US1676627A - Method and means for reducing cross talk in carrier-current signaling systems - Google Patents

Description

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'Patented July 10, 1928,

1,676,627 PATENT oFFlcE.

HAROLD IBL-ACK, OF EAST ORANGE, NEW JERSEY, .AS-SIGNOR TO BELL TELEIHONE LABORATORIES, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK.

MnTHonAND MEANS 'Fon REDUCING cnoss TALK 1N CARRIER-CURRENT sIGNALINa SYSTEMS.

.Application led August 30, i926. Serial No. 132,452.

This invention relates to methods and means for reducing crosstalk incarrier culrent signaling systems.

An object of the invention is to reduce 5 crosstall between adjacent signaling systems.

A related object of the invention is to reduce reflection effects such as are commonly caused by impedance irregularities in electric signaling systems. 1

The invention provides a simple and efficient arrangement fior reducing crosstalk between. adjacent carrier current signaling systems which consists in attenuating reiected waves at one terminal of each system to such an extent that their effect at a distant terminal is negligible, and also in maintaining the currents transmitted in the same direction in each system at substantially the same energy level at all points where the transmission lines rup adjacent each other.

When two or more signaling systems are operated on the same pole line or in the same cable, one of the principal problems encountered is to reduce crosstalk or mutual interaction due to inductive or capacity effects between adjacent systems.

In order to prevent crosstalk in voice frequency signaling syst-ems it is customary to transpose the pairs of wires at intervals which are short relative to the wave lengths of the currents employed. This can usually be done without hardship in such systems since the frequencies are relatively low and the wave lengths correspondingly long. In carrier current signaling systems, however, where the wave lengths involved are relatively short, transpositions are required at much more frequent intervals and the problem of reducing crosstalk becomesA increasingly difficult. y

It has been found that crosstalk can be materially reduced by reducing reflection effects which appear in the form of waves which are refiected from terminal or re-l peater devices which anc connected toa transmission line but do not accurately match the impedance of the line. A method of reducing such reflection effects is dis closed, for example, in a copending application of C. W. Green, Serial No. 132,670, filed Aug. 31, 1926, in which a loss element is-arranged to be traversed by the refiected Wavesagreater number of times than by the signal, so that a discrimination is made in favor ot' the signal. One advantage of this method is that waves which are reflected back over one transmission line due to impedance mismatching are attenuated to such an extent that -they can produce only negligible crosstalk in an adjacent line.

According to a feature of the present invention, a loss element is inserted between atransmission line and a terminal grouping filter' or other device from which the crossta-lk components are reflected so that the crosstalk components will be greatly attenuated in traversing the loss element twice, once before they are reflected and again after they are refiected. By thus reducing crosstalk refiection effects at one terminal of a transmission line their effect at a distant terminal is materially reduced and the problem of transposing the line circuits is greatly simplified. I

According to another feature of the invention, crosstalk is reduced by maintaining similarly directed signaling'currents on adjacent lines at substantially the saine energy level. A special case of this to which the invention applies is where a number of carrier lines are carried on t-he same pole line and terminate at different geographical points. For example, in a single channel carrier currenttelephone system, such as that hereinafter described, one such carrier line may extend from A to B, while another carrier line may come in on the same pole line at some intermediate point C and extend beyond the point B. According to this invention, crosstalk between such systems is reduced by employing the same energy levels on the lines where they run adjacent each other. This is preferably accomplished by raising the energy level of one line by amplification at the transmitter and by lowering the energy level of the other line by a resistance pad at the transmitter, compensation being effected at the receiversby a pad and amplifier, respectively.

The invention will be described in connection with its application to single channel carrier current telephone systems, but it will be evident that it is also applicable to other types of signaling systems.

In the drawing:

Fig. l is a diagranmiatic illustration showing two single channel carrier current telephone sytems having adjacent transmission lines,l and provided with means for re- -ducing reflection from the terminal grouping filters; and

Fig. 2 is a diagrammatic illustration showing two similar systems, in which inutual interaction is reduced by employing the same energy levels on both transmission lines where they rnn adjacent each other.

In Fig. 1,` the terminal circuits of two single channel carrier current telephone systems, located at stations 1V and E, are interconnected by the main transmission lilies ML1 and ML2, respectively, which are located on the same pole line or in the saine cable. The single channel systems per se form no part of the present invention, but may be of the types disclosed in U. S Patent No. 1,602,019, issued Oct. 5, 1926 to C. L. lVeis, Jr., or in my U. S. Patent No. 1,653,837, issued Dec. 27, 1927.

The two systems shown in Fig. 1 may be similar in all respects and hence only the circuits associated with the line MLl will be described in. detail. This system isv composited for simultaneous low frequency and carrier current communication. This is accomplished by carrier composite sets, the set at station 1V comprising the voice frequency filter VF and the carrier frequency filter OF, and the set at station E comprising similar filters VF and OF. VThe filters VF and VF are positioned in the voice frequency telephone branch, and the filters OF and CF in the carrier current branch, of the transmission line. These filters serve to separate the currents corresponding to the frequencies used in their respective types of transmission. Obviously, the system could be used for carrier transmission only, in which case these filters could be eliminated.

The filters of the carrier composite sets, described above, and each of the other filters shown throughouttlie systems may be dcsigned in accordance with the principles set forth in U. S. Patent No. 1,227,113 to A. Campbell, issued May 22, 1917.

The terminal apparatus at station 1V includes a transmitting channel TC and a receiving channel RO. and the terminal apparatus at station E includes a transmitting channel TC and a receiving channel RC. Carrier currents of different frequencies are utilized for transmission in opposite directions over the line MLl, the currents transmitted', from station E to station 1V being chosen of higher frequency than the currents transmitted `from station 1V to station E.

The transmitting channel TC includes an oscillator-modulator OM and a 10W frequency band filter BF while the receiving channel RC includes a 10W pass filter LP, an

oscillator-detector OD and a high frequency band filter BF1. The filters BF and BF1 serve to separate the directional bands of frequencies to the respective terminal transmitting and receiving channels, the'lower band of carrier frequencies being assigned to channel TC and the upper band of frequencies being assigned to channelRC. The corresponding elements .at station E are designed by similar reference characters with primes alliXe-d thereto.

A lowv frequency line L, which may be an ordinary subscriber-s telephone line is associated with channel TC-RC at station "W for communication over the line MLl With a similar low frequency line L associated with channel 'PU-RC at station E. The low frequency lines L and L are provided with balancing artificial lines or networks N and N respectively, and with differential repeating coils H and H, commonly knownl as hybrid coils, for enabling independent transmission in tivo directions between the lines and theterminal circuits.

Voice frequency currents originating in thc low frequency line L at station WV pass through the associated hybrid coil H into the oscillator-modulator OM where they are combined with carrier currents of the frequency assigned to channel TC. Of the components of modulation appearing in the output circuit of the oscillator-modulator OM, the low frequency band filter BF suppresses all but one side band, for example, the lower side band, which it'transmits or passes to the transmission line MLl.

rl`he side band currents incoming at station E pass through the low frequency band filter B191 in the receiving channel RO', and are detected in the oscillator-detector OD. Voice frequency components of demodulation appearing in the output circuit of oscillator-detector OD are selectively transmitted by the low pass filter LP and pass through hybrid coil H to the low frequency line L. Transmission from station E to station W is accomplished in the same manner, except that higher frequency carrier currents are employed.

1n order to prevent near end crosstalk between ,transmission lines ML, and ML2, the Iarrier currents utilized for transmission in each direction are of the same frequency in both systems. That is, the currents transmitted from station 1V to station E are of the same frequency in both systems, and the currents transmitted from station E to station 1V are likewise of the saine frequency in both systems. It will thus be seen that even though a signal modulated carrier Wave of the lower frequency band transmitted over line MLg from station W appears in line ML,L as crosstalk, currents of this frequency are not passed by the band filter BFI, in the receiver channel RO and', of course, cannot pass backwards through the transmitting channel TC. This frequency allocation likewise prevents objectionable'near end cross talk from line- ML, into line ML2. Y

If, however, impedance irregularities exist in the terminal apparatus at station W', for

example, if the band filter BF is improperly termina-ted, signal modulated waves following the path from line ML2 to line ML, and meeting a change in impedance in the filter BF will be refiected over the line ML, and, beine' readily transmitted by the band filter BF, will appear in the receiving channel RC as far end Crosstalk. The same effect is produced when a portion of the lower frequency signalmodulated carrier current transmitted over line ML2 from station W to station E is reected from the low frequency band filter associated with the line ML2 at station E, taking the path 6 directly into the band filter EF1. Similar Crosstalk may occur in' the upper frequency band transi mitted in the opposite direction.

It is very difficult to prevent reflection in the terminal grouping filters for the reason that only a slight error in the impedance of the elements of a filter is sufficient to prO- duce considerable refiection. It is practically impossible with present manufacturing methods to build commercial condenser elements to closer than 0.8% accuracy or to wind coils to closer than 1.2% accuracy, making a total of 2% gross error in the filter elements. This is sufficient to produce an impedance irregularity of as much as 60% near the edge of the frequency scale of the filter.

In order to compensate for the effect of improper filter termination, acording to this invention, a resistance pad K is inserted bctween the terminal grouping filters and the carrier composite set at station W and a similar resistance pad K is inserted between the terminal grouping filters and carrier composite set at station E. Similar resistance pads K2 and K2 are provided -in the line ML2. With the pad Kin the position shown, the energy which is reflected passes through the pad twice, once in each direction, whereas the outgoing and incoming signaling currents pass through the pad but once. This produces a definite discrimination between Crosstalk components and the signaling components. The resistance pads may be of the type disclosed in U. S. Patent Np. 1,591,073 issued July 6, 1926 to O. J. Zobel, having a constant resistance characteristic impedance suitable for terminating the carrier circuits.

Where a large number of single channel systems are operated on the same pole line it is y'often desirable to terminate the systems at different geographical oints, as illustrated in Fig. 2. The line M in Fig. 2, for example, may extend from A. to B, a distance of' say 200 miles, while the line ML2 may come in at some intermediate point C and extend to the point D, some dlstance beyond the point B. Crosstalk between these systems is materially reduced by employing the same energy level for similarly directed currents on the lines ML, and ML2 where they run adjacent each other. This is accomplished by inserting level adjusting pads in certain instances on the drop sides of the carrie-r terminal apparatas, and by the use of terminal amplifiers to control the gain levels. At the terminal A, an amplifier G is inserted in the transmitting -channel TC bet-Ween the two sections of a split band filter BF to raise the outgoing currents to the desired level, and at t-he terminal B a resistance pad P is inserted between the two sections of a similar band filter BF, to reduce the level of the incoming currents to the desired value for reception. At the terminal C a resistance pad P2 is inserted between the sections of the band filter BF2 and serves to bring the energy level of the currents transmitted to the line ML2 down to the point whe-re it is equal to the energy level of the currents transmitted in the same direction over the line ML, at the point X where the line ML2 comes in on the same pole line with the line ML,. At the terminal D an amplifier G2 is inserted between the sections of filter BF3 in the receiving channel RC2 to raise the level of the incoming currents which were reduced at station C in the manner de-4 scribed. Since this. terminal 4is at the far end of the pole line, an lamplifier G2 is included in the transmitting channel TG2 to raise the outgoing currents to the desired level, and a resistance pad P2 similar to the pad P at terminal B is inserted in the receiving channel RC2 at terminal C. A resistance pad P is also provided in the channel TC at terminal B to bring t-he energy level of the currents transmitted to the line ML, down to the same level as similarly directed currents at the point Y on the line ML It2 will be noted that in carrying out the invention described inconnection with Fig. 2, the upper bands of carrier currents are transmitted in the same direction over both transmission lines, while the .lower bands' of carrier currents are transmitted 4in the opposite direction. It is therefore part in the termination of the main transmission lines. These pads and the gain adjusting amplifiers are preferably variable in order to renderthcm suitable for universal application. These elements may therefore be appliedto existing systems to control the energy levels without otherwise changing the circuits. In addition to these pads and amplifiers, the resistance pads K, K', etc., described in connection with Fig. l, are also used to reduce reflection due to mis-matching of terminal impcdances.

The invention is also capable of other modifications and adaptations not specifically referred to but included Within the scope of the appended claims.

What is claimed is:

l. The method of reducing crosstalk between adjacent signaling circuits which comprises attenuating the crosstalk components at the terminals of said circuits a. greater number of times and to a greater extent than the effective signaling currents therein.

2. The method of reducing crosstalk be- Ytween adjacent signaling circuits in which crosst-alk currents are reflected at a terminal, which comprises attenuating the reflected currents at said terminal to a greater extent than the signaling currentstraversing the circuit` to reduce the effect of the reflected currents at a distant terminal.

3. The method of reducing crosstalk between adjacent carrier current signaling circuits in which crosstalk currents are rel flected at one terminal, which comprises attenuating the reflected currents a greater number of times and to a greater extent than the effective signaling currents transmitted from said terminal, to reduce the effect of said reflected currents at a distant terminal. l

- 4. The method of reducing crosstalk between adjacent carrier current signaling systems which comprises maintaining the currents transmitted in the same direction in both systems at substantially the same energy level.

5. The method of reducing crosstalk between carrier current signaling systems having transmission lines which are adjacent each other for only a limited portion of their length, which comprises maintaining substantially the same energy level of tr'ansmission on adjacent portions of said transmission liues.

6. The method of reducing crosstalk between carrier current signaling systems employing substantially the same frequencies and energy levels of transmission in the respective transmitting circuits and having transmission lines which are adjacent each other for only a limited portion of their length,-

which comprises reducing the energy level of the currents at one transmitting terminal to substantially the same value as the current at a point intermediate the terminals of an adjacent transmission line.

7. The method Vof reducing crosstalk between carrier current signaling systems emplo ing substantially` the same frequencies an energy levels of transmission in the respective transmitting circuits and having transmission lines which are adjacent each other for only a limited portion of their length, which comprises reducing the energy level of the currents at one transmitting terminal to substantially the same value as the currents at a point intermediate the terminals' of an adjacent transmission line, and amplifying the currents received at the distant terminal of said firstf mentioned transmission lineto compensate for the reduction in energy level at the transmitting terminal thereof.

8. In a system fortransmitting Waves of different frequencies, a transmission line, a terminal circuit including a filter connected to said line causing certain waves to be reflected, and a loss element between said'circuit and said line to attenuate the reflected waves.

9. In a system for transmitting Waves of different frequencies, a transmission line, a terminal circuit containing an impedance irregularity connected to said line, and a loss element between said circuit and said line to reduce reflection caused by the misn'iatc-hing of terminal impedances.

10. In a system for transmitting Waves of different frequencies, a transmission line, a terminal circuit containing a frequency discriminating device connected to said line and having an impedance which differs from. that .of the line at certain frequencies, and a loss element between said circuit and said line to duce reflection from said device.

11. In a system for transmitting waves of different frequencies, a transmission lino, terminal transmitting and receiving circuits connected to sail line, grouping filters in said circuits for selectively transmitting directional bands of frequencies, said filters con` taining impedance irregularities causing waves in a portion of the frequency band to be reflected, and a resistance pad between said circuits and said line to reduce the rcflected waves..

lf2. The combination with a pair of carrier-current signaling systems having adjacent transmission lines employing substantially the same directional bands of carrier frequencies of means associated with each line to maintain the currents transmitted in one direction thereover at substantially the same energy level as the currents transmitted in the same direction on the other line.

13. The combination with a pair of carrier current signaling systems employing substantially the same frequencies and energy levels of transmission in the respective transmitting circuits and having transmission lines which are adjacent each other for only a limited portion of their length, of a resistance element in the transmitting circuit of one system to reduce the energy level of the currents impressed upon the associated transmission line to substantially the same value as the currents at the same geographical point on the other transmission line.

14. The combination with a pair of carrier current signaling systems comprising terminal transmitting and receiving circuits and employing substantally the same frequencies and energy levels of transmission in f the respective transmitting circuits, of transmission lines which are adjacent each other for only a limited portion of their length, a resistance pad in the transmitting circuit of each system which is adjacent the transmission line of the other system to reduce the energy level of the currents impressed upon each transmission line to substantially the same value as the currents at the same geographical point on the adjacent transmission line, and amplifiers in the associated receiving circuits of said system to compensate for the reduction in energy in the transmitting circuits thereof.

In Witness whereof` I hereunto subscribe my name this 27th day of August A. D.,

HAROLD s. BLACK.

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