US1549477A - Decreasing cross-talk in phantom circuits - Google Patents

Description

Aug. H, 1%25.

K. FISCHER DECREASING GROSS TALK I'N PHANTOM CIRCUITS Filed July 2. 1923 QR W @N lfnfiantor: Kurio N Fischer, R by ,J ZZL-Z.

H i s Attorney.

Patented Aug. 11, 1925.

4 UNITED STATES 1,549,472 PATENT- DFFIE.

KUNO FISCHER, 0F CHARLOTTENBURG, GERMANY, ASSIGNOR T0 GENERAL ELECTRIC COMPANY, A CORPORATION OF NEW YORK.

DECREASING CROSS-TALK IN PHANTOM CIRCUITS.

application filed iuiy 2,

To all whom it may concern:

Be it known that I, KUNO FISCHER, a citizen of the German Realm, residing at Charlottenburg, Germany, have invented certain new and'use'ful Improvements in Decreasing Cross-Talk in Phantom Circuits, of which the following is a specification.

This invention relates to telephone transmission circuits, and more particularly to an arrangement of circuits whereby the socalled cross-talk effect may be reduced.

In telephone cables at the present time, the conductors ,are arranged in so-called quads, consisting of four conductors of which one pair forms one two-way transmis- 'sion circuit and the'other pair forms another two-way transmission circuit, these two circuits being usually denominated.side

' tom. The phantom,

clrcuits. By the use of phantom resistances or choke coils, a third effective circuit may be obtained from the two side circuits. This is accomplished by arranging the circuits so that the two conductors forming one side circuit act in parallel to form one condoctor of the third, or so-called phantom circuit, while the other pair of conductors constituting the other side circuit act in parallel to form the return path for the phanlike the side circuits, is operated as a two-way transmission path, that is, transmission may take place over the circuit in both directions.

It has been the practice to twist the conductors forming each side circuit together. to reduce cross-talk in the side circuit and also to twist the pair of conductors together to reduce cross-talk in the phantom. A similar thing is done in overhead lines by transposing pairs of conductors. While this method serves to reduce the cross-talk to some extent, it has not been entirely satisfactory. In accordance with the present invention, therefore, it is proposed to so arrange matters that outgoing and incoming conversations will take place over pairs of conductors. belonging to different quads, at the same time taking care that both side circuits of-agiven quad are used for transmlssion in the same direction. This involves operation upon a four-wire basis as dlstinguished from two-wire operation, .smce four conductors are necessary or a complete twoway transmission channel, one pair being used for transmission in one directlon and the same direction,

1923. Serial 648,869.

the other pair being used for transmission in another direction. With respect to the phantom circuit, the invention contemplates operating in such a manner that the phantom made up from the conductors forming two side circuits of a quad Wlll act as one side of a four-wire circuit, a similar phantom of another quad constituting the other side of the four-wire circuit, so that one phantom will be used fortransmission in one direction and another phantom for transmission in the reverse direction. The phantom of a particular quad should be so arranged as to transmit in the same direction as the two side circuits of the quad.

If, with matters thus arranged, the various quads which are used for transmission in the same direction are electrically isolated from-quads transmitting in the opposite direction, the amount of cross-talk will be very greatly reduced. In order to understand why the cross-talk is reduced, let us consider two adjacent lines, one of which is transmitting in one direction and the other of which is transmitting in the opposite direction. In both cases the,currents impressed upon the line at one end are greatly attenuated upon reaching the other end. Consequently the cross-talk induced in the receiv ing end of one line from the currents of considerable amplitude which are impressed upon the adjacent transmitting end of the other line, may have values comparable with and even greater than the attenuated currents which are bein received from the transmitting end of t e same line. the other hand, if the two adjacent lines are operated so that transmission takes place in the cross-talk eflect must necessarily be much smaller than the currents actually transmitted over the line, for with respect to a given l ine'the current normally flowing therein w1ll be comparable in Upon I magnitude to the current in the adjacent line which induces the cross-talk, and this holds true at any point along the line. 'Consequently it follows that by grouping circuits so that both the side circuits and the phantoms of one group transmit in one direction while the side circuits and the phantoms of .the other group transmit in t e other direction, the cross-talk produced in any circuit of a group due to induction from another circuit in the group will be of much less consequence than the cross-talk induced by a circuit transmitting in the other direction.

Cross-talk of the latter variety, which is known as near end cross-talk, may be practically eliminated or at any rate greatly reduced by the segregation of the two groups of circuits. The method by which the elecrical segregation of the groups is obtained is well known in the art. For example, in a cable the conductors of one group may be shielded from those in the other group in any one of a number of ways. In the case of open-wire circuits, the two groups may be arranged upon opposite ends of the crossarms so that they are physically separated from each other.

The invention may now .be more fully understood by reference to the following description when read in connection with the accompanying drawing, Fig. 1 of which is a simplified circuit diagram illustrating the principles of the invention, and Fig. 2' of which is a detail showing how loadingmay be applied to the sideand phantom circuits of Fig. 1.

Referring to Fig. 1, twosets of quads are shown. Two conductors of one quad constitute a line LE, arranged for transmis sion from west to east between stations A, and B,. Two conductors of the other quad constitute a line LW, arranged to transmit from east to west between stations B, and A,. In a similar manner the other two conductors of the first quad constitute a line LE for transmitting from west to east between stations A, and 13,, while the remaining pair of conductors of the other uad constitute a line LW for transmitting from east to west between the stations 13, and A It will thus be seen that we have two complete four-wire circuits so arranged that the conductors of each quad all transmit in the same direction.

As is usual in four-wire circuits, oneway amplifiers are included in the lines transmitting in a given direction. For ex ample, amplifiers AE,, AE, and AE, are schematically illustrated in connection with the line LE,, and similar amplifiers are illustrated in connection with the other lines. The. lines LE, and LW,, constituting one four-wire circuit, are connected through the usual hybrid coil at each terminal to the terminal line leading to the stations'A and B, respectively. The ter-' minal lines are balanced by artificial lines or networks N, and N, at the two termi-,

nal stations, so that the lines LE, and LVV are substantially conjugate with respect to each other. In a similar manner, the conductors LE and LW are connected to the terminal lines leading to the stations A; and 3,. A

A phantom circuit PE is obtained from the four conductors constituting the lines LE, and LE of the one quad by means of drops taken from the midpoints of choke coils or resistances bridged across the side circuits as indicated. Similar phantom connections are made at each repeating point, so that a one-way phantom circuit .may be obtained in which amplifiers PAE, PAE and PAE" are inserted as indicated. This phantom circuit operates to transmit from station A, to station E, in a direction from west to east. A similar phantom circuit PW may be obtained from the conductors of the other quad and may be provided with one-way amplifiers for transmitting in a direction from east to West between terminal stations B and A,,. The two phantom lines are connected through hybrid coils to the terminal lines in a man ner similar to that described in connection with the side circuits and are rendered substantially conjugate by balancing the terminal lines with suitable networks. N and N,,. As will be noted, the phantom circuits are arranged to transmit in the same direction as the side circuits from which they "are formed, and one complete fourwire phantom conducting system involves eight individual conductors. Assuming that the quads are electrically segregated, it will be apparent that the cross-talk'in line LE,, for example, will be "that'induced from a line such as LE transmitting in the same direction and will. consequently be much smaller in magnitude than the normal cur rents transmitted over the line LE,.' The cross-talk induced by a line transmitting in the opposite direction, such, for example, as the line LW. may be made negligible by the electrical segregation of the circuits. The same holds true with respect to the phantom. For example, the only cross-talk with which we need be concerned in the case of the phantom PE is cross-talk induced by other phantoms transmitting in the same direction, it being assumed that all phantom circuits transmitting in the opposite direction are electrically segregated from those transmitting in the same direction as the phantom PE. By this method of operation the cross-talk problem is reduced to far end cross-talk only,.the

near end cross-talk being reduced by the grouping of the circuits. Owing to its rel- ,atively small magnitude, the far end crosstalk will be fully taken care of by the twistmg of the conductors in the usual manner.

At first thought it might seem that the system herein disclosed involves a prodigal waste-of conductors in order to obtain a reduction in the cross-talk, since it in-' volves four conductors for each fundamental circuit and. ei ht conductors for each phantom. Upon c oser analysis, however, it

name??? will be seen that the arranigement'is quite economical as compared with the ordinary system in which transmission takes place in two directions over the same circuit. In the first place, two-wire operation involves the use of the 22 type of repeater, so that the lines must be balanced at each repeater point, and, furthermore. since it is not possible to efiect a perfect balance of the lines, the gain which may be obtained from each repeater is limited by the tendency of the repeater to sing as soon as the amplification is increased beyond a certain point. Practically, it has also been found that a limited number of intermediate amplifiers (usually not more than 5) can be inserted in tandem in the same circuit. Two-wire operation also involves a duplication of the band filters used in connection with repeaters at each repeater point.

The four-wire circuit requires in contrast to the two-wire circuit balancing of the line.-

at each terminal only. No balance is required at the intermediate repeater points for the reason that one-way amplifiers may be employed. The amplifiers may be adjust-ed to obtain their maximum galn, therefore, without producing singing at the repeater point. No singing can take place in the four-wire circuit so long as the total attenuation in the loop formed by the two sides in the four-wire circuit is equal to or greater than the total amplification in the loop. Consequently, with the same number of amplifiers it is possible to use conductors of smaller diameter, the amplifiers permitting. (because of their greater gain) agreater attenuation between rephantom when operated in manner with 22 repeaters.

peater points. It has been found in practice that, by the use of the four-wire principle, six wires of the total number of eight wires for the phantom may be obtained Without using any more copper than would be necessary for the four wires used in the the erdinary The additional cost of the remaining two wires of the phantomis made up in the majority of cases by the elimination of a large number of balancing adjustments at the repeater points and by the elimination of the additional band filters, etc, required.

There remains to be considered the matter of loading the circuits. For four-wire operation, the side. circuits may be loaded in the same manner as in the case of twowire operation, and the usual methods may also be employed for loading the quad. For example, the phantom path transmitting in one direction and comprising the four conductors of a quad, may be arran ed at the loading point as illustrated in ig. 2 in which windings 10 and 11 may be taken as the windings of a ring transformer for coupling two adjacent loading sections of one ings 14 and 15 of another ring transformer may be included in the drops for pose of loading the phantom.

It will be obvious that the general printhe pureiples herein disclosed may be employed in many other organizations widely diiferent from those illustrated Without departing from the spirit of the invention as defined in the following claims.

What is claimed is:

1. An arrangement for decreasing crosstalk in telephone lines comprising a quad of four conductors arranged to form two side circuits and a phantom circuit, and means whereby transmission takes place over each of the side circuits and over the phantom circuit in one and the same direction only.

. 2. An arrangement for decreasing crosstalk in telephone lines comprising a quad of four conductors arranged to form two side circuits and a phantom circuit, means for transmitting in one and the same direction only over each of the side circuits and over the phantom circuit, and a oneway return path for each side circuit and.

.or the phantom circuit for transmission in the opposite direction, said. return paths comprising conductors of quads other than said first mentioned quad.

3. An arrangement for decreasin the cross-talk in telephone lines comprising a quad of four conductors arranged to form two side circuits and a phantom, a second uad of four conductors also arranged to orm two side circuits and a phantom, means whereby both side circuit." and the phantom of said first mentioned quad will transmit in one and the same direction only, means whereby the two side circuits and the phantom of the second quad transmit in the opposite direction only, and means whereby each of the three transmission paths formed from one quad may be associated with a correspo ling transmission path in the other quad "is form a complete two-way transmission channel.

4. An arrangement for decreasing crosstalk in telephone lines comprising a plurality of quads of four conductors each, each quad being arranged to form two side circuits and a phantom circuit, means whereby the quads may be arranged in two groups electrically segregated from each other, means to permit transmission to take place over the side circuits and the phantom circuits of the quads in one group in one and the same direction only, means to permit transmission to take place over the side circuits and phantoms of the quads in the other group in the opposite direction only, and means to associate each transmission path in the one group with a corresponding transmission path of the other group to form two-way transmission channels.

5. A telephone cable comprising a plurality of quads of four conductors each, said quads being arranged to form side circuits and phantom circuits, the various quads in the cable being arranged in two groups, means to shield the quads in one group from the quads in the other group, means to permit transmission in one and the same direction only over the. side circuits and the phantom circuits of one group, and means to permit transmission in the opposite direction only over the side circuits ant phantom circuits of the other group.

In witness whereof, I have hereunto set my hand this 8th day of June, 1923.

KUNO FISCHER.

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