US2374567A - Multichannel carrier transmission system - Google Patents

Description

TO FIG. I

April '24; 1945.

R. C. TAYLOR MULTICHANNEL CARRIER TRANSMISSION SYSTEM Filed Sept. 6, 1944 3 Sheets-Sheet 2 FIG. 2

J INVENTOR J R.c. TAYLOR ATTORNEY April 24, 1945. R. c. TAYLOR 5 7 MULTICHANNEL CARRIER TRANSMISSION SYSTEM Filed Sept. 6, 1944 3 SheetsSheet 3 53 INVENTORJ 5 R.C. TAYLOR s V o BY ATTORNEY Patented Apr. 24, 1945 MULTICHANNEL GARRIER TRANSMISSION SYSTEM Roland C. Taylor, Ridgewood, N. J., assignor to The Western Union Telegraph Company, New York, N. Y., a corporation of New Yorlr Application S eptemhcr 6, 1944, Serial Nclssasao 9 Claims. (01. 178-51) This invention relates to a method and means for improving high frequency carrier systems,

suitable for the transmission of carrier telegraph, telephone or facsimile signals, andespecially to such a multichannel system in which substantial advantages result from the mode of grouping the basic channels and the translation l of these channels to and from the respective positions which they occupy in the available frequency spectrum of the transmission line.

The transmission space in the frequency s-pectrum which; may be made available on a given transmission line is in generalpropontional to the expenditure invested in the line. For example, an ordinary pair ,of open line wires, which represents a minimum in line expenditure, will handle satisfactorily the voice range of frequencies; it requires only a relatively simple transposition system, and repeaters may be spaced at long intervals such as a few hundred miles. As the frequency range is extended, however, it becomes necessary to transpose the line more frequently in order to prevent cross talk and noise, and to space repeaters at smaller intervals because of the increased attenuation. Also, if intermediate cable sections occur, loading coils must be inserted at frequent, intervals. Each of these measures expands in cost at a raterough- 1y proportional to the frequencies to be transmitted. It follows, therefore, that the efficient utilization of ithe frequency spectrum rendered available on a given transmission. line through a given expenditure ofmoney is a matter of considerable economic importance. Another general rule applicable, toqhigh frequency carrier systems is that for a given type of quality of filter or selective circuit, the relative sharpness of cut-off is roughly proportional to the frequency, that is, if the characteristic curve of a filter cuttingoff at a frequency of 1000 cycles consumes a space 100 cycles wide in passing from minimum to maximum attenuation, 10% of the frequency space is therefore wasted. At higher frequencies, for example, 30,000 cycles, a 10% wastage amounts to a 3000 cycle band which is a, frequency space of considerable economic value. Because of this ever increasing dissipation of useful frequency space, it is desirable to perform frequency selections, in so far as it is possible, in the low frequency range.

Band pass filters of very narrow range, such as are often used to accommodate carrierztelegraph channels spaced, forexample, 300cycles apart, may be constructed with relatively good Basic carrier telegraphic systems therefore usually comprise a group of from eight to twelve or more channels.

herein, these channels comprisingai total fre quency band approximately 3000 cycles widaand such a band is alternatively suitable for telephone transmission purposes. Suchan arrangement fulfills the requirement of minimuinorlginal cost and convenience of handling in them'ocess of openation. Whengreater messagecapac ity is needed, and since it is uneconomicalto construct channel filters along these linesiorflthe higher frequencies, it has been the usual practice heretofore to bodily step upward the basic nine channel group into the next adjacent'3000 cycle band. If further channels were needed, they were obtainedby a similar process ,of moving other nine channel basic groups into successame rate, at 30,000cycles a band 5100 cycles Proportionately large bands wouldbe wasted at junctions between free quency bands located at still higher frequencies.

sive higher 3000 cycle spaces in the available frequency. spectrum. However, even with these comparatively wide bands this processbecomes very wasteful of frequency space as soonas the relatively higher range is reached, because the essential grouping filters which must beflemployed in various phases in the systemconsume a considerable frequency space at theirtcut-offs.

The instant invention eliminates a large part of the foregoing waste in frequency space. Consider, for example, a thirty-six channel, twowire, 30kt}. carrier telegraph system designed according to, prior practice. each of the frequency bands which accommodates a basic group of nine 300 cycle spacedcarwide would be wasted.

Thus, a two-way four band 36 chamiel system of the type heretofore employed would require a frequency space of approximately 50.5 kc.,whereas in accordance with the. instant invention in a system of this size only 31 kc. would be required. This added space would more than accommodate two additionalgroups of two-way channels, and

efliciencyup to frequencies of 5000 or 6000'cycles.

at higher frequencies many more additional groups :of two-way channels maybe obtained.

Nine pba'sicchannels are fre quently employed, as in the system illustrated This means that and receiving equipment T and R. understood that a full metallic circuit is em- One of the objects of the invention is to reduce waste of frequency space in a high frequency carrier system and to enable a larger number of useful channels to be employed in a given frequency spectrum.

Another object is a system of the character demg drawings, in which:

' Figs. 1 and 2 illustrate schematically a 72 channel two-way carrier sending and receiving system embodying the principles of the invention; Fig.3 shows diagrammatically the cut-off valules and arrangement of basic channel group filters, unit group filters anddirectional separation filters employed for effecting transmission over the system of Figs. 1 and 2; and

Fig. 4 shows diagrammatically the filter arrangementfemployed for effecting two-way com i'municationovera 144 channel carrier circuit.

R eferring to Figs. 1 and 2, which together comprise a, one-line diagram of a '72 channel,

two-way carrier system, Fig. 1 shows the transmitting and receiving equipment T and R at one terminal, and Fig. 2 shows similar transmittin It will be ployed for such carrier systems, and that each circuit shown represents a pair of conductors.

The two terminals may be connected by a line L,

it being understood that suitable repeater points are interposed at various intervals in the line, the number and kind of such repeaters depending upon the length, attenuation and other characteristics of theline.

' Referring to the transmitting side T of the terminal shown in the upper half of Fig. 1, it

will be seen that there are eight basic groups a to h of transmitting channels. Each of the basic channel groups provides nine transmitting channels, the frequencies of which range from 750 cycles to 3150 cycles, the various channels bein spaced 300 cycles apart. The eight basic groups of channels are arranged to form four identical unit pairs by the process of stepping one group above the other by a single modulation, as by a modulator Ml using a 3.3 kc. carrier, with selection of the upper side band for transmission. The first basic group a of nine channels is connectedby a circuit H] to a band pass filter F which passes the frequency range of 600 to 3300 cycles. The other basic group b of nine channels is. translated upward by the modulator Ml, the

output of which passes to a hand pass filter Fl which passes the frequency range of 3900 to 6600 cycles produced by the modulation step. The output of filters F and F1, which comprise a unit group of frequencies from 600 cycles to 6600 cycles,are passed through circuits H and 12 to a modulator M4, hereinafter described. Th nit group thus formed from the basic groups is in- }dicated by thebracket which encloses the two basic groups a. and 2). Immediately below this first unit group is a second unit ground which similarly is composed of two basic groups c and d. The frequencies of the basic group c are applied by means of the circuit 54 to the band pass filter F, which is like the first-named filter F,

' and which passes a frequency range from 600 to 3300 cycles. The second basic group (1 of frequencies of the second unit group is translated upward to the position justabove group c, as by a modulator Ml using a 3.3 kc, carrier, the output of which is passed through a band pass filter Fl. The output of filters F and F1 of the secnd unit group is moved upward, as by a modulator M53 using a 6.9 kc. carrier, so that the channels of the second unit group are in a frequency range from 7500 to 13,500 cycles, this latter range lying just above the frequency range of the first unit group. The output of the modulator M2 is passed through a band pass filter F2 which passes frequencies from 7500 to 13,500 and thence through circuits H and 12 to the modulator M4.

The remaining four basic channel groups 6 to h which are immediately below the first four channel groups are similar thereto. The chan-' nels of the fifthbasic group e are connected by circuit It through a filter F; the sixth basic group j is translated upward by a modulator MI, and the combined fifth and sixth basic groups are applied by circuit i? to a modulator M3. At the same time'the seventh and eighth basic channel groups 9 and h are stepped upward by an ar- 'rangement like that above described with respect to thethird and fourth basic groups, the output of modulator M2 andband pass filter F2 being applied by means of the circuit 11 to the modulator M3. In the latter modulator the frequencies of the last four basic channel groups are 'moved upwardly by a single modulation using a approximately 28,500 cycles is to be used for transmission from the station of Fig. 2 to the station of Fig. l, and therefore the previously assembled group of' seventy-two channels of Fig.

1 is relocated by the modulator'M4 using a 34.5 kc. carrier so that the combined channels will lie within the band of 35,100 cycles to 63,000 cycles, which band is passed by a filter F4. From this band pass filter the frequency band is amplified by a terminal amplifier Al and thence passes through another band pass filter F 3 and circuit 20 to the outgoing line L. I

At the receiving station R of Fig. 2, the band of frequencies transmitted is received from the line L and applied by means of a conductor 30 to a band pass filter F4, the output of which is am-f' plified by the receiving amplifier Al, and the transmitted band of frequencies 35,100 to 63,000

cycles is stepped downward by a first demodulator DM l using a 34.5 kc. carrier. Frequencies within the range 600-13,500 in the output of the first demodulator are applied by means of conductors 32 and 33 to receiving band pass filters F, FI and F2 of the upper pair of unit group but are rejected by F3. From filter F the frequencies 750 to 3150 cycles originally transmitted by group a of Fig. 1 are applied by means of a circuit 34 to 6.9 kc. The band 600 to 3300 cycles in the output" of the design; installation and maintenance of such carriersystems.

which are being transmitted in the same direction From the filter F2 the band of freare at substantially the same level, the problem of separating the various groups and suppressing unwanted frequencies is greatly facilitated. At the line terminals and at the inputand output of repeaters (not shown). directional grouping filters having frequency ranges of. approximately of this demodulator is applied to a band pass filter -'F and by means of a circuit 35 to the third basic channel group 0'. The band 3900 to 6600 cycles in the output of DM2 is applied to a receiving filter Fl and a demodulatorDMl where they are demodulated by a frequency of 3.3 kc. to produce the original frequencies 750 to 3150 in th fourth basic channel group cl.

The band 15,600 to 28,500 cycles in the output of. the demodulatorDM l is applied by means of the circuit 32 to a receiving band pass filter F3, theoutput of which is applied to a demodulator DM3 and demodulated by a frequency of .15 kc, the relocated frequencies thus lying within the band from 600 to 13,500 cycles. The band 600 to 3300 cycles in the output of DM3 is applied by means of a circuit 36 to a filter F and thence to the fifth basic receivingchannel group 6. The

band of frequencies from 3900 to 6600 cycles is applied from circuit 36 through a filter Fl to a demodulator DMl where it is demodulated by a frequency of 3.3 kc. to give th sixth receiving basic channel group J. The band from 7500 to 13,500 cycles in the output of DM3 i applied through a filter F2 to ademodulator DM2 to same manner as transmission from theportion T1;

of the station of Fig. 1, the circuits which interconnect the filters and modulators being designated by the same reference numerals as inFig. 1, with a prime mark added. The station of Fig.

2. differs, however, in that from acircuit 12 the transmitted band of frequencies from 600 to 28,500 cyclesis not stepped upv as in the case of thetransmitting circuit of Fig. 1, butinstead the band is passed through an amplifier A2 and filter F5 and thence by the circuit to the line L. In

the receiving side Rof thestation of Fig. 1 the band 600 to 29,500 cycle is applied to a filter F5 and a terminal amplifier AZ. By means of circuits 32' and 33 a band 600 to6600 cycles isapplied to filters F and Fl. From filter F the band 600 to 3300 cycles applied to the first receiving basic group of channels a, and the band 3900 to 6600 is relocated by the demodulator Ml and applied to the second, receiving group b. i In a l manner similar to that heretofore described with 'respect to the receiving equipment R of Fig.2, the remaining frequencies from 7500 to 28,500

cycles are relocated and applied to the remainin receiving basic groups c to h.

It will be noted that only a very small number of different types of filters, modulators, demodulators and carrier frequenciesare required, and that such devices are relatively few in number. This economy in types and numbers of equipment itemspermits a considerable saving in the cost 600-28,500 kc. and 35,10063,000 kc. separate the transmissions in both directions. At these points a considerable divergence in leyel may exist between the received frequency and the amplified transmitted frequencies, and the requirements for these filters is accordingly more critical, Forthis reason a wider spacing is necessary between the directional groups than at the division points be tween groups which are being transmitted in the same direction. Also, the band widths are in convenientmultiples, as, appears from Figs. .1 and 2, so that a varietyof services may readily be accommodated in which bands of widths most suitable to such services may be employed, such groups of double band width extending from 0 to 7500, andthe like.

The relatively few types of filters required and the economy in frequency space are-readily seen from Fig. 3 of the drawings, which outlines the positions of the bands of fre quencies in the seventy-two channel system illustrated in Figs. 1 and 2, the boundary frequencies of the various grouping and directional separation filters also being indicated. Each of the filters F passes a bas c group of nine carrier telegraph channels spaced 300 cycleslapart. From anybasic group of nine channels in one direction, the next group of nine channels is located in a position just above and with a separation of approximately 600 cycle between the adjacent channels of the adjacent basic groups. The separation between the directional filters F4 and F5 of 28,500 to 35,100 cycles constitutes a 6600 cycle loss of. useful signaling frequency space, or 2.0% of the mean frequency at the point of separation. It will be noted that this frequency space 'is the only one occurring in the entire system in which a substantial waste of frequencies'occurs. If a multi channel system were constructed according to any I one above the other in the frequency spectrum.

Such wastage of frequency space becomes increasingly severe in the higher portions of the spectrum. The cumulative effect of such wastage means that a great many signaling channels must. be omitted which otherwise could be used with a given frequency spectrum. In the instant arrangement, on the contrary, the spacingbetween adjacent basic groups is uniform and is required to be no greater in the upper portions of the spectrum than in the lower portions thereof, and

thus an important saving in frequency space is effected.

In the foregoing description it has been assumed that the upper side bands of each channel were selected for transmission in eachcase.

With such an arrangement the basic channels retain their original order in the group throughout; for example, the 750 cycle telegraph channel remains always at the low frequency end of its group while the 3150 channel lies at the upper frequency end. As is well known, the characteristics of band filters, such as attenuation, im-

Since all the channels 7 pedance'and phase shift, make marked departures from the linear ideal at the edges of the filter pass band. Therefore the 750 and 3150 channels only will consistently encounter these filter defects and thus suffer impairment to the exclusion of the channels located near the center of the band, and circuit layout engineers and traffic dispatchers must keep these characteristics in mind in making certain assignments. It may be preferable in many instances to cause these cumulative impairments to transmission to be concentrated upon the lowest frequency channels rather than be divided between the upper and lower channels, and in this case circuit assignments may be made upon the premise that only the lower frequency channels are markedly inferior. The channels in the instant system may therefore be rearranged so that only the basic low frequency channels would lie in most cases near the edge of the group by employing instead of the 3.3 kc. carrier indicated in the to relocate the channels 750 to 3150 so that they would be received in their initial order. The value of 7.2 kc. is chosen in the instance given in order that the -basic grou 750 to 3150 cycles would be stepped up to the same band 3900 600 obtained whenthe upper side bands are chosen as in the case first described, and thus no other "changes in the remainder of the system need be made. The upper frequencychannels will then be free of the impairments to transmission inflicted by the imperfections of the various band filters and so could be regarded always as preferred channels.

In the system illustrated two basic channel groups, for example, groups a and b, are combined into a unit group, and two unit groups are translated into positions one above the other in the frequency spectrum, but it will be understood that the number of basic channel groups thus combined to form unit groups and the number of unit groups translated to form larger unit groups may be varied, depending upon the number of channels to be made available in the system. Thus, three, four or more basic channel' groups could be combined to'form'unit groups, and three, four or more unit groups could be translated to form larger unit groups which would be translated into positions one above the other in the spectrum. Also. the particular range of frequencies employed may be varied within wide limits, depending upon the character or type of service with which the system is to be used.

Moreover, the cut-oil values of the band pass filters employed may vary within considerable limits, depending upon the efiiciency and type of filters employed. In the case of the basic group filters F these may, for example, be low pass filters 0-3300 cycles instead of band pass filters 600-3300 cycles, although the latter are preferred in order to eliminate/possible interference Dres ent due to extraneous frequencies below 600 cycles. Similarly the directional filter F4 may be a high pass filter for 35,100 cycles and above.

Fig.4 illustrates the advantages of the invention when applied to a 144 channel, two-way carrier system, and it will be noted that with the exception of the necessary separation between the wide band pass filters F4 and F5, and F6 and F1, there is no wastage in frequency space notwithstanding that sixteen basic group filters are employed in each direction and that these basic groups are translated one above the other into high regions in the frequency spectrum. In other words, the only substantial frequency space losses are those occasioned by the directional separation filters F5 and F1 and the wide band group filters F' l andF5.

'In the foregoing description unit groups and successively larger unit groups were composed of pairs of the next smaller groups. however, was illustrated only since the number of groups used to build the next larger group may be any reasonable number and furthermore the'numbers need not all be the same.

Various modifications of the circuit arrangement and apparatus shown, and various equivalents or substitutes in the devices illustrated,

will readily occur to those versed in the art without departing from the spirit and scope of the present invention. The disclosure, therefore, is for the purpose of illustrating the principles of the invention which is not to be regarded as limited except as indicated by the scope of the appended claims.

What is claimed is:

1. In a high frequency carrier transmission system wherein a plurality of basic carrier channels may be transmitted over a-single transmission path, the method of substantially reducing the idle frequency space required to avoid interference between the channels which comprises forming the carrier channels into a plurality of basic channel groups each of which comprises a plurality of carrier channels occupying the same frequency band in the lower portion of the available frequency spectrum of the transmission path, translating certain of the basic channel groups each to an adjacent position above another of said basic channel groups to form. a plurality of unit groups each of which occupies the same portion of the frequency spectrum,

translating certain of said unit groups to an adjacent position above another of said unit groups to form larger unit groups each of which occupies the same portion of the frequency spectrum, translating at least one of said larger unit groups into an adjacent position above another of the larger unit groups to form a still larger'unit group and transmitting the last-named group over said transmission path.

' 2., In a high frequency carrier transmission system wherein a plurality of basic carrier channels may be transmitted over a single transmission path, the method of substantially reducing the idle frequency space required to avoid interference between the channels which comprises form ing the carrier channels into a plurality of basic channel groups each of which comprises a plurality of carrier channels occupying the same frequency band in the lower portion of the available frequency spectrum of the transmission path,

said unit groups to an adjacent position above the other unit groups to form larger unit groups This choice,

path themethod of substantially reducing the idle frequency space required to avoid interference betweenthe channels which comprises forming the carrier channels into a plurality of basic channel groups each of which comprises a plurality ofcarrier channels occupying the same frequency band in the lower portion of the available frequency spectrum of the transmissionpath,

translating alternate ones of the basic channel groups to adjacent positions above the otherbasic channel groups to form a pair of 'unit'groups each of which unit groups occupies the same portion of the frequency spectrum, translating one of said unit groups of each pair to an adjacent position above the other unit group of the pair to form a pair of larger unit groups each occupying the same portion of the frequency spectrum, translating one of said larger unit groups into an adjacent position above the other larger unit group of said pair to form a still larger unit group and transmitting the last-named group over said transmission path.

4. In a high frequency carrier transmission system wherein a plurality of basic groups each comprising a plurality of carrier channels may be transmitted over a single transmission path,

the method of substantially reducing the idle frequency space required to avoid interference between the channel groups which comprises combining at least n basic channel groups into a unit group, translating at least n1 such unit groups into adjacent positions one above the other in the available frequency spectrum of said transmission path to form a larger unit group, and then translating at least n2 such larger unit groups into adjacent positions one above the other ,in said available frequency spectrum, where n,

121 and n2 represent integers each of which is an exact divisor of the total number of said carrier channels to be transmitted in one direction over said transmission path.

5. A high frequency carrier transmission system for the transmission-of aplurality of basic carrier channels over a single transmission path, comprising means for connecting the basic carrier channels into a plurality of basic channel groups each of which comprises a plurality of .basic channels occupyingthe same frequency band in the lower portion of the available frequency spectrum of the transmission path, means i for modulating certain of said basic channel groups to translate them upward to an adjacent position above the remainder of said basic channel groups and form a plurality of unit groups each of which occupies the same portion of the.

6. A high frequency carrier transmission system for the transmission of a plurality of basic carrier channels over a single transmission path, comprising means forconnecting the :basic carrier channels into a plurality of basic channel groups each of which comprises aaplurality of basic channels occupying the same frequency band in the lower portion of the available frequency spectrum of the transmission path, means for modulating alternate 101183 of said basic chan: nel groups to translate them upward t-oran ad jacent ,IJOSitlOl'l above the remainderpf said lbasic channel groups and form a plurality :of unit groups each of which occupies thesameiportion.

of the frequency spectrum, means for modulat- :ing alternate ones ,of said nnit groups to translate them upward to an adjacent position above the remainder of said unit groups and form larger unit groups each of which comprises the same portion of the frequency spectrum, and means for modulating at least one of said larger unit groups to translate it upward to an adjacent position above the other larger unit groups and form a still larger unit group for transmission over said transmission path.

'7. A high frequency carrier transmission system for the transmission of a plurality of basic carrier channels over a single transmission path, comprising means for connecting the basic carrier channels into a plurality of basic channel groups each of which comprises a plurality of basic channels occupying the same frequency band in the lower portion of i the available frequency spectrum of the transmission path, means for modulating alternate ones of said basic channel groups to translate them upward to an adjacent position above the remainder of said basic channel groups and form a pair of unit groups each of which occupies the same portion of the frequency spectrum, means for modulating one of saidunit groups of each pair to translate it upward to an adjacent position above the other unit group of the pair andform a pair of larger unit groups each of which comprises the same portion of 'the frequency spectrum, and means for modulating one of said larger unit groups to translate it upward to an'adjacent position above the other larger unit group of said pair and form a still larger unit group for transmission over said transmission path.

8. 'A high frequency carrier transmission system for the transmission of a plurality of basic carrier channels over a single transmission path,-

comprising means for connecting the basic carrier channels into a plurality of basic channel groups each of which comprises a plurality of basic channels occupying the same frequency bandin the lower portion of the available frequency spectrum of the transmission path, means for modulating at least 11 of said basic channel groups to translate them upward to an adjacent position abovethe remainder of said basic channel groups and form a plurality of unit groups each of which occupies the same portion of the frequency spectrum, means for modulating at least n1 of said unit groups to translate them upward to, an adjacent position above the remainder of said unit groups and ,form larger unit groups each of which comprises the same portion of the frequency spectrum, and means for each'of which is an exact divisor of the total number of said carrier channels to be transmitted in one direction over said transmission path.

9. A high frequency carrier transmission system for'the transmission of a plurality of basic carrier channels over a single transmission path, comprising means for connecting the basic carrier channels into a plurality of basic channel groups each of which comprises a plurality of basic channels occupying the same frequency band in the lower portion of the available frequency spectrum of the transmission path, each of said basic channel groups having a band width of the order of 2500 cycles to 3300 cycles, means for modulating certain of said basic channel groups to translate them upward to an adjacent position above the remainder of said basic channel groups and form a plurality of unit groups each of which occupies the same portion of the frequency spectrum, means for modulating certain of said unit groups to translate them .upward to an adjacent position above the remainder of said unit groups and form larger unit groups each of which comprises the same portion of the frequency spectrum, and means for modulating at least one of said larger unit groups to translate it upward to an adjacent position above the other larger unit groups and form a still larger unit group for transmission over said transmission path.

ROLAND C. TAYLOR.

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