Subscriber's circuit for four-wire-system local switch

A subscriber's circuit for a four-wire-system local switch is provided with a two-wire/four-wire hybrid circuit that includes a two-wire/four-wire converter circuit. Connected between the converter circuit and a time-division switch section is an incoming circuit that amplifies an incoming voice current, supplies a DC current to a subscriber's telephone set, and generates a subscriber's line signal of large amplitude, such as a bell and a howler signal, by repeatedly reversing the polarity of the DC current. An alarm signal is produced by stepwise changing the duration and the frequency of the repetitive polarity reversal in response to digital command signals supplied from the time-division switch section. An outgoing circuit device, which is connected between the converter circuit and the time-division switch section amplifies the output signal of the converter circuit, transmits a signal representative of a subscriber's call or answer to the time-division switch section in response to the state of the DC current supplied to the subscriber's telephone set, and terminates the operation thereof in response to the cut-off of the DC current to thereby prevent singing.

The present invention relates to a subscriber's circuit suitable for a 
four-wire-system local switch and more particularly to a subscriber's 
circuit for supplying a DC current to a telephone set, transmitting a bell 
and a howler signal, detecting a call and an answer and preventing 
singing. 
The conventional local switches are generally limited to a two-wire system. 
This is due to the fact that the two-wire system of the subscriber's line 
makes difficult a low-transmission-loss operation of the four-wire switch 
and that, even if the four-wire switch is possible in such a case, the 
exchange will be uneconomically high in cost. In the case where a 
time-division digital switching system is applied to a local switch, 
however, it is well known that a four-wire switch requires circuit 
arrangement that the receiving and the transmitting section thereof be 
separated.

Assuming that the subscriber's circuit used for the conventional two-wire 
switch is used, as it is, for a subscriber's circuit of four-wire switch 
such as a time division digital switch, the resulting circuit arrangement 
is as shown in FIG. 1. 
In FIG. 1, reference numeral 1 designates a subscriber's premise, numeral 2 
a subscriber's line, and numeral 3 a telephone exchange office. Numeral 4 
designates a telephone set, numeral 5 a hook switch, numeral 6 a bell, 
numeral 7 a handset, numerals 8, 9, 14 and 15 relay contacts of the 
telephone exchange office, numeral 10 a bell signal circuit, numeral 11 an 
answer-detecting relay, numeral 12 a 16Hz power supply, numeral 13 a power 
supply, numeral 16 a howler signal circuit, numeral 17 a DC power supply 
circuit, numeral 18 a balanced type double-coil relay with coils 19 and 
20, numeral 21 an intra-office power supply, 22 and 23 capacitors for 
cutting off a direct current, numeral 24 a hybrid circuit, numeral 25 a 
two-wire/four-wire converter circuit, numeral 26 a balancing network, 
numeral 27 a receiving amplifier circuit, numeral 28 an amplifier circuit, 
and numeral 29 a time-division switch section. 
In this circuit arrangement, when the subscriber picks up the handset 7 for 
making a call, a signal is sent out to the subscriber's line 2 from the 
intra-office power supply 21 through the relay coil 19 and relay contacts 
8 and 14, so that a DC current is supplied to the handset 7 as it flows 
back to the exchange 3 via the hook switch 5 and handset 7 of the 
telephone set 4, the subscriber's line 2, the relay contacts 15 and 9, and 
the relay coil 20. At the same time, the balanced type double-coil relay 
18 is energized. In response to the energization of the balanced type 
double-coil relay 18, the telephone exchange office 3 makes preparations 
for connection at its contacts not shown, followed by receiving the other 
party's number transmitted from the telephone set 4, thereby starting the 
switching operation for connecting the calling party to the other party. 
Upon completing the connecting operation, the voice speech current from 
the telephone set 4 actuated by the DC current supplied from the exchange 
office 3 reaches the telephone exchange office 3 through the subscriber's 
line 2. After passing the relay contacts 14, 15, 8 and 9, the DC component 
of this current is separated by the capacitors 22 and 23 and introduced 
into the hybrid circuit 24. In the hybrid circuit 24, the transmitting 
section thereof is separated from the receiving section thereof by the 
two-wire/four-wire converter circuit 25, and the output current of 
amplifier circuit 28 reaches the called party not shown through a coding 
circuit and a highway circuit not shown after given the exchange operation 
performed in the time-division switch section 29. 
Now, the receiving operation will be explained below. 
When a signal terminates at the subscriber's premise 1, a relay not shown 
in the drawing is energized by the telephone exchange office 3, so that 
the bell signal circuit 10 is connected by the contacts 8 and 9 of the 
relay. A bell signal is transmitted from the 16Hz power supply 12 within 
the bell signal circuit 10 and, through the subscriber's line 2, reaches 
the telephone set 4 thereby to ring the bell 6 thereof. When the 
subscriber picks up the handset 7 and answers, the hook switch 5 is closed 
and DC current is sent out from the intra-office power supply 13 in the 
bell signal circuit 10, which current flows back through the 
answer-detecting relay 11. The fact that the call has been answered is 
detected by the telephone exchange office 3 thereby to proceed with the 
connecting operation. Upon completing the connecting operation, the speech 
current from the other party is introduced into the receiving amplifier 
circuit 27 via the time-division switch section 29 and a highway circuit 
and a decoding circuit not shown. In the receiving amplifier circuit 27, 
the voice level is regulated. The output of the receiving amplifier 
circuit 27 is converted into a two-wire system in the two-wire/four-wire 
converter circuit 25, while a balance with the subscriber's line 2 is 
established in the balancing network 26, with the result that the output 
of the converter circuit 25 is applied to the telephone set 4 through the 
capacitors 22 and 23, the relays 8, 9, 14 and 15 and further through the 
subscriber's line 2. 
By the way, the howler signal circuit is provided for the purpose of 
issuing an audible alarm in the case where the subscriber fails to hang up 
the handset 4. The alarming sound is gradually increased in level. If the 
subscriber still fails to hang up the handset even after the alarm is 
given for a certain period of time, the line is forcibly cut off. The 
circuit arrangement of this circuit is similar to that of the bell signal 
circuit 10 and therefore will not be described. 
This conventional subscriber's circuit system has the following 
shortcomings: 
(1) A countermeasure is required to be taken against the insertion loss due 
to the DC current power supply circuit 17. 
(2) The bell signal and the howler signal require an output of large 
amplitude and cannot pass the time-division switch. It is therefore 
necessary to provide a contactor circuit corresponding to the subscriber 
for connecting the subscriber's line to the signal source side. 
(3) The presence of a group of contactor circuits at the two-wire side of 
the hybrid circuit worsens the echo loss at the time of energization of 
the contactor circuits. 
(4) The presence of inductive and capacitive impedances at the two-wire 
side of the hybrid circuit complicates the construction of the balancing 
network for assuring a balance with the hybrid circuit. 
(5) As far as the subscriber's line is connected to the bell signal circuit 
and the howler signal circuit side, a degree of unbalance of the 
subscriber's line with respect to the earth is generally worsened. 
(6) Since the bell signal and the howler signal are analog signals, neither 
the bell signal circuit nor the howler signal circuit nor the contactor 
circuit for circuit change-over can be produced in the form of economical 
integrated circuitry. 
(7) The subscriber's circuit is provided for every subscriber separately 
and, therefore, requires an economical form suitable for miniaturization 
and integrated circuitry. However, the conventional type of circuit is not 
suitable for miniaturization and integrated circuitry. 
(8) Since electromagnetic parts are involved, a high transient voltage is 
generated at the time of their energization, thus causing breakdown of the 
parts as well as generation of noise. 
The present invention is intended to provide a subscriber's circuit 
adaptable to the four-wire switching and characterized in that: 
(1) The point at which a DC current is supplied to the telephone set is 
moved to a hybrid circuit, and the voice output is superimposed on the DC 
current. 
(2) A bell signal and a howler signal are generated by repeatedly reversing 
the polarity of the DC current sent out. 
(3) By operating the hybrid circuit only when the DC current is supplied, 
the singing which might occur at the time of hooking or immediately after 
hooking on the handset is prevented. 
With the features (1) to (3) described above as an essence, there is 
provided means for improving the transmission characteristics, especially, 
for preventing the singing, thereby realizing a four-wire switch with a 
low-transmission loss. Also, a signal system suitable for a logic circuit 
is employed and electromagnetic parts are eliminated. In this way, 
generation of a high voltage due to a transient phenomenon is prevented 
while at the same time making a compact and economical system possible. 
In FIG. 2, like component elements are denoted by like reference numerals 
in FIG. 1. Referring to FIG. 2, reference numeral 24' designates a hybrid 
circuit, numeral 25' a two-wire/four wire converter circuit capable of 
transmitting a DC signal, and numeral 27' an incoming circuit device 
capable of supplying a DC current to the telephone set 4 and amplifying a 
voice current superimposed on the DC current and repeatedly reversing the 
polarity thereof. Numeral 32 designates a voice input terminal of the 
incoming circuit device 27', numeral 33 a polarity-reversing command input 
terminal of the incoming circuit device 27', and numeral 28' an outgoing 
circuit device, the output terminals which are composed of a voice output 
terminal 30, and a call-answer detecting terminal 31 for detecting the 
condition of the DC current on the subscriber's line which implys the 
call, answer, receiving of dial pulses and the like. 
In FIG. 2, when calling the subscriber's premise 1 from the telephone 
exchange office 3, a 32Hz polarity-reversing command pulse signal is 
applied to the polarity-reversing command terminal 33 of the incoming 
circuit device 27' thereby to generate a bell signal. This bell signal is 
applied to the telephone set 4 via the two-wire/four-wire converter 
circuit 25' and the subscriber's line 2, thus ringing the bell 6. The 
subscriber's picking up the handset 7 causes the hook switch 5 to be 
closed, thus supplying a direct current to the handset 7. The direct 
current supplied to the handset 7 is detected by the outgoing circuit 
device 28', so that an output is produced at the call-answer detecting 
terminal 31 to indicate an answer. The telephone exchange office 3 stops 
sending the bell signal and proceeds with the connecting operation. Upon 
completion of the connecting operation, speech begins, so that the speech 
current from the other party is applied to the voice input terminal 32 via 
the time-division switch section 29, a highway circuit and a decoding 
circuit not shown. This signal is superimposed on the DC current supplied 
from the incoming circuit device 27' to the telephone set 4. Further, this 
speech current reaches the telephone set 4 through the two-wire/four-wire 
converter circuit 25' and the subscriber's line 2. The speech current from 
the telephone set 4, on the other hand, is introduced into the outgoing 
circuit device 28' via the subscriber's line 2 and the two-wire/four-wire 
converter circuit 25', thereby to produce an output at the voice output 
terminal of the outgoing circuit device 28'. The speech current further 
reaches the other party via the coding circuit, the highway circuit not 
shown and the time-division switch section 29. 
Next, the calling operation of the subscriber will be explained below. 
When the subscriber picks up the handset 7 for making a call, the hook 
switch 5 is closed, so that a DC current is supplied from the incoming 
circuit device 27' to the telephone 4 through the two-wire/four-wire 
converter circuit 25' and the subscriber's line 2. Simultaneously with the 
supply of DC current to the telephone 4, the outgoing circuit device 28' 
is energized and produces at its call-answer detecting terminal 31 an 
output indicating the requirement for a call. As a result, the telephone 
exchange office 3 proceeds with the preparations for connection, followed 
by receiving the telephone number of the other party transmitted from the 
subscriber and the operation of connecting the calling party to the other 
party. Upon completing the connecting operation, the voice speech current 
from the telephone 4 driven by the DC current supplied from the telephone 
exchange office 3 is separated into the voice speech currents fed to the 
transmission system and the receiving system in the two-wire/four-wire 
converter circuit 25' through the subscriber's line 2. Through the 
not-shown coding circuit and the highway circuit connected to the output 
terminal of the outgoing circuit device 28', the time-division exchange 
operation is performed in the time-division switch section 29, and the 
resulting signal reaches the other party not shown. 
The systems which form the essential parts of the present invention will be 
specifically explained below. 
(1) A system for DC current supply to the telephone set: 
A system for supplying a DC current at the two-wire side of the hybrid 
circuit is accompanied by an insertion loss on the one hand and requires a 
circuit for separating the voice speech current from the DC current. 
Further, the circuits inserted at the two-wire side complicate the 
construction of the balancing network. 
According to the present invention, the hybrid circuit has a function to 
supply a DC current, and the basic construction as shown in FIG. 3 is made 
possible if coils are used as the two-wire/four-wire converter circuit 25. 
In FIG. 3, like component parts are shown by like reference numerals in 
FIG. 1. The size of this basic construction, however, is not reduced in 
the case where a DC current is superimposed on the hybrid coils. Further, 
the disadvantage still remains unsolved that the transformers and the like 
form a roadblock to integration of the circuit. FIGS. 4 and 5 shows a 
resistor hybrid circuit capable of transmitting the direct current through 
the two-wire/four-wire converter circuit in order to eliminate the 
above-mentioned disadvantages. Specifically, FIG. 4 shows a circuit of 
balanced type, and FIG. 5 a circuit of unbalanced type. In FIGS. 4 and 5, 
like component elements are designated by like reference numerals in FIGS. 
1 and 2. Referring to FIG. 4, reference numeral 27' designates an incoming 
circuit device capable of sending out a DC current. By applying a 
polarity-reversing command to the polarity-reversing command terminal 33, 
the polarity of DC output is reversed. The functions of this incoming 
circuit device 27' may be obtained, in principle, by combining the 
well-known bipolar-pulse transmitting circuit 38 for use in telegraph with 
the voice amplifier 40 and the transformer 39 for coupling the voice 
signal to the circuit 38, shown in FIG. 6. Actually, however, the 
combination of these functions is designed in the form of a circuit which 
does not employ any parts such as a transformer. 
In FIG. 4, the two-wire/four-wire converter circuit 25' is of bridged 
H-type. 
At the bridge section, the voice and the DC signal are taken out by the 
diodes 34 and 35 of two photo-couplers thereby to photo-coupled them to 
the outgoing circuit device 28'. Such DC signals as a call and an answer 
are produced at the call-answer detecting terminal 31, while the voice 
output is produced at the voice output terminal 30. By the way, the two 
photo-couplers are coupled to the outgoing circuit device 28' in such a 
manner that the outputs therefrom make up a sum thereby to erase the 
longitudinal current from the subscriber's line 2. Also, the difference 
between the outputs is taken as required, thereby making possible the 
detection of a line trouble, if any, such as an induction failure or 
single-line grounding. The diodes 36 and 37 connected in parallel to the 
diode sections 34 and 35 of the photo-couplers are provided for purposes 
of protecting the photo-couplers and forming a bypass in the case of the 
polarity reversal of the DC current transmitted. The balancing network 26' 
is for maintaining balance with the subscriber's line against the voice 
output of the incoming circuit device 27'. 
The circuit of FIG. 5 is intended to save circuit elements by forming the 
circuit of FIG. 4 as an unbalanced type and will not be described as it 
has the same operating principle as the circuit of FIG. 4. Incidentally, a 
resistor 41 is employed at the junction point of the subscriber's line 
thereby to regulate the balanced form. 
(2) System for generating and transmitting a bell signal and howler signal: 
According to the present invention, a subscriber's signal is generated by 
reversing the polarity of the DC current supplied to the telephone. Since 
conventional time-division switches could not pass the large power, the 
subscriber's line was connected to the signal source in order to transmit 
a bell and a howler signal to the subscriber. This system has the 
disadvantage that a plurality of contacts 8, 9, 14 and 15 are required for 
transmitting the bell signal and howler signal shown in FIG. 1 as 
corresponding to the subscriber, resulting in an increased amount of 
wiring and a decreased echo loss at the time of connecting to contacts 8, 
9 or 14, 15. 
The present invention overcomes these disadvantages. According to the 
invention, a subscriber's signal is generated by reversing the polarity of 
the DC current supplied to the telephone, and a large power is easily 
generated. And since the generation of the signal represents the operation 
of polarity reversal, the present invention may be suitable for a logic 
circuit, thus facilitating control. 
In FIG. 4 or 5, pulses of, say 32Hz as shown in FIG. 7 are transmitted for 
one second and suspended for two seconds at the polarity-reversing command 
terminal 33 of the incoming circuit device 27' so as to repeatedly reverse 
the transmission polarity of the local power supply -48V. In this way, a 
bell signal is generated and transmitted to the subscriber's line. 
Incidentally, the circuit of FIG. 8 shows an embodiment in which the 
function of polarity reversal is separated from the incoming circuit 
device 27' and an analog switch 42 added to generate a subscriber's line 
signal by applying and a repetitive reversal command thereto. As described 
above, according to the invention, a subscriber's signal is generated by 
the operation of polarity reversal of the DC power supply. If this 
operation is applied to the conventional system for transmitting a howler 
alarm, the circuit configuration is complicated since, in the conventional 
system, the signal is transmitted at a small amplitude at the initial 
stage of alarming while increasing it progressively. The present 
invention, on the other hand, employs a method suitable for a logic 
circuit, whereby an alarming period is progressively lengthened. 
The diagram of FIGS. 9 and 10 are for explaining the method for 
transmitting a howler alarm signal according to the present invention. 
Specifically, a waveform of the howler alarm signal transmitted to the 
subscriber's line is shown in FIG. 9, while FIG. 10 shows a command for 
repetitive polarity-reversing for transmitting a howler alarm signal, 
which is applied to the polarity-reversing command terminal 33 of FIG. 4. 
By the way, in the event that no answer is obtained from the subscriber 
after issue of an alarm signal for a certain period of time, the action is 
taken to disconnect the subscriber's telephone set in the same manner as 
in the conventional system. 
(3) Subscriber's line signal-detecting system: 
In the conventional subscriber's circuit shown in FIG. 1, the detection of 
a subscriber's call or "on-hook" is effected by the balanced type 
double-coil relay 18 while the detection of a subscriber's answer is 
performed by the answer-detecting relay 11 of the bell signal circuit 10. 
Both of the signal detection are based on the monitoring of the outflow of 
DC current from the power supply. This function is utilized by the present 
invention, so that by providing a two-wire/four-wire converter circuit in 
the form capable of transmitting a DC signal, a subscriber's signal is 
monitored and detected by a unified means represented by the outgoing 
circuit device 28' shown in FIG. 4, thus simplifying the circuit 
construction. Referring to FIGS. 4 and 5, the DC current state of the 
subscriber's line is displayed at the call-answer detecting terminal 31 of 
the outgoing circuit device 28'. By so doing, the detection of a call, 
on-hook and answer as well as the transmission and receipt of dial pulses 
are carried out in the telephone exchange office 3. 
(4) Singing-dampening system: 
In effecting the exchange operation on the two-wire channel with a 
four-wire exchange switch, it is generally considered that the generation 
of singing makes difficult an exchange operation with low transmission 
loss. According to the present invention, the operation of the outgoing 
circuit device is turned on and off depending on whether or not the DC 
current is supplied to the subscriber's telephone. Therefore, the singing 
at the time of hooking or immediately after hooking on is dampened, thus 
permitting an exchange operation with low transmission loss. 
Embodiments of the present invention are shown in FIGS. 4 and 5. In FIG. 4, 
only when a DC forward current is flowing in the diode sections 34 and 35 
of the photo-couplers, the two-wire/four-wire converter circuit are 
photo-coupled to the outgoing circuit device 28'. At the same time when 
the subscriber hooks on, the DC current supply is cut off to terminate the 
photo-coupling. As a result, the singing loop comprising the incoming 
circuit device 27', the two-wire/four-wire converter circuit 25' and the 
outgoing circuit device 28' is also cut off. Since the embodiment of FIG. 
5 operates in the similar way to that of FIG. 4, it will not be described. 
It will be understood from the foregoing description that the system 
according to the present invention eliminates the disadvantages of the 
prior art on the one hand and uses no electromagnetic parts at all on the 
other. Thus, the size of the system can be reduced and also it can be 
formed in integrated circuitry. Further, the absence of electromagnetic 
parts prevents a high voltage from being generated, with the result that 
not only the breakdown of the parts is prevented but also local noises are 
reduced. Although the embodiments of FIGS. 4 and 5 are described above 
employing the photo-couplers as elements for coupling the 
two-wire/four-wire converter circuit 25' and the outgoing circuit device 
28' together, they may be replaced by any elements such as operational 
amplifiers or Hall elements capable of detecting the state of DC current 
supply. 
A more specific practical subscriber's circuit system according to the 
present invention is shown in FIG. 11. In this drawing, reference numeral 
25' designates a two-wire/four-wire converter circuit, numeral 26' a 
balancing network, numeral 27'-1 a polarity-reversing section of the 
incoming circuit device, numeral 27'-2 a voice-coupling section thereof, 
numeral 27'-3 a bell signal control section thereof, and numeral 28' an 
outgoing circuit device. Reference characters R.sub.1 to R.sub.28 
designate resistors, characters C.sub.1 to C.sub.2 capacitors, characters 
RC.sub.1 to RC.sub.3 circuit-protecting diodes, characters Q.sub.1 to 
Q.sub.9 transistors, character IC a three-input NAND gate, character N a 
photo-coupler, numerals -48V, -12V and +5V DC power supply terminals, 
character G an earth terminal, characters L.sub.1 and L.sub.2 connecting 
terminals for subscriber's line, character RGP a 16Hz bell signal input 
terminal, character CTLB a bell signal transmission control terminal, 
character CTLF a call-answer detecting terminal, character HYBIN a voice 
input terminal, and character HYBOUT a voice output terminal. 
The operation from the transmission of a bell signal to the answer by the 
subscriber will be explained below with reference to FIG. 11. The terminal 
RGP is kept impressed with a 16Hz logic level with the duty ratio of 50% 
at regular intervals including 1-second transmission and 2-second 
suspension. The terminal CTLB is impressed with a "0" level signal when no 
bell signal is transmitted. Now, assume that the level of the terminal 
CTLB is changed to "1". Since a "1" signal is applied to the remaining 
input terminal of the three-input AND gate IC from the outgoing circuit 
device 28' during the on-hook state of the subscriber, the output of the 
three-input AND gate IC alternates between "1" and "0" in accordance with 
the input at the terminal RGP. As a result, the polarity-reversing section 
27'-1 of the incoming circuit device alternates between the state that the 
transistors Q.sub.2 and Q.sub.3 are ON, while transistors Q.sub.5 and 
Q.sub.6 are OFF, and the state that the transistors Q.sub.2 and Q.sub.3 
are OFF while the transistors Q.sub.5 and Q.sub.6 are ON. In this way, the 
DC current flows into the subscriber's line through the two-wire/four-wire 
converter circuit. When the subscriber answers by hooking off the handset 
in response to the bell signal generated by this repetitive polarity 
reversal of the DC power supply, DC current is supplied from the power 
supply of -48V to the handset. This is detected by the photo-coupler N, so 
that the transistor Q.sub.7 in the outgoing circuit device is turned ON, 
thus changing the logic level of the terminal CTLF from "1" to "0". As the 
result of the output of the terminal CTLF having changed to "0", the 
telephone exchange office proceeds with the connecting operation. Further, 
the output at the terminal CTLF branches into the bell signal transmission 
control section 27'-3 thereby to stop the transmission of a bell signal. 
Upon completion of the connecting operation by the telephone exchange 
office, the speech current from the other party is supplied to the 
terminal HYBIN and coupled to the polarity-reversing section 27'-1 via the 
voice-coupling section 27'-2. Further through the two-wire/four-wire 
converter circuit 25', it is sent out to the subscriber's line by way of 
the terminals L.sub.1 and L.sub.2. Also, the speech current that arrives 
at the terminals L.sub.1 and L.sub.2 via the subscriber's line is coupled 
to the outgoing circuit device 28' by the diode section N1/2 of the 
photo-coupler of the two-wire/four-wire converter circuit 25'. The output 
of the outgoing circuit device 28' is applied to the other party through 
the terminal HYBOUT, thus permitting the subscriber to talk with the other 
party. Under this condition, assume that the subscriber has made hooking 
on the handset. The DC current supply to the subscriber's telephone is 
stopped while eliminating the forward bias current which has been flowing 
through the diode section N1/2 of the photo-coupler. The coupling to the 
outgoing circuit device 28' is also cut off. Therefore, the echo current 
that has been flowing from the terminal HYBIN to the terminal HYBOUT 
disappears. 
In the practical circuit under consideration, a photo-coupler is inserted 
only in one side of the two-wire/four-wire converter circuit of H-shaped 
bridge type. In the event that a long subscriber's line is involved or the 
system is located in an environment easily subjected to external 
induction, however, another photo-coupler may be inserted on the other 
side of the bridge, so that a sum of the two outputs is taken, thus 
erasing the longitudinal current. Furthermore, this practical circuit 
employs a constant-current supply system for supplying a DC current to the 
subscriber.