Apparatus for detecting the closure of a subscriber's line during ringing

Apparatus for detecting the closure of a subscriber's telephone line in the presence of an alternating ringing voltage superimposed on a direct voltage comprises circuitry for producing a unidirectional current from the combined alternating and direct voltages. When the line is open no direct current will flow and hence the peak value of successive half cycles will be equal whereas when the loop is closed and direct current flows the peak values of successive peaks will be different. A capacitor (26) is charged by a voltage (U.sub.1), which is proportional to the combined voltage, through a transistor (24), provided that the voltage (U.sub.1) is greater than the voltage across the capacitor, and discharged through a resistor (27). The time constant of the discharge circuit is selected so that when the line is looped the smaller peak is lower than the voltage to which the capacitor is discharged and hence charging current flows into capacitor (26) on alternate peaks of the voltage (U.sub.1) only. The intervals between pulses of charging current are determined by a comparator (33) which provides an output (S) which indicates whether the subscriber's line is closed.

The invention relates to apparatus for detecting the closure of a 
subscriber's telephone line during the ringing period, the line being fed 
during this period with an a.c. ringing voltage and a d.c. voltage. 
BACKGROUND OF THE INVENTION 
Telephone installations include apparatus for detecting the lifting of the 
handset of a telephone set connected to the subscriber's lines. This 
apparatus being arranged in the interface circuits between the exchange 
and the subscriber's lines. Lifting the handset produces a closure of the 
subscriber's line and this closure can be detected by checking whether a 
d.c. voltage which is applied to the subscriber's line causes a direct 
current to circulate in the subscriber's line. It is comparatively easy to 
detect the closure of the subscriber's line if the subscriber's line 
receives only a d.c. voltage. However, there are some problems in 
detecting the direct current when the subscriber's line receives the a.c. 
ringing voltage superimposed on this d.c. voltage, since the ringing 
voltage has a low frequency (for example, 50 Hz) and generates a 
comparatively high current in the subscriber's line during closure of the 
line. The ringing voltage may be, for example 70 V r.m.s. It should also 
be noted that the apparatus must function correctly in the presence of 
parasitic currents in the subscriber's line, such as longitudinal currents 
or leakage currents between the two wires of the line. 
Conventional apparatus for detecting the closure of a subscriber's line 
have been implemented by means of relays which have for their purpose the 
detection of the direct loop current and are relatively intensitive to the 
alternating ringing current. These relays are rather bulky, costly, 
difficult to adjust and not compatible with contemporary electronic 
equipment. 
Swiss Pat. No. 526,893 describes a device which compares the time intervals 
during which the subscriber's line current is higher or lower than a 
certain threshold value. From this comparison it is possible to deduce 
whether the current in the subscriber's line, which varies with the 
frequency of the ringing voltage includes a direct loop current. This 
rather complicated device has, however, the disadvantage that it is rather 
sensitive to the frequency and amplitude characteristics of the ringing 
current and to various parasitic currents in the subscriber's line. 
It is an object of the present invention to provide a novel device for 
detecting the closure of a subscriber's line during ringing which 
mitigates the above-mentioned drawbacks. 
SUMMARY OF THE INVENTION 
A device for detecting the closure of a subscriber's telephone line during 
the ringing period, the line being fed during the ringing period with an 
a.c. ringing voltage and a d.c. voltage, comprises means for forming a 
signal which is representative of the absolute value of the current in the 
subscriber's line, the signal comprising cyclically varying unidirectional 
voltages having substantially equal peak values caused by the a.c. voltage 
when the line is open and unequal peak values caused by the superimposed 
d.c. voltage when the line is closed; a capacitor; means for charging the 
capacitor when the voltage across the capacitor is less than the 
instantaneous value of the signal; means for discharging the capacitor 
when the voltage across the capacitor is greater than the instantaneous 
value of the signal; and means for producing a loop closure detection 
signal from a characteristic of the charging current for the capacitor; 
wherein the time constant of the discharging means is such that when the 
line is closed the voltage across the capacitor falls by less than the 
difference between the higher and lower peak values so that the capacitor 
is not charged when the lower peak value occurs. 
In this specification the absolute value of an alternating current or 
voltage means the magnitude of the current or voltage without reference to 
its sign, for example that current or voltage obtained by full wave 
rectification of the alternating signal. 
With this device the period of the pulses of the charging current of the 
capacitor is equal to T or (T/2), depending on whether the subscriber's 
line is closed or not closed, so that it is very simple to detect closing 
of the subscriber's line by measuring a period of time which corresponds 
to the interval between these pulses or to the period of these pulses and 
by checking whether this time exceeds a certain threshold.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows apparatus according to the invention associated with a 
subscriber's circuit 1, which is intended to supply a subscriber's set 2 
through the two wires L.sub.A and L.sub.B of a subscriber's line. When the 
subscriber is called under a command from an exchange 3, the subscriber's 
circuit 1 applies between the two wires L.sub.A and L.sub.B a.c. voltage 
pulse trains which have a value of, for example, approximately 70 Vrms and 
a frequency of 50 Hz and which are used to excite ringing of the telephone 
set. 
When the called subscriber lifts the handset, the subscriber's line is 
closed and, in order to be able to detect this closure during the ringing 
voltage pulse trains it is customary to apply to the subscriber's line, 
together with the ringing voltage, a d.c. voltage which will not produce a 
direct current circulation in the subscriber's line until this line is 
closed by the lifting of the handset. 
Supplying the subscriber's line with the a.c. ringing voltage on which a 
d.c. voltage is superimposed may be effected in several ways, which are 
known per se: a conventional means is the use of a transformer whose 
primary winding is fed with a voltage having the frequency 
f=(.omega./2.pi.) of the ringing voltage and whose two secondary windings, 
which produce the voltages of the amplitude V, are connected to the wires 
of the line L.sub.A and L.sub.B, respectively, and also to ground via a 
d.c. voltage source Eo for one of these secondary windings. Thus, the 
voltages V.sub.A and V.sub.B which are applied to the two wires L.sub.A 
and L.sub.B of the line may be at the ringing instant: 
##EQU1## 
As a result thereof the voltage U.sub.S =V.sub.A -V.sub.B applied at the 
ringing instant between the two wires L.sub.A and L.sub.B of the line is 
equal to 
EQU U.sub.S =E.sub.o +2 V sin .omega.t. (2) 
This voltage comprises a d.c. component having the amplitude E.sub.o equal 
to, for example, 12 Volts and an a.c. ringing component having an 
amplitude 2 V equal to, for example, 96 Volts, when V=48 Volts. 
The same result can be obtained without the use of a transformer by means 
of, for example, the subscriber's circuit described in French Pat. No. 
2,406,357. By way of example let it be assumed for the following 
description that the subscriber's circuit 1 is of the type described in 
this French Patent. During ringing there are applied to two control 
terminals 4 and 5 of the subscriber's circuit, digital signals 
.DELTA..sub.A and .DELTA..sub.B, which are obtained in response to the 
coding by means of delta modulation of signals which have the same shape 
as the signals V.sub.A and V.sub.B in formula (1). These signals 
.DELTA..sub.A and .DELTA..sub.B are converted into analog signals in 
decoders 6 and 7 and thereafter applied to the input terminals 8 and 9 of 
a push-pull amplifier which is formed by transistors T.sub.1 to T.sub.4 
which are arranged in the manner shown in FIG. 1. Two output terminals 10 
and 11 of this push-pull amplifier are connected to two wires L.sub.A and 
L.sub.B of the subscriber's line. The push-pull amplifier is supplied with 
direct current by way of two supply terminals 12 and 13 of the 
subscriber's line which are connected to terminals 14 and 15, 
respectively, of a d.c. voltage supply source, not shown. The positive 
terminal 14 of this source carries a voltage E.sub.o +V with respect to 
ground and the negative terminal 15 carries a voltage -V with respect to 
ground. In a subscriber's circuit 1, which is controlled and fed in the 
above-described manner, the voltage U.sub.S defined by formula (2) is 
finally obtained between the wires L.sub.A and L.sub.B of the subscriber's 
line. 
Apparatus according to the invention, which has for its function to detect 
closure of the subscriber's line during ringing is based on the 
characteristic features of a signal of an absolute value of the current in 
the subscriber's line, as will be explained with reference to the diagrams 
2a and 2b of FIG. 2. 
In diagram 2a the current I.sub.e in the subscriber's line during ringing 
is shown as a function of time, that is to say during the period of time 
the voltage U defined in formula (2) is applied to this line. As shown by 
diagram 2a, the current I.sub.e has, up to the instant to where the line 
is closed, a sinusoidal form having an average value zero because no 
continuous current can circulate through the subscriber's line. After the 
instant of the current I.sub.e it has the form of a sine wave with a 
non-zero average value because of the direct current produced by the d.c. 
voltage E.sub.o. The amplitude of the sinusoidal components of the current 
I.sub.e prior to and after closure of the subscriber's line is not the 
same, as the a.c. impedance of the subscriber's line, seen from the 
subscriber's circuit, is different in these two cases. 
The diagram 2b shows the signal .vertline.I.sub.e .vertline., which 
represents the absolute value of the current I.sub.e and which can be 
obtained in practice by full-wave rectification. It is shown that prior to 
the instant of closing the successive peak values of the signal 
.vertline.I.sub.e .vertline. are equal, while in contrast therewith, after 
the closing instant t.sub.o, the successive peak values of the signal 
.vertline.I.sub.e .vertline. have alternately a high value I.sub.e1 and a 
low value I.sub.e2. 
It is possible to calculate the high peak value I.sub.e1 and the low value 
I.sub.e2. Using the notations of the formula (2) it is possible to write 
that: 
##EQU2## 
where Z.sub.ca and Z.sub.cc are the a.c. impedance and the d.c. impedance, 
respectively, of the subscriber's line. 
From this it is easy to derive that: 
##EQU3## 
Before the line closes, Z.sub.cc =.alpha. and consequently I.sub.e1 
=I.sub.e2. 
After the line has closed, Z.sub.cc has a finite value and consequently 
I.sub.e1 .noteq.I.sub.e2 and I.sub.e1 -I.sub.e2 =2E.sub.o /Z.sub.cc. 
According to the invention, to detect closing of the subscriber's line 
during ringing a signal is used which is representative of the absolute 
value .vertline.I.sub.e .vertline. of the current in the subscriber's line 
and it can be demonstrated with a very simple means that a difference 
unequal to zero (I.sub.e1 -I.sub.e2) between the successive peak values 
.vertline.I.sub.e .vertline. exists after closing of the line. 
Therefore the device according to the invention comprises means for forming 
a signal which is representative of the absolute value of the current 
flowing in the subscriber's line. For the case of the subscriber circuit 1 
shown in FIG. 1 such a signal is obtained with little special effort and 
cost. While the subscriber's line is supplied with the ringing voltage via 
the push-pull amplifier T.sub.1 to T.sub.4 a unidirectional current which 
is equal to the current in the subscriber's line after full-wave 
rectification and which is therefore equal to .vertline.I.sub.e .vertline. 
appears on the supply wires of the subscriber's circuit which 
interconnects the terminals 14-12 and 15-13. In order to obtain a current 
K .vertline.I.sub.e .vertline., which is proportional to .vertline.I.sub.e 
.vertline., a well-known circuit 17, called a current mirror, is used 
which is included between the terminals 12 and 14. In the simplified form 
shown in the drawing, this current mirror is formed by a diode 18 and a 
transistor 19, which are connected, as shown in the drawing, to the supply 
terminal 14, to the input terminal 20 and to the output terminal 21 of the 
current mirror. The terminal 21 is connected to ground via the resistor 
22. In response to the current .vertline.I.sub.e .vertline. which appears 
at its input terminal 20, the current mirror 17 produces a current i.sub.1 
=K .vertline.I.sub.e .vertline. at its output terminal 21 and a voltage 
u.sub.1, which is proportional to .vertline.I.sub.e .vertline., appears at 
the terminals of resistor 22. 
This voltage u.sub.1 at the terminals of resistor 22 has the same shape as 
the current .vertline.I.sub.e .vertline. shown in diagram 2b and has the 
same characteristics as explained above as regards its successive peak 
values, prior to and after closing of the subscriber's line. 
This voltage u.sub.1, which is proportional to .vertline.I.sub.e .vertline. 
is used in the circuit 23 to form a logic signal which indicates closing 
of the subscriber's line. This will now be further explained. 
The voltage u.sub.1 is applied in a first stage of a circuit 23 to the base 
of an npn transistor 24. The collector of this transistor is connected to 
the positive supply terminal 14 via a resistor 25. Its emitter is 
connected to ground via a capacitor 26 which has a capacitance C and to 
whose terminals a resistor 27 which has a resistance value R is connected. 
The operation of this first stage will be explained with reference to the 
diagrams 2c and 2d. 
In diagram 2c, the voltage u.sub.1 which has the same shape as the current 
.vertline.I.sub.e .vertline. of diagram 2b is represented by the broken 
line curve. The solid curve represents the voltage u.sub.2 at the 
terminals of capacitor 26. As the capacitor 26 is connected to the 
resistor 22 via the base-emitter diode of the transistor 24, the voltage 
u.sub.2 at its terminals follows the voltage u.sub.1 during a portion of 
the ascending edges of u.sub.1 until the peak value of u.sub.1 has been 
reached. Above the peak value, at the descending edges of u.sub.1, the 
capacitor 26 does not follow the voltage u.sub.1 any longer and discharges 
through resistor 27. Recharging of the capacitor 26 at the ascending edges 
of u.sub.1 starts from the instants at which the decreasing voltage 
u.sub.2 becomes equal to the voltage u.sub.1. 
Prior to the instant at which the subscriber's line closes, the successive 
peak values of the voltage u.sub.1 are equal so that during operation the 
duration of the charging and the discharging cycles of the capacitor 26 is 
equal to (T/2), T being the period of the ringing voltage. Diagram 2d 
shows the charging current i.sub.2 of capacitor 26, which is substantially 
equal to the collector current of the transistor 24. Prior to the closure 
instant t.sub.o this charging current is formed of pulses which are 
produced with the period (T/2). 
After the line closing instant t.sub.o the successive peak values of the 
voltage u.sub.1 are different and by choosing a sufficiently long 
discharge time constant RC of the capacitor 26 through the resistor 27 it 
is possible to arrange as will be obvious from diagram 2c, that the 
decreasing voltage u.sub.2 reaches not more than alternate ascending edges 
of voltage u.sub.1. After the line closing instant t.sub.o the duration of 
the charging and discharging cycles of the capacitor 26 is equal to T and, 
as shown in diagram 2d, the pulses of the charging current of the 
capacitor 26 are produced with the period T. The choice of the charging 
time constant RC is not critical and is not difficult to define; this time 
constant must be sufficiently large to ensure that during a discharging 
period (T/2) of capacitor 26 the voltage drop u.sub.2 at its terminals 
does not reach the difference .DELTA.u.sub.1 between two successive peak 
values of the voltage u.sub.1. This difference .DELTA.u.sub.1 is 
proportional to the difference between the peak currents I.sub.e1 
-I.sub.e2, which as described above is equal to (2E.sub.o /Z.sub.cc) and 
consequently independent of the amplitude 2 V of the ringing voltage 
applied to the subscriber's line and the ac. impedance Z.sub.ca of this 
line. The device according to the invention detects the closure of the 
subscriber's line by using the change caused by this closure in the period 
or in the pulse spacing of the charging current i.sub.2 of the capacitor 
26, and the result of the foregoing is that this device is in theory not 
affected by the variations of these values 2 V and Z.sub.ca. 
In the embodiment shown in FIG. 1 the pulses of the charging current 
i.sub.2 are used in the following manner. The resistor 25 through which 
the current i.sub.2 flows is connected between the base and the emitter of 
the pnp transistor 28. The emitter of this transistor 28 is connected to 
the positive supply terminal 14 and its collector is connected to ground 
via a resistor 29. The transistor 28 amplifies the current i.sub.2 and 
produces at its collector a current i.sub.3 which generates at the 
terminals of the resistor 29 a voltage u.sub.3 which, but for a 
proportionality coefficient, has the same shape as the charging current 
i.sub.2 shown in diagram 2d. 
The resistor 29 is connected between the base and the emitter of an npn 
transistor 30, which is driven to the cutoff or saturated state by the 
voltage u.sub.3. A current source 32 which is based on the d.c. voltage 
u.sub.o produces a constant current which charges a capacitor 31 which 
constitutes an integrating circuit. The capacitor 31 is connected between 
the collector and the emitter of the transistor 30. In this manner the 
capacitor 31 can be charged with a constant current when the transistor 30 
is in the cutoff state, whereas it suddenly discharges when this 
transistor 30 is saturated. 
As shown in diagram 2e, the result is that the voltage u.sub.4 at the 
terminals of the capacitor 31 is zero during the pulse durations of the 
current i.sub.2 and that it increases linearly in the time intervals 
between these pulses. The maximum voltage reached at the end of each of 
these time intervals is a measure thereof. As, during operation, these 
time intervals between the current pulses i.sub.2 are longer after closing 
of the line that before closing, the voltage u.sub.4 at the terminals of 
the capacitor 31 reaches, after closing, a maximum value v.sub.2, which is 
higher than its maximum value v.sub.1 before line closure. 
To detect closing of the subscriber's line, the voltage u.sub.4 is applied 
to the input of a threshold circuit 33, which compares the voltage u.sub.4 
with a reference voltage v.sub.o having a value between the voltages 
v.sub.1 and v.sub.2. Prior to the line closing instant t.sub.o the voltage 
u.sub.4 remains below v.sub.o and the threshold circuit 33 produces at the 
output 34 of the line closure detection circuit a signal S which is shown 
in diagram 2f and which has the logic value "0"; after the line closure 
instant t.sub.o, at the instant at which the voltage u.sub.4 reaches the 
reference voltage v.sub.o, the signal S changes to the logic "1" state 
which indicates closure of the subscriber's line. It can be seen that the 
detection of closure is effected within a period T after the line closing 
instant t.sub.o. 
A person skilled in the art will easily understand that instead of using an 
integrator 31, 32 of the analog type to measure the time interval between 
the current pulses i.sub.2 it is alternatively possible to use a counter 
which counts clock pulses during these time intervals and which is reset 
to zero during the current pulses i.sub.2. Closure will then be detected 
when a counting threshold is reached. Instead of measuring the interval 
between the current pulses i.sub.2 it is alternatively possible to measure 
the period of these pulses which, as shown above, charge from (T/2) before 
closing to T after closing. 
The device according to the invention can be used in a subscriber's circuit 
of a type other than that shown in FIG. 1, which has only been given by 
way of a non-limitative example. It is sufficient that in the subscriber's 
circuit used the subscriber's line is fed during ringing by the a.c. 
ringing voltage and by a d.c. voltage and that a signal representative of 
the absolute value of the current in the subscriber's line is formed. 
In order to obtain a line closure detecting device which is insensitive to 
parasitic longitudinal currents in the subscriber's line, it is 
advantageous to use a device as shown in FIG. 3. 
FIG. 3 shows a certain number of elements which are identical to those 
shown in FIG. 1 and which have been given the same reference numerals. The 
two wires L.sub.A and L.sub.B of the subscriber's line carry the currents 
I.sub.ea and I.sub.eb, respectively, which may be different when 
longitudinal currents are produced inopportunely in the subscriber's line. 
Let it be assumed by way of example that the subscriber's circuit 1 is of 
the same type as that shown in FIG. 1. Then a current .vertline.I.sub.ea 
.vertline., which results from the full-wave rectification of the current 
I.sub.ea, flows between the terminal 14 of the supply source and the 
supply terminal 12 of the subscriber's circuit, and a current 
.vertline.I.sub.eb .vertline. which results from the fullwave 
rectification of the current I.sub.eb flows between the terminal 15 of the 
supply source and the supply terminal 13 of the subscriber's circuit. A 
current mirror 17', which has the same ratio k as the mirror circuit 17 
and which is formed by the diode 18' and the transistor 19' is included 
between the terminals 13 and 15. 
In response to the currents .vertline.I.sub.ea .vertline. and 
.vertline.I.sub.eb .vertline. applied to the input terminals 20 and 20' of 
the current mirrors 17 and 17', the currents k .vertline.I.sub.ea 
.vertline. and k .vertline.I.sub.eb .vertline. appear at the output 
terminals 21 and 21' and are applied to the current adder circuit 35. At 
the output of the current adder circuit 35 the sum current k 
.vertline.I.sub.ea .vertline.+k .vertline.I.sub.eb .vertline. is obtained. 
The longitudinal current components have been suppressed in this sum 
current. It is then possible to make this sum current identical to the 
current i.sub.1 which, in FIG. 1, flows through the resistor 22. In FIG. 3 
the resistor 22 receives the voltage -V. The voltage u.sub.1 at the 
terminals of the resistor 22 is applied to the circuit 23, which may be 
identical to and have the same function as this circuit in FIG. 1. At its 
output 34 a logic signal S is obtained which indicates closure of the 
subscriber's line and which is less affected by longitudinal currents in 
the subscriber's line.