Switching network with a crosstalk elimination capability

A switching network having particular application in telephone exchanges is provided with crosstalk elimination capability. The crosstalk is effectively cancelled by generating signals which are reverse in phase to the crosstalk signals and adding these generated signals to the crosstalk signals.

BACKGROUND OF THE INVENTION 
The present invention relates to a switching network with crosstalk 
elimination capability for use in crossbar and/or electronic 
telephone/data exchange systems. 
In this type of telephone/data exchange systems, crosstalk caused by a 
switching network has a great adverse effect upon performance, e.g., the 
signal-to-noise ratio, of the exchange. For this reason, various methods 
for reducing the crosstalk have been proposed. For instance, such 
crosstalk is caused on speech paths through switching elements such as 
relays or electronic crosspoints and through stray capacitors due to 
wiring. The crosstalk further gives distrubances to other speech paths as 
intelligible crosstalk and/or unintelligible noise. Especially, such 
crosstalk occurs when semiconductor elements such as MOSFET's are employed 
as switching elements. An example of such a switching network employing 
MOSFET's as switching elements is described by Erich Bachle et al, "Fully 
Electronic Space-Division Telephone Exchanges Using Semiconductor 
Crosspoints and Optical Switching," IEEE Transactions on Communications, 
Vol. COM-22, No. 9, pp. 1286-1291. (See particularly FIG. 2 on page 1288. 
) 
SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide a switching 
network capable of eliminating the above-mentioned crosstalk. 
The present switching network with crosstalk elimination capability 
comprises a group of incoming lines connected to subscribers, a group of 
outgoing lines intersecting said incoming line group, a plurality of 
switching elements provided at preselected crosspoints formed between 
these incoming lines and outgoing lines for interconnecting a desired 
incoming line and a desired outgoing line in response to a call from a 
subscriber, a plurality of transformers each having a first winding 
connected to a selected incoming or outgoing line and a second winding for 
generating a signal reverse in phase to that supplied to the first winding 
and a plurality of capacitors each having one end connected to each second 
winding of the transformers and the other end connected to an incoming or 
outgoing line adjacent to the selected incoming or outgoing line.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In order to better understand the present invention, the crosstalk caused 
by a switching network in a conventional telephone exchange system will be 
explained briefly. In FIG. 1, a switching network 105 employing silicon 
controlled rectifiers (hereinafter abbreviated simply as SCR's) as 
switching elements forms one switching stage of the telephone exchange 
system. The network is assumed to have four incoming lines and four 
outgoing lines. Only alternating current signal components of input 
signals fed to incoming terminals 101, 102, 103 and 104 are given through 
transformers 106, 107, 108 and 109 connected to the incoming terminals 
101, 102, 103 and 104, respectively, to the network 105. The terminals 101 
to 104 are connected to subscriber's telephone sets (now shown). This 
network 105 comprises incoming lines 110, 111, 112 and 113 and outgoing 
lines 138, 139, 140 and 141. At crosspoints formed between these incoming 
lines and outgoing lines, SCR's 114-129 are employed as switching 
elements, and by actuating a selected one of these SCR's, a desired 
incoming line and a desired outgoing line are connected to each other so 
as to achieve a desired exchange operation. In order to turn the SCR on, a 
control section (not shown) detects the OFF-hook state (state where the 
handset is lifted up) of each subscriber's telephone set and dial 
information sent from each subscriber, and in response to the detected 
information, a control signal is given to a designated SCR so that an 
ignition current is passed through its gate terminal to turn said SCR on. 
On the other hand, the SCR is turned off by interrupting a sustaining 
current passing through the SCR when the control section has detected the 
ON-hook state (state where the handset is hung up) of said each 
subscriber's telephone set. However, since such an operation has no direct 
bearing on the subject matter of this invention, a more detailed 
description thereof will be omitted here. Likewise, description of the 
controlling method for said operations will be also omitted. Reference 
numerals 150, 151, 152 and 153 represent circuits for feeding direct 
currents to the respective subscriber's telephone sets when they are in 
the OFF-hook state. 
Similar to the incoming line side, outgoing lines 138, 139, 140 and 141 are 
coupled to outgoing terminals 134, 135, 136 and 137, respectively, through 
transformers 130, 131, 132 and 133. A circuit equivalent to the network 
105 of FIG. 1 is shown in FIG. 2, assuming that the SCR's 115 and 120 are 
in the ON-state to connect the incoming terminal 101 to the outgoing 
terminal 135, and to connect the incoming terminal 102 to the outgoing 
terminal 136. 
In FIG. 2, the incoming terminals 101, 102, 103 and 104 of FIG. 1 are 
represented by reference numerals 201, 202, 203 and 204, respectively, and 
similarly, the incoming lines 110, 111, 112 and 113 of FIG. 1 are 
represented by reference numerals 205, 206, 207 and 208, respectively. In 
addition, the outgoing terminals 134, 135, 136 and 137 of FIG. 1, and the 
outgoing lines 138, 139, 140 and 141 of FIG. 1 are represented by 
reference numerals 235, 236, 237 and 238, respectively, and by reference 
numerals 231, 232, 233 and 234, respectively. Also, the SCR's except for 
the SCR's 216 and 221 in the ON-state, that is, the SCR's in the OFF-state 
are represented equivalently by junction capacitors, and they are 
represented respectively by the capacitors denoted by reference numerals 
212-227. In addition, there are stray capacitors 209, 210 and 211 formed 
between the incoming lines and stray capacitors 228, 229 and 230 formed 
between the outgoing lines, which are caused by wiring of those lines. For 
instance, the capacitor 209 formed between the incoming lines 201 and 202 
and the capacitor 229 formed between the outgoing lines 236 and 237 cause 
signals to be leaked out. As a result, crosstalk is caused between the 
incoming lines 201 and 202, and between the outgoing lines 236 and 237, 
respectively. The feature of the present invention is that a signal 
reverse in phase to that leaking out through these capacitive components, 
or more broadly, through impedance components having resistance components 
connected in parallel to the capacitive components is generated, and also 
that by adding these signals reverse in phase to each other, the leakage 
signal components can be effectively cancelled. 
In FIG. 3 which shows a first embodiment of the present invention, a part 
of the rows and columns including the SCR's of FIG. 2 in the ON-state 
(that is, the portion framed by a dash line 240 of FIG. 2) is depicted in 
detail. The differences between a switching network of FIG. 3 and the 
network of FIG. 1 are that transformers 303, 306, 328 and 329 connected to 
incoming terminals 301 and 302 and outgoing terminals 330 and 331, 
respectively, are provided with both windings 304, 307, 322 and 324 for 
supplying or receiving signals to or from normal speech signal paths and 
windings 305, 308, 323 and 325 for generating signal voltages reverse in 
phase to those appearing across the four windings 304, 307, 322 and 324, 
and that for simplicity of the drawings, circuits corresponding to the 
circuits 150 and 151 of FIG. 1 are omitted from FIG. 3. In addition, stray 
capacitors 313 and 320 are formed between the incoming lines and between 
the outgoing lines, respectively. It is assumed here that the turns ratio 
of the winding 304 to the winding 305 is 1:1, and that similarly, the 
respective turns ratios of the winding 307 to the winding 308, the winding 
322 to the winding 323 and the winding 324 to the winding 325 are also 
selected to be 1:1. In this case, since capacitors 309 and 313 are chosen 
to have an equal capacitance, the capacitors 320 and 321 are also given an 
equal capacity, the crosstalk components caused by the respective stray 
capacitors 313 and 320 are cancelled. Also, as soon as the connections 
through switching elements 315 and 319 in the ON-state which are 
represented equivalently by connecting lines are brought to OFF-state, the 
portions of the connecting lines are changed from the connecting lines to 
the junction capacitors. On the contrary, when an SCR in the OFF-state 
equivalently represented by a capacitor 317 is turned on, the capacitor 
317 is changed to a mere connecting line. Although the other rows and 
columns are omitted in the illustrated example, in order to further offset 
crosstalk caused in adjacent lines due to stray capacitors, capacitors 310 
and 326 may be connected to the windings 308 and 325, respectively, and 
the other terminals 332 and 327 of these capacitors may be connected to 
the adjacent lines. In this way, for the adjacent rows and columns, 
crosstalk can be similarly reduced. 
While the turns ratios of the transformers 303, 306, 328 and 329 for 
offsetting the crosstalk caused by the stray capacitors are selected to be 
1:1 in the above-described example, the turns ratios may be varied. In 
this case, the same effect as described above can be expected by selecting 
the capacitance of each of the capacitors at a value inversely 
proportional to the turns ratio as will be understood by those of ordinary 
skill in the art. For instance, if the number of turns of the winding 305 
is twice as great as that of the winding 304 compared with the case of the 
turns ratio of 1:1, the capacitance of the capacitor 309 must be reduced 
by half. 
Though the network of FIG. 3 shows a one stage construction, the present 
invention is similarly applicable to a multi-stage construction. 
In FIG. 4 which shows a schematic diagram of a switching network having a 
two-switching stage construction, the network is formed of two switching 
stages defined by two columns and consisting of unit switching networks 
405, 411, 410 and 412 each having two inputs and two outputs. Also, links 
406, 407, 408 and 409 are coupled between incoming terminals 401, 402, 403 
and 404 and outgoing terminals 413, 414, 415 and 416, respectively, and 
the four unit switching networks form a switching network having four 
inputs and four outputs. 
In FIG. 5, three switching networks extracted from the four unit networks 
of FIG. 4 are shown in detail. Incoming lines 500 and 503 are connected to 
incoming terminals 502 and 501, respectively, and outgoing lines 513, 514, 
519 and 521 are connected to outgoing terminals 516, 517, 523 and 524, 
respectively. An outgoing line 505 of the first switching stage is 
connected through a link to an incoming line 512 of the second switching 
stage, while an outgoing line 504 of the first switching stage is 
connected through a link to the incoming line 518 of the second switching 
stage. Here, links leading to incoming lines 515 and 522 are omitted for 
the sake of simplicity. Similarly, switching elements such as, for 
example, SCR's formed at the crosspoints between the incoming lines and 
the outgoing lines are omitted. Now let us consider crosstalk with respect 
to the outgoing line 504. For instance, crosstalk is caused by a stray 
capacitor C.sub.S1 formed between the outgoing line 504 and the outgoing 
line 505. In addition, with regard to the incoming lines of the next 
stage, crosstalk is caused by a stray capacitor C.sub.S2 formed between 
the incoming line 518 to which said outgoing line 504 is connected and the 
adjacent incoming line 522. In order to offset the crosstalk, the 
following construction is adopted in this embodiment. A speech signal 
appearing on the outgoing line 504 is given to a primary winding 508 of a 
transformer 509 via a direct current blocking capacitor 507. Capacitors 
506 and 520 for offsetting the stray capacitors are connected to a 
terminal of the reverse phase side 510 of a secondary winding 511 for 
generating a reverse-phase signal, and the other ends of the capacitors 
506 and 520 are connected to the outgoing line 505 and the incoming line 
522, respectively. For this reason, the crosstalk caused by the stray 
capacitor C.sub.S1 between the outgoing lines 505 and 504 and caused by 
the stray capacitor C.sub.S2 formed between the incoming lines 518 and 522 
is effectively eliminated by the capacitors 506 and 520. In this case, 
upon determining the capacitance of the capacitors 507, 506 and 520, the 
crosstalk signals are effectively cancelled. For instance, the capacitor 
507 may be selected sufficiently large with respect to the other two 
capacitors so that its value may be neglected in circuit computations, and 
the remaining two capacitors 506 and 520 may be selected at such a value 
that the stray capacitors C.sub.S1 and C.sub.S2, respectively, may be 
directly offset thereby. 
In the above-mentioned description, crosstalk has been described with 
respect to a particular line as affecting the other lines. However, in 
general, though crosstalk is mutually caused by each line, crosstalk 
inversely caused by the other lines can be reduced in a similar manner 
since the transformer and the capacitors for eliminating the crosstalk 
operate in both directions. 
While the present invention has been described above in connection to the 
preferred embodiments, various modifications and alternatives may be made 
within the scope of the invention defined by the appended claims.