GB2061064A - Line extender signalling unit - Google Patents

Abstract

A line extender signalling unit for applying boost voltages to telephone line conductors extending between a telephone exchange and a subscriber loop in order to aid applied d.c. signals is arranged to derive boost voltage supplies from a d.c. potential source, e.g. exchange battery, by way of an inverter, and a rectifier connected to the output of the inverter. An inductive reactance is provided between the inverter and the rectifier stage, in order to reduce the amplitude of the available d.c. boost voltage when the line extender signalling unit is connected to short telephone lines of correspondingly reduced resistance. Means are also provided for controlling the response of amplifying means of the line extender signalling unit to earth recalling, and coin impulsing signals.

Description

SPECIFICATION Line extender signalling unit This invention relates to a line extender signalling unit. Such units are employed in two wire telephone lines to extend the resistive limits over which telephone service can be provided from an exchange. In contrast to line extenders, or so-called repeaters, which are employed to provide audio frequency amplification of voice signals, so-called line extender signalling units are employed for amplification of the d.c. or pulsed control signals.

This is achieved by inserting one or more floating d.c. voltage sources in series with the telephone line, so augmenting the potential applied by the exchange to the line, and increasing the line current on lines of high than normal resistance to an acceptable level to enable both the exchange equipment and subscribers' terminal apparatus to continue to function on such circuits. The polarity and magnitude of these floating d.c. voltage sources must in general be arranged to respond in a carefully controlled way to polarity reversals impressed on the telephone circuit by the exchange equipment, to dialling and other signalling conditions imposed by the subscriber, and to variations in the resistance of the telephone line and subscribers' equipment which is connected.

The resistance of telephone lines varies considerably with variations in conductor gauge, material and line length. Line current must be controlled with-in certain limits to ensure proper performance of exchange and subscribers' equipment. The performance of certain subscribers' equipment is arranged to vary in response to line current so as to compensate for variations in line lengths, and both exchange and subscribers' equipment may be damaged by excessively high currents.It is therefore of paramount importance that any line extender signalling introduced into a telephone circuit regulate the magnitude of the boost voltage applied to ensure that excessive current does not flow on short lines, that the regulation characteristics of certain subscribers' apparatus is not impaired on lines of intermediate length, and that sufficient boost is available on long lines to ensure that proper equipment performance is maintained.

A known type of line extender signalling unit is described, for example, in U.S. Patent Specification No. 4,130,735. Line extender signalling units of this kind, however, have the disadvantage that on short telephones lines a great deal of power is dissipated in the line extender unit itself, since the full boost voltage is maintained available, while only a small portion of this voltage is applied to the line.

Other disadvantages of known line extender units will become apparent from the following description of typical applications of such line extender units.

Referring to Figure 1, there is shown in diagrammatic form, the circuit of a telephone line extending between the public telephone exchange, and a private exchange system. The public exchange comprises an exchange feeding bridge 2 which is served by exchange battery and earth 1 and 3 respectively, which are connected via the telephone line to the exchange terminals of a line extender signalling unit 4. The unit 4 comprises individual voltage boosting means 4a and 4b for the respective line wires, which may be connected to a private exchange line as indicated at 5a and Sb respectively. The private exchange system may include an extension line indicated at 10, and a telephone set 7, which is equipped with an earth recall switch 8, for providing connection to earth 3 by way of a resistor 9.Possible alternative connections of the switch 8 and 9 are indicated respectively at S and RE, and, in broken lines, at S' and RE'. The purpose of the earth recall switch 8 is to provide a calling condition, for example to initiate an enquiry call or to transfer a call, which will be recognised by a differential current sensing device 6 of the private exchange system. Thus when the earth recall switch is closed, the application of earth via the respective resistance 9 to one of the legs of the line loop will cause a difference in the loop currents flowing in the respective lines 5a and Sb of the private exchange system.

This current difference is normally recognised by the detecting device 6, which initiates the required enquiry or transfer calling sequence. However, it should be noted that reversals in line potential occur in certain circumstances, and it is normally not possible to ensure that the earth is always applied at the telephone on the wire which ensures maximum longitudinal or differential current. Unless the operating thresholds of relay RL1 are therefore tightly controlled, signalling may be marginal even on quite short lines.

The introduction of the conventional line extender unit 4 services to further degrade earth recall performance as the boost element 4B in series with line 5B tends to limit the rise in current in this line when the earth calling condition is applied, and the boost element 4A in line 5A may very well continue to provide boost voltage and maintain current in that leg of the pair. As both booster elements have of necessity a regulating characteristic, the differential current which results may be insufficient to operate the device 6.

Figure 2 shows an arrangement in which a line extender signalling unit is employed in a telephone line extending between a public telephone exchange and a coin box telephone. As in the case of Figure 1, the public exchange comprises a conventional line feeding bridge 2 served by exchange battery and earth 1 and 3 respectively. The telephone line also includes a line extender signalling unit 4 which is connected in the telephone line between the exchange and telephone line wires 5A and 5B leading to a coin box telephone. The public telephone exchange incorporates line current sensing relays 11 and 12 which are high speed polarised devices which respond to differential current conditions arising in the telephone line and are associated with coin and fee checking circuitry in the telephone exchange.The coin box telephone has a conventional telephone handset indicated at 13, which in known manner is provided with a dial impulsing contact 14, a dial spark quench capacitor and resistor 16 and 15 respectively, a dial-off normal contact 17 and a coin impulse contact 18 with associated spark quench capacitor 19 and resistor 20. A further resistor 21 is connected in parallel with the coin impulse contact 18.

The relays 11 and 12 perform the function of differentiating between signalling impulses arising from the insertion of coins and all other signalling conditions which may arise on the circuit due to dialling, manipulation ofthe cradle switch or line transient conditions. This requirement is met by setting the operate and release current threshold of the relays 11 and 12 so as to ensure that both relays operate and release together in response to dialling conditions, and to cradle switch manipulation. When the coin impulse contact 18 opens, however, the resistor 21 appears in series with the line, reducing the line current below the release threshold of the relay 12. The relay 11 has a lower release threshold, and therefore remains operated.The coin and fee checking circuitry associated with the relays 11 and 12 is arranged to recognise this condition as a true coin impulse.

When a line extender unit is introduced into a telephone line serving a coin box it is therefore clearly of vital importance to ensure that the correct operation of these current sensing relays is not disturbed. Line extender units employed in the past have several vital limitations in this regard, resulting for example, in the following erroneous conditions arising (i) Both relays 11 and 12 may release during part of the coin impulse train thus simulating dialling rather than coin impulsing. This is due to the "spark quench" capacitor in the coin box becoming charged to a voltage exceeding the exchange battery voltage prior to the line extender boost voltage collapsing in response to the high line resistance seen during the coin impulse. When the boost voltage does collapse, current reversal may occur for a short period as the capacitor discharges.The consequent release of relay 11 results in the coin impulse appearing invalid.

(ii) - Many coin boxes are arranged so as to open circuit the line for a short period following a coin impulse train. During this period both relays 11 and 12 release. When the telephone circuitry in the coin box is subsequently reconnected, relay 11 may operate substantially faster than relay 12, as the latter may require that boost voltage be available from the line extender for sufficient current to flow in the line for its operation. A finite time is required for the line extender to sense that boost is required and apply it. As relay 11 is operated and relay 12 released during this period an invalid coin impulse may be registered as a result.

It is an object of the present invention to provide a line extender signalling unit which overcomes one more of the disadvantages referred to above.

In accordance with one aspect of the invention there is provided a line extender signalling unit for adding a series aided boost voltage to a subscriber loop of a telephone system in response to sensed loop current applied from said telephone system, said unit comprising inverter means for providing an a.c. potential from a d.c. potential source, rectifier means for providing a d.c. boost voltage supply from said a.c. potential, and an amplifier circuit having an active device for connecting said d.c. boost voltage supply in series with a conductor of said loop, said amplifier circuit being so arranged that its gain is varied in accordance with the resistance presented by the said conductor of said loop, and there being connected between the output of said inverter and the said rectifier means, an inductive reactance such that the available d.c. boost voltage is reduced at relatively low resistances of said conductor of said loop.

Preferably, said amplifier circuit is a two stage circuit comprising an operational amplifier as the first stage and the said active element as the second stage, a feedback loop extending from the output of said second stage to the input of said first stage and the operational amplifier being connected as a voltage comparator with its respective inputs connected to receive a feedback signal from said feedback loop and a control signal in accordance with the loop current applied from said telephone system.

Advantageously, the line extender signalling unit according to the invention may be provided with a sensing means for sensing a difference in the currents flowing in the respective legs of the subscriber loop, in order to detect an earth calling condition, and means responsive to said sensing means for reversing the polarity of the boost voltage applied to one leg ofthe line loop, in order to enhance the current differential in the line loop and to ensure operation of any differential current sensing element connected thereto.

In accordance with a further, preferred feature of the invention, the said line extender signalling unit may include means for ensuring that the boost voltage is maintained in at least one line of the telephone circuit throughout a coin impulse train and any subsequent short period of disconnection.

By this means, the line extender signalling unit is prevented from responding to coin impulse signals in such a manner as may cause faulty operation of coin sensing relays of the exchange, or coin and fee checking circuitry connected thereto.

The invention is illustrated by way of example in the accompanying drawings, in which: Figure 1 is a diagrammatic view illustrating a conventional arrangement of a line extender signalling unit in a telephone line extending between a public telephone exchange and a private system, Figure 2 is a similar view of a line extender signalling connected between a public telephone exchange and a coin box telephone, and Figures 3A to 3Ctogether make up a circuit diagram of a line extender signalling unit in accordance with the invention.

Referring to Figures 3A and 3B, it will be seen that each of these Figures shows an amplifying circuit 34, 33 for adding a series aided boost voltage to a respective conductor of a subscriber loop of a telephone system. Thus the circuit of Figure 3A is connected in series between a line conductor 35A from the public telephone exchange, and a telephone line conductor 36A leading to the subscriber loop. Likewise, the circuit of Figure 3B is connected between conductor 358 and 368.It will be appreciated that the respective boost voltages are normally provided in opposite directions in the circuits of Figure 3A and Figure 3B respectively, that is to say if the circuit of Figure 3A provides a series aided boost voltage in the direction from the public exchange towards the subscriber loop, then the circuit of Figure 38 will provide boost voltage in the direction from the subscriber loop to the main public exchange. The circuits of Figures 3A and 38 are generally similar, and operate in the general manner described in more detail in U.S.A. Patent Specification No.4,130,735. Since the circuits are both substantially identical, only the circuit of Figure 38 will be described in more detail.

It will be seen that the amplifier circuit of Figure 38 comprises an operational amplifier A2, which if fed from a balanced power supply comprising positive and negative terminals +Y and -Y, respectively, and a reference potential indicated at COM Y. In accordance with the direction in which boost voltage is to be provided, the amplifier circuit operates to effectively connect the d.c. boost voltage +Y or -Y in series with the respective loop conductor of the subscriber line. The magnitude of the boost voltage applied to terminal 36B of the subscriber line is controlled by a transistor TR7 or TR8, the collector emitter current path of which is connected in series between the boost voltage supply and the terminal 368. The gain of the transistor TR7 or TR8 is controlled by the operational amplifier A2 in the following manner.

As shown in Figure 38, an input resistor R18 for the amplifier A2, and a current sensing resistor R15, are connected in series between the exchange line terminal 35B and the inverting input of the amplifier A2. Another resistor R23 is connected between the exchange line terminal 35B and the non-inverting input of the amplifier A2. Since the tapping between resistors R15 and R18 is connected to the common reference potential COM Y, and since there is negligible voltage drop across resistor R23, the respective inputs of the operational amplifier A2 receive signal voltages determined in accordance with the voltage drop across resistor R15 and resistor R18 respectively.In accordance with the relationship between these signal voltages, the operational amplifier supplies an output voltage to the bases of transistors TR7 and TR8. The voltage drop across resistor R18 is determined by the current flowing in a feedback circuit connected between the emitters of transistors TR7 and the inverting input of operational amplifier A2, and including resistor R19, resistor R17, resistor R16, and diodes D13 and D14.

The precise effect of this feedback network upon the operational amplifierA2 is described in more detail in U.S.A. Patent Specification No.4,130,735, and suffice it to say that this feedback net work varies the voltage drop across resistor R18 in accordance with the gain of the two-stage amplifying circuit formed by operational amplifier A2 and transistor TR7 or TR8. Thus, the gain of the amplifier circuit is adjusted in accordance with the voltage drop across resistor R15,which is in turn determined by the resistance of the loop conductor connected at the terminal 368.

The arrangement as described above provides for adjustment of the inserted boost voltage in accordance with line length, in generally known manner, but, has the disadvantage, if the boost voltage supplies provided at terminals +X, -X and +Y, -Y are constant, that there is undue power dissipation within the circuit at very short line length, when the available boost voltages are not fully utilized. In order to avoid this disadvantage, therefore, there is provided, as shown in Figure 3C, a regulated power supply which provides for reduction of the available boost voltages at low line loop resistances.The power supply circuit can be divided into three basic sections: a high frequency d.c. -a.c. inverter 31, which derives a d.c. power supply from the battery voltage terminals 315 and 315 of the telephone exchange system; a reactive inductance 37 connected to the output of the inverter 31, and a rectifier stage 38 which provides the voltage supply +Y, -Y and +X, -X for the circuit shown in Figures 3A and 3B, in addition to the reference voltages COM Y and COM X.

The inverter 31 comprises transistors TR1 to TR4 resistors R2 to R5 and a transformer T1. The inverter circuit derives power from the exchange battery voltage supply terminals 315 and 316, being connected to the 50 volt battery supply terminal 316, by way of resistor R53 and resistor R1, an electronic switch indicated diagrammatically at S1, which is controlled in known manner by way of a line loop detector circuit of the line extender signalling unit.

When the line loop condition is detected, the inverter circuit accordingly receives power from the exchange battery, and a current path is completed via resistors R4 and R5, the base-emitter junction of transistor TR1 and resistor R2. Transistor TR1 is thus turned ON, whereby current also flows via the winding W1 of transformer T1 and the collector current path of transistor TR1.The current flowing in winding W1 induces a corresponding current in the winding W2, the sense of which is such that transistor T81 is rapidly driven in a direction towards saturation. As the current in transistor TR1 increases, current flows in the base-emitter junction of transis torTR1 whereby the latter becomes conductive and robs current from the base of transistor TR1.Transistor TR1 now turns OFF, and the collapse of the current flowing in winding W1 now causes current to flow in winding W2 in a sense to turn ON transistor TR3. The inverter thus establishes an oscillating cycle and provides an a.c. voltage at its output terminals, which is applied to the primary winding of transformer T2, from the respective secondary windings of which corresponding output voltages are fed to the rectifier circuit 38 for the provision of the boost voltages previously referred to. The transformer T2 has an inductive reactance resulting from its leakage inductance, which is determined by the physical construction of the transformer and the juxtaposition ofwindings 39,310 and 311. Windings 310 and 311 are bi-filar wound on one section of the transformer bobbin and winding 39 is wound on the other section. The leakage inductance so created appears in series with the inverter 31 and the rectifier assembly 38.

As the resistance of the telephone line connected to terminals 3a and 36b is reduced, line current increases, as does the direct current drawn from rectifier assembly 38. The load reflected by transformer T2 on to the inverter assembly 31 is likewise increased and the inverter assembly responds by increasing its frequency of operation. This increases the inductive reactance introduced by the transformer T2 resulting in boost voltages Y and X reducing.

The overall voltage in series with the telephone line is therefore reduced and line current is limited on very short lines. Similar results may be achieved by inserting carefully chosen inductors between the inverter and rectifier assemblies, or by re-arranging the winding configuration of the inverter output transformer T1.

Referring back to Figures 3A and 3B, it will be seen that the circuit is provided with further features for overcoming the above described disadvantages of known line extender signalling units, as regards the response of the circuit to earth recall, or coin pulsing signals.

At the outputs of the respective legs 34 and 33 of the line extender signalling unit, there are connected windings 35a and 35b respectively of a sensitive differential-current sensing relay, the windings in each case being arranged in series between the output of the second stage of the previously described amplifier, and the corresponding line conductors 36a and 36b. Thus, when a difference occurs in the currents conducted by the respective relay windings 315a and 31 sub, the relay responds to close contacts 315c and 315d.

The relay contacts 31 sic and 315d control the response of the operational amplifiers Al and A2 to earth recalling, and/or coin-pulsing signals, in a manner to be described below, in conjunction with supervisory circuits which comprise transistors Tri 1 and TR12, together with integrated circuits IC3 and IC4, and their associated components. The respective circuits are powered from the rectifier circuit 38, by way of voltage stabilising circuits respectively comprising resistors R39, R40, diode D32 and Zener diodes Z4 and Z5, and resistors R48 and R49, diodes D34 and D37 and Zener diodes Z6 and Z7. The latter circuits provide stabilised operating voltages X'+, X'- and Y'+ and for the supervisory circuits.

The construction of both supervisory circuits is substantially identical, although the supervisory circuit associated with ampl if ier A2 contains some additional components the function of which will become apparent from the following description.

Accordingly, for brevity only the construction and operation of the circuit associated with amplifier A2 will be described in detail.

Although the circuit of Figure 3A operates in substantially identical manner during the application of both earth recall signals and coin pulsing signals to the line extender signalling unit, the circuit of Figure 3B comprises switch links SWB, SWC, and SWD, which are optionally connectable in accordance with the function of the signalling unit. Thus, if the signalling unit is to serve a private exchange which will provide earth recalling signals, the links SWB and SWD are closed, in which case the configuration of the circuit of Figure 3B corresponds substantially identically to that of Figure 3A. If the line extender unit is to serve a coin box, on the other hand, the link SWC is closed and the links SWB and SWD are broken.For the purposes of an initial description of the circuit operation, it will be assumed that the line extender unit is to serve a private exchange, and that accordingly links SWB and SWD will be closed.

The emitter of transistor Tor 12 is connected to the output of the operational amplifier A2, whereas the collector is connected, via resistor R45 and diodes D35 and D36, to the stabilized voltage supply Y'+.

The base oftransistorTR12 is also connected to Y'+, by way of resistor R54. A capacitor C18 is connected in series with resistor R54. A capacitor C18 is connected in series with resistors R46 and R47, to the tapping between diodes D35 and D36, and can thus become charged by way of the emitter/collector current path of transistor TR 2, when the latter is conductive. The tapping between capacitor C18 and resistor R47 is connected to the input of an inverter Il of the integrated circuit IC4, the output of which is connected via a further inverter 12 and diode D39 to the input of a further inverter 13. The output of inverter 13 is connected via diode D40 to the non-inverting input of amplifier A2.The relay contact 31 sod is connected in series between the Y'+ voltage supply and the input to an inverter 14, the output of which is connected, via a further inverter 15 and resistor R43, to the input of the inverter 13. A resistor R41 is connected between the negative supply voltage Y'- and the input to inverter 14.

When the output from the amplifier A2 is negative, then transistor TR12 will be non-conductive, and the capacitor C18 will be discharged by way of resistor R47, diode D44, resistor R52 and diode D45. A positive voltage will therefore appear at the output of inverter 11, so that a negative voltage appears at the output of inverter 12, and the voltage at the input of inverter 13 will also be pulled to a negative value, by way of diodes D39. Thus the output of inverter 13 will be positive and diode D40 will be rendered non-conductive. The integrated circuit IC4 will thus have no effect on the signal input at the noninverting input of amplifier A2, and the latter will operate precisely in the manner as previously described above. It will be noted atthis pointthat although a further inverter 16 is connected to the non-inverting input of the amplifier A2 by way of a diode D41,this latter inverter is effectively disabled by way of the previously described link SWD which connects the input of the inverter 14 to the Y'+ voltage source.

When the voltage output from amplifier A2 reaches a positive value, following completion of a subscriber loop, the transistor TR12 is rendered conductive, and the capacitor C18 becomes charged by way of the emitter/collector current path of the transistorT12. The output of inverter 12 goes positive, rendering diode D39 non-conductive, and thus removing the inhibiting signal from the input of the inverter 13. The time constant provided by resistors R46, R47 and capacitor C18 is such as to ensure that this removal of the inhibiting signal occurs only after a predetermined delay, to ensure that the circuit does not respond to differential current conditions occurring during an initial call setting operation and invalidly recognising such conditions as an earth recall signal.

When the differential current sensing relay now senses the presence of the earth calling condition, the contacts 315c and 315dare closed. Thus, for example in the circuit of Figure 3B, closure of the contact 31 sod causes a positive voltage signal to be applied at the input of inverter 14, in place of the negative signal previously applied by way of resistor R41. A positive voltage signal is correspondingly applied from the output of inverter 15 to the input of inverter 13, so that the voltage at the non-inverting input of amplifier A2 is pulled to a negative value by way of diode D40. This overrides any other input function seen by the amplifier A2 and forces the amplifier A2 to apply a negative voltage boost to the line conductor 36b.The corresponding reversal of the polarity of the voltage applied to the conductor 36b thus greatly accentuates the current differential in the subscriber loop and ensures positive and reliable operation of ali known differential current sensing means employed in private exchange telephone systems currently in service.

It will be noted that, during a normal line looped condition, only one of the amplifiers Al or A2 will be providing a positive voltage at its output, and correspondingly only that amplier will be caused to reverse its polarity, since the supervisory circuit associated with the other amplifier will prevent any input signal reaching the latter as a result of closure of the corresponding contact of the differential current sensing relay.

To cater for coin box signalling requirements, the links SWB and SWD of the above described circuit are removed, and the link SWC is inserted. Thus, the input of inverter 14 is connected to the exchange line wire 35a via resistor R42, and the positive voltage previously provided at the input of inverter 14 via link SWD is also removed. During dialling conditions, the exchange equipment applies a negative voltage to line 35b and a positive potential to line 35a. Boost voltage control elements 34 and 33 respond by applying positive and negative boost respectively.

The potential at the output of amplifier A2 is therefore negative and C18 is discharged. An inhibiting signal thus appears at the input of the inverter 13 as previously described. In addition the inverter 14 is similarly inhibited by way of diode D38. The boost control element thus continues to function normally in response to line current as sensed by resistor R15.

When the called party answers, the exchange equipment reverses the polarity of lines 35a and 35b, and the boost control elements 33 and 34 reverse the polarity of the voltage boost applied to the respective legs of the line which they serve. The polarity at the output of operational amplifier A2 now becomes positive and the capacitor C18 is charged in the manner described above, removing the inhibitfunc- tion from inverters 13 and 16. The boost control element 33 is now controlled directly by line voltage polarity via resistor R42, inverters 14, 15, 13 and 16 and diodes D40 and D41. This forces the boost control element to maintain a series aiding boost voltage to line 36b throughout the coin impulse signalling train.

Should line polarity again reverse and so be restored to the original state where line 35b is negative with respect to line 35a, the boost control element 33 will reverse in sympathy and remain under direct line voltage control until the inhibit function discussed above is restored following the discharge of capacitor C18. This discharge is arranged to be slow enough to allow for the completion of any coin signalling sequence.

Claims (10)

1. A line extender signalling unit for adding a series aided boost voltage to a subscriber loop of a telephone system in response to sensed loop current applied from said telephone system, said unit com prising inverter means for providing an a.c. potential from a d.c. potential source, rectifier means for providing a d.c. boost voltage supply from said a.c.

potential, and an amplifier circuit having an active device for connecting said d.c. boost voltage supply in series with a conductor of said loop, said amplifier circuit being so arranged that its gain is varied in accordance with the resistance presented by the said conductor of said loop, and there being connected between the output of said inverter and the said rectifier means feeding said amplifier an inductive reactance such that the available d.c. boost voltage is reduced at relatively low resistances of said conductor of said loop.

2. A unit as claimed in Claim 1, in which said amplifier circuit is a two-stage circuit comprising an operational amplifier as the first stage and the said active element as the second stage, a feedback loop extending from the output of said second stage to the input of said first stage and the operational amplifier being connected as a voltage comparator with its respective inputs connected to receive a feedback signal from said feedback loop and a control signal in accordance with the loop current applied from said telephone system.

3. A unit as claimed in Claim 1 or 2, including two of said boost voltage controlling amplifiers ranged for connection in respective legs of a subscriber loop, differential current sensing means for sensing a difference in the line currents flowing at the outputs of the respective amplifiers, and means controlled by said differential current sensing means for varying the output signal from one of said amplifiers in a sense such as to increase the said difference in line currents.

4. A unit as claimed in Claim 3, in which said output signal varying means is arranged to reverse the polarity of the signal at the output of said one amplifier.

5. A unit as claimed in Claim 4, when appended to Claim 2, wherein said differential current sensing means comprises a differential current sensing relay having a relay current connected in each case between a voltage source of a predetermined polarity and one of the inputs to a respective one of said operational amplifiers, means is provided for sens ing the polarity of the output signal of each amplifier and for blocking the signal path between said relay contact and the respective amplifier input when said output signal has a first predetermined polarity and for opening said signal path when said output signal has a second polarity, opposite from the first, and the arrangement being such that when said relay contact is closed effectively to connect said amplifier input to said voltage source the polarity of said output signal is caused to change from said second polarity to said first polarity.

6. A unit as claimed in Claim 5, wherein a timing circuit is provided for delaying reponse of said polarity sensing means to changes in the polarity of said output signal.

7. A unit as claimed in Claim 1 or 2, including two of said boost voltage controlling amplifiers arranged for connection in respective legs of a subscriber loop, one of said amplifiers being associated with means for sensing a reversal of the polarity of its output signal with respect to a predetermined polarity of normal operation, and for maintaining said reversal in polarity during a train of coin impulsing signals.

8. A unit as claimed in Claim 7 as appended to Claim 2, wherein said means for sensing a polarity reversal is arranged to effect a connection between that input of the respective amplifier which is connected to sense the line current signal of a respective line wire, and the other one of said line wires, by way of a signal inverter, whereby the polarity of both amplifiers is controlled by the signal in a single line wire.

9. A unit as claimed in Claim 7 or 8, wherein said polarity reversal sensing means is coupled to a delay circuit for delaying response thereof to changes in the polarity of said output signal.

10. A line extender signalling unit substantially as described herein with reference to the accompanying drawings.