US1595135A - Carrier-current signal system - Google Patents

Description

H. AFFEL CARRIER CURRENT SIGNAL SYSTEM Augo ma 19%.; www

Filed Jan. 27, 1922 3 Sheets-Sheet 1 INVENTOR.

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ATTORNEYS. I

Aug i 9 192%o H. A AFFEL.

CARRIER CURRENT SIGNAL SYSTEM Filed Jan. 27, 1922 3 Sheets-Sheet 2 :1 BF m 14-15000 A} 5 m0 R w m lfatented r tngm l0,

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application filed January transmission over a given line, and separating a; frequency band into narrower hands and transmitting them separately. These and other objects of my invention he made apparent on consideration of a few specific examples which I have chosen for presentationin this specification. it will he understood that the followingdescription refers to thseexamples and that the invention is defined in the appended claims.

The drawings are diagrams. The symholr i iwill he understood to mean cycles per second. Figure 1 shows a system for shiftmga frequency hand of 5000 7000- to aloand of 300015000-. Figs. 1 and i are diagrams of wave; filters. Fig. 2 shows a two-way repeater system with a moderate frequency shift between the input and the output. Fig. 1i shows a two-way repeater.

system with a frequency inversion between the input and output. Fig. 4 shows a repeater system between two parallel cir cuits on one side and a single circuit on the other side, the single circuit carrying a total frequency range corresponding to douhle the range of one of the two parallel circuits, and Fig. 5 shows a system for carrying a voice frequency range of 02000- over five lines, each carrying 0-400.

Referring to Fig. 1, this shows a one-way system by which alternating currents of various frequencies from 50O0- to 7000- are shifted [to corresponding frequencies lying between3000 and 5000 The modulator M .modulates 20000- frequency from the generator E with 1000 frequency from the generator 1%. This modulator h/ 1 comprises two three-electrode vacuum tubes connected as shown in the diagram. This connection serves toeliminate the carrier frequency (namely 200005 from the output of the modulator system. The output frequencies from the modulator M} will accordingly he l,-000-, i0000- and These go to a .highpass filter whose critical-frequency is 5000-, which shifting a frequency, hand to. a range of values most'suitahle for ices. serial no. traces.

accordingly suppresses the l000- frequency. This high pass filter of critical frequency 5000- helongs to-the general type of filters shown in Fig.1 having series impedances Z and shunt irnpedances Z in alternation. When impedance Z is represented physically by a condenser C and impedance Z is represented physically hyj'an inductance L, the filter takes the form shown in Fig. 1 and becomes a high pass filter, with critical frequency @ther well-known combinations give the low pass filter and the hand pass filter. The remaining frequencies passed, by this filter ofrcritical frequency 5000- are separated into respective branchconductorsi hy the two filters of critical frequency 20000-, one heingj low-pass and the other high-pass.

Accordingly, the 10000- frequency goes.

over the conductors as shown in Fig. i, as the carrier current for the modulator M and the 2l000- frequency rier current for the modulator M The 19000- frequency in the modulator:

goesas the oarll/lg is modulated by the input currents or various frequencies from 5000 to 7000'.

Accordingly, the output of the modulator ll I2 will be made up of frequency bands of .50U07000-, l2000l4000-, and 2l000 2eooo-. V

This composite output going to the highpass filter whose critical frequency is 15,000-, the output of this filter will he only the last mentioned hand, viz, 24000 to 2fi000w. This frequency range is applied to modulate the 2l000- frequency in the modulator M and accordingly in the output from modulator M -there will he found the three frequency hands 3000-5000 24000-26000 and l5000l7000-. This composite output goes to the low-pass filter whose critical frequency is l5000-, and accordinglythe final output is only 3000 to 5000' it will be seen that the output frequency hand 3000 to 5000- lies adjacent to the input frequency band 5000 to 7000-. Further, it will be seen that this shift of a frequency band to an adjacent frequency hand is accomplished by shifting first to a distant hand and then back to the adjacent hand. More specifically, the band 5,000

icu

7,000- is shifted to 24000-26000-, which is thenshifted back to 30005000-.

If it had been attempted to make the shift directly from the input frequency band to the output frequency band, that is, from" obtained. It will be seen that the general plan is to modulate a 'carrier frequency large in comparison with the input frequency or band of frequencies, and then to demodulate by asecond carrier whose frequency is equal to that of the original carrier plus or minus the net frequency displacement which it is desired to effect. In other words, it may be said in this case that the most effective method of producing a small shift of a frequency band is to make a large shift and then asoi newhat greater or smaller shift in the opposite direction, resulting in a differential small shift, as desired. In this way, at

the various steps of modulation, the by-prodnot harmonic terms and undesired side bands are readily separated out by suitable filters, siiiee thevarious ranges of frequencies involved at each step aresubstantially nonoverlapping and non-adjacent.

The method of Fig. 1 will be seen to involve the local generation of twb carrier currents differing moderately in frequency, viz, in this particular case, '1-9000 and;2lO00-.

These are generated by generating the intermediate or average frequency, viz,

2000O-, and modulating it by a frequency equal to half the difference between the two desired frequencies. In this case it will be seen that half the difference between 19000- and 21000- is 1,000-. Even if there should be a little variation in the frequency of the generator F so that its fre uency varies a little from 20000-, neverthe ess, if the frequency of the generator F remains constant at 1000-, the two output frequen-' cies 19000- and 21000- may vary a little but will keep the constant difference of 2000-."

Referring toFig. 2, this illustrates a. two i way .device by which alternating currents of 40006000-, coming from the west, are shifted to 30005000-, going east, and on the other hand, alternating currents of 3000-5000- coming from the cast are shifted to 40006000- going west. The operation of the unit shown in Fig. 2 will be readily understood fromthe discussion that has been substantial currents in the other.

given of Fig. 1. Input eurrents.from the west pass to the primary of the three winding transformer T of well known design whose input and output branches are conjugate, so that currents in one such branch produce no The cur-. rents from the west are assumed to lie in the range 40006000- and go through the two band filters whose transmitting ranges are 4000 to 6000-,, but these currents are blocked by the two band filters whose transmitting ranges are 14000 to 16000-. Accordingly, these frequencies. 4000 to 6000 go V respectively to the audion grids and plates of the modulator M But the audions being unilateral devices, the effect on the plates of the frequencies considered is nil. The modu lator M is connected with a 10000- cycle generator and the frequencies 40006000- on the grid side are modulated with the frequency 10000-, so that the output of the modulator M comprises the frequencies 10000-', 4000600 0'- and l400016000-..

Of these ranges, only that of 40006000- is passed to the west by the upper left band filter, and only the last range, namely 14000-16000-, is passed to the east through the upper. middle band filter'of Fig. 2. This frequency range 1400016000- is blocked I by the upper righthand band'filter of range 3000-5000 input of the modulator M where it is combined with the locally generated frequency 11000-, giving an output of 1l000-, 30005000-, and 25000.27000-. All of these ranges are blocked by the lower middle band filter of range 14000,16000-. Only the 30005000- band .is passed by the lower righthand band filter of Fig. 2. frequency range is applied at the three-winding transformer T at the neutral points so that it goes equally to the line to the east and to its balancing network, and not at all into the secondary circuit. Accordingly it will be seen that the 40006000- range from the west is repeated to the east as a 3000:5000- range. The full-line arrows indicate the course of the essential currents through the apparatus for this repeating operation. The dotted arrows indicate the repeating operation from east to west by which incoming currents of a frequency range 3000-5000 are transformed to output 'currents of a range '40006000-. The repetition from east to west is the same in principle as from west to east and will be understood without.

further description.

Thus it will be seen that any frequency entering the circuit from\ the east within the range of 3000 to 5000- will appear in the west increased by 1000- and reciprocally, any frequency entering from the west within the range 4000 to 6000- will appear in the east decreased by 1000-.'

A use for a unit'such asthat shown in Fig. 2 is toinstall it at the ends of alineover This Accordingly it goes to the 1 stations will make which it is desired that telephone transmisslon shall besecre't; thus, instead of an ordinary repeater station which repeats without changing the frequency, the repeater-Sta tions of Fig. 2 may be installed at the ends of astretch of line. Assuming that voice frequency currents are put in at one end and are shifted by the device, their altered frequency on the stretch between the two them unintelligible for direct reception.

It would be still more effective to invert thefrequencies and a unit that accomplishes this purpose is shown in Fig. 3. It will be seen that in general structure the arrange ment is the same as for Fig. 2." .Whereas in Fig. 2, the band filters transmitting from H000 to 16000- are transmitting an upper. side band, in Fig. 3 theband filters transmitting from 8000 to 10000- are transmitting a lower side band. The result isv that the frequencies are inverted. For example, a frequency of 500 from the westgoes to the modulator M and appears in its output as a component, 95006 of the lower side band which goes through the band filter transmitting from 8000 to 10000-. This 05500- frequency is combined in the modulator M; with the locally generated frequency 8000- and-gives 1500- in the lower side band of the output of modulator M This is passed by the lower righthand band filter whose range is 0 to 2000- and appears in the line eastas a frequency of 1500-. ln' short any frequency nbetween 0 and 2000- in the line west gives an output frequency of (2000n)- in the'lineeast, and vice versa. Accordingly, the substitution of two repeater stations like Fig. 3' for the ordinary repeater stations in a long transmission line secures the result that between those stations all the voice currents will be of inverted frequency and quite unintelligible for direct reception. 7

lt has been suggested to attain secrecy by transmitting with the voice currents a pure tone loud enough to drown out the voice and to separate this tone from the voice currents at the receiving end. However, it is difficult to separate out the pure tone lying within this frequency range without suppressing a whole range of adjacent frequen cies and seriously distorting the voice currents. By my method for example, a pure tone of lO0- might be put on the line with the voice currents so thatanywhere on the line, by direct reception, this tone would competely mask the voice currents. At the 0 receiving station the received currents would be displaced down 400 thus causing the 400- loud tone current to appear at approximately 0-.' it could then beeasily filtered out and the frequency range shifted back by 400 giving the voice frequency range substantially unimpaired as transmitted at the outset.

indicate the resultant frequency shift.

It is known that loaded cables have a tendencyrto attenuate high frequencies relatively more than low frequencies, so that transmission of a wide frequency range over a loaded cable may be impractical on this account. A much wider range can be trans I mitted on an. open wire line because the at.- tenuation does not fall ed with increasing frequency so rapidly in that case. It is common that a long distance transmission will' be partly by cable in a region of dense trafiic and partly by open wire in an outlying region. The direct connection of a cable in series with an open wire therefore cuts down the capacity'of the open wire to the frequency range which may be transmitted over the cable. lnFig. 4,1 have shown an arrange ment by which one open wire line can be made to serve for connection in series with each of two cables. This figure illustrates a carrier current telegraph system with s'm, channels each Way on an open wire line connected serially to each of two cables, each cable having hree channels corresponding to three of the six of the open wire line. In Fig. 4, the rectangles marked (Fig. 1) represent the system of Fig. 1 by which oneway frequency shift is obtained. The respective numerals in these rectangles,-viz, 300-, 12o0-, +1500-, and +3400 11 this connection it will be understood that the corresponding numeral for Fig. 1 would be 2000-. It will be seen that from west to east on the open wire line there are six carrier current frequencies ranging by 300- intervals from 3300 to 4800-. These are separated into two groups by the band filters BF and BF Then the frequencies are shifted down as indicated and as already mentioned, and each group of three carrier frequencies is put on a respective cable, each group comprising 8000, 3300 and 3600-. Other frequencies namely 4000, 4300 and 4600 come in over the cables from east to west and go through the band filters BFg and BF; to the devices like Fig. l, where the frequencies are raised by 1500- for the group from cable No. 1,"and by 3400- for the group from cable No. 2. This gives the set of six west-bound carrier frequencies, ranging by 300 cycle intervals, from 6500 to 8000 Thus it will be seen that the greater frequency range of the. open wire line as compared with the cable is utilized by making one open wire line'serve in series with each of a plurality of cables.

. In cases for which it'is desired to transmit voice currents over a line of limited frequency range, my invention may be employed to divide the voice range up and apportion the parts thereof to conductors each having a frequency range suflicient to carry its respective portion. lt is well known that transmission through ocean cables is possible only for the relativelyv low frcquencies involved in telegraphy and not for the higher frequencies involved invoice currents. Referring to Fig. 5, sup ose that there are five lines, L L L L an L each 5 of which will carry effectively. a frequency range of only 0 to 400-, and that it is desired to transmit between their common terminals a voice-frequency range of 0 to 2000-. At one end a set of five band filters will separate the. voice frequencles into five consecutive bands, each of range 400. These various frequency ranges will then be'depressed by the devices of Fig. 1, and by amounts indicated by the numerals -400-, -800-, etc., so that the output frequency ranges for each of the five devices will be from 0 to 400-. These ranges will be transmitted over the five respectivelines and at the other end the respective frequency ranges will be increased by +400 +800-, etc.,and 'thenrecombined to give a correct voice frequency range.

With the simplest types of carrier modulating circuits difficulty has been experienced .using carrier frequencies lower than about 4000-. In this case, the harmonics of the .voice frequency ange, produced by the normal action of the modulating circuits, are transmitted in the carrier band itself, 4,000 to 6,000-, and cause effective noise and quality distortion in the demodulating circuit at the receiving end. For such low carrier frequencies, and even lower carrier frequencies, when desired, "it will be seen that my system will be advantageous. Such low carrier frequencies might be useful for loaded cable. nel might be operated at say 2500- for the carrier frequency. in addition to a voice range channel.

The amount of amplifying gain at voice frequencies is often limited by the incidental noise produced in the vacuum tube circuits, as, for example, by the vibration of the vacuum tube grid. By introducing the amplification in the high frequency part of the circuit much of this trouble may be avoided. 'Accordingly, it may be desirable to shift the frequencies to a high frequency merely to secure this advantage in cases 0 where no ultimate frequency displacement is desired.

I claim:

1. The method of producing a modula tion of a comparatively low frequency by a 55 range of frequencies Without intermediate transmission, which consists in modulating a comparatively high frequenc by such range and then directly demodu ating by a frequency differing from said high frequency by the desired frequency.

2. The method of shifting a frequency band at a single station by a comparatively small frequency displacement, which consists in first shifting it by a wide frequency displacement and then at the same station A carrier telegraph chan memes shifting it back to the desired degree 0 displacement.

3. The method of producing a comparati vely small displacement of a frequency band, which consists in shifting it by modulation over a comparatively wide displacement, then filtering. out undesired frequency bands, then directly at the same station shifting back to the desired point of displacement by another modulation, and againfiltering out undesired frequencies.

4. The method of shifting the frequency range of a wave by a comparatively small frequency displacement, which consists in modulating it with one comparatively large frequency and then demodulating with another frequency differing therefrom by the ultimate displacement desired, and generating the two modulating frequencies by modulating their average with half their difference.

5. The method of repeating a frequency range which consists in shifting such range by a comparatively large frequency displacement, anddirectly at the same station shifting back by a range differing slightly from the first range, and'transrnitting the resultant frequency range.

' 6. A two-way repeater comprising means both to transmit and receive throughout one "frequency range on one side, means both to transmit and receive throughout one different frequency range on the other side, and means to shift the frequency up one way and down the other way .from one'said range to the other, the said two ranges being near to-' gether in frequency, so as to effect transmission on each side each way within and throughout the respective frequency ranges.

7. A two-way repeater comprising two three-winding transformers, two opposite modulators, one to step a frequencyup by a certain number of cycles and the other to step it down. by a number of cycles slightly different, respective generators of different frequency for these two modulators, and filters connected for two-way transmission whereby the transmission on one side of the repeater both ways is' at a slightly. different frequency than on the other side of the repeater both ways.

8. A two-way repeater comprising two three-winding transformers, two opposite modulators, one to step a frequency up by a comparatively wide frequency range and the other to step it'back by a range nearly but not quite the same, respective generators for the two modulators each of frequency high compared to the frequencies in said transformers but the two generators being of slightly different frequency, and filters connected for two-way transmission whereby the transmission both ways on one side of the repeater is at a frequency slightly different from the transmission both ways on the other side of the repeater.

til

lit

r ea-rec either transformer to a comparatively distent range and then back not far from the original-range but inverted at the other transformer. v7

11. The method of transmitting a band of waves over a conducting path which ofiers an excessive attenuation to the higher frequency components thereof, which method comprises reducing the frequencies of all thecomponents to frequencies well the eficient transmission range of said path.

12. The method of transmitting a band of waves lying between two definite frequency limits over a conductor within the transmission range of which one of said limits does not fall, which method comprises reduc-- mg the frequencies of each component of.

said band by a predetermined amount to cause said frequency limits to fall within said transmission range, transmitting said reduced frequency band over said conductor and thereafter increasing the frequency of each of the transmitted components by the same predetermined amount by which'it was reduced before transim'ssion.

13,, The method of transmitting between two points aband of waves of broader frequency extent than the transmission rangeof a practical transmission circuit which can be used to connect said points, comprising separating said band into a plurality fill of nonoverlapping frequency sub-bands of narrower extent, transmitting each of said sub-bands over an individual transmission circuit, connecting said two points, receiving said sub-bands, and re-assembling them to produce said original band of waves. 7

M. The method of transmitting between two points a band of waves of greater fre quency extent than the practical transmission range of a circuit thatmay housed to connect said points, comprising dividing said band into a plurality of non-overlapping frequency bands, reducing the component frequencies of each of said v plurality of bands, transmitting each of said reduced frequency hands over an individual circuit connecting said points, restoring each of said transmitted bands to its original frequency position, and re-assembling said restored frequency bands to reproduce the original band.

15. The method of transmitting over a conductor waves having components higher than the practical upper frequency transmission limit of said conductor, which com prises so combining said waves with a wave of a single frequency as to reduce the component frequencies by said single frequency, transmitting said wave of reduced frequency components over said conductor, and thereafter so combining said transmitted wave with a wave of said single frequency as to increase the component frequencies of the -transmitted wave to their original magnitudes. v

16. A. transmission system for transmitting a band of waves over. a conducting path which ofiers excessive attenuation to the of elicit the componentsof said band to frequencies well within the emcient transmission range of said path, and transmitting said reduced frequency components over said path.

l'Z.- A system for transmitting a band of waves lying between two definite frequency limits over a conductor within the transmission range of which one of said limits does not fall, which comprises means for reducing the frequency of each component of said band by" a predetermined amount to cause said frequency limits to fall within said transmission range, means for transmitting said reduced frequency band over said conw ductor to a distant station, and means at .said station for increasing the frequency of each of the transmitted components by the same predetermined amount by which it was reduced before transmission.

18. A system for transmitting between two points a band of waves of broader frequency extent than the practical transmission range of a circuit which can be used to connect said points, comprising means for separating said band of waves into a plurality of non-overlapping frequency sub-bands of narrower extent, an individual transmission circuit for each of said sub-bands connecting said two points, means for supplying each sub-band to its individual transmission cir fill higher frequency components thereof, comprismgmeans for reducing the frequencies rot cuit, means for receiving said sub-bands at a distant point, and means for re-assembling them to produce said original band of waves. 19. A system for transmitting between two points a band of waves of greater frequency extent than the practical transmission range of a circuit that may be used to connect said points, comprising means for dividing said band into a plurality of nonoverlapping frequency bands,.means for reducing the component frequencies of each of said plurality of bands, means for trans mitting each of said reduced frequency hands over an individual circuit, means for restoring each of said transmitted bands to its original frequency position, and means for re-assembling said restored frequency bands to reproduce the original hand.

20.. A system for transmitting over a conductor waves having components higher than the practical upper frequency trans wave after transmission with a wave of said mission limit of said conductor, which comsinglefrequency to increase the component 10 prises means for combining said waves with frequencies thereof to their original frea wave of a single frequency to reduce the quency magnitudes.

component frequencies by' said single fre- In testimony'whereof, I have signed my quency, 'means for transmitting said wave name to this specification this 26th day of of reduced fre uency components over said January, 1922.

conductor, an means for combining said HERMAN A AFFEL,

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