BE903442A - Telephone-exchange subscriber-line circuit interface - has amplifier providing basic line functions and only operative during off hook condition and ringing generation - Google Patents

Abstract

The circuit has a two wire line with each line coupled to a central office battery via a separate feed resistor. A controllable amplifier has a floating switching converter with a capacitor coupled across the line to supply an auxiliary voltage. - A circuit monitors the voltage across the capacitor to provide a feedback control signal for the amplifier. An adder has an output coupled to the converter for controlling the auxiliary voltage across the capacitor. It has inputs coupled to the feedback control signal and to input signals for the line circuit. - A detector provides a signal, when the line goes off-hook, to control the amplifier to operate during the off-hook condition. A detector produces an output on detecting a ring control signal to cause the amplifier to generate a ringing signal. The amplifier is operative only during the presence of the ring control signal and during an off-hook condition of the line.

Description

       

  TELECOMMUNICATION LINE CIRCUIT

  
The present invention generally relates to

  
telephone line circuits and, in particular, to

  
a supply circuit for such a telephone line circuit using a floating voltage supply together with a standard voltage supply from the central office.

  
 <EMI ID = 1.1>

  
digital techniques for making communications between different subscribers. these systems have become extremely sophisticated and allow speech and data to be transmitted quickly and securely. A typical example of such a system is that described in the Revue des Télécommunications, No 1/2, 1985. A major consideration in the development of such switching systems is

  
in the construction of the line circuit and the aforementioned publication includes an article on an analog line circuit on pages 43 to 47, the latter indicating various references.

  
In a telephone system, line circuits are responsible for a substantial part of the cost of the system since their number corresponds to that of the subscribers. Consequently, in this area, specialists have concentrated their efforts on simplifying the line circuit while

  
making it more efficient. Thus, the literature shows various structures intended to reduce the dissipation of energy and the number of components in a line circuit.

  
 <EMI ID = 2.1>

  
890061 and 886385.

  
These patents are part of a large number relating to line circuits. Essentially, as a result of technical advances in the field of integrated circuit technology, a goal of the prior art has been

  
to provide a maximum of components in the form of integrated circuits in order to achieve a line circuit structure.

  
It is also known that various functions associated with the latter cannot easily be accommodated using

  
integrated circuits. This is the case, for example, of the main line control functions which include the application of a direct voltage to the subscriber line, the use of a high-voltage ring signal, the transmission

  
and speech reception, as well as remote metering signals.

  
The prior art has attempted to achieve

  
these functions through the use of different devices, many of which were not capable of being integrated. Recently he

  
there have been a series of developments which have made it possible to accommodate such functions by the use of amplification structure

  
in a large band. We can refer for example to the request

  
European patent published under N [deg.] 0158290.

  
In the latter, a high-voltage, high-power broadband amplifier that employs a switching converter with the capacitive storage element coupled

  
on the subscriber line has been described. The subscriber line is characterized as having a predetermined load impedance which is mainly resistive and which appears in

  
parallel on the storage capacitor of the converter to

  
 <EMI ID = 3.1>

  
an extremely efficient, relatively inexpensive, safe line circuit capable of withstanding ringing signals and other high voltage signals is obtained.

  
An important consideration in the development of a line circuit is the problem of power dissipation since the number of subscribers in a typical telephone system is very high, i.e. 10,000 or more. As a result, even small savings <EMI ID = 4.1>

  
substantial power savings from a system perspective.

  
It follows that one of the objects of the present invention is to provide a line supply circuit configuration which can be used in a line circuit to substantially reduce power dissipation and to improve operations.

  
This goal is accomplished, at least in part, by

  
a line circuit comprising a voltage source selectively operated and which is coupled to the line so, when it is operational, supplying the latter with an auxiliary operating current.

  
Since the line circuit is used to interface with each subscriber, whether a telephone set or a trunk interface for a private office, the problems

  
among the essential functions of the circuit

  
line.

  
In addition to the patents already cited, we can also

  
here refer to US Patent 4,349,703 as well as Belgian Patent 890,440, the latter describing a subscriber line interface circuit where current is supplied to a subscriber line from a voltage source shared, such as a DC / DC converter which is regulated to allow

  
providing regulated line current to many lines from the shared voltage source.

  
As already indicated, the line circuit must perform various basic line control functions. So, apart from providing the conversation battery,

  
line circuit must be able to process signals

  
ringing signals, as well as voice signals, remote charging and test signals. In particular, based

  
current requirements, ringing problems are

  
very important because the application of a ring signal * &#65533;

  
to a line circuit requires the use of contacts which can pass high voltages. Thus, in conventional circuits, the ring signal requires components that can support a high voltage. They are not easily integrated and constitute an expensive part of the line circuit. These components include high voltage isolating transformers, optical couplers, high voltage transistors and other devices.

  
It is therefore another object of the invention to provide an improved line circuit which uses a unique configuration of high operational amplifier

  
 <EMI ID = 5.1>

  
of high voltage integrated circuit.

  
It is yet another object of the invention that

  
to provide an improved line circuit capable of handling all basic line control functions including those related to ringing.

  
According to a preferred embodiment of a front-end circuit structure for a subscriber line circuit interface to be used in a telephone office,

  
the line circuit having a two-conductor line

  
(T, R) with each conductor coupled to a central office battery (VB) by a separate supply resistor

  
 <EMI ID = 6.1>

  
include a switching converter, including,

  
as a component thereof, a capacitance plugged into the two-conductor line and operating to supply an auxiliary voltage to the line, means coupled to the latter for monitoring the voltage on the capacitance in order to provide a counter control signal reaction

  
for the amplifier, an adder having an output coupled to the switching converter for controlling the auxiliary voltage on the capacitance, with a first input of the coupled adder; to the feedback control signal and a second input adapted to receive the input signals

J

  
line circuit input, detachment signal detecting means operating to provide an output signal when the subscriber line is off-hook in order to control said amplifying means to cause their operation during the off-hook condition, ringing control means coupled to said amplifiers and operative to detect the presence of a ringing control signal as applied

  
to the line to provide an output in order to control the amplifying means so that they supply the ringing signal from which, the amplifying means, as controlled, operate only during the presence of the ringing and during an off-hook condition of the line subscriber.

  
Other objects and advantages of the invention will appear more clearly to specialists on reading the following detailed description to be read in relation to the claims and the drawings accompanying the description.

  
Figure 1 is a schematic diagram showing a dual supply arrangement for a line incorporating the principles of the present invention; Figure 2 is a schematic diagram of an alternative embodiment; Figure 3 is a detailed schematic circuit of a dual power arrangement for a line circuit in accordance with the principles of the present invention; Figure 4 is a detailed schematic diagram of a line circuit employing an amplifier with a switching converter according to the invention; and Figure 5 is a schematic diagram of the amplifier used in the line circuit.

  
Referring to FIG. 1, there is shown a floating continuous source 10 also designated by VA.

  
Essentially resistance 11 constitutes the impedance of the line and is also designated by IL. In the off-hook condition, the subscriber line impedance is a function of the line length and the typical impedance of such lines can vary between a very low value up to 2000

  
ohm or more.

  
As seen in Figure 1, the floating DC source 10 is coupled to the terminals of lines A and B

  
by equal resistors 12 and 13 of RB value. The

  
central battery conventionally provided in a switching system is designated by VB and its voltage is usually of the order of 48 or 60 Volts depending on the country where

  
the telephone office is located. The power supply by the central battery is supplied at the terminals of the line circuit by the supply resistors 14 and 15.

  
The value (R1) of these resistors is chosen

  
 <EMI ID = 7.1>

  
For the circuit of FIG. 1, the central battery VB is that which is usually found in the office and the floating DC source 10, for example a DC / DC switching converter. They can be used respectively for the supervision of the line and for its supply.

  
Such converters have previously been used in line circuits. One can refer for example to the Belgian patent 886385 quoted above where the figure

  
5 shows a DC / DC converter in conjunction with a line circuit.

  
Anyway, as will be explained, in the on-hook condition, the converter 10 is disconnected and supervision of the stall is accomplished only by the battery VB of the central office. Disconnection of the floating voltage supply VA is accomplished by means of

  
of the switch 20 actually in series with this supply VA as shown in FIG. 1.

  
This technique has the effect of eliminating the residual power dissipation and the high frequency noise which is associated with a switching converter in the quiescent state. After the off-hook condition has been detected, the switching converter 10 is activated to then supply the proper supply current for the line. On the basis of the circuit shown in FIG. 1, it will be understood that the voltage source V can be constituted by an amplifier having a bandwidth and a power capacity allowing it to match the DC, ringing, voice and signal signals. remote charging with a frequency which can for example be between 12 and

  
16 KHz. In this way, the amplifier is a single device capable of handling all control functions

  
basic line circuit.

  
As will be explained later, it may have

  
the basic structure of a switching type converter.

  
Referring to FIG. 1, when the switch 20 is open and the line is at rest,

  
 <EMI ID = 8.1>

  
can be likened to an infinite resistance, the voltages at the junctions of resistors 12 and 15, and 13 and 14, respectively indicated by V .. and V, are

  
 <EMI ID = 9.1>

  
 <EMI ID = 10.1>

  
since there is no charge resulting in a flow of the battery VB whose positive pole on the side of the resistor 15 <EMI ID = 11.1>

  
On the other hand, when the switch 20 is closed, the two voltage sources VA and VB are put in series and half of their voltage difference is found on each of the resistors 14 and 15, giving
 <EMI ID = 12.1>
  <EMI ID = 13.1>

  
or the condition of the line since its impedance RL is short-circuited by the voltage source VA in series with the switch 20.

  
But as long as the switch 20 is open,

  
 <EMI ID = 14.1>

  
central office battery which supervises the stall. The RB value of resistors 11 and 12 is chosen in the range from 25 to 200 ohms, while that of R1 for resistors 14 and 15 can range from

  
5,000 to 50,000 ohms. As indicated, for the on-hook condition, the voltage source 10 is disconnected by the opening of the switch 20. In a concrete embodiment, this source is a DC / DC converter or a

  
amplifier. So the VB central office battery is used mainly to detect the condition

  
stall. For this, the floating power supply 10 is activated to supply power to the loop allowing conversation.

  
As seen in Figure 1, the

  
 <EMI ID = 15.1>

  
hanging up the subscriber line and after detecting

  
this condition, it operates the switch 20 to place the voltage source 10 (VA) in the circuit and cause the flow of an additional loop current (I1). The latter supplies the conversation current necessary for the line. This can be accomplished by the telephone system software. As will be described in conjunction with FIG. 3, the stall detector 25 has a conventional circuit configuration and detects the flow of current in the two-wire line following the stall by the subscriber, the current being initially supplied by the battery VB of the central office.

  
Depending on this arrangement, there is therefore a very low power consumption both in the on-hook condition and in the off-hook condition.

  
Referring to FIG. 2, a circuit similar to that of FIG. 1 has been shown there, but where the resistors 21 and 22 corresponding respectively to 15 and 14 in FIG. 1 are this time directly connected between the earth and wire A and secondly VB and wire B, so that when the two voltage sources VA and VB are interconnected by closing the switch
20, such a loop includes not only the two resistors of value R. but also, in series with them,

  
 <EMI ID = 16.1>

  
Referring to Figure 3, there is shown the dual power arrangement according to the invention where the auxiliary voltage source comprises an amplifier configuration with a switching converter. Such amplifier configurations have been described in detail in the European patent application mentioned above.

  
In any case, as can be seen from FIG. 3, the subscriber line which is normally associated with a subscriber station 30 is coupled to the conductors T and -R by means of supply resistors 31 and 32 of value RB . The subscriber line constitutes a two-wire line with a length depending on the distance from the central office and it

  
 <EMI ID = 17.1>

  
also a resistive component.

  
As shown in Figure 3, the battery of the central office VB whose positive pole is connected to the earth, is coupled to the line by means of resistors 34 and 35 whose

  
 <EMI ID = 18.1>

  
tances 31 and 32, for example 20 Kohm instead of 100 ohm.

  
The subscriber station 30, the data terminal or another device is normally associated with a cradle contact 36.

  
Consequently, when the subscriber picks up the handset in response to the ring or initiates a call, a hang-up condition exists when the cradle contact:

  
 <EMI ID = 19.1>

  
provides loop current through the subscriber line and

  
through the closed cradle contact. This loop current is detected by the line circuit by means of the stall detector 40 (25 in Figures 1 and 2).

  
Essentially, this detector has the function of reacting to a flow of current in the subscriber line which indicates the closing of the cradle contact 36. As an example, reference may be made to Belgian patent 881581.

  
When the subscriber picks up, the loop current

  
 <EMI ID = 20.1>

  
the off-hook is detected, the auxiliary voltage VA is applied to the line to supply the conversation current to the loop and other signals such as for example the ringing voltage. Following this circuit based on the use of an auxiliary voltage source activated only during the off-hook condition, this results in less power dissipation following the operation of the amplifier using a switching converter as described in the aforementioned European patent application.

  
In FIG. 3, the reference 10 designates the switching converter part of the amplifier circuit. Phone

  
 <EMI ID = 21.1>

  
a substantially standard component. A capacitor 41 is placed in parallel on the subscriber line and is supplied by a diode 42 which is coupled to the secondary of a power transformer 43. The primary of the latter has a terminal coupled to the central battery VA and another to earth through a transistor 44 of the FET type. An amplifier 52 serving as a buffer (A1) has its input coupled to the capacitor 41 and its output goes to an adder 53 whose other input, designated by VIN constitutes the signal terminal <EMI ID = 22.1>

  
 <EMI ID = 23.1>

  
may be the ring signal, the talk signal or a relatively high charging signal.

  
The output of the converter is coupled to the input of a second

  
 <EMI ID = 24.1>

  
suitable frequency for amplifier with converter

  
switching.

  
The output of amplifier 54 is coupled to an input of a pulse width modulator 55 whose other input is coupled to a reference voltage or clock source 56. The pulse width modulator 55 can understand a comparator using for example a source

  
of reference to triangular waveform. The output of the modulator 55 is coupled to the gate electrode of the transistor 44 of the FET type. As indicated, the latter has its source and drain electrodes coupled in series with the primary winding of the power transformer 43. The secondary winding of this transformer has a terminal coupled to the anode of the diode 42 with the cathode of the latter coupled to the junction between the resistor 31 and the capacitor 41. Essentially, the voltage on the capacitor, that is to say the auxiliary voltage source, is a function of the operation of the pulse width modulator

  
as controlled by gate 44.

  
As already pointed out, the circuit described is only put into operation during the line circuit stall condition. The latter is detected by the detector 40 which can be used to supply the various modules of the circuit using an additional transistor ( not shown) of the FET type. In particular, it controls the signals applied to the gate of transistor 44 by means of

  
55 and 56 passing through door 20. This exit from the detec &#65533; 40 also goes to a control microprocessor which is also connected to the other input of door 20.

  
We will now refer to FIG. 4 which shows a similar circuit but with more details as regards the operations linked to the ringing. Line circuit interface includes auxiliary voltage source
(VA) produced here in the form of an operational amplifier

  
 <EMI ID = 25.1>

  
it therefore corresponds to the circuit designated by VA in FIG. 3.

  
 <EMI ID = 26.1>

  
applied to the wires T and R of the subscriber line by the resistors of 20 Kohm while the supply resistors to these same wires and coming from the amplifier 30

  
have an RB value of 100 ohm. The input of this amplifier is connected to the common part 49 of the electronic equivalent of a change-over switch. For the position shown, the input of the amplifier is coupled to the adder.

  
53 while in the other position of the switch 49
(dotted line). the entry of 30 is

  
coupled to the ringing reference signal which essentially consists of an AC component superimposed on a DC voltage.

  
As will be explained later, the amplifier 30 is polarized to be conductive when

  
the subscriber line is off-hook as well as during the ringing condition. The switch 49 is operated by a ringing logic circuit 50 which allows the switching in

  
the position shown in dotted lines when the ringing control signal is present and the subscriber has not yet answered. So the amplifier will be active

  
both in the off-hook condition and in the ringing condition in order to allow it to generate the ringing signals.

  
When the subscriber answers, the ring detector
39 and triggered, signaled to the ringing logic 50 to restore the switch 49 to the contact position indicated by lines &#65533; &#65533; &#65533; &#65533; ns to inhibit the generation of ring and allow normal line feed functions. At this stage, the stall detector 38 maintains the polarization on the amplifier 30.

  
Other line circuits of this type exist and in particular that of the Belgian patent 8815.81 already mentioned.

  
The line circuit operation in Figure 4

  
 <EMI ID = 27.1>

  
as a line current detector. An example of such a circuit is shown in the Belgian patent cited last. This amplifier circuit 35 monitors the line current flowing in the subscriber line to determine when the subscriber has picked up. Consequently, the line current detection circuit compares the output voltage between the conductors T and R of the line with that at the output of the operational amplifier. In this way

  
circuit A., provides a voltage which is proportional to the line current.

  
The output of this line current detector 35 is applied to a bandpass filter 36, to a lowpass filter 37 and also to an input terminal of the cradle contact and dial pulse detector 38

  
The latter provides a signal to the switching system when the subscriber goes off-hook and further provides a signal indicating the dial pulses generated. For an ordinary dial a pulse is generated by the opening and closing of a contact in series with that of the cradle and the detector 38 will follow such transitions to provide an output of dial pulses.

  
The output of the low-pass filter 37 is coupled to

  
a ring pickup detector input terminal

  
39. Essentially, such detectors are known and operate to send a signal to the central office when a subscriber has answered the ring and to inhibit

  
ringing generation by switch control

  
49. The central office will cut the ringing control signal to this line to prevent a regeneration of the ringing signal when the call ends. The two detectors 38 and 39 have another input terminal to which a reference voltage is applied which essentially establishes a threshold for the operation of the detector.

  
We can for example refer to the US patent 4161633. In the latter, typical line circuits are shown as well as the detectors 38 and 39.

  
 <EMI ID = 28.1>

  
which is a benchmark filter for remote charging. During a charging operation, a relatively high frequency signal is applied to the line circuit.

  
The output of module 40 is coupled to the adder
41 which essentially receives input from a. balancing network 42. Such circuits are well known in connection with the couplings between the bi-directional two-wire lines and the uni-directional lines with four wires and are found in particular in various prior patents.

  
On the transmission side, the output of the adder 41 passes to the module 43 which is constituted by a filter and

  
an analog / digital converter, by means of an attenuator 42 allowing a good adaptation of the impedances. Examples of analog / digital converters are found in previous publications and in particular in American patent 4270027.

  
On the receiver side of the line circuit, there is a module 45 consisting of a digital / analog converter and a filter, the function this time being to receive digital signals from the remote office and to convert them into analog signals for transmission to the line. subscriber

  
 <EMI ID = 29.1>

  
45 is coupled to the rest of the circuit through an attenuator 46, and more particularly to the input of the balancing network 42 as well as to one of the inputs of the adder 55.

  
The latter receives the analog signals for transmission to the line circuit and can, in addition, receive at another input a remote charging signal, which is transmitted to the line circuit when the switch is closed.
51 through the high pass filter 52. In the remote charging mode, a 12 or 16 KHz signal is transmitted to the line circuit. The filter 40 prevents this signal from being returned by the line circuit during the remote charging procedure. The adder circuit 55 also receives an input from the bandpass filter 36.,. Which allows

  
at the central office to offer a programmable impedance

  
for audio signals.

  
The output of the adder 55 is applied to an input of an additional adder 53. The latter also receives an input from the low-pass filter 37 and to a third input, a continuous reference signal (DC).

  
This feedback is therefore analogous in terms of continuous signals to that for the synthesis of the impedance.
(AC signals) involving the bandpass filter 36.

  
Referring to Figure 5, there is shown a schematic diagram of the operational amplifier appearing in Figure 4. The same reference numbers have been used to designate the same components. Essentially, as noted, the operational amplifier 30 uses a switching converter. As shown, there

  
has two inputs to the amplifier which can be selected by the switch 49 controlled by the ringing logic 50. They consist of the ringing reference for ringing generation and by the voice input for normal line conditions. The output of switch 49 goes to an input terminal of an adder

  
60 whose output terminal is coupled to the input of a pulse width modulator 61 controlled by a clock signal. The output of this modulator is applied to the electrode of this gate of a field effect transistor 63. This transistor has its source / drain path in series with the primary of the power transformer 64 which receives at one terminal a bias potential indicated by VA. The secondary 65 of this power transformer is coupled by the diode 66 to

  
a shunt filtering capacitor 67 to produce the voltage VA on the line circuit. The output voltage on the capacitor 67 is monitored by means of a differential amplifier 68. The latter has two input terminals

  
coupled respectively to associated potentiometers 71 and

  
73 to monitor the VA voltage and thereby provide an error voltage at the amplifier output based on the difference

  
 <EMI ID = 30.1>

  
 <EMI ID = 31.1>

  
through a feedback network 69 which essentially provides a signal of suitable shape applied to the adder

  
60 and which will specify operational characteristics

  
of the line circuit. For examples of a particular amplifier configuration you can refer to the request

  
of previously cited European patent.

  
As can be seen, the amplifier described comprises

  
the switching converter which supplies an auxiliary voltage to the line circuit. This is developed

  
on the output capacitor 67 and supplies the current necessary for the line circuit during the conversation. The amplifier 68 is actuated by means of the stall detector 70 which is itself controlled by a circuit

  
in bridge connected to the line and which provides an output when the line is calling. Such circuits are also known from the prior art and a recent version

  
is found in the European patent application 85200774.9 filed on 17.5.1985.

  
In this way, this stall detector 70

  
and the associated circuits provide a control signal

  
during the stall. This signal applies suitable polarizations to the various modules included in the amplifier assembly. Thus, the circuit described above operates only when the line is lifted with the

  
 <EMI ID = 32.1>

  
power supply for the line circuit at this time.

  
This control signal can also be supplied by external command to impose conditions such as a

  
line blocking and preventing the amplifier from being

  
activated even during stall conditions.

  
When ringing is required for the circuit

  
line, ringing signal is detected

  
by the ringing logic 50 of FIG. 4. It operates

  
to activate the amplifier during this period. So,

  
in Fig. 5, the output of the ringing logic can be applied to the stall detector 70 to activate it

  
and thus, bias the amplifier to generate the ring.

  
In this case, the switch 49 associated with the amplifier 30 is operated to be placed in the condition shown in dotted lines in FIG. 4. It then reacts to the ringing reference signal and applies the ringing voltage

  
on the line by the switching converter.

  
Although the principles of the invention have been described above with reference to specific examples, it is understood that this description is made only

  
by way of example and in no way constitutes a limitation on the scope of the invention.


    

Claims (1)

1) Telecommunication line circuit comprising a DC power source for the line, characterized by the use of a second DC power source which can be selectively connected to the line. 2) Line circuit as under 1, characterized in that none of the two poles of the auxiliary source <EMI ID = 33.1> 3) Line circuit as in 1, characterized by the presence of means for detecting the on-hook or off-hook state of the line and providing a control signal for the off-hook state of the line, this signal putting the auxiliary source in circuit. 4) Line circuit as under 1, characterized by the fact that each of the two sources is or can be connected to the line via a separate pair of impedances. 5) Line circuit as under 4, characterized by the fact that the resistance of the impedances connecting the second source is much weaker than that of the other two impedances. 6) Line circuit as under 1, characterized by the fact that the second source comprises a DC / DC converter. 7) Line circuit as under 6, characterized by the fact that the DC / DC converter is constituted by amplifier means comprising a frame return converter whose storage capacitor is coupled on the line. 8) Line circuit as under 4, characterized by the fact that each of the poles of the two sources is directly connected to one of the conductors of the line by a single impedance. 9) Telecommunication line circuit comprising controllable amplifier means constituting a floating power source for the line and comprising a DC / DC converter with a feedback circuit from the line and which includes an adder for inserting input signals for the line circuit, characterized by means detecting the on-hook or off-hook condition of the line in order to activate the amplifying means during the off-hook condition and by means of detection of the ringing condition coupled to the amplifier means for detecting the presence of a ringing control signal applied to the line and for providing an output controlling the  <EMI ID = 34.1> the presence of the ringing control signal, hence the controlled amplifier means are only activated during the presence of a ringing control signal and during the stall of the line. 10) Line circuit as under 9, characterized by the fact that the converter is of the frame return type with a power transformer whose secondary is connected to an output capacitance via a serial diode and whose primary is connected in series with a controllable switch.  <EMI ID = 35.1> by the fact that the output of the adder is coupled at the input of a pulse width modulator having another input receiving clock pulses, to provide at its output a signal whose pulses are modulated in duration following an input signal applied to the adder, means being provided for coupling the output of the modulator to the controllable switching means. 12) Line circuit as in 11, characterized in that the second input of the adder is coupled to controllable switching means, with the output of the operating bell control means for controlling the switching means for placing them in a first condition allowing to receive the ring reference signal during generation of The ring. 13) Line circuit as under 9, characterized by the fact that it includes a line current detector providing a signal proportional to the current flowing in the line. 14) Line circuit as in 13, characterized in that it comprises a band-pass transducer whose input is coupled to the line current detector and whose output is coupled to an input of an adder circuit. 15) Line circuit as under 13, characterized in that it comprises a detector of the on-hook or off-hook condition of the line having a coupled input at the output of the line current detector to provide an output indicating the condition of the line. 16) Line circuit as under 9, characterized by the fact that it includes a digital / analog converter fed by the signals entering towards the line and the output of which is coupled to a second input of the adder. 17) Line circuit as under 16, characterized by the fact that it includes a remote charging signal adder having an input supplemented to the output digital / analog converter and a second input coupled to a source of remote zero-rating signals, the output of the signal adder being coupled to the second input of the adder suitable for receiving the input signals. / 18) Line circuit as under 13, characterized in that it includes a low-pass transducer having an input coupled to the output of the line current detector, with the output of the low-pass transducer coupled to the input of a ring trigger detector to provide at its output a indicative level of a line reaction to a condition off-hook during ringing generation and to inhibit it and provide line supply control conditions, the low-pass transducer output being further coupled to an adder and providing line feed signals. Subject: Correction of the Belgian patent application N [deg.] 2/60816) filed on October 15, 1985 in the name of: INTERNATIONAL STANDARD ELECTRIC CORPORATION Gentlemen, Please correct the text of the patent application mentioned under the heading below:  <EMI ID = 36.1> The undersigned is aware that no attached document in the file of a patent for invention cannot be of a nature to bring, either to the description, or to the drawings, substantive modifications and declares that the content of this note does not bring such modifications and other than reporting one or more hardware errors. It recognizes that the content of this note may not have the effect of rendering fully or partially valid the patent application N [deg.] 2] 60816) if it was not wholly or partly under the legislation currently in force. He authorizes the administration to attach this note to the patent file and to issue a photocopy. In the appendix we send you a tax stamp of FB 100, - as adjustment charge for this correction.  <EMI ID = 37.1>