US1565091A - Wave-transmission system - Google Patents

Description

Dec. s, 192s.

E. V. GRIGGS WAVE TRANSMISSION SYSTEM 4 Sheets-Sheet l Original Fileq lfeb. 1'7. 1922 -8. um E S MN :E mwuhw Zoom oomh. s NN 2 a; L Q w .n 2 2 E om@ a mm1 com aoco. mmi @Si w QW .www mh E www@ E awww 232 -OSL-AML 20Go. nQo- Room- ODE Illv Boom.

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E. V. GRIGGS WAVE TRANSMI S S I ON SYSTEM- oriigl Filed Feb. 17. 1922 4 sheds-sheet 4 White Plains, in the county of YVestchester,

Patented Dec. 8', 1925.

UNITED` STATES PATENToF-FICE.

ELMER v. GRIGes'oF WHITE PLAINS, NEW YORK, AssIGNoR To WESTERN ELECTRIC COMPANY, INCORPORATED, OF NEWYORK, N. Y., A CORPORATION OF NEW YORK.

WAVE-TRANSMISSION SYSTEM.

Application mea February 17, 1922, serial Ne. 537,156. Renewed :une 2, was.A

T o all whom it may concern Be it known that I, ELMER V. Games, a citizen of the United States, residing at State of New York, have invented certain new andl useful Improvements in -Wave- Transmission Systems, of which the following is a full, clear, concise, and exact description. -i' v l This invention'relates towave transmis-v sion systems andmore'particularly to transmission of the energy of given'waves or impulses by means of waves of'lower frequenc than those whose energy is to be transmitte An important eldot' electric transmission is that involving the, use of unidirectional or alternating currents'modified in certain of their characteristics, such as continuity, intensity or frequency. In this field, which includes electric signaling and' other teledynamic electric operation, efficiency from an ener standpoint is important but is frequengtl'y much less important than faithful transmission of the wave forms involved.

" In tllecase'of telephony there is involved transmission'of a wide rang of essential frequencies and for certain circuits, such as those of submarine cables, the distance to which such transmission may be had is limitedbecause of the dierence in transmission.-vl

ot the various frequency components involved. Various expediente have been re sorted to in an attempt to improve the trans; mission of such circuits for the higher fre-f quency components and to equalize the at-v` tenuation experienced by the different fre- V quen'cy components. These expediente in general involve change in the characteristics of the transmission circuit itself or special terminal apparatus associated therewith. From a practical standpoint loading of the,-

circuit, particularly in the case'of submarine ponente after transmission to the same -p'ortional amplitude which they had before circuits is feasible only for limited distances. lnductance loading which is commonly employed for such circuits also imposes an upperfrequeney limit'upon the transmission range o f the circuit lower than that possessed by the unloaded circuit. Attenuation equalization may be resorted to in lieu of loading in order Vto reduce each of the comprotransmission. This requires elaborate teri' minal apparatus and high amplification and lis moreover subject to the difficulty that any disturbance or interference is likewise highly magnified in the amplifying system. The

ymethod of the present invention attacks the art, such-r as loading and attenuation equal-` izers.

Since transmission of speech involves waves extending throughout a wide band of frequencies, it is necessary, if the intelligibility andA naturalness ofthe speech are to be preserved to make'the transmission such as to retain each of the initial frequency elements. `Where the highest essential frequency elements exceed the upper -limit of the frequency transmission range of the circuit, successful transmission thereover is ,not possible. An Object ofthe present invention is to effectively reduce the frequencies of the various components of a band of currents so as to make them fall within thetransmission range of commercial transmission circuits. f Y

-' With this general view of the problem involved the principles kof the invention may be outlined. Assume that a band of currents v rangingifrom a definite lower frequency up 'to a fixed upper frequency is to be transmitted. According` to `the present invention there is subtracted from the frequency of each component of the-band. the frequency of the lowest frequency component. Thel band remains. 4of the same extent but its lower` frequency limit has become zero and.

its. upper frequency limit'has beenv so reduced that the bandmay fall within the transmission range of a circuit which could not successfully transmit 'the original band. After transmission over` the circuit the frequency of each com onent is augmented b v the original lower imiting frequency and the components are thus restored to their original frequency position.

If the original band, `which it is desirable or necessary to transmit, is not continuous, but consists lof a number of smaller subbands separated by blank or unessential frequency intervals. the operation upon the lowest frel i quency subband will obviously be to subtract from the frequency of each ofits components the original frequency of its lowest frequency component. This operation reduces the first or lowest frequency subband to a range from zero to a new upper limiting frequency. There may be subtracted from the frequency of each component ofthe second or next highest subband a frequency sufficient to reduce its lowest frequency component to approximately the new upper limiting frequency of the first subband. The third subband is similarly reduced in frequency so as to make its lowest frequency approximately that of the highest frequency of the reduced second band. This procedure is re eated with each subband in turn and the filial result is a continuous band from zero to a fixed upper limit. This continuous band may be transmitted and, after trans-v mission, maybe separated into its various subbands, and each subband may be stepped up in frequency to its original frequency position.

It may happen with the original band reduced in frequency in the manner just described toa continuous band extending up from` zero frequency, that its upper limiting frequency is still outside the frequency transmission range of circuits commercially available. In this case instead of reducing the entire band to one continuous band, each of the original subbands may be treatedindependently and reduced in frequency by the original frequency of its own lowest component. yThere vwill result a number of subbands each extending from zero frequency up and each may be transmitted over an individual transmission channel or circuit. The number of subbands and channels will be the same and. if this number is.

made suiiiciently'large the highest frequency of the original band may be reduced to any desired degree. After transmission over their separate channels each of the various subbandsmay be stepped up to its original frequency position, and the bands may then be reassembled -to reproduce .the original band.

The result of modulation of a carrier wave by waves of a band of frequencies is to produce two so-called side bands. See, for exam le, the article entitled Carrier current te ephony and telegraphy by Colpitts and Blackwell, pages 307 to 310 inclusive, Journal of the American Institute of Electrical Engineers, Volume 40, April 1921, No. 4.-. Of these, the upper side band may be considered as the original band of waves having' the frequency of each of its com` gonentsincreased by the carrier frequency.

ince instead of a high carrier frequency there may be impressed upon the modulator a frequency changing wave of any definite frequency it is obviously possible to increase the effective frequencies of each component of the band by this definite amount. If the frequency ofthe wave applied to the modulator is lower than that of the lowest component of the original band, the lower or difference frequency Side -band resulting from the modulating action may be considered as the original band with its component frequencies, each reduced by the definite frequency. There are accordingly available simple methods of stepping up the frequencies of bands of waves or stepping them down at will.

The operation of both modulators and demodulators may be looked upon as that of frequency changers. The modulator operates to change the fre uency ,by the frequency of the so-called carrier wave applied thereto. The demodulator in general changes the frequency of a band of modulated wave components by the frequency of the unmodulated carrier wave component which is usually present with the side bands. In both instances upper and lower side bands are produced by the frequency changing operation and it is necessary to select the desired side band by means of filters or other selective circuits.

If in addition to producing the sum and Vdifference frequency components the frequency changer were to transmit components of the original wave frequencies as they occur before combination, there might be confusion if any components of the original waves fell within the same frequency range as the desired combination frequency components or side bands. In order to prevent this, use may be made of modulators and demodulators of the so-called balanced type disclosed and described in connection with Figs. 20 and 49 of the article Carrier cnrrent telephony and telegraphy. The characteristic of these balanced combining devices or frequency changing devices is such thatl they may be -made to substantially prevent transmission of unchanged frequency components and to transmit only components 1of the combination frequencies.

In order to insure that the frequenv)r increase 'at the receiving terminal may be the same as the frequency reduction at the transmitting terminal, it is desirable to derive the frequency changing waves at both terminals from a common source which may be located at anyconvenient point in the system. Where several bands or snbbands undergo frequency changes of dilhncnt amount it is convenient to derive the fr quency changing waves from one base fri-- quency wave. by harmonic generatnm. Any well-known system of harmonic generation as, for example, that illustrated and described in connection with Fig. i9 of the article Carrier current telephony and telegraphy may be used.

The novel .features which are considered characteristic of the invention are pointed llO .ilustrates a telephone system particularly adapted for a single loaded cable circuit; Figs. 2 and 3, a telephone system for multiple circuit submarine cable operation; Fig.

4', 'a detail of the system of Figs. 2 and 3;` and Fig. 5 an alternative of this detail.

Referring to Fig. 1, an ordinary tele- -plione circuit 1 is shown connected to al second telephone circuit 2 by means of a cable, loaded circuit, or other toll link 3. The telephone circuit 1 is connected to the cable 3 through a frequency changing apparatus at station A and circuit 2 is connected to the cable by similar frequency changing apparatus atstation B. Speech currentsoriginating in or transmitted over circuit 1 may comprise components from very low frequencies up to frequencies of several thousand cycles per second. It is frequently assumed that the essential speech frequency lie within the range of 100 to 2200 cycles. It is of course, possible to transmit intelligible speech by much more limited bands, lt is, moreover, not essential that the entire range be retained as portions of it may be omitted without serious loss. An improvement may be made vby transmitting` certain currents of higher frequenciesrepresenting such sounds as the sibilan-t's which are usually poorly transmitted, particularly when circuits of limited frequency transmission range are employed,

Let it be assumed that good quality speech-I ti'ansinissionniay be had Aif the baiidsfrom 100 to 1000 cycles, 1500 to 2200 cycles. and@ 3600 to S800-cycles are retained and used.

Assume further that the cable or loa-dedcircuit 3 has a' transmission range lying `substantially entirely below 2150 cycles,- 'l`he function of the apparatus at station' A is to reduce the frequencies ofthe bands chosen for good qualtiy transmission to .such

an extent that they may be faithfully,trans-fl initted by circuit Speech currents frenicii'cuit l are supplied by the hybrid coiljo'r' balanced transformer 4 to outgoing channel 5 which includes three band filters 6, T, and 8, having their respective input' end sections connected in series. These filters which may bc of the type described in ythe aiticle by Colpitts and Blackwell have transmission frequencv ranges, the limits of which are indica-ted iii the. drawing. The

- filter 6 selects its band viz. of 100 to 1000 cycles inclusive.l and supplies it directly to f and telegraphy,7 by Colpitts and Blackwell,

supplies oscillations of its base frequency, e.

`g., 50 cycles, through a low pass filterll to circuit 9. The harmonic 'generator sup- Aplies harmonic oscillations of the 50 cycle base frequency w-ave to an output circuit 40. Any other type of harmonic generator as, for example, that of Fig. 1 of vBritish Patent v131,426 may be employed. Harmonic' oscillations of 500 cycle frequency are selected vfrom output circuit 40 by tuned circuits 12 and impressed on a, frequencyv the operation' of, the frequency changer 13 is to produce a band of changed frequency in which -the original frequencies of the band passed by filter 7 are each 'diminished' by 500 cycles. This diminished frequency bandis selected by band filter 14, the

transmission frequency limits of which-y range of the cable 3.

The currents transmitted to station B areA supplied by 'a hybrid coil 4 to incoming circuitl'f includin 21 transmits only currents of frequency-well below 100 cycles, and according-ly selectsthe 50 cycle base frequency ,current and supner similar to thaty described inthe article by Colpittsandvlackwell. This harmonic l band filters 18,119 and 20 and-low -pass fi ter 21 having respective .end sections Viiiseries. .The low pass filter f changer or modulator 13 together with the I lband, transmitted by filter 7. One result of 'plies it to harmonic.reprodueer 22 in a maiireproducer vmay be identical` in circuit 'arrangement-with element 144` of Fig. 3. Repro neer 22 supplies harmonic oscillations of various multiple frequei'icies to circuit 23. ',Tuned circuits 24 select-1900 cycle oscillations and supply tliemto frequency changer 25 to increase the frequencies of the band of' oscillations selected by filter 18 to their original magnitudes and the original frequency subband of 3600 to 3800 cycles inclusive is then separated from resulting currents of other frequencies by filter 26 and supplied to circuit 2. Similarly, 500 cycle oscillations are selected and used to increase the frequencies of the band selected by filter 19 to their original `frequency magnitudes. This band is then selectively transmitted by filter 27 to circuit 2. Filter 20 transmits the unchangedV band received fromt filter 6. Each of the various bands is thus restored to its original frequency position and together villv yield intelligible speech signals in a tele'- phone receiver associated with circuit 2.-

ATransmission. in .the lopposite direction, viz., from circuit 2 to circuit- 1 is similarly accomplished. Filters 36, 37 and38 correspond to filters 6, 7 and 8 'and serve toseparate out three subbands for transmission the outgoin from the entire range of currents originating in circuit 2. -The subband selected by filters 36 is sent direct to line 3. That selected by filter 3 7 isreduced in frequency by 500 cycles before transmission.l The third subband, that passed'byfilter 38, is reduced by 190D-cycles'. The 500 cycle and 1900 cycle currents used to reduce the frequencies of bands are selected from the output circu1t'23 of harmonic reproducer 22 by tuned circuits. 'lhe operation at station A in separating the various subbands received from station B and restoring each to 'its Aoriginalfrequency position Will be obvlous from the foregoing explanation. The

- various harmonic frequency currents neces- Fig. 1, the characteristic of the transmission circuit 3 and the range of electric waves which are to be transmitted thereover are matched by operation upon the Waves to be transmitted.v It may happen, as in the casel of submarine cables over which speech is to be transmitted`,that operation' upon the waves insuch manner as'to retain the elements essential for -intelligibility at the distant receiver will not reduce the frequency range to. that which can be successfully transmitted by a single' submarine cable or other transmission circuit. Figs. 2 and 3 together, illustrate a system adapted to overcome this difficulty. A telephone line 101v terminating at station X is shown connected with adistant telephone line 102 terminating -at station Y by means of a system of cable lcn'cuits 103, 104, 105, etc. By way of example, it may beassumed that these cablev circuits have such transmission characteristics that each of them may be used to transmit currents of :tr-ange of.0 to 200 cycles frequency 'without excessive distortion or prohibitive f attenuation. The number of such cablecircuits-necessary' will be dependent upon-this transmission range and will increase as the-practical transmission band of each circuit decreases. The cable circuits may be metallic or may be ground return and the drawing is intended to indicate diagrammatically either type of circuit. Moreover, the conductorsof these circuits may be placed in individual cables or may all be placed in the same cable, depending upon circumstances. The latter form of construction is preferable from the. standpoint of economy in the first cost Wherever it is permissible from the standpoint of transmission requirements. Each of these cables may be of the periodically loaded (Pupin) or continuously loaded (Krarup) types. Attenuation equaliz'ers may also be employed to counteract the effect of different line attenuations for different frequencycomponents by giving such attenuation to each component that the added attenuations caused by the. line and the equalizer are the same for each component. Various attenuation equalizer circuits are Well known. Although any desirable type may be used, that preferred in this system` and illustrated, is of the type disclosed at Figs. 9 and 10 of U. S. patent to Hoyt No. 1,453,980, dated May 1, 1923. Its advantage as an element of a system in which filters are employed is obvious. It is to be understood that the various receiving circuit-devices referred to as band filters, such as 18, 19, etc. in Fig. 1 and the corresponding filters in the system of Figs. 2 and 3 comprise not only frequency selectingA elements as in the case of the ordinary band filter, but Aalso elements designed as set forth in the Hoyt patent to give an attenuation complemental to that of the main or toll transmission link.

Line 101 terminatesa't station X in a net N which simulates its impedance throughout the entire frequency range of the currents transmitted thereover. A hybrid coil or balanced three winding transformer, similar to element 4 of Fig. 1 serves to connect line 101 to incoming and outgoing channels 10T and 108, respectively. Channel 108 is provided Wit-h transformers 109, 110. 111, 112, etc.,.separated respectively by high pass iiiters 113, 114, 115, ete. Transformer 100 asses outgoing currents from line 101 to a ow pass filter which selects those of frequency below 200 cycles and transmits them to line' 103. High pass filter 113`readily --transmits currents of all frequencies higher than 200 cycles to transformer 110 which supplies them to a band lilter having a transmlssion range extending from -200 to 400 cycles. This band filter impresses ils selected bandupon frequency changer 120 which is of the same general type as the modulating,r element illustrated at Fig. 20 of the article Carrier currenttelephony and telegraphy.

A harmonic generator 1.30 of the type illustrated at Fig. 49 of the article Carrier current telephony and telegraphy comprising an electric discharge oscillator having coupled input and output circuits and a high resistance element in its space current circuit produces base-frequency oscillations of 100 llt) lil)

dit

device which serves cycles fre uency and harmonics thereof and supplies t em through a thermionic amplier also having a high resistanceelement in itsspace current circuit to circuit 131 from which tuned circuits 132 select 200 cycle currents for use with frequency changer 120 in reducing the frequencies of the outgoing band transmitted thereby. Frequency changer decreases the frequency of the transmitted subband4 by 200 cycles. This decreased frequencysubband is then selected by a band filter 133 having a transmission range-from 0 to 200 cycles and is impressed upon outgoing line 104. Currents trans-v mitted over line 104 are supplied at station Y to a frequency changer 140,l which in'- creases the frequency of the received subband by 200 cycles,"fand this increasedfrequency subband is then selected by ai band filter and impressed upon incoming channel 151 by which it is transmitted 'to line 102. The frequency changing operation of the device 140 is accomplished by combining the received subband with 200 cycle b Waves'l supplied by tuned circuits 141 through an amplifier. Circuit 106 is used to transmit 100 cycle base frequency current from station X to the remote station rllhis current is selected b a lovv` pass filter 143 and is supplied to a liarmonic reproducer 144 of any 'desired type which supplies harmonic frequency currents to its output circuit 145. 'lhe harmonic reproducer comprises three electric discharge devices in tandem. The first of these receives the base frequency current and amplifies it to such an extent that the amplified base frequency Waves impressed upon the second electric discharge device cannot be faithfully repeated thereby. This produces a distortion of the base frequency Waves as repeated in the output circuit of the second electric discharge device and a consequent generation of harmonics thereof. In order to increase the distorting action of the second electric discharge device, it is provided with a high resistance in series with its space current The distorted base frequency wave charge device is impressed upon the input circult of a three element electric discharge ics and supply them to circuit 145.

In a similar manner currents originating in line 102 are separated into various frequency subbands each of 200 cycles extent and each subband is reduced to the range cf 0 to 200 cycles before transmission to station X. After reception at station X, it is again restored to its original frequency position. It is of course to be understood that the frequency range of the currents transmitted over each individual cable will be determined by the circumstances.y Moreoveigas `supplied to the input circuit 176 is "aSSllmE a various harmonics in the' output -clrcuit of the second electrlc d1sto amplify the harmon-v in the system of Fig. 1, certainportions of thedspeech frequency spectrum may-"be discar e Circuit 106 is associated at station with a. band pass -ilter which transmits currents of a narrow band of frequencies in- Tlus circuit serves there- .of the incoming subbands. The gain control device 200 at station Y is associated with the input circuit 17 6 of the harmonic reproducer 144 so as to be controlled by the asel frequency wave of 100 cyclesfrequency transmitted from station X. K f

Fig. 4 illustrates the operation of the gain control element tiain control current of 100 cycles impressed] upon the grid-lament circuit of a three.- element electric discharge device 191 which includes a series blocking condenser-177, a polarizing source 178 and a high resistance grid leak 179. ri`he space current in the output circuit of the electric discharge ,Y de vice will be determinedin the absence of gain control currents largely by the polarization, potential impressed onk the grid by source 178. A solenoid having ak Winding 180 connected in the space currentcircuit will cause spring retracted plunger 181 toA position determined by the strength of this space current. v.As the gain'control current increases, the potential of the grid becomes more negative -in the same proportion in accordance with the Well-known operation of the grid lealv demodulators and theV space current falls. As a result of this action, the plunger 181 movesand carries a circuit closer 183 '.to a position in engagement with. a different -contact 184. The circuit of the circuit closerl may be traced from ground by way of'battery 182, circuit'- closer 183, one of the contacts 184, and a winding 4of the corresponding relay 1851to ground.

The relay 185 uwhich energizes', lclosesits armature 186, thus determiningthe portion of the` potentiometer resistance 187 to be included in the input circuit of amplifier 188 `or 189, as the case vmay be, 4and thus in the system of Figs. V2

to determine the amplification ofthe sub-` bands received-at station Y, v

Fig. '5 illustrates a modification of the 'arrangement of Fig. 4, involving no moving elements. Only so much of the circuits are shown as are necessary to explain theap-l plicatioii of this -modified arrangement to one for each of the transmission circuits' 103, 104, 105, etc.

Element 190 is substantially identical with electric discharge device 191 of Fig. 4, except for the substitution of manner' that the a resistance network for the v'solenoid 180 and its associated elements. This network, comprises resistances 192 and 193, each `of very high magnitude, as of the order of a half megolim, a resistance 194 Iin `shunt thereto and a capacity element 195 which serves as a low impedance for all alternating currents to divert them from the resistance paths. Space current-passes mainly by Way of resistance 194 which maybe of a few thousand ohms to match the internal space current resistance of the, device 190. Re-

sistance 193 is of such magnitude as to effectively separate the input circuit of ampliiier 188 and the output circuit of gain control element 190 so far as alternating current is concerned. The only action of device 190 will therefore be to impress a direct current E. M. F. across resistance 192. The resistance 192 is included in the polarizing gridcathode circuit of amplifier 188 in such a difference of potential caused by the flow of space `current therethrough. tendsy to oppose the polarizing source 196. The result is that the grid potential of amplifier 188- may be only slightlynegative or even positive and the amplifying action of the device 188 will be large. As the energy received over circuit 176 increases, the space current of device 190 falls and likewise the potential difference across resistance 192. amplifier 188 therefore becomes more negative and the amplifying action of thisfa-mplifier decreases, thus compensating for the increased transmission on circuit 103.

The transmission` of the base frequency current from stat-ion X to station Y serves a vdouble function in that it enables the frequency change at station Y to be made exactly theconverse of that of station X and moreover, thatit may serve to determine the operation of the gaincontrol apparatus.

In the circuit arrangement of Figs. 2 and 3, the attenuation equalization o eration may be-combined with the select-ing unction of the receiving band filters such` as filter 150. The ensuing amplification must of course be sufficient to augment the attenuated current, but it should be borne in mind that if the frequency changer is of the electric discharge type illustrated, it introduces an amplification factor.

Although the invention'has been disclosed as embodied in certain specific arran ements,

the principles of the invention are o )viously The grid of 'to reduce the component frequencies by a predetermined amount, transmitting said wave of reduced frequency components and said single frequency wave to a distant station, and utilizing said single frequency wave to increase the component frequencies to their original magnitudes.

2. The method of transmitting a broad band of Waves which comprises dividing saidband into va plurality of subbands, reducing each of said subbands by the frequency of a harmonic of a given base frequency wave,'transmitting said reduced frequency subbands to a distant station, transmitting said base frequency wave to said distant station, deriving harmonics of said base frequency wave, and increasing the frequency of each of said subbands by combination with a wave of the same frequency by which it was reduced before transmis- 100 sion.

8. The method of transmitting a broad band of waves which comprises dividing said band into a plurality of subbands, combining each of said subbands'with a har- 105 inonic of the same base frequency wave to reduce the effective frequencies of said subbands, transmitting. said reduced frequency subbands and energy of the base frequency wave to adistant station, deriving haimonies of said transmitted base frequency wave, and combining each of said transmitted subbands with one of said harmonics to restore the subband to its original frequency position.

4. 'A system foi` transmitting a band of waves between two stationscomprising a source of Waves of a definite frequency, means for supplying waves from saidsource to each of said stations, means for combining waves derived from said source with said band atthe first of said stations to reduce the effective frequency of said band, means for transmitting said reduced, frequency band to said second station, and means at said secondstation for combining said received band'with waves derived from the waves from said lsource to restore said reduced frequency band to its original frequency position.

5. 'A system comprising two separated stations, a source of base frequency currents at the first of said stations, means for supplying base frequency currents therefrom to the second station, means for producing a broad band of waves at the first station, means for dividing said band into a plurality of non-overlapping frequency subbands, means for deriving harmonics of said base frequency Wave vand reducinfi the effective frequency of each of said su bands by the frequency of one of said harmonics, means for transmitting each of said reduced f requency subbands to said second station, means at said second station for producing harmonics of the base frequencywave transmitted thereto, and means for combining each of the transmitted subbands with one of said harmonics at said second station to restore said subhand to its original frequency position.

6. Means for transmitting between two y stations a wave having components of a given range of -frequnecies which comprises means for reducing the frequency of each of said components by a predetermined amount, means for transmitting said reduced frequency components from one station to the other, means at said second station for increasing the frequency1 of each component of said received wave, means for transmitting between said stations a wave of a definite frequency, and means for causing said denite frequency wave to control the `frequency change of said transmitted band.

7. A system for two-way transmission between distant points comprising means for transmitting base frequency energy therebetween, means for causing -said base frequency energy at each of said points to reduce the effective frequencies of signal energy to be transmitted and to increase the effective frequencies of signal energy received by like amounts.

8. A, continuously loaded cable circuit, means for transmitting currents of a' plurality of frequencies within a range smaller than the effective frequency transmission range of saidcable but extending between frequency limits at least one of which is higher than tlie upper limit of said effective transmission range, and a frequency changing means connecting said transmission means to said cable to reduce the effective frequencies of all of said currents to magnitudes within thel limit/sl of said -effective transmission range.

In witness whereof, I hereunto subscribe my name this 10th day-of February A. D., 192

`ELMEVR v. caress.

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