JP2005176038A - Communication device and communication method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To suppress emissive noise which is caused by a communication line as the degree of balancing of the communication line is lowered. <P>SOLUTION: The communication device 10 establishes communication by using a power line 1 including conductive wires 1a and 1b. The communication device 10 detects the degree of balancing of the conductive wires 1a and 1b of the power line 1 by a balancing detection circuit 20 before starting communication or when it is out of communication. A controller in a digital processing circuit 15 judges whether there is a frequency band whose degree of balancing is less than a predetermined reference value or not. If there is such a frequency band, it stores it as an abnormal band. The controller controls communication so that the level of transmission signals in the abnormal band may be lower than that in other frequency bands when the communication device 10 establishes communication. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a communication apparatus and a communication method for performing communication using a communication line including two conductive lines.

  In recent years, there has been an increasing need for information communication in the home for the purpose of sharing peripheral devices of computers, sharing of information such as documents, still images, and moving images, games, and the Internet. Therefore, there is a demand for communication network systems not only in offices but also in general homes. As communication methods that can be selected when constructing a communication network system in the home, there are a communication method using wireless, a communication method using wire, and a power line communication method using power line. Among these, the power line communication method has advantages such as no wiring work cost is required because an existing power line is used as a communication line, and the appearance in the home is not impaired. In the present application, a power line used as a communication line in power line communication is referred to as a power line communication line.

  The power line communication method has a problem in that communication failure may occur due to noise generated by devices connected to the power line communication line. Therefore, conventionally, various countermeasures against noise in power line communication systems have been proposed.

  By the way, in a power line communication system, for example, an orthogonal frequency division multiplexing (hereinafter referred to as OFDM) method is used as a communication method. In this OFDM system, data is transmitted in parallel using a plurality of carriers (carrier waves) having different frequency bands. This OFDM method has an advantage that data can be reproduced even if data is lost due to the influence of noise in some bands by using an error correction technique or the like.

  Further, in Patent Document 1, when performing data communication by a multicarrier modulation / demodulation method, only carriers other than a band that is affected by noise or a band that is used for communication by other systems among a plurality of carriers are included. A technique for selecting and transmitting data is described.

  In Patent Document 2 and Patent Document 3, when performing data communication by the multicarrier modulation / demodulation method, frequency bands of a plurality of carriers are set so as to avoid a band affected by noise or a band already used. The technology to move is described.

JP 2000-216752 A JP 2001-7737 A JP 2001-7779 A

  By the way, in a power line communication system, generally, a communication device that performs power line communication is connected to a power line communication line via a power line communication modem. The power line communication modem inputs / outputs a normal mode communication signal to / from the power line communication line.

  The modem for power line communication is, for example, a live line (hereinafter referred to as L line) and a neutral line (hereinafter referred to as N line) which are two conductive lines constituting the power line communication line. A balanced signal composed of two signals having a voltage waveform symmetrical with respect to a potential (hereinafter referred to as a ground level) is transmitted. When this signal passes through the L line and the N line, a radiation electric field is generated from each of the L line and the N line. Here, consider the case where the power line communication line is balanced, that is, the case where the impedance between the L line and the earth or ground is equal to the impedance between the N line and the earth or ground. The impedance between the L line and the earth or ground is due to stray capacitance between the L line and the earth or ground. The impedance between the N line and the earth or ground is due to stray capacitance between the N line and the earth or ground. Since the L line and the N line are close to each other, when the power line communication line is balanced, the radiated electric field generated from the L line and the radiated electric field generated from the N line cancel each other, and radiated noise from the power line communication line. Does not occur.

  However, if the power line communication line is unbalanced, that is, if the impedance between the L line and earth or ground is different from the impedance between the N line and earth or ground, it is generated from the L line. Since the absolute values of the radiated electric field and the radiated electric field generated from the N line are different, they are not canceled out. In this case, radiation noise having a magnitude corresponding to the difference between the absolute values of the two radiation electric fields is generated from the power line communication line. Accordingly, the lower the balance of the power line communication line, the more radiation noise is generated from the power line communication line.

  Note that the above problem applies not only to power line communication lines, but also to all communication lines that transmit balanced signals.

  Conventionally, effective countermeasures have not been taken against radiation noise generated with such a decrease in the balance of the communication line.

  The present invention has been made in view of such problems, and an object of the present invention is to provide a communication apparatus and a communication method capable of suppressing radiation noise generated from the communication line as the balance of the communication line decreases. Is to provide.

The communication device of the present invention
A communication means for performing communication using a communication line including two conductive lines;
A balance detection means for detecting the balance of the communication line in the frequency range used for communication by the communication means;
Based on the detection result by the balance detection means, the level of the transmission signal transmitted from the communication means is lowered in the frequency band where the balance of the communication line is less than the predetermined reference value compared to other frequency bands. And a control means for controlling the communication means.

  In the communication device of the present invention, the balance degree of the communication line is detected by the balance degree detection means in the frequency range used by the communication means for communication. Based on the detection result, the balance degree of the communication line is determined by the control means. In the frequency band that does not satisfy the reference value, the communication means is controlled so that the level of the transmission signal is lower than in other frequency bands.

  In the communication apparatus of the present invention, the communication means may perform communication using a plurality of carriers having different frequency bands. In this case, the control means may not transmit data in a frequency band in which the balance of the communication line is less than a predetermined reference value. At this time, a carrier may or may not be transmitted to a frequency band in which data is not transmitted. In the communication device of the present invention, the communication line may be a power line.

  In the communication apparatus of the present invention, the balance degree detecting means may send a common mode signal to the communication line and detect the potential difference between the two conductive lines at that time.

The communication method of the present invention is a method of performing communication using a communication line including two conductive lines,
A balance detection procedure for detecting the balance of the communication line in the frequency range used for communication;
A communication procedure for performing communication so that the level of the transmission signal is lower in the frequency band where the balance of the communication line is less than a predetermined reference value compared to other frequency bands based on the detection result of the balance degree detection procedure. It is equipped with.

  In the communication method of the present invention, the communication procedure may be performed using a plurality of carriers having different frequency bands. In this case, the communication procedure may be configured not to transmit data in a frequency band where the balance of the communication line is less than a predetermined reference value. At this time, a carrier may or may not be transmitted to a frequency band in which data is not transmitted. In the communication method of the present invention, the communication line may be a power line.

  In the communication method of the present invention, the balance detection procedure may send a common mode signal to the communication line and detect the potential difference between the two conductive lines at that time.

  In the communication device or the communication method of the present invention, the balance of the communication line is detected in the frequency range used for communication, and based on the detection result, in the frequency band where the balance of the communication line does not satisfy a predetermined reference value. Communication is performed so that the level of the transmission signal is lower than in other frequency bands. Thereby, according to this invention, there exists an effect that the radiation noise generated from a communication line with the fall of the balance degree of a communication line can be suppressed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a circuit diagram showing an example of a configuration of a power line communication system including a communication device according to an embodiment of the present invention. A communication device 10 according to the present embodiment is a device that performs communication using a power line communication line 1 including two conductive wires 1a and 1b. The power line communication line 1 corresponds to the communication line in the present invention. Conductive lines 1a and 1b transport electric power and transmit a normal mode communication signal in power line communication. This communication signal is a balanced signal composed of two signals having a symmetrical voltage waveform with respect to the ground level. The communication device 10 inputs and outputs the communication signal with the power line communication line 1. In FIG. 1, symbol Z1 represents the impedance between the conductive wire 1a and the earth or ground. The symbol Z2 represents the impedance between the conductive wire 1b and the earth or ground.

  The communication device 10 includes input / output terminals 11a and 11b connected to the conductive lines 1a and 1b, a coupling capacitor 12a having one end connected to the input / output terminal 11a, and a cup having one end connected to the input / output terminal 11b. And a ring capacitor 12b.

  The communication device 10 further includes a coupler 13 connected to each other end of the coupling capacitors 12a and 12b, an analog front end unit 14 connected to the coupler 13, and a digital unit connected to the analog front end unit 14. A processing circuit 15, an interface controller 16 connected to the digital processing circuit 15, and an interface unit 17 connected to the interface controller 16 are provided. The communication device 10 further includes a balance degree detection circuit 20 connected to each other end of the coupling capacitors 12 a and 12 b and the digital processing circuit 15.

  The coupler 13 has a winding 13a connected to the other ends of the coupling capacitors 12a and 12b, and a winding 13b coupled to the winding 13a. The winding 13 b is connected to the analog front end portion 14. The coupler 13 cooperates with the coupling capacitors 12a and 12b to prevent the commercial frequency from passing and allow the communication signal to pass. A transmission signal output from the analog front end unit 14 is sent to the power line communication line 1 as a communication signal through the coupler 13 and the coupling capacitors 12a and 12b. A communication signal input from the power line communication line 1 to the communication device 10 is input to the analog front end unit 14 as a received signal via the coupling capacitors 12a and 12b and the coupler 13.

  The analog front end unit 14 performs digital-analog conversion on the digital transmission data from the digital processing circuit 15 to generate an analog transmission signal, and outputs the transmission signal to the coupler 13 through the filter circuit and the amplification circuit. To do. The analog front end unit 14 passes the analog reception signal from the coupler 13 through the filter circuit and the amplification circuit, and further performs analog-digital conversion to generate digital reception data. Output to the processing circuit 15.

  The digital processing circuit 15 performs, for example, OFDM modulation and demodulation processing. The configuration of the digital processing circuit 15 will be described in detail later.

  The interface unit 17 inputs and outputs data with a device that performs power line communication. The interface controller 16 controls data exchange between the interface unit 17 and the digital processing circuit 15.

  The balance detection circuit 20 detects the balance of the power line communication line 1 in a frequency range that the communication device 10 uses for power line communication. The balance degree detection circuit 20 corresponds to the balance degree detection means in the present invention. Further, in the communication device 10, the part other than the balance degree detection circuit 20 corresponds to the communication means in the present invention.

  Next, the configuration of the balance degree detection circuit 20 will be described with reference to FIG. The balance detection circuit 20 has input / output terminals 21a and 21b connected to the coupling capacitors 12a and 12b, respectively, and an input signal Vi input from the digital processing circuit 15 via the analog front end unit 14. Ends 22a, 22b and output ends 23a, 23b for outputting an output signal Vo output to the digital processing circuit 15 via the analog front end unit 14 are provided.

  The balance detection circuit 20 is further connected to a transformer 24 connected to the input terminals 22a and 22b, a transformer 25 connected to the output terminals 23a and 23b, and the transformer 24 and the input / output terminals 21a and 21b. Common mode signal injection circuit 26, common mode signal blocking circuit 27 connected to input / output terminals 21a and 21b, and normal mode signal detection circuit 28 connected to common mode signal blocking circuit 27 and transformer 25. It has.

  The transformer 24 has a winding 24a having one end connected to the input end 22a and the other end connected to the input end 22b, and a winding 24b coupled to the winding 24a. One end of the winding 24 b is connected to the common mode signal injection circuit 26. The other end of the winding 24b is grounded. The transformer 24 superimposes the input signal Vi input from the digital processing circuit 15 via the analog front end unit 14 on the power line communication line 1.

  The transformer 25 has a winding 25a having one end connected to the output end 23a and the other end connected to the output end 23b, and a winding 25b coupled to the winding 25a. The transformer 25, in cooperation with the coupling capacitors 12a and 12b, passes the output signal Vo output to the digital processing circuit 15 via the analog front end unit 14, and is transported by the power line communication line 1 Block the passage of power.

  The common mode signal injection circuit 26 has windings 26a and 26b coupled to pass the common mode signal and prevent the normal mode signal from passing. One end of the winding 26a is connected to one end of the winding 24b, and the other end of the winding 26a is connected to the input / output end 21a. One end of the winding 26b is connected to one end of the winding 24b, and the other end of the winding 26b is connected to the input / output end 21b. Although not shown, an impedance matching resistor having an appropriate resistance value may be disposed between the transformer 24 and the common mode signal injection circuit 26 as necessary.

  The common mode signal blocking circuit 27 has windings 31a and 31b coupled so as to pass a normal mode signal and block the passage of the common mode signal. One end of the winding 31a is connected to the input / output end 21a, and one end of the winding 31b is connected to the input / output end 21b. Common mode signal blocking circuit 27 further includes windings 32a and 32b coupled to pass the common mode signal and block the passage of the normal mode signal. One end of the winding 32a is connected to the other end of the winding 31a, and the other end of the winding 32a is grounded. One end of the winding 32b is connected to the other end of the winding 31b, and the other end of the winding 32b is grounded. The common mode signal blocking circuit 27 further includes windings 33a and 33b coupled to pass the normal mode signal and block the passage of the common mode signal. One end of the winding 33a is connected to the other end of the winding 31a, and the other end of the winding 33a is connected to one end of the winding 25b. One end of the winding 33b is connected to the other end of the winding 31b, and the other end of the winding 33b is connected to the other end of the winding 25b.

  The normal mode signal detection circuit 28 includes, for example, 50Ω impedance elements 28a and 28b, respectively. One end of the impedance element 28a is connected to one end of the winding 25b, and one end of the impedance element 28b is connected to the other end of the winding 25b. The other ends of the impedance elements 28a and 28b are connected to the midpoint of the winding 25b and grounded.

  Here, the operation of the balance degree detection circuit 20 shown in FIG. 2 will be described. An input signal Vi input from the digital processing circuit 15 via the analog front end unit 14 is input to the input terminals 22a and 22b. This input signal Vi passes through the transformer 24 and is converted into a common mode signal by the common mode signal injection circuit 26. The common mode signal is sent to the conductive lines 1a and 1b via the input / output terminals 21a and 21b, the coupling capacitors 12a and 12b, and the input / output terminals 11a and 11b. Here, when the conductive lines 1a and 1b are not balanced, a potential difference is generated between the conductive lines 1a and 1b. This potential difference is detected by a common mode signal blocking circuit 27 and a normal mode signal detection circuit 28. That is, in the common mode signal blocking circuit 27, the common mode signal is prevented from traveling toward the windings 32a and 32b by the windings 31a and 31b and the windings 32a and 32b. In the common mode signal blocking circuit 27, the normal mode signal passes through the windings 31a and 31b and the windings 33a and 33b. The normal mode signal is output as an output signal Vo from the output terminals 23a and 23b via the normal mode signal detection circuit 28 and the transformer 25. The output signal Vo is sent to the digital processing circuit 15 as a digital signal via the analog front end unit 14. The output signal Vo represents the potential difference between the conductive lines 1a and 1b when a common mode signal is sent to the conductive lines 1a and 1b.

  The digital processing circuit 15 obtains a ratio Vi / Vo between the input signal Vi and the output signal Vo. The ratio expressed in decibels (dB) is called longitudinal conversion loss. This longitudinal conversion loss serves as an index representing the degree of balance of the conductive lines 1a and 1b. The greater the longitudinal conversion loss, the higher the balance.

  Next, the configuration of the digital processing circuit 15 will be described with reference to FIG. The digital processing circuit 15 includes, as a transmission system configuration, an encoding unit 41 that inputs data from the interface controller 16, a modulation unit 42 that is serially connected in a subsequent stage of the encoding unit 41, and a serial-parallel conversion unit. (Referred to as S / P converter in FIG. 3) 43, inverse fast Fourier transform processor (referred to as IFFT processor in FIG. 3) 44, parallel-serial converter (referred to as P / S converter in FIG. 3). .) 45 and a guard interval adding unit (referred to as a GI adding unit in FIG. 3) 46.

  Data from the interface controller 16 is subjected to encoding processing such as interleaving by the encoding unit 41 and then modulated by the modulation unit 42. The output data of the modulation unit 42 is converted into parallel data by the serial-parallel conversion unit 43. Here, the data is assigned to a plurality of carriers (carrier waves) having different frequency bands for each predetermined unit. The output data of the serial-parallel converter 43 is subjected to inverse Fourier transform by the inverse fast Fourier transform processor 44 and converted to time domain data. The output data of the inverse fast Fourier transform processing unit 44 is converted into serial data by the parallel-serial conversion unit 45. The output data of the parallel-serial conversion unit 45 is output to the analog front end unit 14 after the guard interval is added by the guard interval adding unit 46.

  The digital processing circuit 15 includes a guard interval removing unit (referred to as a GI removing unit in FIG. 3) 51 for inputting data from the analog front end unit 14 and a subsequent stage of the guard interval removing unit 51 as a receiving system configuration. A serial-parallel converter (indicated as S / P converter in FIG. 3) 52, a fast Fourier transform processor (indicated as FFT processor in FIG. 3) 53, and a parallel-serial converter (in FIG. 3 includes a P / S converter.) 54, a demodulator 55, and a decoder 56.

  The data from the analog front end unit 14 is converted into parallel data by the serial-parallel conversion unit 52 after the guard interval is removed by the guard interval removal unit 51. The output data of the serial-parallel converter 52 is subjected to Fourier transform by the fast Fourier transform processor 53 and converted into frequency domain data. The output data of the fast Fourier transform processing unit 53 is converted into serial data by the parallel-serial conversion unit 54. The output data of the parallel-serial conversion unit 54 is demodulated by the demodulation unit 55 and then subjected to decoding processing such as deinterleaving by the decoding unit 56. The output data of the decoding unit 56 is output to the interface controller 16.

  The digital processing circuit 15 further includes a control unit 60 that controls the entire digital processing circuit 15. The control unit 60 sends an input signal Vi to the balance detection circuit 20 via the analog front end unit 14, and outputs an output signal Vo from the balance detection circuit 20 as a digital signal via the analog front end unit 14. input. The control unit 60 is configured by a microcomputer, for example. The control unit 60 corresponds to the control means in the present invention.

  FIG. 4 is a block diagram illustrating an example of the configuration of the control unit 60. 4 includes a microprocessor unit (hereinafter referred to as MPU) 61, a read only memory (hereinafter referred to as ROM) 62, and a random access memory (hereinafter referred to as RAM). 63), an input / output interface unit 64, and a bus 65 for connecting them to each other. The control unit 60 functions as the control unit 60 when the MPU 61 executes a program stored in the ROM 62 using the RAM 63 as a work area. The input / output interface unit 64 inputs and outputs data between the control unit 60 and the outside.

  Next, the operation of the communication apparatus 10 according to the present embodiment and the communication method according to the present embodiment will be described with reference to the flowchart of FIG. The communication device 10 detects the degree of balance between the conductive lines 1a and 1b of the power line communication line 1 before starting communication or when communication is not performed (step S101). At this time, the control unit 60 sends the input signal Vi to the balance degree detection circuit 20 via the analog front end unit 14, and based on the input signal Vi and the output signal Vo output from the balance degree detection circuit 20. Find the balance. The input signal Vi includes signals having a plurality of frequencies within a frequency range used by the communication device 10 for communication. The plurality of frequencies include at least frequencies in each frequency band of all carriers. The balance is given by, for example, the longitudinal conversion loss obtained as described above. In this way, the degree of balance of the conductive lines 1a and 1b is detected for each frequency band of all carriers.

  Next, the control unit 60 determines whether there is a frequency band in which the degree of balance does not satisfy a predetermined reference value, and if there is such a frequency band, stores it as an abnormal band in the RAM 63 (step) S102). The predetermined reference value is, for example, 20 dB expressed as a longitudinal conversion loss. In this case, the frequency band in which the longitudinal conversion loss is less than 20 dB is determined as the abnormal band.

  Next, when the communication device 10 performs communication, the control unit 60 controls communication so that the level of the transmission signal is lower in the abnormal band than in other frequency bands (step 103).

  Here, with reference to FIGS. 6 to 8, a specific method for controlling communication so that the level of the transmission signal in the abnormal band is lower than that in other frequency bands will be described. Here, an example will be described in which the serial-parallel converter 43 of the digital processing circuit 15 is controlled to control the above communication.

FIG. 6 is an explanatory diagram showing the operation of the serial-parallel converter 43 when there is no abnormal band. Here, as an example, it is assumed that the communication apparatus 10 transmits data using eight carriers. In FIG. 6, f _{1 to} f ₈ represent the center frequencies of the carriers. As shown in FIG. 6, when there is no abnormal band, the data A to H input to the serial-parallel converter 43 in time series are respectively transmitted to the carriers corresponding to the center frequencies f _{1 to} f _8. Assigned.

FIG. 7 is an explanatory diagram showing an example of the operation of the serial-parallel converter 43 when the frequency bands of the two carriers corresponding to the center frequencies f ₄ and f ₅ are abnormal bands. In the example shown in FIG. 7, the two carriers corresponding to the center frequencies f ₄ and f ₅ are not used for communication, and data is allocated to the other six carriers.

FIG. 8 is an explanatory diagram showing another example of the operation of the serial-parallel converter 43 when the frequency bands of the two carriers corresponding to the center frequencies f ₄ and f ₅ are abnormal bands. In the example shown in FIG. 8, data is not allocated to the two carriers corresponding to the center frequencies f ₄ and f ₅ .

  In either case of the example shown in FIG. 7 or the example shown in FIG. 8, data is not transmitted in the abnormal band, thereby reducing the level of the transmission signal in the abnormal band compared to other frequency bands. Can do.

  In addition, as another method for controlling communication so that the level of the transmission signal in the abnormal band is lower than that in other frequency bands, the data level allocated to the carrier corresponding to the abnormal band can be detected on the receiving side. A method of lowering the level of data allocated to other carriers within a range of different levels is conceivable. In this case, the level of the transmission signal can be lowered in the abnormal band as compared with other frequency bands while using the carrier corresponding to the abnormal band.

  As described above, in the present embodiment, the balance of the conductive lines 1a and 1b is detected in the frequency range used for communication, and the balance of the conductive lines 1a and 1b is determined based on the detection result. Communication is performed in a frequency band that does not satisfy the reference value so that the level of the transmission signal is lower than in other frequency bands. Thereby, according to this Embodiment, the radiation noise generate | occur | produced from conductive wire 1a, 1b with the fall of the balance degree of conductive wire 1a, 1b can be suppressed.

  In addition, this invention is not limited to the said embodiment, A various change is possible. For example, the present invention can also be applied to a communication device and a communication method that perform communication using a communication line other than a power line. Further, the present invention can be applied to communication apparatuses and communication methods that use frequency division multiplexing.

It is a circuit diagram which shows an example of a structure of the power line communication system containing the communication apparatus which concerns on one embodiment of this invention. It is a circuit diagram which shows the structure of the balance degree detection circuit in FIG. It is a block diagram which shows the structure of the digital processing circuit in FIG. It is a block diagram which shows the structure of the control part in FIG. It is a flowchart which shows operation | movement of the communication apparatus which concerns on one embodiment of this invention. It is explanatory drawing for demonstrating the method to control communication in one embodiment of this invention. It is explanatory drawing for demonstrating the method to control communication in one embodiment of this invention. It is explanatory drawing for demonstrating the method to control communication in one embodiment of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Power line communication line, 1a, 1b ... Conductive line, 10 ... Communication apparatus, 13 ... Coupler, 14 ... Analog front end part, 15 ... Digital processing circuit, 20 ... Balance degree detection circuit, 60 ... Control part.

Claims (10)

A communication means for performing communication using a communication line including two conductive lines;
A balance degree detecting means for detecting a balance degree of the communication line in a frequency range used by the communication means for communication;
Based on the detection result by the balance detection means, the level of the transmission signal transmitted from the communication means is higher in the frequency band where the balance of the communication line is less than a predetermined reference value compared to other frequency bands. A communication apparatus comprising: a control unit that controls the communication unit so as to decrease.   The communication apparatus according to claim 1, wherein the communication unit performs communication using a plurality of carriers having different frequency bands.   The communication device according to claim 2, wherein the control means does not transmit data in a frequency band in which a degree of balance of the communication line is less than a predetermined reference value.   The communication apparatus according to claim 1, wherein the communication line is a power line.   5. The communication device according to claim 1, wherein the balance degree detection unit transmits a common mode signal to the communication line and detects a potential difference between the two conductive lines at that time. . A communication method for performing communication using a communication line including two conductive wires,
A balance detection procedure for detecting the balance of the communication line in a frequency range used for communication;
Based on the detection result of the balance detection procedure, communication is performed such that the level of the transmission signal is lower in the frequency band where the balance of the communication line is less than a predetermined reference value compared to other frequency bands. A communication method comprising: a communication procedure.   7. The communication method according to claim 6, wherein the communication procedure performs communication using a plurality of carriers having different frequency bands.   8. The communication method according to claim 7, wherein the communication procedure does not transmit data in a frequency band in which the degree of balance of the communication line does not satisfy a predetermined reference value.   The communication method according to claim 6, wherein the communication line is a power line. The communication method according to claim 6, wherein the balance degree detection procedure transmits a common mode signal to the communication line and detects a potential difference between the two conductive lines at that time. .

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