US1910977A - Wave transmission system - Google Patents

Description

May 23, 1933. c 1 was, JR 1,910,977

WAVE TRANSMI SS ION SYSTEM Filed Aug. 14, 1931 3 Sheets-Sheet 1 492.25- I II 494.75 ll ll C 4920 KC INVENTOR C. L. WE/S JR.

A TTO/PNEY May 23, 1933. c. L. was, JR 1,910,977

WAVE TRANSMISSION SYSTEM Filed Aug. 14, 1931 5 Sheets-Sheet 3 FIGS INVENTOR CL. WE/S JR.

ATTORNEY fatentecl May 23,

UNITED, STATES PATENT wE I a m us Were in Q1 1 ers mea an. New oax. ss m e BE L ere PHONE LABORATQIQIES, rnoonrongnrnn, OF NEW YORK; N. Y., A ooaronarron or NEW Y RK r WA E. B NsM S r' SYSTEM Application mea a st 14, 133;. are; No. 556,949.

This invention relates to; Wave transmise sicn sy tems and. more p rticularly to Wav transmission systems utilizing a plurality of fr quen y ranges o multiplex transmis- It is an objectof theinvention to increase the number of Wave transmission channels that may be superposed Within a given range of frequency. More specifically the object of .thepresent invention is to decrease the frequency interval between adjacent chain nels in amultiplex carrier wave signaling systemvvithout detrimentally affecting the' quality of transmission. 4

In another aspect of theinvention its object is to increase the effectiveness of free quency selective devices.

In carrier Wave communication systems in which a plurality of bands of high frequene cy signals are transmitted over a common circuit, it is desirable from the standpoint of efficient utilization of the frequency range which the circuit iscapable of transmitting that the frequencyinterval between the seVQ eral signaling-bands be as small as possible.

It isinot feasible withapparatus and circuits now available to eliminate entirely this inter-band Waste space. The lack of selectivity in the filtering apparatusfor sep arating different frequency bands at the terminal stations has been a decided limitation. A perfect filter would haveno attenuating effect on waves in the signal frequency band it was designed to transmit; atall frequencies outsidethat band it would have infinite attenuation. In any filter that mayactually be constructed, however the attenuation within the selective band is not zero, and at frequenciesconsiderably removed from the band the attenuation is finite and relatively high, but betweenthese regions the attenuation increases only gradually as the frequencydeparts from the extreme frequencies of the band. I Therefore, if the frequency interval between bands in a multiplex system is very small, Waves near the edge of one band will be passed to a certain extent by the receiving filter in an adjacent frequency channel and interference will be set up. Simi-,

larly, here several channelsare appliedto common transniission circuit.

a common conductor each channel filter passes not only a desired bandof signals but also above and below that band, undee sired waves, such as, in the direction of transmission, a part of a signal side band that is tobe suppressed, or in the opposite direction, signals from the adjacent, parallelconnected channel. i

I .Where an attempt is made toplace the channels very closely together another detrimental effect arises. lVitha number of filters at a transmitting station connected in parallel toa common output circuit, the

impedance presented by the filter in one 1 channel to the filter in an adjacent channel may be" quite. low and irregular within the passband of the latter. Since each filter is designed to work into a particular impedance, reflection phenomena and distortion .luay arise,especiaily at frequencies near edges of the bands of waves. At. a receiving station a similar dlstortion may occur. A

band of aves in a given frequency range,

aifect the quality of transmission. 7

In a multiplex system in accordance with applicants inyention the frequency interval between adjacent channels may be consider.-

ably decreased without introducing the detrimental effects briefly pointed out above.

In general, applicants system is character ized by a division into two ormore groups at the terminal stations of the channels that tend to interfere with each other. Each group is isolated from the others but connected in energy transfer relation with the Where the channels are separated into two groups, each group may contain channels in alternate frequency :b'ands. lVith the channels connected in" parallel in each group thus separated from each other by the Width of a channel,

the mutual shunting effect of the filters within the group is practically negligible. A filter in a channel of one group can have no effect on signals in either of the two immediately adjacent frequency channels, as the latter are in the other group and are isolated therefrom. Where the filters in each group are connected in series instead of in parallel, advantage is also derived by isolating the groups in accordance with the present invention.

The group isolating means may take any one of several forms. In one of its embodiments applicants invention is featured by unilaterally conducting devices inserted in the respective group connections to isolate one group from another. A space discharge amplifier can be used to good advantage for this purpose. Waves in a given frequency channel in one group are freely transmitted either to or from the common circuit. They are not affected by the filter in either of the two adjacent frequency channels, these filtersbeing in another group, because they are isolated therefrom by virtue of the unilaterally conducting properties of the device.

In accordance with a further feature of 'applicants invention certain relations are maintained between the extreme frequencies of each channel and the frequency of a carrier wave with which the waves in said channel may be combined for the purposes of modulation or demodulation. The frequency range in which modulation products of certain orders will fall is determined by the relation of these frequencies. Third order products, which are especially troublesome, will fall within the signal band if the lowest frequency of the band to be modulated is less than half the highest frequency. When the lowest frequency is exactly half the highest one, the band of third order products will lie on both sides of the modulated signal band and be contiguous therewith. Since waves of such frequencies are attenuated incompletely bythe filter, it is preferred that a ratio somewhat less than 2 to 1 be observed between the extreme band frequencies. The third order products may thereby be removed to a sufficient extent from the borders of the signaling bands that they are effectively suppressed by thefilter. In the demodulating stage it is likewise desirable that the lowest frequency of the band as demodulated be more than half the highest one so that third order products will not fall within it.

Other features and objects of the present invention will appear in the following detailed description of a specific embodiment thereof. It will be obvious that while the invention may be employed to reduce the frequency intervals between signaling channels, it may be used alternatively to decrease the selectivity requirements imposed on the selective devices. In the drawings,

Fig. 1 shows the present invention as incorporated in the transmitting circuit of a carrier wave transmission system employing successive stages of modulation and successive stages of demodulation;

Fig. 2 shows the receiving circuit associated with the transmitting circuit of Fig. 1;

Figs. 3 and 1 show alternative forms of the group separating means;

Fig. 5 shows the impedance characteristics of the frequency selective devices employed in the channels of the first stage of modulation and the second stage of demodulation of said system; and

Fig. 6 shows the impedance characteristics of the frequency selective devices employed in the second stage of modulation and in the first stage of demodulation of said sys tem to pass groups of doubly modulated waves.

Referring now to Fig- 1, there is shown schematically a terminal station for translating the waves in a multiplicity of low frequency circuits to their respective positions in the wide frequency spectrum that is used for transmission purposes. The frequency translation is accomplished in two steps. The low frequency circuits are first divided into a plurality of groups A, B, C, etc. Within each group the signal waves are then transferred to respective positions in a band of carrier signal frequencies. These several groups of carrier signals are next translated, as groups,'to positions in a still wider range of frequencies and applied to a transmission line.

Within each group, as in group A, for example, there may be represented perhaps sixtylow frequency lines Z Z etc., adapted to transmit telephone, telegraph or other signaling waves. For each of these lines there is an individual modulator M of any suitable type for applying the signaling waves to respective high frequency or carrier waves supplied by the respective generatorsG G etc. The modulators are preferably of the carrier suppressing type such as disclosed in J. R. Carson Patent 1,343,306, June 15, 1920. The relative frequencies of the carrier wave generators may be accurately maintained by controlling them with a common base frequency wave, as shown, for example in B. W. Kendall Patent 1,773,901, August 26, 1930. For the transmission of speech, a band of frequencies ranging from 250 to 2750 cycles may be required. The band passing filters CBF CBF etc., to which the two signal side bands from the preceding respective modulators are applied are accordingly designed each to transmit a band of modulated carrier waves 2500 cycles in width. One side band, preferably the upper, is suppressed in order to conserve the available frequency range; Th filters. are pr e ab y of the t pe ut liz ng piezoelec ri crys al s h s eseribed in a pp t n of W. P. Me on bearing Serial- No. 489,268, filed October 17, 1930, although any other suitable type of lecti g r uit m y be us in ac anc with this invention. The frequency separation of the carrier wavesmay be {1:000 cycles. With s spacing n wi h the car ier of the lowest frequency at 263 kilocyclesper sec nd a d the hig at 4 eyeles per second, sixty channels may be accommodated. I a h group, t oll ct g bus s, .31 and CB are provided. GT0 each of these buses are connected the band passing filters in alternate frequency channels. Thus, to CB1 are connected n para le t e u p t t r inals of fi te s CB 1 173, s, c, while to CB; are connected the terminals of filters. CBF 6BR etc. Signals from the first bus are passed through a vacuum tube amplifier A to one of the group. output lines L L etc, Signals from the other bus are passed through a similar amplifier A to the same outgoing line. A shunt resistance across the input circuit of each amplifier may provide a suitable terminating impede an ef r the filte l a The characteristic impedance-frequency curves Of the filterseonnected to the respective co lecting u e Biand CB2 a h w n'Fig, .5- F equ ncy i p e n ed along h erizontel axis, th ra sti or minal impedance, along the vertical axis, The signal bands in eachof the "two groups, a eprese e lby the tw o o aded rectangles, areseparated' from each other by somewhat more than a single band width," viz., by a band width plus twicethe final inter band spacing interval, or 5,500 cycles.

" A filter in one channel, filter CBF for example, has but little efiect, therefore, on the band .of signals issuing from the filter CBF in the nex para le n t d nne t th lowe t freque cy f the latter be it will be noted th t th ra teristic imp an e offilte C' i1 ishigh and t at is filter thereforehas a negligible effect on the impedance presented to the filterin the adj acent channel, A similar condition obtains in the group represented by the lower row of rectangles. Since the vacuum tube .am-.

)lifiers A and A are connected in the leadsfrom the collecting buses, the signals from filter S13E or from any other filter of that group, are not affected by the filters CBF and QBF for example, which are in the other group. The latter filters are faced by the practically uniform input impedance of the space discharge device in amplifier A Any unilaterally conductive device could be made to serve the same purpose, as it would revent the reflection of the filter characteristicsthrough it.

If the impedance rise of the filters outside their hands were found to be so gradual that a serious overlapping of their characteristics occurred even though the channels in each groupwere more than oncremoved from each other, three or more groups could be formed, each isolated by an amplifier and each containing only every third or higher numbered channel. This would provide afrequency interval between channels of several times the width of a channel and would eitherpermit the use of less selective filters or provide a higher degree of separation of he ann ls.

Had these same groups of filters all been connected inparallel to a single bus, a quite different situation would exist. Superposing the characteristics of thetwo groups of filters as hasbeen done in Fig. 5, it is seen that because of the overlapping of these characteristics the signals from filter CBFia, for example, would be seriously distorted. They would be shunted especially at the upper ahd at the lower frequencies by the filters CBF and, CBE whose impedance is not only low throughout the signal band between them, but also is not. uniform throughout that band. Any shunting effect at all is undesirable, but when distortion occurs also the difficulty cannot be remedied by amplification alone and an equalizer required. y a i Referring againto Fig. 1, there is shown at T a circuit for translating the wide bands of singly-modulated carrier waves arriving from the several identical groups A, B, C, etc,, over lines L L etc, to respective positions in a second carrier frequency range for application to transmission line LE. The apparatus required for this second stage of modulation may be the sameas that used for the first stage except asto the frequency range for which it is designed. The band of waves applied to each of the modulators M extends from26025 to498/75 kilocycles per second .in the embodiment herein described. With the frequencies of the carrier wave generators C G etc., spaced 240 kilocycles apart, the frequencyinterval between the upper band of one translated group and the.lowerband ofthe adjacent translated group will be the same as that between the bands within the groups, via, 1500 cycles.

accommodated Where the highest carrier fre- 1 quency is 5160 kilocycles per second as shown. The group band-passing filters GBF GBF etc., are designed topass only the lower ofthe side bands created. They arepreferably of the electricaltype shown in G. A. Campbell Patent 1,227,113, issued May 22, 1917. I

I The group modulators M are divided into two groups each containing alternate frequency bands. The output filters GBF GBF etc., are connected to the'collecting bus TCB while filters GBF GBF etc., are associated with bus T013 The first group is connected to a vacuum tube amplifier A the second to a similar amplifier A A shunt resistance may again be used as a terminating impedance. These amplifiers may be connected directly to the transmission line LE, or through a power amplifier A as illustrated. For efiicient transmission of frequencies of the order of 5000 kilocycles per second, it is preferred to use a pair of coaxial conductors, a structure such as shown in H. R. Nein Patent 1,781,124, November 11, 1930, being'suitable. It is not essential that the channel modulating circuit and the group modulating circuit be immediately associated with each other, and the lines L L etc., connecting them may be transmission lines of coaxial or any other suitable type.

Fig. 6 shows the typical impedance-frequency characteristics of the group bandpassing filters GBF,, GBF etc. For purposes of illustration only, four signal bands. represented by the shaded rectangles, are shown in each group, although in the embodiment herein described, the sixty chan- .els of each of the groups A, B, (J, etc., would actually be present. Because of the alternate grouping arrangement, there is a wide frequency separation between the groups of channels connected to either bus. Fig. 6 shows that the impedance of the filters, GrBF for example, rises rapidly in this frequency interval. At even the lowest frequency of the e group passed by filter GBF the characteristic impedance of filter GrBF is so high that the latter filter has practically no shunting effect. The same holds true as to the effect of filter GBF on the highest frequency of this same group passed by filter GBF By virtue of the vacuum tube amplifiers A and A connecting the two collecting buses to the common transmission line, the shunting effect of a filter in. one group on signals from a group connected to the other bus is eliminated. This isolation of the groups is identical with that provided in the channel circuits by the amplifiers A and A Preferably, the amplifiers are of the push-pull type and capacitively balanced against feed-back effects.

If the group band filters of the two groups were connected to the same collecting bus, the effect on signals in adjacent frequency groups would be much more serious. As shown by the dotted lines of Fig. 6 the impedance of one group filter, such as GBF very low in the frequency ranges of the groups immediately above and below it and distortion wouldtherefore be introduced, especially in the frequency ban'ds'at the edges of those groups;

In Fig. 2 isshown a'receiving terminal circuit for separating the carrier frequency signals arriving over transmission line LE and for reducing these signals to telephone frequencies for application to a multiplicity of telephone circuits. vThe first stage or group separation and'demodulation occurs in the apparatus V; Each separated and demodulated group is'then completely divided into its component signaling bands, which are then finally demodulated. The grouping arrangement of these receiving circuits may follow that of the transmitting circuits, as illustrated here, although this is not essential. This arrangement is preferable, however, since it permits the use of a common carrier wave source for modulation and demodulation and reduces the number required.

Signals from line LE are passedthrou 'h an amplifier A and through two pat s containing amplifiers A and A respecof different selective circuits tively. To amplifier A is connected a diswave generators in the modulating circuits.

The waves issuing from each of demodulators DM represent in the embodiment shown, sixty carrier wave signal channels extending in frequencyfrom'260.25 kilocycles to 498.75 kilocycles.

The characteristics of the several group filters GBF GBF etc, may be the same as those of filters GBF GBlI', etc.,- shown in Fig. 6. Because of the wide frequency separation of the groups connected to either bus, there is practically no tendency for signals in one frequency group to be affected by the band filters of the adjacent groups connected to the same bus. There is no tendency for the signals transmitted through amplifier A to filter GBFQ, for example, to be shunted by the filters GrBF and GBF in the adjacent frequency channels since the latter are isolated by amplifier A The selectivity requirements on the group filters may be considerably reduced in View of the would ordinarily attend this broadening of the attenuation-frequency characteristic;

The course of the carrier wave band applied to the-groupsA, B, C, etc., of the channel circuits, is.- similar to the course of mthe groupsof bands through the circuit V.

In group A", for example, 'the incoming waves are applied tothe two amplifiers A, and Am, the firstof which is connected to the distributing bus DB and the second, to

5 distributing bus DB To bus DB are conternate frequencychannels arepassed by the filters CBF (lBF' etc., connected to bus DB The succeeding demodulators DM and the high frequency iwave sources G G g, etc., connected thereto, may be similar to the 1 elements DM' of the first demodulating stage and to the? corresponding elements G1, G etc, of the first modulating stage, respectively. The'low frequency signals resulting from this seco'nd stage of demodulation .may be applied to telephonecircuits, which as in thecase of Fig. 1,may lead to telephone exchange system. The bandpassingj filters in thechannel circuits at the receiving terminal are preferably of the piezo-electric crystal type referred to hereinbefore, although other types may be employed Within the scope of this invention.

In Figs. 3 and 4 are shown circuits alternative to the amplifier circuits used-to separate the groups ofalternate channels in a transmitting or areceiving station. In Fig. 3 a resistancepad 7, Sis inserted'm each of the'group connections instead of an amplifier. For atransmitting station, the leads 9 ofpad 7 and leads of pad 8 are connected to the respective collecting buses with whlch the two groups of filters are assoclated. Leads "19 may'beconnected to the common transmission lines. L ,L etc., in the 'embodiment shownl in. Fig. 1. Signals in a given frequency band applied, for example, to leads ware-attenuated by the resistance pad 8' and pass to the transmission line through leads'19. The adjacent frequency channels are in the group connected to leads 9 and the filters therein are practically isolated, so farastheir effect on signals from pad 8 is concerned, by pad 7. Although these latter filtersjmay tend to have a high shunting effect in the frequency range 'of o the band issuing from pad8, this effect is made negligible byvirtue of the attenuation of pad 7. An attenuation of decibels was found a satisfactory value for the pads 7,8 in one particular case. The isolating action may be expressed also in terms of reflected waves. Onehomponent of the waves issuing from padf8 passes out through leads19; another component passes through pad 7 is reflected and returns at a level at least decibels lower than that at which it entered. Any distortion in this reflected wave due to the action of. the filterconnected to leads 9 will have a negligible effect on the resultant wave. It is obvious that addi tional resistance pads may be connected to leads '19 to accommodate other groups should lack'of selectivity in the filters make it necessary to divide the preceding circuit.

into more than two groups. Similarly, at a receiving terminal, waves arriving through leads 19 are substantially unaffected by the characteristics of the filtering devices connected to leads 9 and 10. r

In Fig. 4 is shown a hybrid coil for isolating one group from a second one. With a network 24 to balance the common transmission line 23, thenominal input and outputleads 22 and21 respectively of the hybrid coil are in conjugate relation with each other but in bilateral energy transfer relation with leads 23. Signals to 'or from a filter group connected to either of the leads 21 or 22, are therefore not affected by the filters of the other group.

Another feature of the present invention comprises the proportioning of carrier and signal frequencies observed in its preferred embodiment. Besides the two groups of signal side bands, there are created by the groupmodulators other and undesired modulation products. Third order products are usually of high level and difficult to suppress. Their position in the frequency spectrum is, determined by the frequency 0 of the carrier wave and by the highest and low est frequencies, fh and fl, respectively, of the signal Waves applied to the modulator. One

groupof products ranges from a frequency ,Zcif to a frequency 201 f ,another.liesbetween c 2f .and c 2f and another beislesjs than half f some of theseproducts will fall in the desired signal band, which, in a lower side band system, ranges from c f]. to 0-72. I Where the ratio between f and f; "is exactly 2 to 1, these products do not fall within the signal band, but on just either side of it. The attentuation of the filters is not great at these frequencies and accordingly the waves are readily passed. Toremove thethird order products to a frequency position where the filter can effectivelysuppress them, a ratio somewhat less than 2 to l m ay be employed. 1 Accordingly, the band of Waves applied tothe group modulators M of Fig. 1 is limited to an approximately .240 kilocycle band extending from 260.25 kilocycles per second to 498.75 kilocycles per second, these extreme frequencies having a ratio of 1.92 to 1. Similarly, at the receiving station, it is desirable that naling channels. The control over the ex treme frequencies of the telephone signal bands in the channel modulating circuits can, of course, be only limited. Third order products must, therefore, be controlled in some other manner, as by modulating at low power levels.

lVhile the present invention has been described as embodied in a particular carrier wave transmission system, it may also be applied to systems differing widely there from within the spirit and scope of the ap pended claims.

What is claimed is:

1. In a system transmitting waves in a plurality of frequency bands, a common transmission circuit for said Waves, a plurality of filters adapted to pass respective ones of said bands of waves, said filters being connected in a plurality of groups, the frequency bands represented in each of said groups being respectively intermediate those in the other of said groups, and means to connect said groups simultaneously to said transmission circuit while effectively isolat ing said groups from each other.

2. In a multiplex signaling system, a plurality of selective devices adapted to transmit waves in respective frequency ranges, a circuit adapted to transmit waves in all of said frequency ranges, bilaterally conducting circuits each connecting together a plurality of said devices lying in non-adjacent frequency ranges, and respective means simultaneously connecting saidvbilaterally conducting circuits and said first mentioned circuit, said means being adapted to reduce the mutual effect of selectve devicesin adjacent frequency ranges. a Y

3. In a mult iplex wave transmission system, means to transmit waves in a plurality of frequency channels, a plurality of means to transmit waves in respective ones of said frequency channels, means or separating said latter means into groups, each of said groups being comprised of transmitting means in alternatefrequency channels, and means for associating said groups with said first mentioned transmitting means including meansimpeding the transmission of waves between said groups.

4;. A combination as in claim 3 in which said impeding means comprises u -ilaterally translating device.

5. In combination, a plurality of circuits for translating waves in individual frequency ranges, a pair of conductors for transmitwaves in a plurality of said frequency ranges, bilaterally conducting means to connect said plurality of circuits in a plurality of groups, the circuits included in each of said groups being adapted to transmit ing said groups with said pair of conduc-,

tors, said translating means comprising a space discharge device.

6. In a multiplex signaling system, a plurality of circuits adapted to transmit waves in-individual frequency ranges, frequency selective means in each of said circuits, said means being of such selectivity that they are capable of transmitting with relatively small attenuation waves in adjacent frequency ranges, means to connect said circuits in a plurality of groups, the frequency ranges in each group being respectively intermediate those of another of said groups, a multiplex transmission circuit and respective means simultaneously connecting said groups thereto, said latter means each being adapted to reduce the effect of the frequency selective means in the group associated therewith on waves from others'of said groups.

I 7 A combination in accordance with claim 6 in which the means to connect said groups with said multiplex circuit comprises unilaterally translating devices.v

8. In a system transmitting waves in a plurality of frequency bands, a common transmission circuit for said waves, a group of filters adapted to pass respective ones of said bands of waves, a filter adapted to pass a band of waves intermediate in frequency bands passed by said group of filters, and means simultaneously connecting said filters in energy transfer relation with said transmission circuit, said means providing apath of low transmission efliciency between said filter and said group of filters.

9. A plurality of signaling circuits, modulating means for applying the waves in each thereof. to individual carrier waves, frequency selective means adapted topass respective bands of the waves produced by said modulators, each of said selective means passing in common with the corresponding means for adjoining frequency bands frequencies lying in the range between signal bands, means to connect said circuits inparallel in a plurality of groups, the signal bands represented in each of said groups being separated from each other by at least the width of one single band, a common signaling circuit and individual means to connect said groups thereto, each of said last mentioned means having a high attenuation in at least one direction of transmission, the range of frequencies applied to each of said modulators being substantially less than 2:1.

10. In combination with a circuit for transmitting carrier frequency signals in a plurality of frequency hands, a plurality of signaling circuits, frequency selective means in eachof said. latter circuits adapted to pass respective ones of said plurality of frequency bands, said selective means passing in common with the corresponding frequency bands, fre quencies lying in the range between signal means for adjoining bands, means to connect said plurality of circuits in parallel in a plurality of groups, the signal bands represented in each of said groups being separated by at least the width ofone signal band, individual means to connect said groups to said first mentioned circuit, each of said lastmentioned means having a high attenuation in at least one direction of transmission, means in each of said plurality ofcircuits for combining the signal waves therein with a demodulating wave, the range of frequencies produced by each of said demodulating means being substantially less than 2:1. 7 v p a 11. In a signaling system, a plurality of circuits adapted to translate waves in different frequency bands, means to connect said circuits in two groups, the frequency bands translated by the circuits of eac of said groups being respectively intermediate those in the other of said groups, a circuit adapted to transmit waves in all of said frequency bands, and means to; connect said groups in energy transfer relation with said last mentioned circuit and in conjugate relation with each other. i

12. A combination as in claim 3 inwhich said impeding means-comprises a network of resistance elements.

13. In a multiplex signaling system, a ciradjacent frequency groups,

, for

cuit translating waves in a plurality of frequency channels, plurality of filters for dividing said waves into a plurality of ad acent ones of said filters transmitting certain intergroup frequencies in common, means for connecting certain of said filters together and to said first mentioned circuit, means for similarly connecting filters in frequency groups respectively intermediate those of said certain filters, saidconnecting means being adapted effectively to isolate each group from adjacent frequency groups, means associated with each of said filters partially demodulating the waves passed thereby, the range of frequenciesof said waves as demodulatedbeing substantially less than 2: 1, and means, for separating and finally demodulating the waves in each of said demodulating groups. 1

In witness whereof, I hereunto subscribe my name this 12th day of August 1931. CHARLES L. WEIS, JR.

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