpi-ng, in which --m and 'n may. have any or all integral values, or zero values, and in which the symbol: indicates that the sum,

' the difference,- or both the sum and differ-A uantities may be present. This statement will. be proved later when ence, of the two the complete equation expressing the modulated current is derived.

When either of the coefiicients 'm or n has zero value, the current definedA by this expression is a direct current if the other coefficient is zero. If the other coeficient is instead of side frequency.

not zero, the current will have components the frequency of which is p or g, as the case may be, or various harmonics thereof. When neither of the coefficients has zero value there are obtained combination frequency components, that is, side bands. The order of modulation is conveniently given as the sum of m and lVhen both m and nare unity wel have the familiar case of second order modulation in which the side frequencies are pi -75 Third .order modulation may be correspon ingly represented by 21otg or 32129. There are accordingly four possible third order side frequencies. If r is a number expressing the order of modulation (that is, a number which is the sum of two numbers) it is evident that there are r-1 combinations'of different numbers' which add up to equal 7,-

so that, considering bot-h the sum and difference frequencies, there ma be a maxi- 85 mum of 2 (1f-1) different si e frequencies for each of the respective orders of modulation.

In what follows, it will be assumed that g represents a band of frequencies as, for example, a voice current (the so-called modulating current) and that p represents a fixed carrier frequency having a value greater than the greatestvalue of y. The expres-4 sion side band will accordingly be used 95 Also, only the side bands in which occurs once, that is, thosehaving frequencies pig, 23919, Bpiq, etc., will be consldered, since obviousl onl by the transmission of side bands of this type 10o can the modulated component be reproduced" l at the receiver by modulatinfr such lside bands with a wave having the frequency of the .unmodulated carrier or a harmonic theremodulation is obtained by substituting inthe general equation 'of the type y= am 6x2 -lom?l 115v` values of simultaneously impressed cur` rents (or potentials). ,Sup'posefthat the 4input currents' are? co'spit and -Q cos glt,

in which j), and g, equal respectively- 2n-y and 277g: cos gli?. (-No material change would result if an initial phase angle between'the two impressed Waves were assumed.) l This value of w should be substituted in the general equation.- The first term a w 125 yields merelyamplified waves of the impressed frequenciesv p and g. The term 6x2 yields waves of' frequencies 2p, 2q, and

pig, as iswell known. The second order l slde bands .pig result from the trigono- 139 A metric expansion of the vproduct bPQ cos I the binomial theorem,'it will be ound' that pj cos glt. and 2g result from trigonometric expansions of, respectively, IDP2 cos2 y pla and V bQ2 cos2 glt. f y

If each of the remaining termsof the eneral equation'are algebraically ex ande by the expanded equation comprises terms in powers of cos plt and cos gli, andv terms l which contain the productlcos plt cos g1t as a factor. The first two obviously yield waves whose frequencies' are, respectivel harmonics of p and g. The last obvious y,

yields combination frequency waves, that is,

' side bands.

Consi'deringfthese terms which produce c ombination- 4frequency waves,l it will be found that alternate terms, beginning with the second, in the expansion of even power terms of the general equationare of this' type and are further characterized in that the exponents ofcos p1t and cos g1 t are each odd'. These terms in vtheir regular order, u tov and including ,the expansion of the sixt power term of the general equation, are as follows, only those-coefficients which are necvessary in this discussion being retained:

The frequency determining quantities in these forms are each in the form cosine @1t cos gp5- or this quantity times one or more cosine squared quantities. Since cosa=1/+1/ cos 2a the development of these quantities each contains one term of theform of 1/2 cos plt .cos glt. This demonstrates that the terms of (l) each yields a pair of second order side bands. These side ands for theseveral terms-are superposed to comprise resultant side bands.

v The amplitude of each of rthe resultant upper and lower side bands may accordingly be expressed as a series the termsof which are proportional to PQ, PBQ, PQ, PQ3,.PQ,

A f P'aQs-:all being products of even order.

These terms will he multipliediby the co# efiicients b, d, f, etc.,`of the general equation.I

In general these coeicients decrease in mag -nitude asV the power of the terms of the general equation increases,'that is,y asl the order of the above products increases. 'the characteristic curve may be caused -to -be substantially square so that substantially-only the term PQ, which is linear in Q, is present. For other cases the other termsv will be presentV in relatively small amounts and will introduce somedistortion on account 'of nonlinearity of certain 'of the The harmonic frequencles 2pcoefficients in Q. However, a number of the terms as PQ, PBQ, PQ, etc., hence their sum, will be linear inQ. The magnitude of the distorting terms, that is, PQ3, PQ, PSQ,

etc., may be minimized by making P large as compared with Q, lso that substantial linearity may be obtained, as is necessary for distortionless transmission. This is true even though the characteristic curve departs, widely from its square configuration.

It may be shown, in a manner similar to the above, that other side bands of a diiferent eveuorder may be obtained from other-terms than those indicated in (1 in the expansion oftheeven power'terms of the general e ua-l 'tion. However,-as will be more evi ent later, only those of the form mpi g will have coefficients which are linear in Q and, therefore, useful. n s

From the odd power terms of the general equation odd order side bands-may be similarly obtained. This will be demonstrated for the case of third order side bands.

From the expansion of these odd ower p terms (except the linear term) it will be found that alternatev terms, beginning with the second, have even powers of cos4p,t and odd powers of cos glt. These terms 1n their regularorder, up to and including those for the seventh powerterm of the general -equation, are as follows, retaining, as in (1) only the necessary coeiiicients;

`ment of each of the terms in (2) contains a lll) term of the form cos 212,1? cos glt. -This form' is similar to the form cos pltcos q,t and 1n an analogous manner yields upper and lower side bands of 2p. This demon strates that the terms inl (2) each denotes a In fact third. order side band of the itype Qpig.`

The amplitudes of each of the vresultant upper and lower side bands may accord-- ingly be expressed as a series, the terms of which are pro ortional to PLQ, PQ, PQ, P2Q', PQs, 2 Q". The sum of these terms is linear under the conditions discussed above lfor second order modulation.

That is, when there is only the third power third order side bands ofthe type 'piQg may beobtained from othersterms than those indicated under (2), of the development of the odd power termsof the general equation. The quantities expressing the amplitude of these side bands is made upl of terms none of which are linear in Q, sopthat the sum is not linear and cannot be made linear in Q. This means that third order modulation of this type cannot yield a faithful reproductionv of the signal. The same thing is true of other higher orders of modulation, even or odd, in which n is greater than one. These side bands may, however',v

. be used in signaling where accurate reproduction ofv the modulating wave is not essential.

Side bands of a higher odd order than third of the type 'mpg can also be ob-A tained fro'mthe oddpower terms-of the general equation. `These side bands, as well'as the higher even order side bands of the same type, can be made substantially linear in Q.

Although telephone systems now in use depend upon second 'order modulation and demodulation, experience has shown that certain higher orders, especially the third, are substantially as suitable for the production of side bands and for the reproduction `of speech. As illustrating the practicability of using the higher orders of modulation itwas-recentlyfound that in an actual carrier current telephone system arranged for optimum second order modulation condican be demodu'lated to reproduce a signal by veither second or thirdorder demodulatiom tionsan'd for transmission of the unmodulated carrier component, the amplitude of the third order side bands could, by adjustments not affecting the conditions of second order modulation, be made twice as great as l that of the second It is apparent that theterm carrier fre-v quency must fbe re-deined for use in describingA systems of modulation of higher Aorder than the second. For example, a third order side band having frequencies Qpig,

depending uponwhether a current of frequency 2p' or p is available. Accordingly,`

depending on lthe order of demodulation,

" e'itherof these-two frequencies vmay play thel rle-playedl by the carrier frequency in a system using second order modulation (and accordingly second order demodulation). In

i this specification the terms carrier current" and ccarrier wave will be applied to any current or wave thatmay be combined in a distorting device with the transmitted Side band or side bands to produce a signal, and the frequency of such current or wave will accordingly be designated a carrier ,frequency. A modulated Wave ofa high order may accordingly have ascarrier frequencies both the frequencies of the' impressed high frequency wave and certain of its harmonics. The impressed high frequency wave -will be designated as the impressed origi- Conditions favorable to the productionA Fig. 1 in which A andB ara-respectively,

the Ec-Ib characteristic curve and the curve of the second order side band output current of a modulator recently tested. The modu-l lator circuit was'of the general type illustrated in U. patent to Vander Bijl, No. 1,350,752, issued August 24, 1920. A tuing-k sten filament carryinga current of 1.35 amperes was used. The plate potential was 220 volts.V The important thing to notice about this figure is that when the grid is given a polarizing potential of -18 volts, thev second order side band becomes substantially zero. This value of grid potential marks a point of symmetry of the characteristic curve.

This condition, which is unusual and difficult to obtain with oxide coated filaments, Was made possible byreason of the choice of filament material and the critical values of the constants used. The ,other `even order side bands would also be found to be substantially 4zero lif modulation occurred about the same point.- Although the odd order side band output is not shown in the curve, itis nota minimum at that point."

a point is. represented by a power Iseries,

this series will have no even. power terms,

fthat is, thel coeilicients b, d, f, etc.,lof the generalequation will-each have zero values.

Although it is 'not necessary, inorder' to -produce odd order side bands, to` operateA about a point of symmetry, such method of 4 operation makes possibleodd order modulation with suppression of even'harmonics of 20 p the impressed carrier frequency'and 'with suppression` ofeven order side bands since these components result from the termsin` lthe general equation having the vsame ex-l 'l lponents as the numbers indicating thev frequency multiple. This expedient is of value,

in its economy of energy, in the avoidance ofinterferencedue to the presenceat'the receiver o feven orderl side bands, and,v on account of' its secrecy. `Such aniysystemis relatively secret, since it insl'lrel tha Fig. 15 and which will be describedlater)l sup an Y' means of a llilter.- l Since the odd. order si e-..

the 'expedient insures, a -vgreater se arationi between message carrying side ban I t f' Economy: #nd Sel'fci-al'e--Pmmotd inf"- odd' order modulation y suppressionfhat is, .byV Suppressinsth 'im' ressed carrler as well as its-even harmonics. ismay-be' accomplihed, for example, by

'.suppressing'fthe. impressed carrier b bands are 'far removedfrom the impression carrier, separation" can easily be 'effected by ,the filter. -V Suppression of the carrierl frequencies may be accomplished' in this simple4 manner without resort -to rbalanced' arrangements of tubes.A This is of importancein instances where, ifthe usual second order modulating methods were to-v be used with carrier suppression, the carrier frequency would be too high to rmit separation from the second order side ands'without resort to` such balanced arrangements.4 Y

Where lt-is diilcult to provide a single tube having the requisite symmetry of characteristic to produce, exclusively, odd order Side bands, a push-pull arrangement-l of modulator tubes may be used to produce such an effect.v Fi 2 illustrates such an arrangement. In t is figure, p and q .indicate, respectively, the sources ofthe corresponding imfpressed carrier and modulating currents. I

same direction, and if coils 3 and 4 .are

wound in'relatively opposite directions, asA

indicated, this arrangement of-.tubes will produce odd `order. lside bands and will suppress the' modulated even order -side bands and odd carrier frequency harmonics.

` Filter Fmay be used to sup ress the impressed carnerand its even armonics up to that harmonic'whichcorresponds to the particular'orderof modulation used. For

example, if only third order .side bands 'were to be used the filter would be desi yto vsuppress only the second harmonic o the impressed earner, if fifth orderV side bands 'were to be used the filter would su press' the Vsecond andl fourth harmonics. v ither of thesefconditions would effectively correcomplete carrier su pression.

wind to s arrangement 4of circult, as we 1 as other balanced minuitarrangements to be described below, is' quite similar to certain 'l forms of push-'pullmodulators' discussed in U. S. patent to Carson No. 1,343,306, issued June 15, 1920, which also contains a general complete'. carrier ressmg the even harmonicsas above,.'

charge modulator, the

v device.

the ,tubes have identical char- -acteristics,if coils 1 and 2' are wound in the wel@ Alternatively',i`the two sources could be interehanged.; IThis arrangement would not v produce higher order side bands in the sense that this expression is used in this specification, lthatis,- o f the fornijmgvL-g. The arrangement .therefore would not bel useful vin h igherordermodulation.

Byr'ev'ersing either coil 3 or coill 4 in the arrangement of Fig. 2, even order vside bands ma 'similarly be produced.

or demodulation in which, depending o n the adjustment of .the quantities concerned,

pronounced even ...orodd .orderside bands. ma be obtained. l

ig..3 illustratesl a system of modulation ssuming for the present that the-switch which short .circuits the transformer 6. isl

closed so that this transformer is ineffective, circuit' 5 is connected on one hand to the 'source of currents to be modulated or delmodulated and on'the other hand through transformer 6 to devices 7 and 8. The

formerv may be' any type of' electric disatter a rectifierfor example, a two-electrode electric discharge Reference ynumeral 9 indicates a grid 'leak impedance for preventing too great an accumulation of negative charge on the grid of tube 7;. This impedance a resistance in the'case illustrated) is preferably high, for example of `the order of a megohm. The output currentl from; the combination may be used as'required in circuit 10.

The theory of the operation of the circuit lll() is best explained with reference to Figs, 4, 5

and' 6. Curve abc of Fi .`5 is characteristic curve of tube 7 plotte between grid-filament potential and plate-filament current.

If the rectifier tube were not present so that the potentials would be impressed directly between the rid andfilament of tube 7, the variations o impressed otentials would occur over the ortion bc o the curve 'in tlfe usual way an sincethis portion of the curve exhibits ,only slightvariations of impedance, the form of the plate-v ilament current would be a substantialre- .production of the form of the impressed potentials.

With the rectifier in circuit, the characteristic curve is modified, so as to assume the form bd, over the operating range. The platefilament current, plotted on a time axis will be as indicated in Fig. 4, if a sine wave of impressed potentials is assumed. In this ligure, the time periods represented by intervals between 1, 2, 3, 4, etc are equal.

the non-horizontal portion of the characteristic curve were straight, the non-horizontal portion of the curve of. Fig. 4 would have a sine Wave configuration. With the' characteristic disclosed, this portion of the curve nevadas- Ar p 1 i Aofi-tig. -4 is somewhat less pointed thanl a" sine wave curve. -Accordingjto thej'criterion establishedabove, the` modification of the impedance varies by a very large factor, one

hundred in a known instance, when the polarity of the potentials .impressed are reversed. When positive half waves are impressed on the combination of tubes, that is, when the grid of tube 7 is positive` with respect to the anode-element-of tube 8, agridlament current tends to flow in tube 7 since i its grid is positive relative to its filament. The impedance of theinput circuit will correspondingly decrease. The total potential-l will divide according to the respectivel impedances of this input circuit and of the rectifier. The result will be that substantially al1 of the applied potential is impressed in eiect across the rectifier. The potential impressed across the input of the tube.7 will correspondingly be` very small or zero and the plate-lament current will uniformly, throughoutthe half cycle, have the value corresponding to zero grid. potential. When negative halfwaves are impressed on the combination of tubes, the conditions are the converse. The rectifier will have substantially zero impedance, the input circuit will have substantially infinite impedance, and the potentials will divide accordingly. This condition corresponds to the normal condition of operation of modulator tubes with negative grid potential and the operation will therefore occur over the normal portion b ofthe characteristic. y

If the variations of potentials are sufficient to cause operation beyond the foot of the characteristic curve so that the modified characteristic assumes the form e a o d, the conditions are favorable to the production of odd order side bands. Fig. 6 indicates the corresponding plate-'iilaxncntcurrent curve plotted to a time basis. In this'curve time periods indicated by intervals between I points l, 2, 3, 4, etc., are equal, as in Fi 4.

It is.v apparent that'. the modified c aracteristic would be changedif the maximum amplitude of the impressed potentialsl were changed, since this would result in the oper-- ation over a greater or less extent of the horizontal portions of curve e al o d or, otherwise stated, Iit would result'in the inclined portion of the characteristic curve lbecoming relatively greater or smaller. The corresponding effect `on the current-time curve would be to change the slope of the sides of the curve. An increase in the maximum amplitudewould cause the curve4 to approach a rectangular configuration. A rectangular configuration would be characternent curves. Fig. 68,

iaedbyl symmetry between consecutive half t waves, since the time periods corresponding to the intervals between points 1. 2 3 4., etc., and 1, 2, '3, 4, etc., would all be equal. This limiting condition, which corresponds to an infinite maximum impressed potential,

defines 'a situation in which there is an entire g absence of even-harmonics (if amodulating wave is superposed with the carrier wave on the -modulator, an entire absence of even order side bands). The'corresponding com pletesymmetry inthe characteristic curve`- which should define this condition, is that becomes relatively `zero as compared with the infinitely extending horizontal portions.

-whichis attained when the inclined portion I so Byv using a moderately largel amplitude of.

carrier potential, this y limiting condition may be -quite .closely approximated.

A symmetry not dependent on the amplitudes of the impressed potentials, may be obtained by adding the other circuits of Fig.- 3 shown between 5 und 10, as by opening'the switch which short circuits transformer 6.v

There is no mutual inductance between the two sets of transformer windings 5. 6 and 6a so that the .openingy or closing of the switch does not aEect 'the operation of the transformer 6.` The prlmary and secondary windings of transformera should be oppositelyrelated as compared with the corresponding windings of transformer 6. This result may he accomplished by reversing lthe direction of winding of one of the coils 'of transformer l 6l with respectto the corresponding coil of transformer 6.

The `addition of the lower set of circuits'will result in a currentl indi` j i cated by curve ea/ b d of Fig. 5 in the lower half of the primarytransformer during the time in which the current in the upper half is as indicated lby curve e a o d of the saine figure. The curve of resultant current in the primary, and ac-l cordingly the form of the currentwave inl the secondary, of the output transformer, with reference to thepotentials -a'p liedto the combined primary windings o? of the output 'i transformers 6 and 6a, is the sum of these curves.

n order to avoidunduly complicating the' figure this curve is omitted. -It is apparent, however,

straight about the origin andwould have i.

pronounced bends' at the ends, if the operaf tion extended that far', vcorresponding lto the bends at the feet of the respective compo-r Figs. 4 and 6 illustrates the current .in the primary winding of the output transformer y.for this method of operation. Because of which corresponds to .115.- that .it would `be substantially' y windings of the primary of the out ut transformer are in aiding relation'. I they are oppositely wound, even order modulation side bands may be produced to the exclusion of odd order side bands and odd power carrier harmonics. p

A' result similar to that'obtainable by one pair of the sets of tubes of Fig'. 3 can be obtained by substituting for the rectifier of` Fig. 3, a resistance in the grid lead of a single three-electrode tube, suoli as resistance 11 of Fig. 7. If the potential variations across the secondaryottransformer 6 (Fig.

7) causes the potential of the grid to tend toA become positive, the potential drop in resistance l1 will modify this portion of the characteristic curve so as to cause an-abrupt las ' bands.

bend like that between b and al of Fig'. 5.

Although' electric discharge deviceshave so farbeen considered the number of kinds of devices that may vbe used for higher order modulation or deii'iodulation is limited only.

by the number of those capable of modulation generally. The method in which modulation is effected by varyingthe -inductance of a coil by correspondingly variably saturat-ing its magnetic core, is especiallyrapplicable t0 suppressed carrier, odd order, modulation. Fig. 8 illustrates one modulating arrangement of this kind and also a system in which it, or any equivalent means, may be used.

In Fig. 8 numeral 12 indicates a modula-l tor of this type which consists of a toroidal core wound with a single coil. The use of the condenser shown in dotted lines is alternative and will be referred to later when considering Fig. 12. The function of modulator 12 is to distort the currents flowing therethrough in the mannerre uiredto prodescribed in U. S. patent to Blackwell, No.

1,261.096, issued April 2, 1918.

Modulator 12 suppresses even-harmonics of the impressed carrier and even order s ide The modulated wave is impressed by transformer '16 on filter FB which selects thel side bands of the order desired or "a single side band ot that order. The transmittedv currents are impressed on the transmitting conductor 18 through three-winding` Y transformer 19.v Device 17, which may be similar to 15, maybe usedl if desired, to

.regulate the amplitude of` the transmitted side band. -It may equally well be used at the receiving station. 'Circuit 18 may be a transmission line connecting two communicating stations or pair of radio antennae,

lone at the transmitting station andA another at the receiving. station.

Carrier current for demodulation is sup' plied throughv circuit 20, the switch 123 being closed, to points in the transformer 19 which' are balanced with respect to the modulated wave input circuit. 21 balances the line 18. The amount of carA- An artificial net-work n l l rier current energy required tor the par ticular modulating and deniodulatiiigg; condition may be critically adjusted by device l22. The use of 'the three-Winding transf` the distant stations serve to receive and uti-- transmitted thereto. Assume, for the instant, thatswitches 125 and 126 are open.

ylVith the particular arrangement so far described, since the carrier is transmitted, deinodulator DM must be arranged for deinodulation of the saine order as the order of modulation used at the transmitting station. ny one of the specific types of vInodulators or demodulatorsvdescribed in this specification or their equivalents in function may be used. i j

The signal component is separated vb low pass filter FL and impressed on telep ones 23. this and other figures of the drawing may be of .any type capable-of vthe functions indicated. U. S.- patent to Campbell, No.

llize the wave impressed on conductor 18 and rl'ilie filters shown in several places in 1,227,113, issued May 22, 1917, describes varions types of `filters' from which selection transmission characteristic desired. i

The operation of the magnetic core modulator of Fig. 8 will be explained by refer- .may be made 'according to the particular i ence to Fig. 9. The curvefand g in thisy figure are the familiar B-f tween the magnetizing force H and magnetic induction B. .Curve f is the -mag-l netization curve for positive variations of` H which would be obtained if a magnetic core, as, for example, the toroidal core in the magnetic modulator, having no initial netized by positive values of current. The

' residual magnetization, were variablyv mags.V

curve g/ would be similarly obtained if neg-- ativev values of magnetomotive force were used. Curves f and g, taken together, ex-

hibit perfect symmetry about the origin.

The permeability of the core is. measure the ratio of B and H, that is, by the slope- 'Y ofthe chords of the magnetization curve between the origin and the referencepoints.

masias The curves which-express the absolute values of permeability corresponding to'curves f and g have approximately the shaperespec-y tively of curves z, and z', which obviously anceof the magnetizing winding, the ing ductance varies 1n accordance with these curves. If the variable.saturation of* the toroidal core with variable positive land negative values of -impressed potentials, is assumed to follow these curves, the characn teristic curve 'between the" impressed potentials and. current -owing through" the coil would be obtained by dividing these positive and negative values by theordinates of z and e'. y would correspondfor example, tothe characteristic curve of Fig. 5, and would deterY mine the modulating properties, of the cirn cuit.

The curve would obviously havevsymmetry o the same type as that of curve f g, that is, it will have symmetry. about a .point. Accordin ly, under the assumed conditions odd vor er modulation would l be achieved with suppression of the even order side bands and of even power harmonics of the impressed carrier'frequency.. This result 1s achieved wlthout the use of a polarizing means.

As a practical matter, the variation of under conditions assumed above of variable a the symmetry ofthe loop insures that these permeability curves are symmetrical.A Accordingly, the loperation of the magnetic core modulator, even when there is -h s!v teresis, has the desirable features 'of odd order modulation with suppression of the even order eiiects.

' Of course, the hysteresis loop disclosed .would result only if the magnetizlng current has not-.more than one maximum and minimum per cycle, as for example, a sine wave current. In the actualcasethe magnetizingA current consists` of superposed carrier an modulating current, and the resulting wave lform accordingly has numerous irregularities and reversals of slope. For'this case the hysteresis loop disclosed illustrates, for example, only the uniformly recurrent loop corresponding tol the carrier current.y The irregularities due to the modulating current would properly be shown as both large and The resultant characteristic curve l may be considered broadly as a variation of small hysteresis loops having their origins ilu the loop disclosed.4 f

It is. desirable to-use a core whichjsaturates ata small value of magnetizing current so that the characteristic curve has pronounced bends at its two extremes, and accordingly so that there arejmore pronounced high. order modulation. eiects. From anf other point of view, the use of; a saturated .core makes possible the-production of .highv A I `order effects with the ,smallv valuesotim'` must often be used. l v

Although in thel above the effect of varia'- tion of inductance has been described, the corresponding variation of leffective resist. ance contributes,"A although usuall to a smaller extent, tothe result, so thatt e eiect pressed' potential, 4that for y'practical reasons,

im edance.

n practice, 1t has aoy ssi"

the core markedly improves the operation of the device. This metal is an alloy containing two y elements lof the magnetic group, such group beingv made vup'of manganese, iron, cobalt, nickel and copper.l In its usual form the metal is an alloy of nickel and iron.

Whatever its specific composition, it is dis#` tinguished by its pronouncedmagnetic propv i ertles and especlallythe remarkably low value of magnetomotivepfor -"2715e"quired to' produce saturation. These magneticiproperties depend j-ulargely onf-thespecial heat treatment whiclrislgiven to :the alloy-*during its process of manufacture."-'-'A` 'A' Emagnetic core modulator, made of a core; und`-with a ribbon ofy thatmaterial, the wholegicoilbeing a proximatelythe size of ningen-ring, was" ound tov operatein the system'o Fig. 8,-w1'th substantially the same eic'ienoy and to give substantially A as 'good reproduced y speech as. wlth' the'use of the conventional. I carrier currentv systems using vacuum tubes' l and second order modulation. .Referencefis madeto a'paper by Arnold and Eline'n in the Journal of the llranklinInstitute for May,1923,'forafdetaileddescripuonof this -alloy and its properties.

' AlthoughnFg- Slthe ptenti'als'areshown .Y i I' `impressed on the modulator in series, the'y' may equally well be impressedin parallel, the

particular arrangement of circuits depending upon the impedances of the lters outside the transmission bands, andtherefore being dev'termined'by the laws governing the eilicient connection of circuits generally.I In general filters having low impedance outside the -v band shouldI b efconnected in series. If the impedances outside the bands'are high, theyl should be connected inparallel. A- conductive instead of an inductive connection be-V tween the modulator circuit and filter FB may be used; 'I4i'gs. 10y and 11 illustrate tir-rangements; which cnductivs tonnes' modulation; and ,des instit tions sie liittd,-. anti iiliistjftte further the of lseries and. parallel connections of circuits. figures illustite; the case' of 'eithermodulatin 6r demodalstjion and.

also the use. of the .same fof both .Fer this interesse, ex' ien-tymnyufge thegnse' t @deferment-designed" i one,

-. 'signin wave tmpiitude inail-sw sgml-.wave frequencygandtheaotheg .thetn. mater,-

" ing the arrangement' lfi Vi beingJ desifned'for low modulated wave' ntilde en higiiiiiiiaituted ime- .lgreqllend Modulationjis' here 'eil' mary *windingY as in theabove age). Further, the capacitive' vbe vt "ix'1creaae .-thea'liux de`ns ity .for a oad .ful 'the ecient operation ofthe lnodutoligb'yfle'asonA of the'sel'e'ctive characteristic of thev -circuit referred tothe'priinaryiif seconda '.is tuned .toy resonanceA the', carrier 1 u'enc. The 's'electivecharactenst ic t at' this feature..

The modulating carrier vand side band -currents, or certain of Y to flow through separate .coi son the core, Vas. in Figs.y 13 and 14.l i

appropriately denotedv longitudinal magnetization, since the dunes' resulting,"l`

from the two magnetomo'tive forces follow arallel, that is, the same paths. Ity ,has n found that a. similareif'ect isl produced bywhatma be called cross v"magnetization Iin -whic the windings are 'so arranged 'that 'the two fluxes. tend to lbejno'rinaljto eachother. The lthree circuits'v for. the

`modulating, 'oarrie'r, and 'sid'e band currents may be otherwise related in eitherof the ways above described.' .use of. cross magnetization has an advantage, in a modulatoror a demodu1ator,. that mutual indue# tion between the 'modulating and carrier cir` cuits 'may be made substantially zero', there-y fore avoiding the necessity 'of the use of filters in these circuits.' In*l a' combined Insta statista* itititaiiiitafwiiiafi .l

qluency characteristic, filter` Fn described, together.' with the varying 'mutual'. e (varyingle secondar` load, by reducing the demagnetf'" 'izing e ect of ,the mndsrygeufrenajnsy' .be' similarl theinJna even be caused modulator anddemodulator, however, only the carrier filter vmay be omitted. The effect Thear'rangemen may ibe; varied .considera ly.. from that shown.. -Forlexample, the unmodulated car- -rvier maybe lntlfoducedat the receiving sta- 'tion-,f-.fin'stead' 'off being transmitted thereto vfrom thev transmitti i .source 12'?,v switch' 1 5 being c osed. The carrier. may, of course, betransmitted from station, b means of the transmitting station by an independent mit-modulated current of a particular'fremay be transmitting Vconductor` 18. If even order demo'dulati-on' isr used corres onding. even harmonics ofthe unmodulate carrier may transmitted or locally introduced at t e receiver.

t ofi-,thas stem of Fig.. 8f

For example, the Asignal may be reproduced by second order -dc'modul'ation if ythe double carrier 2p is "'ci'rcuitQ When 1t is not important'to transself-inductanoe and' "of the i-.p'n-ir used. The 'double'y carrier may be conveniently derived from source 14, is by thecir- 'cuit described in U. S. patent to Kendall, No. 1,446,752, issued February 27 '1923, by ning' 'switch 123, so that current from circuit4 v20 is supplied froni source 14 through harmonic generator GH. It may be derived from double carrier frequency stliur 124 at the receiver, switch 126 being c ose A` substantial vdegree of secrecy may be obtained with the system of Fig. 8 by producing lalower third order side band, or a side bandhavingsimilar frequency characteristics, su pressing the double carrier fre- '.The examples `of magnetic coil modulatorsdescribed'above illustrate what 'might method, that is, second order demodulation,

HNI

llo

will give an audible frequency component represented by (2p-q)p which 1s substantially inverted speech. Any 'signal obtained by incidental third order of modulation will be effectually masked by this inverted speech. Only by using a demoduator which gives no second order side band as. for example, by using the magnetic demodulator of :the type shown in Fig. 8, may undistorted speech be obtained. The same princi lc lmay be applied if other orders of modu a tion and'demodulation are used.

Multiplex signaling may be achieved will: the system of Fig. 15 by using different orders of modulation, or at least using except vfor the specifc features of the present invention, may be similar to that of conventional multiplex carrier current systems, thesystem is illustrated diagrannnatically'. Referencenumerals 24 and 25 indicate two transmitting' circuits of a multiplex carrier system.' The corresponding' receiving cirl cuits are indicated by 26 and 27 and 28 indifrequency carrier through filter F2.

cates the line circuit. -Low frequency signals from lines 29 and 30 modulate carrier c urrent derived from source 31 in modulators M1 and M2. The former produces second, the latter third, order side bands or side bands of similar frequency characteristics if produced by other methods which are selectively transmitted by liltcrs F1 and F2. The transmitted side bands are selectively received by filters RF1 and RF2 and demodulated by demodulators DM1 and DMB, the former being of the second, the latter ofthe third order type or of other types producing similar demodulation products. If the un modulated carrier is transmitted, it may be utilized in the usual manner in the respective demodulators. It, however, may be supplied toboth demodulators, or to either, by separat-e local source 31. or amplilied `carrier from demodulator DM1 may be supplied to demodulator DMZ, 'through filter F2. .Demodulator DM2 may, if desired,be of the second 'order type and be supplied with doull course, other orders of demodulation may be used for the two channels disclosed. Additional channels using additional orders may be used. In particular, even order modulation (odd harmonic carriers) may be used for all the channels. If the harmonic frequency carriers are transmitted second order demodulation may be used, which provide-sa simple means for deriving from a'single/carrier source the various frequencies used in conventional multiplex harmonic systems. Since the carrier'frequencies are odd militiples of the basic carrier frequencv they cannot produce interfering effects by intera ctien, as would occur if even multiples were used.

The result achieved is similar to that of a multiplex harmonic system emploving the usual second order modulation. However, besides the advantages which inliere in high order modulation as pointed out above, the svstem as a whole is characterized by a sim-- plicity which is not found in other harmonic systems. especially on account of the avoidan'e of the use of harmonic generators.

Fig. 16 illustrates the use of a similar principle in a two-way signaling system in which the frequencies for the two directions are fixed by a single source. The two Oneway channels may be joined to a single line in each station by aconjugateconnection 'as shown. i The channel identified by'M TF1, 28, RF DM1 may be identical with the correspondingly labeledr channel in Fig, 15.

Similarly channel M2, TF2, 28, RF2 and I DM2 may be identical with the other chancurs in tie opposite direction. Source 81 supphesthe current for,l thefirstchannel nel of Fi 15, except that transmission 0c.

and demodulated`carrier for demodul'ator *I DM2 may be selected by .filter Fzand supplied to the second channel.l Obviously the two-way channel of. Fig. .16 may be duplicated to constitute a multiplex signaling system having allof the features of the system of Fig. 15.- l

Although, of the higherorders, the application of odd order modulation has been emphasized as compared with' even order,

modulation. this' is only because lthe range of odd. orders of modulation includes the lowest of these even higher orders. The amplitude of the `side band of the several orders in gene-ral varies 'inversely' as the order. as has been explained. This means practi^ally that in general, :third order modulation is -to be preferred. A second reason for the emphasis on odd order modulation is that this type of modulation can conveniently be accomplished to the exclusion of even order modulation, a result frey quently desired. ,The converse result, thaty is` even` order modulation -to the -exclusion of odd order mod-ulation,is not readily accomplislied.

It is realized in view of the similar physi'al operation involved in modulation .and demodulation, thatthere should be a single Word `toinclud'e v both or* either .of these operations. Accordingly, in the claims the word modulation will be understood, where applicable, to cover vboth modulation' and demodulation as ordinarily used.

tically the .same frequency val-ues vas the high order side bandsproduced'by.themethod de scribed in this specification :may'be,producedA i It is also realized that vside 'bands of lideni,

cessive steps performed in `di-iferent.modul lating devices. For example, side 'bands of vfrequencies Qpfl-fg .may "be produced by doubling the carrier .frequency lin -one step i of second order modulation ,and then isepa.-

ratelv combining Y.the wave of `.this double carrier frequency with 'the wave of frequencies f] inthe same yor `anotherfdevice byA another step of second order modulation. This process lis yold .in the prior Vart and' differs fundamentally froml'thisfinvention as hcreinbefore described. Consequently, it is" vnot-'desired to claim.' herein as newper se rocessof,ol"v means4 for pr ucing side andsv of ,thej form mping, mtl-'n greater than 2, vwhere theirproduction invo v devices; although -the inventive idea illustrated in Figs. and lincludes-the promethods and Jbylother 'methods capable Vof producing" side bandsof` similar frequency produced', are -alike' in j their quency mp.'

Since a be combined with` either the original carrier fixed freor any multiple thereof,"depending. on the l' onthe same distorting medium, and utilizingusefull band order of demodulationused, 'a word is similarly required toi express' fthese concepts. Consequently,v the word multiple will be used in the claims in a generic sense to include the single as well as4 plural multiples unless such an interpretation .would be inconsistent with. the context." 25

What is claimed i's:

1. The method hich comprises jointly impressing 'a carrier wave and a signal wave on the same distorting medium, and utilizingr one or more' of the resultant sidebands of higher than the second-order. f 2. Thef" ethod lwhich comprises jointly y impressing acarrier wave and a signal wave resultant modulated componentscorresponding to at least one order of modulation higher than the second. i

The method, using a distorting device which vhas yan input-potential-output-current y l characteristic, the general equation of which includes'odd power terms, which consists in jointly limpressing waves of diireent frequencies on said device and utilizing theresultant combination `frequencies of higher -order than the second resultingifrom said odd order terms. 4. The method using av distorting device which consists-in jointly impressing waves of different frequencies on said device, combining said frequencies in a single step -to 4 produce side-bands of higher order lthan the second, and leading oi from said device -and employing currents of saidv siderequencles. 5. The method of signaling which comprises jointly impressinga carrier wave and ,a signal 'wave on the same distorting med1um,.transmitting the resultant modulated components having higher order thany second, and demodulating said components 'to reproduce the signal.

f 6.l The method'of signaling which com-v A prises jointly impressing a carrier waveand a sional-wave on a 'd istorting'deviee, transmitting at least one side band corresponding "to at least one order of modulationhigher es successive separate steps of modula- 'tion or distortion yin the sameor ldiii'erent.

however highI order modulated wave .mayy

'prises generatlng by hlgher order modulaband with a wave whose frequency ris 'amul- ".tiple of the unmodulated carrier frequency in a distorting device, toreproduc'e the signal.

7. In a systemincluding a single distorting aling Vwhich comdevice the method-.of si prises generatingl by higher j order modulation and transmitting a modulated wave 'y having lthe frequencies mping, in which fm.

and -nare integers, at least one of which has a value greater than unity, is the unmodulatedcarrier frequency, and g is the modulating frequency.

8. he method of signaling which com tion a modulated Wave having frequencies mpi-g in which m is an integer'having a i value greater than one, and p and g are rerespectively theunmodulated carrier and modulating frequencies, transmitting said '-wave, and combining it at the receiver with a wave having a frequency which is a submultiple of m to reproduce the modulating wave.

9. In a system including a single distort. ing device the method of signaling which comprises generating by higher 'order modul lation and transmitting a modulated wave having the frequencies 2pig, in which p and g are respectively the unmodulat'ed carrier and modulating frequencies. l0. The method of signaling which `Colnprises generating a modulated wave of the third order, transmitting said wave, and

modulating it at the receiver with a carrier r current' to reproduce the modulating current.

11. The method of signaling which comprises generating modulated side bands of the third order, transmitting said side bands, transmitting a wave of the. original carrier frequency, intermodulating said transmitted third order side bands, and indicating the third order side band which corresponds to .the modulating wave.v

13. The method of signaling iwhich com- `side bands and carrier wave in such a manprises generating modulated side'bands of the third order, transmitting said side bands, intermodulating at the receiver said transmitted. side bands with a wave having double the4 original carrier frequency inl such a manner as to produce second lorder modulatedv side bands, and indicating the second ordery side 'band which corresponds to the modulating wave. i

14. The method ofv signaling which comprises generating and transmitting a modulated wave 'having only the frequencies mpig, in Which p' and g are respectively the carrier and modulating frequencies and 'm is an integer greater than one, and reproducing the modulated wave at the receiver by combining said transmitted wave with a carrier wave.

15. The method of signaling which comprises generating a carrie;1 wave, generating a modulating Wave, producing odd order modulated side bands of said carrier wave, andat the same time suppressing the'even order side bands and even power carrier terms, suppressing the unniodulated carrier and transmitting the resultant waves.

16. A secrecy carrier signaling method which consists in generating and transmit.

ting a Wave having only the frequencies 2pg and p, in which g `is the frequency of the modulating current and p is the unmodulated carrier frequency, which is so chosen that p-g is within the audibleirange, pro- -ducing third order modulated side bands from the received currents of said frequencies, and indicating that side band which corresponds to the modulating current.

1 7. A multiplex` harmonic carrier trans-l mission. system comprising, means for gen erating a carrierwave, a single modulating device individual to ea'ch channel for modulating a portion of said carrier Wave with one of a'plurality of signal waves to pro- 5' duce, with all of said devices, a plurality of sets of side band waves of the type mpiq in which m is different for each such set of side band waves, means for transmitting said side band waves, and means at thereceiver for combining said side band waves with a multiple frequency carrier current to reproduce the respective signal currents.

18. A multiplex harmonic carrier transmission system comprising mcans for generating acarrier wave, a single modulating device individuall to each channel for' modulating a portion of said carrier wave lwith one of a plurality of signal waves to produce, with all of said devices, a plurality of sets of side band Waves of the type mpi g in which m is different for each such set cf side band waves means for transmitting said side band waves together with va carrier wave whose frequencyV isa multiple of theoriginal carrier frequency, inclu ing such frequency; means at the receiver for-modulating one of said setsof side band waves with said multiple frequency waves to re-v produce one signal wave; means for selecting afportion of the amplified multiple fre quency wave fromthe modulated output of said modulating means, and means for reproducing other signal waves by .modulating the other side band waves with said selected multiple frequency carrier wave.

isV

bands with some of the unmodulated carrier to the other station, means for modulating said side bands with the unmodulated carrier at the other station to reproduce the signal, means for selecting a portion of the amplified carrier resulting from demodula. tion at the other station, a single modulating" means for intermodulating said selected carrier and a signal wave at the other station to produce side bands having a fixed frcquency which is a multiple, greater than one of the Iixed frequency of the wave 'receive at the other station, transmitting at least one of the last mentioned side bands to the iirst station, and means for modulating the side bands received at the first station with a portion of said generated carrier wave.

20. A signaling system comprising a plurality of one-way channels for different transmissions and means whereby'- each channel employs 'an order of modulation different from the others.

21. A signaling system comprising two one-way channels for different transmissions each employing a different order of modulation. 22. A duplex system in'which the oppo sitely directed, channels employ different orders of modulation. n 4 23. A signaling system comprising, a. plurality of one-way channels, a carrier waveA ico".

is different from the corresponding xed frequency of the side bands in thelother l channels anda multiple of the frequency of vsaid carrier wave includingv said carrier (wave. f

. 24. A signaling system comprising two one-way channels and a carrier wave source,

each ofsaid'channels comprisino' a 'singlev modulating device for mtermodui-atmg said carrier waves with signalwaves to produceA -side bands the fixed frequency of which is different from that of the corresponding side bands in the other channel and a multiple of saidcarrier frequency, including the frequency of the carrier.

. 25. The method of modulation which comprises impressing the modulating and carrier waves uponla distorting medium, and

controlling the medium to have a sharply f curved input-voltage-output-current characteristic, whereby pronounced high order modulation side bands are produced.

26. The methodof producing odd order lv.side by aA distorting device which'con- V said y'double curvature;-

sifstsinimpressmg the carrier 'and modulat ingI waves o'na' distorting device, the y'characteri'stic curve between the'input andA out- -put quantities of which has a double curva- ,'even'order modulation effects, using a disopposite directions and which are symmetritorting device, which consists in impressing the carrier and modulatin'g'waves on a'distorting device whose operating characteristic has two pronounced bends in relatively cal about a--poin-t in the curve, and operating uniformly .over said bends inbothdirections from said point.

28."'1he ,method of producingodd order which consists in variably saturating the ance with` modulatin f magnetic-core in accordance with impressed alternating carrier and modulating currents,

and variably inducing alternating potentialsin 'jaccodance with said vari-able saturation.

29,]Themethod of producing odd order modulation side bands using a magnetic core, 'a magnetizing winding, and an inductor winding, which consists in varying the permeability of the core by and in accordcurrents flowing in the magnetizing winding, through maximum values injtheltwo directions, and thereby invariableE. M. F.s in sai Vinductor Win ing.

30. The method of ymagnetic core modulation using a'magnetic core and a combined magnetizing and inductor Winding which consists in varying the permeability of the lcoreyby and in accordance with carrier and modulatin currents flowing through said winding, t rough maximum values in the two directions, and thereby correspondingly varying the impedance of said Winding whereby odd order modulated side band currentsv are caused to flow in said winding.

31. A magnetic core modulator comprising in confibinationa toroidal magnetic core, a magnetizing coil therefor, means for caus- 4to a maximum ineither direction.

modulation side bands using a magnetic coreing carrier and modulating cuirents1to flow in said coil, the values of said currents an the magnetic proportions of said core being 'i such lthat the core is periodically substans4 .ti-ally saturated for opposite directions of flux, and an inductor circuit wherein there` is induced variable E. M. \F. 's corresponding to the variable saturation of said core.

32. `A 'magnetic core modulator .compris. ing in combination a magnetic core, means whereby carrier and modulating currentsproduceva magnetic iux in said core, and4 means whereby said currents together periodically vary the permeability -of said core 33. The magnetic core modulator specified in-cl'a-im 32 in which the magnetic core Y tied in claim 32 in which the core is of a magnetic material comprising two elements of the magnetic group.

35. The magnet-ic core modulator "specif fied in claim 32 in which the core is of a magnetic material comprising nickel and another member of the magnetic group.

36. The magnetic core modulator specified inclaim 32 in which the core isof a magnetic material comprising nickel iron.

37. The magnetic core modulator specilied in claim 32 in which the core is of a magnetic material comprising nickel and another element of the magnetic group, the nickel component predominating and the material having a permeability higher than that of iron at low magnetizing forces.

38. The magnetic core modulator specified in claim 32 in which the core is of a magnetic material comprising nickel and iron in which the nickel component predominates and having a permeability higher than that of iron at magnetzing forces of the order of a few tenths of a gauss.

In-witness whereof, I hereunto subscribe my name this 26 day of December, A. D.

and

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