Off-hook detector with reduced sensitivity to common mode

An off-hook detector for use in subscriber line interface circuits to indicate status of the subscriber line, or loop, exhibits reduced sensitivity to common mode or longitudinal currents. A central signal from the exchange switches the transmission bridge for the subscriber line from a balanced to an unbalanced condition. In the balanced condition a difference detector indicates loop status. In the unbalanced condition, a comparator compares the voltage on one side of the transmission bridge to a reference voltage to indicate loop status.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention relates to an off-hook detector for use in a subscriber line 
interface circuit comprising a transmission bridge in one of whose 
branches an asymmetry may be introduced or not introduced depending on the 
value of a control signal, and a difference detector whose inputs are 
connected to the two terminals of the transmission bridge from where the 
said branches are connected to the central battery and whose output 
supplies a logic signal whose states indicate the opening or closing of 
the loop. 
2. Description of the Prior Art 
The off-hook detector is an element which detects the establishment or the 
interruption of the line direct current or transverse current. This 
transverse current may be disturbed by longitudinal parasitic currents 
induced on the line by power mains frequencies and which have the same 
direction at a given instant in the two wires of the line. When they flow 
through the transmission bridge, each longitudinal current is thus added 
to the transverse current in one of its branches and is subtracted in the 
other branch. If the bridge is symmetrical, that is to say if the two 
branches have the same resistances, the effects of these currents are 
cancelled and a difference detector connected to the transmission bridge 
terminals then produces a correct indication of the state of the loop; 
this detecting mode is of common usage in electronic telephone exchanges. 
But if an asymmetry is introduced in the bridge, the longitudinal currents 
cause the appearance at its terminals, superposed on the applied direct 
current, of an alternating voltage which is equal to the product of the 
current intensity and the difference of the resistance in the two branches 
of the bridge. In the event of a bridge which is considerably 
asymmetrical, the value of this alternating voltage reaches, and even 
exceeds, the value of the direct voltage at the terminals of the bridge, 
for example 30 Volts for a value of the central battery voltage equal to 
48 Volts, which thus gives rise to inadvertent activation of the 
difference detector, which may then furnish a faulty indication about the 
state of the loop. An asymmetry might, for example, be introduced in the 
transmission bridge when the resistance value in one single branch is 
increased in order to reduce the power consumption in the line in the case 
of a "false call", that is to say when a subscriber keeps the handset too 
long from the hook before dialing or when he forgets to put the handset 
back on the apparatus at the end of the conversation; it would therefore 
be sufficient if the central exchange conveys, after a specified time 
interval, a control signal which causes an electromechanical or electronic 
interruptor connected to the terminals of a high-value additional resistor 
in one of the branches of the bridge to close. But the correct detection 
of the state of the loop by means of the prior art method of differential 
detection entails the necessity to incorporate in the same manner an 
additional resistor of the same value in the other bridge, which is a 
particularly costly solution. 
SUMMARY OF THE INVENTION 
The invention has for its object to provide an off-hook detector with 
reduced sensitivity to common mode when an asymmetry is introduced in the 
transmission bridge, and which thus makes it possible to realize the 
savings which result from modifying the resistive circuit of one single 
branch of the bridge, for example in the case of a "false call", while 
maintaining the difference detection in a symmetrical transmission bridge 
having a low ohmic value. 
According to the invention such a device is characterized in that the said 
detector comprises a comparator whose first input is connected via a 
low-pass filter, to the said terminal of the bridge which terminal belongs 
to the said branch in which an asymmetry may be introduced or not 
introduced, the second input of the said comparator being connected to a 
voltage threshold and its output producing a logic signal whose states 
indicate the openings or closing of the loop, the loop signal in the 
subscriber line being either the output signal of the difference detector 
or the output signal of the comparator according as the said control 
signal puts the transmission bridge in its symmetrical state or in its 
asymmetrical state. 
By means of the following description which is given by way of 
non-limitative example with reference to the accompanying drawing, it will 
be better understood how the invention can be put into effect.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In this FIGURE, the wires 1 and 1' of a subscriber's line lead to the 
respective terminals 2 and 2' of the transmission bridge via the 
half-windings 3 and 3' of a transformer whose winding 4 conveys the speech 
currents to the exchange. The terminals 2 and 2' are interconnected by a 
capacitor 5 whose value C must be sufficient to effect a quasi 
short-circuit at the speech frequencies, and are also connected to the 
positive terminal 6 and to the negative terminal 6' of the central battery 
each one via a branch of the transmission bridge, the positive terminal 6 
being connected to ground and the negative terminal 6' to the potential 
-E. The branch which is connected between the terminals 2 and 6 and the 
branch connected between the terminals 2' and 6' comprise the respective 
identical resistive circuits 7 and 7' having the value R.sub.1, and 
additional resistor 8 having value R.sub.2 being arranged in series with 
the resistive circuit 7' in the branch 2'-6'. An electromechanical or 
electronic interruptor device 9 controlled by the logic signal F supplied 
by the central exchange is connected to the terminals of the resistor 8 in 
order to make the bridge symmetrical or asymmetrical. The well-known 
off-hook detector formed by a difference detector 10, whose inputs are 
connected to the terminals 2 and 2' of the bridge, produces at its output 
a logic signal B.sub.1. 
For that value of the control signal F which causes the interruptor 9 to 
close, the bridge is symmetrical and the voltage difference at its 
terminals is substantially insensitive to common mode effects which cancel 
each other in its two branches. The states of the output signal B.sub.1 of 
the difference detector 10 correctly indicate opening or closure of the 
loop. If the value of the control signal F causes the interruptor 9 to 
open, the resistor 8 is connected into the bridge which then becomes 
asymmetrical. If the potential at the terminals 2 and 2' of the bridge are 
designated V and V', respectively, the line direct current is designated I 
and the longitudinal current having the maximum value I.sub.l induced in 
the line at an industrial pulse frequency .omega. is designated I.sub.l 
e.sup.j.omega.t the difference potential between the terminals of the 
bridge in response to the ohmic drop in its branches is written at the 
instant t: 
EQU V-V'=E-(2R.sub.1 +R.sub.2)I-R.sub.2 I.sub.l e.sup.j.omega.t. 
It will be noted that the alternating component is important if the value 
of R.sub.2 must be high. But the above expression does not take account of 
the presence of the capacitor 5 having the value C which, for alternating 
current, is arranged in parallel with the resistor 8 so that the 
expression for the difference of the alternating potential at the 
terminals of the bridge is actually: 
##EQU1## 
This expression shows that the asymmetry of the bridge is partially 
attenuated by the influence of the coupling capacitor of the half-windings 
of the transformer but to an insufficient extent to render the difference 
detection by the loop substantially insensitive to common mode. In order 
to be able to adopt the economical solution of the asymmetrical bridge 
when, for example, in the case of a "false call" the resistor 8 is 
introduced in the branch 2'-6', the present invention joins to the 
difference detector 10 a comparator 11, one input of which is connected to 
the terminal 2' of the transmission bridge via a low-pass filter 12 and 
the other input of which is connected to a voltage threshold -kE which is 
proportional to the battery voltage, the proportionality coefficient k 
depending on the ohmic drop in the branch 2'-6' of the bridge. The output 
of a comparator 11 produces the logic signal B.sub.2. The logic signals F, 
B.sub.1, B.sub.2 are conveyed to a logic device 13 which realizes the 
logic function B=FB.sub.1 +FB.sub.2. In this manner, the loop is correctly 
detected by the output signal of the difference detector when the bridge 
is symmetrical and by the output signal of the comparator when the bridge 
becomes asymmetrical; in the first case the detection is substantially 
insensitive to the influence of common modes because of the symmetry of 
the bridge and in the second case it is substantially insensitive to this 
influence thanks to the efficiency of the low-pass filter 12 whose cut-off 
frequency has a value of a few Hertz. 
It should be noted that the difference detector 10 which is active during 
normal functioning of a subscriber line must have a delay time which is 
sufficiently small, of the order of some milliseconds, in order to enable 
detection of loop interruptions owing to dialling. In contrast therewith, 
the comparator 11, which becomes operative when an abnormal situation 
occurs which might continue for a certain period of time, for example in 
the case of a "false call", may allow a low response period.