Method of operating a hearing aid and hearing aid

Abstract

For compensation of an interference signal coupled from an interferer line (8) into a receiver line (14) as a result of parasitic capacitances, an electronic circuit (4) of a hearing aid (2) has a compensation part 18 which has at least one coupling line (20) with an adjustable capacitor (22) arranged therein, a compensation interference signal being coupled into the receiver line (14) via the coupling line (20) during operation. The compensation interference signal is in particular in phase with the interference signal.

Description

[0001] Description

[0002] Method of operating a hearing aid and hearing aid

[0003] The invention relates to a method of operating a hearing aid and to a hearing aid.

[0004] The hearing aid is in particular a hearing aid intended to compensate for a hearing impairment of a hearing-impaired person.

[0005] Hearing aids of this kind, but also hearing aids in general, have an electronic circuit via which a receiver in particular is controlled. The receiver is typically designed as a loudspeaker. The receiver is regularly connected to an amplifier part of the circuit via a two-wire connection line.

[0006] Hearing aids regularly also have input transducers, in particular microphones, which convert for example an acoustic sound signal into an electrical signal which is then further processed by the electronic circuit and transmitted to the receiver as a processed output signal. For example an analogue microphone connected with the electronic circuit and there in particular with an analogue-digital converter.

[0007] Furthermore, antennas, especially inductive antennas, are often provided for wireless communication. Especially in binaural systems, so-called NFMI (near-field magnetic inductive) antennas are used, with the help of which the two hearing aids of a binaural system communicate with each other. The signal of the antenna is amplified by an amplifier, especially an LNA (low noise amplifier).

[0008] Due to the compact design of the hearing aids, there is generally the problem that interference signals can be transmitted between the connection lines due to coupling effects. In particular, the connection line leading to the receiver acts as a source of interference and therefore can be designated as an interferer line. Data signals transmitted via the connection lines from the antenna and / or from the microphone are very sensitive to interference. Various measures are known to avoid interference, such as shielding or laying the various connection lines as spatially separated as possible.

[0009] An object of the invention is to avoid or at least reduce such unwanted interference effects.

[0010] A method for operating a hearing aid and a hearing aid are provided to solve this problem. The hearing aid has the following features:

[0011] - an electronic circuit with several circuit parts,

[0012] - a first electronic component, in particular a receiver, which is connected to a first circuit part of the electronic circuit, in particular an amplifier part of the electronic circuit, via a first connection line referred to as an interferer line,

[0013] - a second electronic component which is connected to a second circuit part of the electronic circuit via a second electrical connection line referred to as a receiver line, wherein

[0014] - parasitic capacitances exist between the two connection lines, so that a parasitic interference signal is coupled into the receiver line during operation.

[0015] For compensation of the parasitic interference signal, the electronic circuit has a compensation part which has at least one coupling line that connects the two connection lines to each other. A capacitor is arranged in the coupling line, so that during operation a compensation interference signal is coupled into the receiver line via the at least one coupling line.

[0016] When operating the hearing aid a normal signal, generated by a signal generating unit (digital signal processing unit (DSP)) of the hearing aid, is transmitted via the interferer line. This normal signal is especially a so called drive signal form an amplifier part of the signal processing unit. Due to the parasitic capacitances present, the unwanted parasitic interference signals is generated and coupled into the receiver line. At the same time the compensation interference signal is - due to the coupling line with the capacitor- generated when the normal signal is transmitted via the inter- ferer line and also coupled into the receiver line. So due to the coupling line the compensation interference signal is an additional interference signal. This additional interference signal is therefore in some way deliberately generated. This compensation interference signal compensates the unwanted interference signal.

[0017] The advantages and preferred further embodiments listed below in connection with the method are also applicable to the hearing aid and vice versa. The hearing aid is generally designed to carry out the method.

[0018] The invention is based on the consideration that parasitic capacitances are responsible for the undesired coupling of interference signals into the second connection line (receiver line). A basic concept of the invention generally envisages mapping these parasitic capacitances in the circuit by means of suitable circuit components, so that in addition to the parasitic interference signal caused by the parasitic effects, an additional interference signal is generated, namely the compensation interference signal, which is also coupled into the second connection line (receiver line).

[0019] The coupling via the parasitic capacitances is therefore reproduced by the at least one coupling line, which connects the interferer line with the receiver line electrically and by wire. At the same time, a capacitor with a predetermined, defined capacity is arranged in this coupling line. The circuit components of the compensation part, i.e. in particular the at least one capacitor, is selected in such a way that the coupled compensation interference signal compensates for the parasitic interference signal.

[0020] Compensation of the parasitic interference signal is generally understood to mean that an effect caused by the parasitic interference signal or the interference signal itself are at least reduced and preferably cancelled. In the following the phrases “asymmetrical parasitic interference signal” and “parasitic interference signal” may be used interchangeably, since in most normally occurring situations the parasitic interference signal will be asymmetrical when coupled into e.g. a two-wire receiver line.

[0021] In the following the phrase “resulting parasitic interference signal” or “resulting compensation interference signal” is generally construed to be a resulting signal composed of a superposition of a plurality (especially two) interference signals (e.g. two parasitic interference signals or two compensation interference signals) which are coupled into different line wires of the receiver line, in cases where the receiver line consists of multiple lines wires. The resulting interference signal in such cases can be regarded especially as a parasitic differential signal between the (two) parasitic interference signals or as a compensation differential signal of the (two) compensation interference signals, coupled into the respective line wires.

[0022] In the following the phrase “compensation interference signal” may consist of a single compensation interference signal or consist of multiple compensation interference signals dependent on the number of line wires the receiver line consists of.

[0023] Depending on the configuration, there are different design options.

[0024] According to a first variant, a compensation interference signal is coupled in as an antiphase signal compared to the parasitic interference signal, so that the parasitic interference signal is reduced or eliminated by the antiphase compensation interference signal. An antiphase signal is generally understood as a signal with an amplitude opposite to that of the reference signal (parasitic interference signal), i.e. one signal has a positive amplitude and the other signal has a negative amplitude. The compensation signal preferably has the same amplitude in terms of amount as the parasitic interference signal.

[0025] In a second variant, and especially in the case of a two-wire receiver line into which an unbalanced, asymmetrical parasitic interference signal is coupled during operation, an in-phase compensation interference signal is preferably coupled in. This reduces the asymmetry of the parasitic interference signal and preferably eliminates it. In this embodiment, the compensation interference signal therefore ensures that symmetrical, in-phase interference signals with preferably identical amplitude are present on both line wires of the receiver line. This means that, in the case of an amplification with a differential amplifier, the interference signal is no longer contained in the amplified signal or is at least reduced compared to a variant without the compensation interference signal.

[0026] In a preferred embodiment, the electronic circuit is part of an IC and in particular part of an ASIC.

[0027] Generally, the interferer line and the receiver line are designed as single-wire or two-wire lines. The interferer line is usually designed as a two-wire line. Due to the parasitic effects, there is in principle the danger that a parasitic interference signal is coupled from each wire of the interferer line into each wire of the receiver line, so that there can be a large number of individual interference paths.

[0028] Preferably, a separate coupling line is formed for each of these interference paths, each with a capacitor arranged therein. Therefore, if both the interferer line and the receiver line are a two-wire line, a total of four coupling lines are provided. This embodiment reproduces the possible parasitic coupling paths, so that an efficient compensation is achieved.

[0029] With regard to the second variant described above, in a preferred embodiment the receiver line is designed as a two-wire line with two line wires into which the already mentioned parasitic interference signal is coupled as an asymmetrical parasitic interference signal. By asymmetrical interference signal it is understood that different parasitic interference signals, which differ in particular with regard to their amplitude, are coupled into the two line wires of the receiver line. The two parasitic interference signals coupled into the two line wires are therefore not symmetrical to each other, so that there is an overall asymmetry which forms the (resultant) asymmetrical (parasitic) interference signal. In a preferred embodiment, the compensation interference signal reduces this asymmetry, which results in no or less amplification of the parasitic interference signal in a subsequent differential amplifier.

[0030] Therefore, in order to reduce the asymmetry, an in-phase compensation interference signal is coupled (only) into one of the line wires. This in-phase compensation interference signal is especially a resulting in-phase compensation interference signal. Similar to the parasitic interference signals also different compensation interference signals can be coupled into the two line wires of the receiver line. However these different compensation interferences signals define the resulting in-phase compensation interference signal, which therefore is present only in one line wire. So the resulting compensation interference signal is especially a differential signal of the different signals coupled into the different line wires.

[0031] This embodiment is based on the consideration that - as a result of the asymmetrical coupling of different parasitic interference signals into the two line wires - a resulting parasitic interference signal is present in one line wire. This resulting parasitic interference signal is in particular a differential signal between the two parasitic interference signals. For example, in the case of two coupled parasitic interference signals, having the same phase but different amplitude, the resulting parasitic interference signal is formed by the amplitude difference.

[0032] This resulting parasitic interference signal is reproduced by the compensation part and is coupled as a resulting in-phase compensation interference signal into the other wire. This therefore achieves a symmetrisation.

[0033] Alternatively, it is also possible to feed a resulting antiphase compensation interference signal into the line wire on which the resulting parasitic interference signal is present. In this variant, the asymmetry component (and only this component) is eliminated so that only mutually symmetrical interference signals are contained in the two wires of the receiver line. As already mentioned, this design is based on the fact that the receiver line is connected to a differential amplifier, which only amplifies the differences between the signals. Symmetrical components are not amplified. Due to the symmetrisation, the amplified signal therefore contains no or only a small interference signal. In a preferred embodiment, the second circuit part is therefore a differential amplifier.

[0034] A particular advantage of this design with symmetrisation is that only comparatively small capacitances are required for the capacitors used, since only the asymmetry component has to be compensated. Accordingly, in a preferred embodiment, the capacitance of the at least one capacitor corresponds to only a fraction of a maximum parasitic capacitance. The fraction is preferably less than 50% and in particular less than 30%, especially the capacitance is only 20% of the maximum parasitic capacitance. The maximum parasitic capacitance depends on the configuration of the hearing aid with the components used and is - based on the current configuration of the hearing aid - the maximum possible capacitance of a capacitive parasitic coupling between the interferer line and the receiver line. This maximum possible capacitance can be calculated or at least estimated based on the design of the hearing aid.

[0035] This embodiment is based on the consideration that only the differences caused by the asymmetry have to be compensated, so that the capacities also only have to be a fraction of the maximum parasitic capacities to be expected.

[0036] Based on the preferred design with a two-wire interferer line connecting an amplifier with a receiver and with a two-wire receiver line, the parasitic capacitances typically lie for example in a range between 2fF and a maximum of 100fF (fF = femto-Farad). The maximum parasitic capacitance to be expected is therefore 100fF.

[0037] The capacitance of the at least one capacitor is overall preferably in a range of 5fF to 40fF and in particular between 15fF and 25fF. If there are several coupling lines each of the capacitors has a capacitance as mentioned.

[0038] As already mentioned, the first circuit part is in particular an amplifier part, for example an H-bridge and in particular a so-called class D amplifier. Alternatively or additionally, the first circuit part is a signal generator, which feeds an especially digital, preferably clocked signal into the interferer line . The first circuit part can, for example, also be a so-called charge pump or have other circuit components, especially components with short transition or switching times of, for example, less than 1 ns and / or components which generate a high-frequency data signal (greater than or equal to one megahertz). For example, a differential data signal is transmitted via the interferer line.

[0039] Furthermore, the first component is preferably a receiver, especially a loudspeaker.

[0040] The second electronic component is preferably an antenna, in particular an inductive antenna and especially an NFMI antenna. The receiver line is designed in particular as a two-wire line.

[0041] Furthermore, in this variant with the antenna, the second circuit part is designed in particular as a differential amplifier (LNA).

[0042] The parasitic effects typically vary from hearing aid to hearing aid and can also change over time, for example due to external influences or also due to ageing effects. According to a preferred embodiment, the capacitance of the at least one capacitor is adjustable. The capacitor therefore is an electronic component with a variable capacitance. Preferably, when using several coupling lines in each coupling line an adjustable capacitor is arranged.

[0043] The above mentioned values for the capacitances of the at least one capacitor are therefore the maximum capacitance of each of the capacitors of the individual coupling lines. The capacitance of the adjustable capacitor can therefore preferably be set between 0 and the maximum capacitance

[0044] To enable a sufficiently fine adjustment, the respective capacitor is preferably adjustable in steps between 0.5fF and 1 fF.

[0045] According to one variant, the respective capacitors are preferably adjusted at the manufacturer's factory as part of an initial configuration, so that hearing aid-specific parasitic effects are compensated.

[0046] With regard to a simple adjustment, an adjustment process is provided in a preferred embodiment, wherein the electronic circuit has an adjustment part for this purpose, which is designed to detect at least an effect of the interference signal and to adjust the at least one capacitor. In particular, a setting element is arranged for the adjustment, which can adjust the capacitance of the respective capacitor.

[0047] An effect of the interference signal is to be understood in general as any disturbance or amendment of the normal data signal transmitted via the receiver line due to the parasitic interference signal. An effect is for example noise or a signal-to- noise ratio on the receiver line or downstream of the second circuit part.

[0048] To detect the effect of the parasitic interference signal, a measuring element is formed as part of the circuit. Alternatively the measuring element for example directly measures the parasitic interference signal. Preferably, however, a noise and in particular a signal-to-noise ratio is determined as a parameter for the parasitic interference signal. The measuring element can consist of various electronic components.

[0049] The adjustment process is in particular an iterative process in which one or more of the capacitors are repeatedly adjusted and the effect of this adjustment is checked on the basis of the measurement results, i.e. specifically on the basis of the noise. In the course of the adjustment process, the noise is preferably reduced to a minimum. Due to the unknown parasitic effects, which can lead to complex mutual reactions that complicate the adjustment process, a defined test signal is preferably first fed into only one of the conductors of the interferer line. No signal is fed to the other line wire. Then one or more capacitors are adjusted, especially to reduce noise. Subsequently, another test signal is fed into the other wire of the interferer line and the capacitors are again adjusted and optimised. Preferably the test signals are fed alternately into each of the line wires.

[0050] Preferably, the electronic circuit is designed to carry out the adjustment process automatically. The adjustment process is therefore carried out automatically and, in particular, also initiated automatically. The adjustment process is preferably initiated at predefined states, for example when the hearing aid is switched on and / or after defined time intervals or defined operating times of the hearing aid.

[0051] An example of an embodiment is explained in more detail below with reference to the figure. This shows a partial representation of a hearing aid by means of a circuit diagram.

[0052] The hearing aid 2 is in particular a hearing aid in which an electronic input signal is generated via an input transducer, in particular a microphone. This electronic input signal is processed in a signal processing unit (typically a digital signal processing unit (DSP) that may be part of e.g. an IC or ASIC). The processed and especially amplified signal is fed to an output transducer, in particular a receiver I loudspeaker. The hearing aid is especially designed and adjusted to compensate for a hearing impairment of a hearing-impaired person. This signal generated by the signal processing unit is also called a normal or drive signal.

[0053] The hearing aid 2 generally has an electronic circuit 4, which is formed in particular by a microchip, an IC or in particular a so-called ASIC. This electronic circuit 4 has as a first circuit part an amplifier part 6, which in particular has an output bridge circuit, for example a so-called H-bridge. The amplifier part 6 is designed in particular as a so-called class D amplifier part. This is connected to a first electronic component via a first connection line, which is referred to as an inter- ferer line 8. In the shown embodiment, this is a receiver 10 (loudspeaker). The amplifier part 6 controls the receiver 10 via a digital control signal (drive signal) which is transmitted via the interferer line 8.

[0054] In addition, the circuit 4 has a second circuit part, which in the shown embodiment is designed as a differential amplifier 12 (LNA).

[0055] The amplifier 12 is connected with a second electronic component via a second connection line, which is referred to as the receiver line 14. In the shown embodiment of the Figure this is designed as an antenna 16, in particular as a so-called NFMI antenna. The differential amplifier 12 receives a signal from the antenna 16, amplifies it and outputs it for further processing.

[0056] The hearing aid 2 is in particular a hearing aid 2 of a binaural system and a communication signal of a second hearing aid of the binaural system is received via the antenna 16.

[0057] Both the interferer line 8 and the receiver line 14 are each designed as two-core lines with two line wires each.

[0058] The electronic circuit 4 further comprises a compensation part 18. The compensation part 18 has several coupling lines 20, whereby an individually adjustable capacitor 22 is arranged in each of the coupling lines 20. Each line wire of the interferer line 8 is connected to each line wire of the receiver line 14. In this embodiment, a total of four coupling lines 20 are therefore arranged, each with an adjustable capacitor 22. In principle, the capacitor 22 can also be formed by several individual capacitors.

[0059] Finally, the electronic circuit 4 further comprises an adjustment part 26 . This comprises as essential components a measuring element 28, i.e. a special arrangement of circuit components, and a setting element 30. In the shown embodiment, the measuring element 28 is used to determine a signal-to-noise ratio, namely for the amplified output signal of the amplifier 12. The setting element 30 is connected to each of the adjustable capacitors 22 and is used to adjust and to set the capacitance of these capacitors 22 in a controlled manner.

[0060] The circuit 4 is formed by at least one electronic component like a microcontroller, a micro-chip, an IC and an ASIC.

[0061] The figure also shows interfaces 32 of this electronic component with the various connection lines 8, 14. Inside of this electronic component, the connection lines are designed in particular as conductor tracks and outside the component, for example, as sheathed conductors.

[0062] During operation of the hearing aid, the typically high-frequency control signal (drive signal) transmitted via the interferer line 8 causes interference signals to be coupled into the receiver line 14 due to existing parasitic capacitances. These coupling effects thus lead to a coupling between the individual line wires of the interferer line 8 and the line wires of the receiver line 14. This capacitive coupling is represented by dashed parasitic coupling lines 34 and by parasitic capacitors 36. These are not real components. They only serve to describe the parasitic effects.

[0063] In an ideal configuration, a symmetrical coupling of interference signals from the interferer line 8 to the receiver line 14 would be expected. However, due to, for example, tolerances in the components, interference signals are usually coupled asymmetrically, so that a parasitic interference signal coupled into one line wire of the receiver line 14 differs from the parasitic interference signal coupled into the other line wire. Overall, this produces a resulting parasitic interference signal which can be regarded as a differential signal between the parasitic interference signals coupled into the two wires of the receiver line.

[0064] Due to the differential gain in amplifier 12, the amplifier 12 would amplify the resulting parasitic interference signal. To compensate for this resulting parasitic interference signal, the compensation section 18 is set in such a way that a resulting compensation interference signal is coupled into the receiver line 14, which ensures symmetrisation in the two receiver line wires of a resulting interference signal consisting of the resulting compensation interference signal and the resulting parasitic interference signal. For generating the resulting compensation interference signal different compensation signals can be coupled into the different line wires of the receiver line 14. The resulting compensation interference signal is especially a differential signal of these signals.

[0065] Specifically, an in-phase, i.e. non-inverted, resulting compensation interference signal with preferably the same amplitude as the resulting parasitic interference signal is coupled into the receiver line. This resulting compensation interference signal is fed into the line wire in which the resulting parasitic interference signal is not present. By coupling in the resulting compensation interference signal, it is ensured that the same interference signals, i.e. with the same phase and with the same amplitude, are present in both line wires. During the subsequent differential amplification, these are eliminated by the differential amplifier. Overall, the compensation part 18 compensates for the parasitic interference signal so that it is no longer contained in the amplified output signal of the amplifier 12.

[0066] With the help of the adjustment part 26, a suitable adjustment of the capacitances of the capacitors 22 is carried out. For this purpose, the signal-to-noise ratio is measured and in several iteration loops the capacitances of the capacitors 22 are varied until an optimal adjustment, especially an optimal signal-to-noise ratio is achieved. Overall, the parasitic effects are reproduced by the compensation part.

[0067] At the end of the adjustment process, especially identical interference signals are present on the input side of the amplifier 12 on both line wires.

[0068] For a simple adjustment of the capacitors 22, during the adjustment process a first test signal is preferably fed to one line wire of the interferer line 8, then an optimisation is carried out with the aid of the adjustment part 26. Subsequently, a second test signal, which can be identical to the first test signal, is fed to the second wire of the interferer line 8 and optimisation is carried out again. This process is preferably repeated several times until the best possible setting is achieved.

[0069] List of reference signs

[0070] 2 Hearing aid

[0071] 4 Electronic circuit 6 Amplifier part

[0072] 8 Interferer line

[0073] 10 Receiver

[0074] 12 Differential amplifier

[0075] 14 Receiver line 16 Antenna

[0076] 18 Compensation part

[0077] 20 Coupling line

[0078] 22 Adjustable capacitor

[0079] 26 Adjustment part 28 Measuring element

[0080] 30 Setting element

[0081] 32 Interface

[0082] 34 parasitic coupling line

[0083] 36 parasitic capacitor

Claims

Claims1 . Method of operating a hearing aid (2), the hearing aid (2) comprising- an electronic circuit (4) with several circuit parts (6,12),- a first electronic component (10) which is connected to a first circuit part (6) of the electronic circuit (4) via a first connection line designated as an inter- ferer line (8),- a second electronic component (16) which is connected to a second circuit part (12) of the electronic circuit (4) via a second connection line designated as a receiver line (14), wherein- a parasitic interference signal is coupled into the receiver line (14) due to parasitic capacitances existing between the two connection lines when a signal is transmitted via the interferer line (8), characterized in that- at least one coupling line (20) connects the two connection lines (8, 14) to one another, a capacitor (22) being arranged in the coupling line (20),- wherein a compensation interference signal is coupled into the receiver line (14) via the at least one coupling line (20) when the signal is transmitted via the interferer line (8) and wherein- the compensation interference signal compensates the parasitic interference signal.

2. The method according to claim 1 , wherein the receiver line (14) is a two-wire receiver line (14) with two line wires into which the parasitic interference signal is coupled as an asymmetrical parasitic interference signal and wherein the compensation interference signal is adjusted in such a way that the asymmetry of the parasitic interference signal is reduced.

3. Method according to the preceding claim, wherein a resulting parasitic interference signal is applied to one line wire and the compensation interference signal in phase with the resulting parasitic interference signal is coupled into the other line wire.

4. A method according to any one of the preceding claims, wherein the second circuit part is a differential amplifier (12).

5. A method according to any one of the preceding claims, wherein the capacitance of the at least one capacitor (22) is only a fraction of a maximum parasitic capacitance, the fraction being less than 50% and preferably less than 30% and in particular being 20%.

6. A method according to any one of the preceding claims, wherein the capacitance of the at least one capacitor (22) is in the range of 5fF to 40fF and in particular in the range of 15f F to 25fF.

7. Method according to one of the preceding claims, wherein the first circuit part is an amplifier part (6) and / or a signal generator which feeds a data signal, in particular a digital data signal, into the interferer line (8).

8. Method according to any one of the preceding claims, wherein the first electronic component is a receiver (10) and / or wherein the second electronic component is an antenna (16), in particular an inductive antenna and especially an NFMI antenna.

9. A method according to any one of the preceding claims, wherein the capacitance of the at least one capacitor (22) is adjustable, the capacitance preferably being adjustable in steps of 0.5fF to 1 fF.

10. Method according to the preceding claim, wherein an adjustment process for adjusting the capacitance of the at least one capacitor (22) is provided and the circuit (4) has for this purpose an adjustment part (26) which is designed to detect at least an effect of the parasitic interference signal and to adjust the at least one capacitor (22).

11. A method according to the preceding claim, wherein noise on the receiver line (14) is measured and the at least one capacitor (22) is varied such that a reduction of the noise occurs.

12. Method according to one of the two preceding claims, in which a defined test signal is fed into only one of the line wires of the interferer line (8), with a test signal preferably being transmitted alternately on the two line wires.

13. Method according to one of claims 10 to 12, wherein the electronic circuit (4) is designed to automatically carry out the adjustment process, the adjustment process being activated repeatedly, for example when the hearing aid (2) is switched on or after defined time intervals or operating times of the hearing aid (2).

14. Hearing aid (2) with- an electronic circuit (4) comprising several circuit parts (6, 12),- a first electronic component (10) which is connected to a first circuit part (6) of the electronic circuit (4) via a first connection line designated as an interferer line (8),- a second electronic component (16) which is connected to a second circuit part (12) of the electronic circuit (4) via a second connection line designated as a receiver line (14), wherein- parasitic capacitances exist between the two connection lines which, in operation, lead to a parasitic interference signal being coupled into the receiver line (14), characterized in that- for compensation of the parasitic interference signal, the electronic circuit (4) has a compensation part (18) which is designed to generate and couple a compensation interference signal into the receiver line (14),- the compensation part (18) having for this purpose at least one coupling line (20) connecting the two connection lines and a capacitor (22) arranged in the coupling line (20).

15. Hearing aid (2) according to the preceding claim, in which the receiver line (14) is a two-wire receiver line (14) and the compensation part (18) is designed in such a way that, in operation, when a resultant parasitic interference signal is present on one line wire of the receiver line, a compensation interference signal in phase with the resultant parasitic interference signal is coupled into the other line wire.

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