US1613951A - Carrier-wave signaling system - Google Patents

Description

JQs, JAMMER CARRIER WAVE SIGNALING SYSTEM Filed Nov. 5,- 1924 Jan. 11, 1927.

/n vemor Jacob idammer: by Afi'a Patented Jan. 11, 1927 UNITED STATES 1,613,951 PATENT OFFICE.

JACOBS. JAMIVIER, OF NEW YORK, N. Y., ASSIGN OR TO WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

' CARRIER-WAVE SIGNALING SYSTEM.

Application filed November 3, 1924. Serial No. 747,513.

This invention relates to carrier wave signaling systems and more particularly to terminal arrangements for associating the sending and receivingbranches-of one or more two-way carrier wave signaling circuits with a common transmitting line or medium. I

One object of the invention is to provide circuit arrangements for such a system, whether of the single or multiple channel type, which combines relative great ciliciency, simplicity, and low cost with adequate protection against the various types of interference which are incidental to the operation of such systems.

In a common case, justification for the installation of a carrier wave signaling system, and particularly that type which utilizes the transmission of high frequency carrier waves over wires, and hereinafter called "a carrier current signaling system, arises under conditions where the traflic over an existing low frequency system is such that prospective increases in tratlic cannot be pro' vided for by a mere proportional extension of its facilities, but would require a more or less complete reorganization of the system. The situation can frequently be bet ter and more economically taken care of by the installation of a carrier current system. If there is no existing signaling system, the prospective traflic must be sufficient to justify the considerable first cost and operating cost for such type of carrier wave system or of a carrier wave. signaling system of the other common type in which etheric space instead of a conductor is used as the transmission medium.

A carrier Wave signaling system and especially one associated with a line wire has usually connot-ed a multiplextsystem, and

properly so; in other words. the use of a single channel system (single channel as used here and throughout the application is intended to define a two-way circuit, that is, a complete conversational unit) has-required the use of circuits and devices that could be used in common for a plurality of such single channel circuits, so that considerations of economy has urged the unitary use of a plu 'al'ity of such circuits in a single multiplex system. The practical result has been that the installation-ofjcarrier wave systems has had to be postponed to the time when-the"installation of a multiplex system would be justified. I

A further object of this invention is to make the economy and other advantages of carrier wave signaling systems realizable in smaller units than hasheretofore been possible, and specifically to provide a single channel carrier wave signaling system in which the efliciency of operation, first cost, sinq llcity, etc, approximates that of an equivalent single channel circuit comprised wlthin a multiplex carrier current. system.

Another object is to provide a single channel system which satisfies. the requirements of the above object and which is adaptable to extension into an equally eflivention as embodied in a multiplex system,

a plurality of two-way carrier current communication channels having the unmodulat ed carrier suppressed are associated with a common transmitting circuit between 0pposite termini, channels, and the portions of said "channels used for each direction of transmission, being distinguished from each other by the use of different frequency ranges. Each two-way circuit -(channel) utilizes at each terminus a single translating device which combines the functions of an oscillator, bal

anced modulator and balanced demodulator. The carrier frequencies for each two-way circuit are the same for the two directions. However, to provide for transmission of different frequencies in the two portions of each two-way channel, such as is required for the individualization of the functions occurring in such portions, the modulated side bands used in one direction are of the type the individual twoway' represented by pig, in which p is the carrier frequency, 1] is the modulating frequency and the'symbol i indicates that the sum, the difference or both the sum and difference, and modulated waves having frequencies indicated by Qpig are used for the opposite direction. The translating devices at the termini accordingly utilize the same basic carrier frequencies for modulation but transmit different selected modulated products and correspondingly when they are being used as demodulators.

One'way in which -side bands of the type Qpig may be produced in a single translating device is by the process known as third order modulation. The corresponding process of reproducing g from such side bands is known as third order demodulation. In order to use these processes in accordance with this invention, the translating device at one tcrminusis adaptable to produce, by modulation, second order side bands of the frequency pig and also to act on received third order side bands of the frequency 21919 to reproduce by third order demodulation the modulating frequency 9. The

translating device at the other terminus functions in the same way except that the modulating and demodulating processes are accomplished with reference to the complementary frequencies, since the received frequencies at one terminus corresponds to 'thetransmitted frequency at the other terminus. I

The use of a single channel of the multiplex system described above, insures the most eflicient use of material for the required functions and makes possible the practical realization of the benefits of multiplex carrier current transmission in a single channel unit. On account of the feature ofusing widely different order transmissions for different directions, such a single channel circuit may be considered as being on a group frequency basis. Such a single channel system is accordingly adaptable to extension into a multiplex system using the group frequency arrangement. In extending the system in this manner, the additional side bands transn'iitted from one. terminus will 'be comprised within the frequency interval between the carrier frequency of the first mentioned circuit and the double carrier frequency corresponding to its one-way return channel.

A particular advantage of such a resulting multiplex system not realized, for example, in another type of system using the group frequency-arrangement in which the various carrier frequencies both for transmission and reception at a terminus are derived by harmonicgeneration from a single base frequency, is the resulting increased spacing between the frequency correspond-- mg to the one-way channels having the higher frequencies. For example, where third order side bands are transmitted in one direction and second order in the other, the spacing between the former frequencies, that is, the higher ones, will be twice that of the frequencies in the latter, the lower frequencies. The system accordingly provides in a simple manner a spacing arrangement which approximates the desired ideal in which the spacings are progressively. increased by equal percentage increments, such Variation 0 spacing being necessary in order to make the most eflicient use of the frequency selecting means in the system.

The invention both as'to its principles and its practical embodiment will be better understood by reference to the following detail description taken with the accompanying drawing in which: v

Fig. 1 illustrates diagrammatically a complete single channel system adaptab e to extension into a multiple channel system of the group frequency type the terminal portions of additional channels of the system as extended being indicated;

Fig. 2 illustrates in detail one form of the circuit OMDW and OMDE of Fig. 1 which embodies a common translating device which functions as a balanced'self-oscillating modulator-demodulator and Fig. 3 illustrates in detail a. circuit alternative to that of Fig. 2 which utilizes a separate local carrier source.

In the description which follows the elements of the system will be identified, as far as practically possible, by designations which are suggestive of their respective functions. For example, in the drawing the labels OMINV, TFlV, RFVV and HFWV will be used to identify respectively an oscillatormodulator-deniodulator used at the western terminus of the system, a filter used for transmission at the western terminus, a filter used for reception at the western terminus and a high frequency band pass filter at the western terminus. Elements which are duplicates of each other functionally, so as to require the use of the same literal designations but which are differently located or utilize different frequencies,.wil'l be distinguished by the use of different subscripts.

Referring to Fig. 1 the circuits at the left of this figure are related to one terminus, designated for convenience as the west, of a single channel or multiplex system and the circuits at the right of the figure are correspondingly related to the other, the east, terminus of the system; The circuits at the respective stations are each connected to the conductive transmission path 1. The system is composited for simultaneous low frequency and carrier current communication. This is accomplished by carrier composite sets comprising low frequency filters LFW and LFE and carrier frequency filters HFW and HFE, The first two are positioned in the low frequency telegraph or telephone branch, and the last two in the carrler current branch, of the transmission line. These filters accordingly effectually separate the currents corresponding to the frequencies used in these respective types of transmission. Obviously, the system could be used for carrier current transmission alone, in which'cas'e these filters could be dispensed with. i

One complete single channel system is disclosed. This channel comprises the following elements which may function in like sequence in a two-way conversation initiated at the-west terminus; low frequency line 2, oscillator-modulator-demodulator ()MDlV, transmission band pass filter TJWV, groupingtransmission band pass filter TFVV, the high-pass filter HFlV of the composite set, the common transmission line 1, the high .pass filter HFE of the composite set at the eastterminus, the grouping receivingfilter.

RFE, the receiving band pass filter RI E, the OMDE'and the low frequencyline 3.

Let itbe assumed that second order modulator side bands, which may be represented asabove by pig, are used for transmission east. The operation of this channel is as follows: The low frequency modulating currents in line 2, which for example may be connected to a central telephone ofiice, modulate a carrier curr nt in circuit OMDVV, which as will appear more fully from the description of Figs. 2 and 3, may embody a unitary balanced self-oscillating modulatordemodulator or a combination of a balanced modulator-demodulator and a separate local source. The resultant'modulated side bands of frequencies pi are transmitted through filters T FW, TFWand HFVV to line 1 and then to the east terminus where they are transmitted througlrfilters HFE, R'FE and'R FE 'tb circuit OMDE which may be simi1arto circuit OMDW where they are combined by demodulation with a locally generated carrier wave to reproduce the modulating wave 9, which is transmitted over low frequency line.3. For the opposite direction of transmission, a similar sequence of opcratioris occurs. However, for this case the circuit OMDE functions as a modulator instead of as a demodulator. Correspondmgly the c1rcu1tODMW functions as a dcmodulator instead of as a modulatofi Further, for this direction of transmission, side bands of the frequency Qpiq are used. These side bands are accordingly produced by modulation in the circuit OMDE and are impressed upon-circuit ()MDW to reproduce byjd'emo'dulation the$modulating wave 7,

There are at least two ways in which side bands of frequencies 2pig may be produced. For example, it may result from third order modulation or dem0dulation.

oscillator modulator demodulator 6P cos p t and 6Q? 0082 91 stead of the even power terms in the general equation of this curve.

The following is a theoretical demonstration of third order modulation or demodulation. It must necessarily involve much of the basic theory that underlies modulation generally, including second order modulation.-

.In order to prove that various multiple order side bands occur-and to determine the extent to which they occur, an extension of the method of analysis used in UJS. patent to Carson, Serial No. 1,449,382, issued March 27, 1923, will be used.

According to that method a statement of the current (or potential) resulting from modulation is obtained by substituting in the general equation of the type values of the simultaneously impressed currents (or potentials). Suppose that the input currents are P cos 17,25 and Q cos 9 15, in which ,0, and respectively, equal 211- and Qwg, so that mzP cos p,tl-Q cos g t. (No material change would result if an initial phase angle between the two impressed waves were assumed). By substituting this value of a), the first term are yields merely amplified waves of the impressed frequen cies p and q. The term (m yields waves of frequencies 2p, 2q, and pig, as is well known. The second order side bands pig result from the trigonometric expansion of monic frequencies 2 and 29' result from trigonometric expansions of, respectively,

If each of the remaining terms of the general equation are algebraically expanded by the binomial theorem, it will be found that the expanded equation comprises terms in powers of cos 77,25 and cos 9,15, and terms which contain the product cos pg? cos 7,1

as a factor. The first twov obviously yield waves whose frequencies are, respectively, harmonics of and 1 The last obviously yields combination fifei'luency waves,that is, side liands. Considering these product terms it will be found that those contained in the expansions ot' the odd power terms (except the linear term) of the geiieralequation produce o'dd order side band si Specifically,'it will be found that alternate terms, beginning with the second, of the expansion of each of these odd power terms of the general equation have even powers of cosine p 1: and odd powers of cosine- 9 2f. These 'terms in their regular P Q cos p t cos g t PQ cos p t cos q t' P G} cos p t cos g t PQ cos p 25 cos g t P Q cos 10,2? cos 9,25 I Q' cos p,t cos 9 The frequency determining quantities in these terms are each of the form cos p fi cos g,t times one or nrore cosine squared quantities.

Since cos a: cos 2a, the development of the portion cos p cos g t contains the term A; cos p cos q ,t. The product of this term with the development of the remaining cosine squared quantities results in a term made up of the product of this term and Accordingly, the development of each of the terms in (1) contains a term of the form cos-21M cos q t. This form is similar to the form cos ),t cos t and in an analogous manner yields upper and lower side bands of 27). This demonstrates that the terms in (1) each denotes a third order side band of the type Qpiq.

The amplitudes of each of the resultant upper and lower side bands may accordingly be expressed as a series, the terms of which are proportional to P Q, PQ, P Q, P 62 PQ, P QF. In order that the side band x-orrrponents may have amplitudes such that the modulating wave which is reproduced by demodulation may be the same as the original modulating wave used in the modulating operation, the sum of these coetlicients should be linear as to the quantity Q.-

In this connection it should be noted that such sum would be linear in Q, except for the last three coefficients which contain higher powers of However, these last coefiicients correspond to the higher power terms of the general equation. In general the c'oetlicients decrease in n'ragnitude as the power of the terms of the general equation increases. In fact the characteristic curve may be caused to be substantially cubic, so that substantially only the coefficients P Q which is linear in Q. is present. Consequently, for practical purposes, the sum of these coefficients may be made linear in Q. Any distortion due to the presence of methci'ents containing higher powers of Q, may be minimized by making P large as com-- pared with Q.

Although third order modulation may be used, as described, the method of the invention resides in the production of side bands havinga certain frequency characteristic regardless of the particular physical operation by which they are produced. Another method of modulation and demodulation which can equally well be used to pro of the carrier is produced by distortion accompai'rying the oscillatory function of the device, the resultant double frequency car rier resulting from a principle similar in all theoretical aspects to second order modulation. The side bands of frequency 239:9, are then produced by second order modulation involving the modulating wave and the wave of the double carrier frequency.

As a practical matter, thetwo sets of operations may go on simultaneously so that it is ordinarily difficult, or impossible, in a specific case to determine the relative parts played by each of them, although by special design one type of rrrodulatron can be caused to result, to the substantial exclusion of the other.

It should be noted that for either method of modulation, the translating devices oscillate, at least in their fundamental modes, at the same frequency. If third order modulation is used to the exclusion of cascade modulation, only third order demodulation or cascade demodulation may be used at the other station, since third order modulation per se is not attended by the production of a double carrier frequency component, which, if present and if transmitted, could be utilized at the other station in second order demodulation to reproduce the modulating wave. If cascade modulation is used and the double frequency carrier is transmitted, a simple type of second order demodulator, not requiring the use of locally produced oscillations, could be used at the other station. In the operation of the specific type of circuit disclosed, it is contemplated that third order modulation and demodulation will be used, the original carrier 1) being suppressed by the balanced type arrangement. for reasons of economy of energy and of frequency range utilized.

By reason of the fact that in the operation of the single channel system as described, widely different frequencies are used for opposite direction of transmission, such a system can be considered as being on a group frequency basis. This means that the system is adaptable to extension into a multiplex system merely by adding other similar single channel circuits, the frequencies transmitted in one direction being adjacent to that of the corresponding carrier of the single channel system described and the third order sidev bands of the additional channels used for transmission in the opposite direction being similarly arranged adjacent to the corresponding third order side bands in such single channel system. Each rier frequency from west to east may be 7,000 cycles, 10,000 cycles and 13,000 cycles, the corresponding double carrier frequencies for the opposite direction being accordingly 14,000 cycles, 20,000 cycles and 26,000 cycles. If these particular frequencies are used there is no overlapping of the carrier ranges for the two directions. However, in order to prevent an overlapping of the side bands at the inner boundaries of the two groups, it is necessary to transmit only the upper side bands from east to west and only the lower side bands from west to east. Other arrangements of carrier frequencies than those mentioned will obviously be suggested, in some of'which it would not be necessary to transmit only one side band,

although as a practical matter it is always-in the interest of economy of energy and of frequency range to do so.

Grouping filters TFV, 'RFW, TFE and RFE'can be dispensed with. However, they are almost universally used in multiplex systems of the group frequencytype, their use being justified on account of their contribution to the ease of impedance matching in various parts of the system, which matching is necessary for the most eflicient transfer of energy between sections of the circuit having different impedances. The filters l-lFlV and HFE may be similarly dispensed with on occasion, although their use is justified by reasons similar to those above. The group frequency arrangement is only one of several alternative frequency arrangements that are commonly used in carrier current communication. When such an arrangement can be used, balancing circuitscan be' dispensed with entirelyand frequency selec- ',tion alone relied upon to separate the fretinguished from the ordinary type of crosstalk which measures the interference between receiving circuits where there is insufiicient discrimination between a wave intended for a particular receiving circuit and that intended for an adjacent receiving circuit, and as distinguished from side tone, which measures the interference betweenthe transmitting portion of a two-way circuit and the receivingportion of the same. In situations where multiplex carrier systems are now justified, it is found that the group frequency arrangement is ordinarily preferable to these alternatives.

This invention as so far disclosed therefore provides a single channel carrier current system which is readily capable of extension into a preferred form of multiplex system and one which embodies a frequency arrangement which approaches more nearly to the ideal in which the frequency spacing progressively increases by ,equal percentage increments than other multiplex systems of comparable simplicity or first cost.

tion of the circuits OMDW and OMDE, it 1 will also be obvious that, by reason-of the economies effected by the use of elements having combination functions, the single channel system is remarkably simple, efficient and inexpensive. The invention as awhole, accordingly, to a great extent provides means for realizing the economies of carrier wave communication for a wide range of trafiic conditions.

- It should be noted that in transmission from the West station any unmodulated carrier that is not completely balanced out can be suppressed only by adjusting the cut-off frequencies of the filters TFW, etc., so that the filters discriminate against this frequency 9. It has been found to be a difiicult mat- .ter to suppress this carrier without at the same time distorting the side hands. This difliculty of course is not specific to the system illustrated, but is common to all suppressed carrier systems using second order modulation. The use of a transmission of the type 21219, as in transmission from the east station in the system illustrated completely'avoids this difficulty on account of the different orderv of spacing of the carrier 12 and the side bands 211:9. Asa result it is possible to extend the transmission range of the filters to transmit all the side band frequencies equally, with attendant avoidance of distortion of boundary side band frequencies.

Fig. 2 illustrates the details of onecexample of the translating device OMDW or OMDE of Fig. 1 This'translating circuit consists of a self-oscillatory-modulator-demodulator arranged to oscillate at the carrier frequency 30 that is used for both directions of transmission. It functions as a modulator to combine this frequency with a As will appear from the followingdetailed descripwave having the modulating frequencies q to produce side bands of frequencies pig] or 2piq, depending on the particular one of the two translating devices that is referred to. It correspondingly functions as a demodulator to combine the incoming side bands, whether characterized by a fixed frequency p or 2 with the carrier p,,to reproduce the modulating wave q. As has been pointed out in the description of Fig. 1, the modulating and demodulating functions involving the fixed frequency 2 may result from either or both of two quite dissimilar physical operations. However, in the particular example illustrated in Fig. 2. by reason of the balanced arrangement of circuits the modulation must be of an even order such as second order modulation on the double carrier as above described that is, although third order side bands can be produced in the output circuits of the individual tubes, they cannot be transmitted to the circuit related in common to both tubes. In addition to its functions as pointed out above, the translating device also, by reason of this use of balanced arrangement of circuits, insures that the output currents do not contain the unmodulated carrier component. If the balance were not complete, if there were any 'diss'ymmetry of the tubes or their associated circuits, or if a single tube were used, third order modulation 'or demodulation could be used, although, of course, with a sacrifice of the advantage of completely suppressing the unmodulated carrier component.

The common translating circuit is shown as comprising a balanced vacuum tube circuit including the two vacuum discharge tubes 4 and 5 which may be of the usual cathode-gri'd-anode type. The cathodes of these tubes are connected together and to the midpointof the secondary of the input transformer 6, the grids of the tubes being connected to the respective opposite tenninals of the secondary. The cathodes are also connected, through tuned feed-back circuit 7 andthrough circuit 8, the function of -which will be described later, to the midpoint of the primary of the output trans-' former 9, and thence through the two halves of this primary to the anodes of the respective tubes. Space current -is supplied to these anodes from direct current source 12, through circuit 8 and the two halves of'the primary of transformer 9. Condenser 10 and high'frequency choke 11 insure that the space current and the high frequency feedback current are confined to their proper paths. The common portion of the grid circuits included between the cathodes and thesecondary of input transformer 6 concuits of the balanced arrangement, and the secondary winding 14 which similarly cou ples circuit 15, the function of which will be described later, to-this common input circuit. This common portion of the grid circuits may also include grid polarizing battery 16, the grid polarizing potential being impressed on the respective grids through secondaries 13 and 14 and the two halves of the secondary winding of transformefG. A grid resistance 17 may be inserted, if desired, in each of these grid polarizing paths. The circuit 15 provides a path whereby the incoming modulated side bands are impressed on the translating devices, Circuit 8 similarly provides a path for impressing the resultant demodulated. low frequency currents on the low frequency line 2. Filter- 18 in this circuit insures that the current thus impressed will not contain demodulated components-other than that corresponding to the modulating frequency Q. Since this filter is of the low pass type, circuit 8 provides. a conductive path for the space current. I

v It is a characteristic of a properly balanced circuit. of the type illustrated in this figure that the output coil constituted by the primary winding of transformer 9 and riations impressed on the common portion of the grid circuits are repeated in amplified form into the two anode branches which includes the common portion comprising tuned circuit 7 and circuit 8 and the portions 1ndividual to the two anodes comprising the respective halves of the primary winding of transformer 9. By tuning the circuit 7 to the desired carrier frequency, the translating circuit may therefore be caused to oscillate independently'at this frequency, since the output energy in the tuned circuit is returned in part to the input circuits, where it is again amplified. Since the path which the oscillations follow is, as above stated. neutral with respect to the input and output circuits connected to transformers 6 and 9 the waves locally generated are confined to the translating circuit and are therefore prevented from passing into either of these circuits.

The operation of the translating circuit a modulator and demodulator will be clear from the following description of the path taken by currents in the course of transmission and reception, with relation to the path of the oscillating current.

Modulating currents of frequencies 9 (if telephone communication is assumed or of a single frequency if telegraph communication is assumed) from low frequency circuit 2,

are impressed on the primary winding of transformer 6 and, on account of the connection of the secondary to the grids, cause the potentials of the grids of tubes 4 and 5 to vary in an opposite manner to each other,

to transformer 9 as variations in the ampli-' tude of the high frequency current produced by the translating device by reason of its oscillatory function, which high frequency currents themselves are balanced so that they do not appear in'the output circuit. As long as controlling currents are impressed on the device upper andlower modulated side bands of a high frequency por 2;; are produced in the, output circuit. When the devicefunctions as a second order modulator, these modulated side bands will have the frequencies 10:9. If-the device functions. as a cascade modulator the side bands will have the frequencies 21719 and similarly if, by

reason of an incomplete balance or the use of only one of the tubes, third order side bands may be produced. Other components of modulation, and the amplified modulating currents, appear in the output circuit of the device but are prevented from flowing in the transmitting circuit 1 by the high pass filters between the device and said circuit and illustrated in Fig. 1.

Incomin side bands which pass through the high frequency recelvlng filters illustrated in Fig. 1 are impressed through c1rcuit 15 on the common portions of the grid circuits. These side bands are combined with the carrier frequency p in the same manner as if currents of frequency p were.

' tentials impressed on these common portions of the grid circuits, the common portions of the output circuits are not similarly balanced. Therefore, combination frequency components resulting from interaction of the carrier frequency p and the sideband frequencies in the tubes appear inthis common portion which includes circuit 8. By reason of the fact, and to'the extent that, the individual portions of the output circuit are balanced with respect to the common portion, frequenciesimpressed on the common portion of the grid circuits and their com binations are therefore superposed in additive relation in circuit 8. Filter 18 suppresses all of these components except the desired reproduced modulating wave of fre- "the tubes.

conducting properties of the tubes, these quency q and transmits this component to circuit'2. Outgoing modulating currents which flow through filter 18 are not transmitted to the circuits connected to the secondary of transformer 9 on account of the balanced relation of the output circuits of By reason of the unilaterally currents are prevented from flowing to the input c rcuits of the tubes to set up a sing- .ing condition of the circuits related to the translating device.

It should be-notcd that in addition to the economies effected by the use of a common translating-device-for the three functions,

other economies are effected on account of the particular relation of portions of the circuits related to the device. For example, a portion of the feedback path is used in common for that purpo'e, for transmitting the demodulated low frequencies, and'for providing a path for the space current from source 12. This considerably simplifies the circuits by elimination of details otherwise required for relating these circuits to the anodes of the tubes. Al"o it should be noted that when ,the balanced tube arrangement is used for modulation, the modulating wave is impressed differentially on the grids and the resultant modulated 'wave is similarly derived diflerentially from the anodes while, when the same tubes are rred for demodulation, the common portions of the gridcathode'and anode-cathode circuits are correspondingly used. This means that the balancingarrangements which insure carrier suppression during the modulating operation are also utilized to-in'ure that the input and output circuits are balanced with respect to the incoming side bands so that these side band currents cannot flow in the low frequency circuit 2 or be transmitted without demodulation to the output circuit connected to the secondary of transformer 9. Although a common translating device V is used for the modulating and demodulating functions the circuits as a whole which are used for these respective functions are therefore effectively distinct and individual.

This relation of the circuitsobviates the necessity for the H79 oflow pass filters in circuit 2 or somewhere in th system between circuit 2 and circuit 15. t

Fig. 3 illustrates a circuit that can be used alternatively to that of Fig. 2 in the system of Fig. 1. This circuit differs principally from the circuit of Fig. 2 in that the translating device is not self-oscillatory, aseparate local carrier source being used. Only so much of the circuit of Fig. 3 as differs materially from that of Fig. 2 will-be described in detail. The portions of the circuit which are functionally similar to the corresponding portions of the circuit of. Fig. 2 are correspondingly labeled.

The carrier frequency used for modulation and demodulation is supplied from source 19. The circuit 20 which supplies the carrier from source 19 is connected to the opposite terminals of resistance 21 which is included in the common portions of the grid circuits. The'incoming side band currents are supplied to these common portions through the same circuit. Hybrid coil 22 renders circuit and source 19 conjugate to each other with respect to the common circuit and therefore insures that carrier and side band currents can be impressed in common on the translating device, but are otherwise confined to their individual circuits. The design and function of such hybrid coil arrangements'are now well understood in the art on account of their common use in repeater circuit arrangements. If this, or

an equivalent arrangement, were not used, carrier current would, to some extent, be

transmitted through the receiving high pass filter shown in Fig. 1, to the transmission line 1, since these receiving filters can discriminate betweenthe carrier and its side band only with difiiculty. This would be particularly true at the station where the incoming side bands are of the type pig. At the other station, where the incoming side bands would be of the type 2pig, the filters would ordinarily be adequate to suppress the currents of the frequency 1). In the circuit of Fig. 3 the space current for the two tubes. instead of flowing through the circuit carrying the demodulated low frequency, as in Fig. 2, are directly impressed on the primary of transformer 9 through the primary winding 23 of transformer 24L which is used for coupling the common portion of the anode circuits to low frequency circuit 8. This means is alternative to the corresponding means of Fig. 2 and the two expedients are equally applicable to the arrangements of Figs. 2 and 3.

While the invention has been illustrated as embodied in a limited number of forms it will beunderstood that it is equally applicable to other'forms and to variations in details of the form illustrated without departing from the spirit of the invention as defined inappended claims.

In the claims a pure modulated Wave is a modulated wavewhich contains no unmodulatedcomponent. It may consist of one or both side hands. a

What is claimed is: p

1. A two-way single-channel carrier wave signaling system, characterized by having a common means at one station for generating and transmitting a carrier modulated wave having a fixed frequency characteristic of frequency p .and for demodulating incoming carrier waves comprising single side band components exclusively and-having a.

fixed frequency characteristic of frequency 2p to reproduce the signal.

2. A. two-way single-channel carrier wave signaling system, comprising a common means at one station for generating and transmitting a carrier modulated wave having a fixed frequency characteristic of frequency 2p and for demodulating incoming carrier waves comprising single side band components exclusively and having a fixed frequency characteristic of frequency 1) to reproduce the signal.

3. A two-way single-channel carrier wave signaling system, comprising a common means at one station for generating and transmitting a. carriermodulated wave having a fixed frequqncy characteristic of frequency p and for demodulating incoming carrier modulated waves comprising single side band components exclusively and having a fixed frequency characteristic'of frequency 27) to reproduce the signal, and a common means at the. other station for correspondingly generating and transmitting a carrier modulated wave having a fixed'frequency characteristic of frequency 2;) and for demodulating the incoming carrier modulated waves having a fixed frequency characteristic of frequency p to reproduce the signal.

4. A two-way single channel carrier Wave signaling system comprising means for generating and transmitting in one direction carrier modulated waves comprising single side band component-s exclusively and having a fixed frequency characteristic of frequency 127,, means for generating and transmitting in the other direction carrier-modulated waves comprising single side band components exclusively having a fixed frequency characteristic of frequency n, the frequencies m and n having a commensurable relation, characterized by an oscillator-modulator-demodulator means at each terminal, common to both directions of transmission.

5. A multiplex carrier wave signaling system, comprising means for generating and transmitting in one direction a plurality of modulated waves, "each having frequencies of the type pig and each having a different frequency p, and means for generating and transmitting in the other direction modulated waves, each having frequencies of the type 2p lq, in which the values of p of the Ill) different waves is thesame as for the waves signaling system comprising at each "sertion a source of modulating waves and a self-oscillatory modulator-demodulator device comprising a single pair of electric discharge devices for generating and transmitting a pure modulated wave and for demodulatinga pure modulated wave incoming from the other to reproduce the modulating wave, the device ht one station being adapted to generate a modulated wave the frequency of whose fixed frequency characteristic is p and the device at the other station being adapted to generate a modulated wave the frequency of whose fixed frequency characteristic is 2p. o

7. A multiplex carrier wave Signaling system, comprising means at each station for generating and transmitting a: pure modulated waves and for demodulating in pure modulated waves, the frequency of each of the fixed frequency characteristics of the 2m transmissions being difi'erent from each of the others, said means at each station comprising sources of locally generated high frequency waves.

8. A multiplex carrier Wave signaling system, comprising means at each station for generating "and transmitting a: pure modulated waves and for demodulating a: pure modulated waves, the frequency of each of the fixed frequency characteristics of the 2m' transmissions being different from,

each of the others, said means at each station comprising a: functionally independent devices adapted to be used in common for generating and transmitting a modulated wave and for demodulating an incoming modulated Wave.

9. A multiplex carrier wave signaling system comprising means at each station for generating and transmittin a: pure modumodulated waves, the frequency 0 .eac of the fixed frequency characteristics of the 2m transmissions being, different from each of.

the others, said means at each station comprising for each pair of transmitted and received waves, a common high frequency generating, modulating and. demodulating means. V

10. A multiplex carrier wave signaling system, comprlsing means for generating 'and transmitting in one direction a plurality of modulated waves, each having the frequencies p559 and in which the carrier frequency differs .for. each one-way channel, means for generating and transmitting in the other direction modulatedwaves each having frequencies 2p::g,flin which p has the same yalue as for theop osite direction of transmission, the frequencles of the waves transmittedin'each direction taken together being comprised within a continuous indi-' vidual range, each said signaling channel comprising at each station a common-source ure 11. The system of claim 10, in which each I signaling channelcircuit comprises a common translating device adapted to function join'tl as an oscillator-modulator, and clemodulhtor.

' 12. The system fof claim 10, in which the modulated waves transmitted in one direcquencies transmitted in the other direction are substantially 14,000+q, 20,000+g and 26,000+q cycles.

' 14. The system of claim 10 in which each single-channel circuit comprises a common translating device adapted to function jointly as a local carrier oscillator, modulator, and demodulator, in which the frequencies transmitted in one direction are substantially 7 ,000-g, .10,000-- and 13,000-/] cycles, and in which the requencies transmitted in the other direction are substancycles:

15. A combined balanced-modulator-demodulator, comprising in combination a pair of electric dischar e translating devices, each having a catho e, an anode, and a controlling element, means associating the cathode-controlling element circuits including a common portion and a portion individual to tially 14,000+g, 20,00 +q and- 26,000-j-q each controlling element, means associating the cathode-anodecircuitsincluding a common portion and a portion individual to each anode, the im edance constants of the devices and of t e two associatin means being such that the controlling e ements and the anodes are each balanced with. respect to currents impressed on the common cath-,

tion's of't-he cathode-anode circuits, means I JACOB s. JAMMER.

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