Patent Publication Number: US-10777867-B2

Title: Transmission line

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention is a Continuation of application Ser. No. 15/561,719, filed Sep. 26, 2017, which is a 371 National Stage Entry of International Application No.: PCT/JP2016/062088, filed on Apr. 15, 2016, which in turn claims priority from Japanese Application No. 2015-097930, filed on May 13, 2015, the entire contents of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The disclosure relates to a transmission line that transmits signals. 
     BACKGROUND ART 
     With higher functionalization and multifunctionalization of electronic apparatuses in recent years, the electronic apparatuses are equipped with various devices such as semiconductor chips, sensors, and display devices. Among these devices, data exchange is performed in a large amount. The amount of data has been increasing in accordance with the higher functionalization and the multifunctionalization of the electronic apparatuses. Accordingly, the data exchange is often carried out with the utilization of a high-speed interface that is able to transmit and receive data at a speed of, for example, several Gbps. 
     Regarding methods of increasing transmission capacity even more, various techniques have been disclosed. For example, PTLs 1 and 2 disclose communication systems that transmit three differential signals with the utilization of three pieces of transmission lines. 
     Now, in communication systems, high communication quality is desired. Enhancing communication quality involves appropriate formation of, for example, a wiring pattern of transmission lines. For example, PTL 3 discloses a wiring pattern in a differential transmission line. 
     CITATION LIST 
     Patent Literature 
     PTL 1: Japanese Unexamined Patent Application Publication No. H06-261092 
     PTL 2: Specification of U.S. Pat. No. 8,064,535 
     PTL 3: Japanese Unexamined Patent Application Publication No. 2006-128618 
     SUMMARY OF THE INVENTION 
     As described, in communication systems, high communication quality is desired, with expectation of further enhancement in communication quality. 
     It is therefore desirable to provide a transmission line that makes it possible to enhance communication quality. 
     A first transmission line according to an embodiment of the disclosure includes a first line, a second line, and a third line. The second line has characteristic impedance higher than characteristic impedance of the first line. The first transmission line as mentioned above transmits a symbol that corresponds to a combination of signals in the first line, the second line, and the third line. 
     A second transmission line according to an embodiment of the disclosure includes a first line, a second line, and a third line. The first line is formed in a first layer. The second line is formed in a second layer. The third line is formed in the first layer. The second transmission line as mentioned above transmits a symbol that corresponds to a combination of signals in the first line, the second line, and the third line. 
     A third transmission line according to an embodiment of the disclosure includes a first trio line and a second trio line. The first trio line includes three lines and transmits a first symbol corresponding to a combination of signals. The second trio line includes three lines and transmits a second symbol corresponding to a combination of signals. One of the three lines in the second trio line is disposed between two of the three lines in the first trio line as mentioned above. 
     In the first transmission line according to the embodiment of the disclosure, provided are the first line, the second line, and the third line. The symbol corresponding to the combination of the signals in these lines is transmitted. The characteristic impedance of the second line is higher than the characteristic impedance of the first line. 
     In the second transmission line according to the embodiment of the disclosure, provided are the first line, the second line, and the third line. The symbol corresponding to the combination of the signals in these lines is transmitted. The first line and the third line are formed in the first layer, whereas the second line is formed in the second layer. 
     In the third transmission line according to the embodiment of the disclosure, provided are the first trio line and the second trio line. Moreover, in the first trio line, transmitted is the first symbol corresponding to the combination of the signals in the three lines. In the second trio line, transmitted is the second symbol corresponding to the combination of the signals in the three lines. One of the three lines in the second trio line is disposed between two of the three lines in the first trio line. 
     According to the first transmission line of the embodiment of the disclosure, the characteristic impedance of the second line is higher than the characteristic impedance of the first line. Hence, it is possible to enhance the communication quality. 
     According to the second transmission line of the embodiment of the disclosure, the first line and the third line are formed in the first layer, whereas the second line is formed in the second layer. Hence, it is possible to enhance the communication quality. 
     According to the third transmission line of the embodiment of the disclosure, one of the three lines in the second trio line is disposed between two lines out of the three lines in the first trio line. Hence, it is possible to enhance the communication quality. 
     It is to be noted that effects of the disclosure are not necessarily limited to the effects described above, and may include any of effects that are described herein. 
    
    
     
       BRIEF DESCRIPTION OF DRAWING 
         FIG. 1  is a block diagram that illustrates one configuration example of a communication system according to one example embodiment of the disclosure. 
         FIG. 2  is a descriptive diagram that illustrates voltage states of signals transmitted and received by the communication system illustrated in  FIG. 1 . 
         FIG. 3  is a descriptive diagram that illustrates transitions of symbols transmitted and received by the communication system illustrated in  FIG. 1 . 
         FIG. 4  is a block diagram that illustrates one configuration example of a transmission unit illustrated in  FIG. 1 . 
         FIG. 5  is a table that summarizes one operation example of an output unit illustrated in  FIG. 4 . 
         FIG. 6  is a block diagram that illustrates one configuration example of the output unit illustrated in  FIG. 4 . 
         FIG. 7  is a circuit diagram that illustrates one configuration example of a driver illustrated in  FIG. 6 . 
         FIG. 8  is a block diagram that illustrates one configuration example of a reception unit illustrated in  FIG. 1 . 
         FIG. 9  is a waveform chart that illustrates one example of signals transmitted and received by the communication system illustrated in  FIG. 1 . 
         FIG. 10  is a descriptive diagram that illustrates one example of reception operation of the reception unit illustrated in  FIG. 8 . 
         FIG. 11  is a cross-sectional view of one configuration example of a transmission line according to a first embodiment. 
         FIG. 12  is a characteristic diagram that illustrates one characteristic example of the transmission line illustrated in  FIG. 11 . 
         FIG. 13  is a characteristic diagram that illustrates another characteristic example of the transmission line illustrated in  FIG. 11 . 
         FIG. 14  is a characteristic diagram that illustrates one characteristic example of a transmission line according to a comparative example. 
         FIG. 15  is a characteristic diagram that illustrates another characteristic example of the transmission line according to the comparative example. 
         FIG. 16  is a cross-sectional view of one configuration example of a transmission line according to a modification example of the first embodiment. 
         FIG. 17  is a cross-sectional view of one configuration example of a transmission line according to another modification example of the first embodiment. 
         FIG. 18  is a cross-sectional view of one configuration example of a transmission line according to another modification example of the first embodiment. 
         FIG. 19  is a block diagram that illustrates one configuration example of a communication system according to another modification example of the first embodiment. 
         FIG. 20  is a descriptive diagram that illustrates one configuration example of a transmission line illustrated in  FIG. 19 . 
         FIG. 21  is a table that summarizes one characteristic example of the transmission line illustrated in  FIG. 20 . 
         FIG. 22  is a table that summarizes another characteristic example of the transmission line illustrated in  FIG. 19 . 
         FIG. 23  is a descriptive diagram that illustrates one configuration example of a transmission line according to another modification example of the first embodiment. 
         FIG. 24  is a cross-sectional view of one configuration example of a transmission line according to a second embodiment. 
         FIG. 25  is a block diagram that illustrates one configuration example of a communication system according to a modification example of the second embodiment. 
         FIG. 26  is a cross-sectional view of one configuration example of a transmission line illustrated in  FIG. 25 . 
         FIG. 27  is a cross-sectional view of one configuration example of a transmission line according to another modification example of the second embodiment. 
         FIG. 28  is a block diagram that illustrates one configuration example of a communication system according to a third embodiment. 
         FIG. 29  is a schematic diagram that illustrates one operation example of the communication system illustrated in  FIG. 28 . 
         FIG. 30  is a block diagram that illustrates one configuration example of a communication system according to a modification example of the third embodiment. 
         FIG. 31  is a schematic diagram that illustrates one operation example of the communication system illustrated in  FIG. 30 . 
         FIG. 32  is a block diagram that illustrates one configuration example of a communication system according to another modification example of the third embodiment. 
         FIG. 33  is a schematic diagram that illustrates one operation example of the communication system illustrated in  FIG. 32 . 
         FIG. 34  is a block diagram that illustrates one configuration example of a communication system according to another modification example of the third embodiment. 
         FIG. 35  is a schematic diagram that illustrates one operation example of the communication system illustrated in  FIG. 34 . 
         FIG. 36  is a block diagram that illustrates one configuration example of a communication system according to another modification example of the third embodiment. 
         FIG. 37  is a perspective view of an external appearance and a configuration of a smartphone to which the communication system according to the example embodiment is applied. 
         FIG. 38  is a block diagram that illustrates one configuration example of an application processor to which the communication system according to the example embodiment is applied. 
         FIG. 39  is a block diagram that illustrates one configuration example of an image sensor to which the communication system according to the example embodiment is applied. 
         FIG. 40  is a cross-sectional view of one configuration example of a transmission line according to a modification example. 
         FIG. 41  is a cross-sectional view of one configuration example of a transmission line according to another modification example. 
     
    
    
     MODES FOR CARRYING OUT THE INVENTION 
     In the following, some embodiments of the disclosure are described in detail with reference to the drawings. It is to be noted that description is made in the following order. 
     1. First Embodiment 
     2. Second Embodiment 
     3. Third Embodiment 
     4. Application Example 
     1. First Embodiment 
     [Configuration Example] 
       FIG. 1  illustrates one configuration example of a communication system including a transmission line according to a first embodiment. A communication system  1  performs communication with the utilization of a signal having three voltage levels. 
     The communication system  1  includes a transmission device  10 , a transmission line  100 , and a reception device  30 . The transmission device  10  includes three output terminals ToutA, ToutB, and ToutC. The transmission line  100  includes lines  110 A,  110 B, and  110 C. The reception device  30  includes three input terminals TinA, TinB, and TinC. Moreover, the output terminal ToutA of the transmission device  10  and the input terminal TinA of the reception device  30  are coupled to each other through the line  110 A. The output terminal ToutB of the transmission device  10  and the input terminal TinB of the reception device  30  are coupled to each other through the line  110 B. The output terminal ToutC of the transmission device  10  and the input terminal TinC of the reception device  30  are coupled to each other through the line  110 C. Characteristic impedance of the lines  110 A to  110 C is about 50 [Ω] in this example. Specifically, as described later, the characteristic impedance of the line  110 B is higher than the characteristic impedance of the lines  110 A and  110 C. Hence, in the communication system  1 , as described later, it is possible to enhance communication quality. 
     The transmission device  10  outputs a signal SIGA from the output terminal ToutA, outputs a signal SIGB from the output terminal ToutB, and outputs a signal SIGC from the output terminal ToutC. Moreover, the reception device  30  receives the signal SIGA through the input terminal TinA, receives the signal SIGB through the input terminal TinB, and receives the signal SIGC through the input terminal TinC. The signals SIGA, SIGB, and SIGC each make transitions among the three voltage levels (a high level voltage VH, a medium level voltage VM, and a low level voltage VL). 
       FIG. 2  illustrates voltage states of the signals SIGA, SIGB, and SIGC. The transmission device  10  transmits six symbols “+x”, “−x”, “+y”, “−y”, “+z”, and “−z” with the utilization of the three signals SIGA, SIGB, and SIGC. For example, in a case of the transmission of the symbol “+x”, the transmission device  10  brings the signal SIGA to the high level voltage VH, brings the signal SIGB to the low level voltage VL, and brings the signal SIGC to the medium level voltage VM. In a case of the transmission of the symbol “−x”, the transmission device  10  brings the signal SIGA to the low level voltage VL, brings the signal SIGB to the high level voltage VH, and brings the signal SIGC to the medium level voltage VM. In a case of the transmission of the symbol “+y”, the transmission device  10  brings the signal SIGA to the medium level voltage VM, brings the signal SIGB to the high level voltage VH, and brings the signal SIGC to the low level voltage VL. In a case of the transmission of the symbol “−y”, the transmission device  10  brings the signal SIGA to the medium level voltage VM, brings the signal SIGB to the low level voltage VL, and brings the signal SIGC to the high level voltage VH. In a case of the transmission of the symbol “+z”, the transmission device  10  brings the signal SIGA to the low level voltage VL, brings the signal SIGB to the medium level voltage VM, and brings the signal SIGC to the high level voltage VH. In a case of the transmission of the symbol “−z”, the transmission device  10  brings the signal SIGA to the high level voltage VH, brings the signal SIGB to the medium level voltage VM, and brings the signal SIGC to the low level voltage VL. 
     The transmission line  100  transmits a sequence of the symbols, with the utilization of the signals SIGA, SIGB, and SIGC as mentioned above. Specifically, the transmission line  100  transmits the sequence of the symbols, with the line  110 A transmitting the signal SIGA, with the line  110 B transmitting the signal SIGB, and with the line  110 C transmitting the signal SIGC. In other words, the three lines  110 A,  110 B, and  110 C serve as a single trio line that transmits the sequence of the symbols. 
     [Transmission Device  10 ] 
     Referring to  FIG. 1 , the transmission device  10  includes a clock generator unit  11 , a processor unit  12 , and a transmission unit  20 . 
     The clock generator unit  11  generates a clock signal TxCK. A frequency of the clock signal TxCK is, for example, 2.5 [GHz]. The clock generator unit  11  is constituted by, for example, a PLL (Phase Locked Loop), and generates the clock signal TxCK on the basis of, for example, a reference clock (not illustrated) supplied from outside of the transmission device  10 . Moreover, the clock generator unit  11  supplies the clock signal TxCK to the processor unit  12  and the transmission unit  20 . 
     The processor unit  12  performs predetermined processing, and thereby generates transition signals TxF 0  to TxF 6 , TxR 0  to TxR 6 , and TxP 0  to TxP 6 . Here, a single set of the transition signals TxF 0 , TxR 0 , and TxP 0  indicates a transition of the symbol in the sequence of the symbols to be transmitted by the transmission device  10 . Likewise, a single set of the transition signals TxF 1 , TxR 1 , and TxP 1  indicates the transition of the symbol. A single set of the transition signals TxF 2 , TxR 2 , and TxP 2  indicates the transition of the symbol. A single set of the transition signals TxF 3 , TxR 3 , and TxP 3  indicates the transition of the symbol. A single set of the transition signals TxF 4 , TxR 4 , and TxP 4  indicates the transition of the symbol. A single set of the transition signals TxF 5 , TxR 5 , and TxP 5  indicates the transition of the symbol. A single set of the transition signals TxF 6 , TxR 6 , and TxP 6  indicates the transition of the symbol. In other words, the processor unit  12  generates seven sets of the transition signals. In the following, the term “transition signal TxF” is utilized, as appropriate, to denote any one of the transition signals TxF 0  to TxF 6 . The term “transition signal TxR” is utilized, as appropriate, to denote any one of the transition signals TxR 0  to TxR 6 . The term “transition signal TxP” is utilized, as appropriate, to denote any one of the transition signals TxP 0  to TxP 6 . 
       FIG. 3  illustrates relation between the transition signals TxF, TxR, and TxP, and the transitions of the symbol. A three-digit numerical value assigned to each of the transitions indicates values of the transition signals TxF, TxR, and TxP in the order named. 
     The transition signal TxF (Flip) allows the symbol to make the transition between “+x” and “−x”, allows the symbol to make the transition between “+y” and “−y”, and allows the symbol to make the transition between “+z” and “−z”. Specifically, in a case in which the transition signal TxF is “1”, the transition is made so as to change polarity of the symbol (e.g., from “+x” to “−x”). In a case in which the transition signal TxF is “0”, no such transition is made. 
     The transition signals TxR (Rotation) and TxP (Polarity) allow the symbol to make the transition between “+x” and other than “−x”, between “+y” and other than “−y”, or “+z” and other than “−z”, in a case in which the transition signal TxF is “0”. Specifically, in a case in which the transition signals TxR and TxP are respectively “1” and “0”, the transition is made in a clockwise direction (e.g., from “+x” to “+y”) in  FIG. 3 , with the polarity of the symbol being maintained. In a case in which the transition signals TxR and TxP are respectively “1” and “1”, the transition is made in the clockwise direction (e.g., from “+x” to “−y”) in  FIG. 3 , with the polarity of the symbol being changed. Moreover, in a case in which the transition signals TxR and TxP are respectively “0” and “0”, the transition is made in a counterclockwise direction (e.g., from “+x” to “+z”) in  FIG. 3 , with the polarity of the symbol being maintained. In a case in which the transition signals TxR and TxP are respectively “0” and “1”, the transition is made in the counterclockwise direction (e.g., from “+x” to “−z”) in  FIG. 3 , with the polarity of the symbol being changed. 
     The processor unit  12  generates the seven sets of the transition signals TxF, TxR, and TxP as described. Moreover, the processor unit  12  supplies the seven sets of the transition signals TxF, TxR, and TxP (the transition signals TxF 0  to TxF 6 , TxR 0  to TxR 6 , and TxP 0  to TxP 6 ) to the transmission unit  20 . 
     The transmission unit  20  generates the signals SIGA, SIGB, and SIGC, on the basis of the transition signals TxF 0  to TxF 6 , TxR 0  to TxR 6 , and TxP 0  to TxP 6 . 
       FIG. 4  illustrates one configuration example of the transmission unit  20 . The transmission unit  20  includes serializers  21  to  23 , a transmission symbol generator unit  24 , and an output unit  27 . 
     The serializer  21  serializes the transition signals TxF 0  to TxF 6  in this order, on the basis of the transition signals TxF 0  to TxF 6  and on the basis of the clock signal TxCK, to generate a transition signal TxF 9 . The serializer  22  serializes the transition signals TxR 0  to TxR 6  in this order, on the basis of the transition signals TxR 0  to TxR 6  and on the basis of the clock signal TxCK, to generate a transition signal TxR 9 . The serializer  23  serializes the transition signals TxP 0  to TxP 6  in this order, on the basis of the transition signals TxP 0  to TxP 6  and on the basis of the clock signal TxCK, to generate a symbol signal TxP 9 . 
     The transmission symbol generator unit  24  generates symbol signals Tx 1 , Tx 2 , and Tx 3 , on the basis of the transition signals TxF 9 , TxR 9 , and TxP 9  and on the basis of the clock signal TxCK. The transmission symbol generator unit  24  includes a signal generator unit  25  and a flip flop  26 . 
     The signal generator unit  25  generates the symbol signals Tx 1 , Tx 2 , and Tx 3 , on the basis of the transition signals TxF 9 , TxR 9 , and TxP 9  and on the basis of symbol signals D 1 , D 2 , and D 3 . Specifically, the signal generator unit  25  obtains a symbol after the transition, as illustrated in  FIG. 3 , on the basis of a symbol indicated by the symbol signals D 1 , D 2 , and D 3  (a pre-transition symbol) and on the basis of the transition signals TxF 9 , TxR 9 , and TxP 9 . The signal generator unit  25  outputs the symbol after the transition as the symbol signals Tx 1 , Tx 2 , and Tx 3 . 
     The flip flop  26  performs samplings of the symbol signals Tx 1 , Tx 2 , and Tx 3  on the basis of the clock signal TxCK, and outputs results of the samplings as the symbol signals D 1 , D 2 , and D 3 , respectively. 
     The output unit  27  generates the signals SIGA, SIGB, and SIGC on the basis of the symbol signals Tx 1 , Tx 2 , and Tx 3  and on the basis of the clock signal TxCK. 
       FIG. 5  illustrates one operation example of the output unit  27 . For example, in a case where the symbol signals Tx 1 , Tx 2 , and Tx 3  are “1”, “0”, and “0”, the output unit  27  brings the signal SIGA to the high level voltage VH, brings the signal SIGB to the low level voltage VL, and brings the signal SIGC to the medium level voltage VM. In other words, the output unit  27  generates the symbol “+x”. Moreover, for example, in a case where the symbol signals Tx 1 , Tx 2 , and Tx 3  are “0”, “1”, and “1”, the output unit  27  brings the signal SIGA to the low level voltage VL, brings the signal SIGB to the high level voltage VH, and brings the signal SIGC to the medium level voltage VM. In other words, the output unit  27  generates the symbol “−x”. Moreover, for example, in a case where the symbol signals Tx 1 , Tx 2 , and Tx 3  are “0”, “1”, and “0”, the output unit  27  brings the signal SIGA to the medium level voltage VM, brings the signal SIGB to the high level voltage VH, and brings the signal SIGC to the low level voltage VL. In other words, the output unit  27  generates the symbol “+y”. Moreover, for example, in a case where the symbol signals Tx 1 , Tx 2 , and Tx 3  are “1”, “0”, and “1”, the output unit  27  brings the signal SIGA to the medium level voltage VM, brings the signal SIGB to the low level voltage VL, and brings the signal SIGC to the high level voltage VH. In other words, the output unit  27  generates the symbol “−y”. Moreover, for example, in a case where the symbol signals Tx 1 , Tx 2 , and Tx 3  are “0”, “0”, and “1”, the output unit  27  brings the signal SIGA to the low level voltage VL, brings the signal SIGB to the medium level voltage VM, and brings the signal SIGC to the high level voltage VH. In other words, the output unit  27  generates the symbol “+z”. Moreover, for example, in a case where the symbol signals Tx 1 , Tx 2 , and Tx 3  are “1”, “1”, and “0”, the output unit  27  brings the signal SIGA to the high level voltage VH, brings the signal SIGB to the medium level voltage VM, and brings the signal SIGC to the low level voltage VL. In other words, the output unit  27  generates the symbol “−z”. 
       FIG. 6  illustrates one configuration example of the output unit  27 . The output unit  27  includes a driver control unit  28  and driver units  29 A,  29 B, and  29 C. 
     The driver control unit  28  generates control signals PU 1 A to PU 5 A, PD 1 A to PD 5 A, PU 1 B to PU 5 B, PD 1 B to PD 5 B, PU 1 C to PU 5 C, and PD 1 C to PD 5 C, on the basis of the symbol signals Tx 1 , Tx 2 , and Tx 3  and on the basis of the clock signal TxCK. Moreover, the driver control unit  28  supplies the control signals PU 1 A to PU 5 A and PD 1 A to PD 5 A to the driver unit  29 A, supplies the control signals PU 1 B to PU 5 B and PD 1 B to PD 5 B to the driver unit  29 B, and supplies the control signals PU 1 C to PU 5 C and PD 1 C to PD 5 C to the driver unit  29 C. 
     The driver unit  29 A generates the signal SIGA on the basis of the control signals PU 1 A to PU 5 A and PD 1 A to PD 5 A. The driver unit  29 A includes, in this example, five drivers  291 A to  295 A. The drivers  291 A to  295 A each set a voltage of the output terminal ToutA on the basis of a signal supplied to a positive input terminal and on a basis of a signal supplied to a negative input terminal. The driver  291 A includes the positive input terminal supplied with the control signal PU 1 A, the negative input terminal supplied with the control signal PD 1 A, and an output terminal coupled to the output terminal ToutA of the transmission device  10 . The driver  292 A includes the positive input terminal supplied with the control signal PU 2 A, the negative input terminal supplied with the control signal PD 2 A, and an output terminal coupled to the output terminal ToutA of the transmission device  10 . The driver  293 A includes the positive input terminal supplied with the control signal PU 3 A, the negative input terminal supplied with the control signal PD 3 A, and an output terminal coupled to the output terminal ToutA of the transmission device  10 . The driver  294 A includes the positive input terminal supplied with the control signal PU 4 A, the negative input terminal supplied with the control signal PD 4 A, and an output terminal coupled to the output terminal ToutA of the transmission device  10 . The drive  295 A includes the positive input terminal supplied with the control signal PU 5 A, the negative input terminal supplied with the control signal PD 5 A, and an output terminal coupled to the output terminal ToutA of the transmission device  10 . In this way, the output terminals of the drivers  291 A to  295 A are coupled to one another and are coupled to the output terminal ToutA. 
     The driver unit  29 B generates the signal SIGB on the basis of the control signals PU 1 B to PU 5 B and PD 1 B to PD 5 B. The driver unit  29 B includes, in this example, five drivers  291 B to  295 B. The driver  291 B includes the positive input terminal supplied with the signal PU 1 B, the negative input terminal supplied with the signal PD 1 B, and an output terminal coupled to the output terminal ToutB of the transmission device  10 . The same applies to the drivers  292 B to  295 B. In this way, the output terminals of the drivers  291 B to  295 B are coupled to one another and are coupled to the output terminal ToutB. 
     The driver unit  29 C generates the signal SIGC on the basis of the control signals PU 1 C to PU 5 C and PD 1 C to PD 5 C. The driver unit  29 C includes, in this example, five drivers  291 C to  295 C. The driver  291 C includes the positive input terminal supplied with the signal PU 1 C, the negative input terminal supplied with the signal PD 1 C, and an output terminal coupled to the output terminal ToutC of the transmission device  10 . The same applies to the drivers  292 C to  295 C. In this way, the output terminals of the drivers  291 C to  295 C are coupled to one another and are coupled to the output terminal ToutC. 
       FIG. 7  illustrates one configuration example of the driver  291 A. It is to be noted that the same applies to the drivers  292 A to  295 A,  291 B to  295 B, and  291 C to  295 C. The driver  291 A includes transistors  91  and  94 , and resistors  92  and  93 . The transistors  91  and  94  are, in this example, N channel MOS (Metal Oxide Semiconductor) FET (Field Effect Transistor). The transistor  91  includes a gate that corresponds to the positive input terminal of the driver  291 A and is supplied with the control signal PU 1 A, a drain supplied with a voltage V 1 , and a source coupled to one end of the resistor  92 . The transistor  94  includes a gate that corresponds to the negative input terminal of the driver  291 A and is supplied with the control signal PD 1 A, a drain coupled to one end of the resistor  93 , and a source that is grounded. The resistor  92  includes the one end coupled to the source of the transistor  91 , and another end coupled to another end of the resistor  93  and coupled to the output terminal ToutA of the transmission device  10 . The resistor  93  includes the one end coupled to the drain of the transistor  94 , and the other end coupled to the other end of the resistor  92  and coupled to the output terminal ToutA of the transmission device  10 . In this example, a sum of ON resistance of the transistor  91  and a resistance value of the resistor  92  is about 200 [Ω]. Likewise, a sum of ON resistance of the transistor  94  and a resistance value of the resistor  93  is about 200 [Ω]. 
     With this configuration, the driver control unit  28  sets the voltage of the output terminal ToutA at one of the three voltages (the high level voltage VH, the low level voltage VL, and the medium level voltage VM) with the utilization of the control signals PU 1 A to PU 5 A and PD 1 A to PD 5 A. Specifically, for example, in a case where the voltage of the output terminal ToutA is to be set at the high level voltage VH, for example, four out of the control signals PU 1 A to PU 5 A are set at “1”, while a remaining one and the control signals PD 1 A to PD 5 A are set at “0”. Thus, in the driver unit  29 A, the four transistors  91  whose gates are supplied with “1” are turned on. As a result, the signal SIGA is brought to the high level voltage VH, while output termination resistance of the driver unit  29 A is brought to about 50 [Ω]. Moreover, for example, in a case where the voltage of the output terminal ToutA is to be set at the low level voltage VL, for example, four out of the control signals PD 1 A to PD 5 A are set at “1”, while a remaining one and the control signals PU 1 A to PU 5 A are set at “0”. Thus, in the driver unit  29 A, the four transistors  94  whose gates are supplied with “1” are turned on. As a result, the signal SIGA is brought to the low level voltage VL, while the output termination resistance of the driver unit  29 A is brought to about 50 [Ω]. Moreover, in a case where the voltage of the output terminal ToutA is to be set at the medium level voltage VM, for example, two out of the control signals PU 1 A to PU 5 A are set at “1”, with remaining ones being set at “0”, while for example, two out of the control signals PD 1 A to PD 5 A are set at “1”, with remaining ones being set at “0”. Thus, in the driver unit  291 A, the two transistors  91  and the two transistors  94  whose gates are supplied with “1” are turned on, which provides Thevenin termination. As a result, the signal SIGA is brought to the medium level voltage VM, while the output termination resistance of the driver unit  29 A is brought to about 50 [Ω]. In this way, the driver control unit  28  sets the voltage of the output terminal ToutA at one of the three voltages with the utilization of the control signals PU 1 A to PU 5 A and PD 1 A to PD 5 A. 
     [Reception Device  40 ] 
     As illustrated in  FIG. 1 , the reception device  30  includes the reception unit  40  and a processor unit  32 . 
     The reception unit  40  receives the signals SIGA, SIGB, and SIGC, and generates transition signals RxF, RxR, and RxP and a clock signal RxCK on the basis of the signals SIGA, SIGB, and SIGC. 
       FIG. 8  illustrates one configuration example of the reception unit  40 . The reception unit  40  includes resistors  41 A,  41 B, and  41 C, amplifiers  42 A,  42 B, and  42 C, a clock generator unit  43 , flip flops  44  and  45 , and a signal generator unit  46 . 
     The resistors  41 A,  41 B, and  41 C function as termination resistance in the communication system  1 . The resistor  41 A includes one end that is coupled to the input terminal TinA and is supplied with the signal SIGA, and another end that is coupled to another end of the resistor  41 B and is coupled to another end of the resistor  41 C. The resistor  41 B includes one end that is coupled to the input terminal TinB and is supplied with the signal SIGB, and the other end that is coupled to the other end of the resistor  41 A and is coupled to the other end of the resistor  41 C. The resistor  41 C includes one end that is coupled to the input terminal TinC and is supplied with the signal SIGC, and the other end that is coupled to the other end of the resistor  41 A and is coupled to the other end of the resistor  41 B. 
     The amplifiers  42 A,  42 B, and  42 C each output a signal that corresponds to a difference between a signal at a positive input terminal and a signal at a negative input terminal. The amplifier  42 A includes the positive input terminal that is coupled to the negative input terminal of the amplifier  42 C, is coupled to the one end of the resistor  41 A, and is supplied with the signal SIGA, and the negative input terminal that is coupled to the positive input terminal of the amplifier  42 B, is coupled to the one end of the resistor  41 B, and is supplied with the signal SIGB. The amplifier  42 B includes the positive input terminal that is coupled to the negative input terminal of the amplifier  42 A, is coupled to the one end of the resistor  41 B, and is supplied with the signal SIGB, and the negative input terminal that is coupled to the positive input terminal of the amplifier  42 C, is coupled to the one end of the resistor  41 C, and is supplied with the signal SIGC. The amplifier  42 C includes the positive input terminal that is coupled to the negative input terminal of the amplifier  42 B, is coupled to the one end of the resistor  41 C, and is supplied with the signal SIGC, and the negative input terminal that is coupled to the positive input terminal of the amplifier  42 A, is coupled to the resistor  41 A, and is supplied with the signal SIGA. 
     With this configuration, the amplifier  42 A outputs a signal that corresponds to a difference AB (SIGA−SIGB) between the signal SIGA and the signal SIGB. The amplifier  42 B outputs a signal that corresponds to a difference BC (SIGB−SIGC) between the signal SIGB and the signal SIGC. The amplifier  42 C outputs a signal that corresponds to a difference CA (SIGC−SIGA) between the signal SIGC and the signal SIGA. 
       FIG. 9  illustrates one example of the signals SIGA to SIGC to be received by the reception unit  40 . In this example, the reception unit  40  receives the six symbols “+x”, “−y”, “−z”, “+z”, “+y”, and “−x” in this order. At this occasion, a voltage of the signal SIGA changes as follows: VH, VM, VH, VL, VM, and VL. A voltage of the signal SIGB changes as follows: VL, VL, VM, VM, VH, and VH. A voltage of the signal SIGC changes as follows: VM, VH, VL, VH, VL, and VM. In accordance therewith, the differences AB, BC, and CA also change. For example, the difference AB changes as follows: +2ΔV, +ΔV, +ΔV, −ΔV, −ΔV, and −2ΔV. The difference BC changes as follows: −ΔV, −2ΔV, +ΔV, −ΔV, +2ΔV, and +ΔV. The difference CA changes as follows: −ΔV, +ΔV, −2ΔV, +2ΔV, −ΔV, and +ΔV. Here, ΔV is a difference between any two adjacent voltages out of the three voltages (the high level voltage VH, the medium level voltage VM, and the low level voltage VL). 
       FIG. 10  illustrates one operation example of the amplifiers  42 A,  42 B, and  42 C, in a case where the reception unit receives the symbol “+x”. In this example, the voltage of the signal SIGA is the high level voltage VH. The voltage of the signal SIGB is the low level voltage VL. The voltage of the signal SIGC is the medium level voltage VM. In this case, a current Iin flows in the following order: the input terminal TinA, the resistor  41 A, the resistor  41 B, and the input terminal TinB. Moreover, the positive input terminal of the amplifier  42 A is supplied with the high level voltage VH, while the negative input terminal is supplied with the low level voltage VL, causing the difference AB to be positive. Accordingly, the amplifier  42 A outputs “1”. Moreover, the positive input terminal of the amplifier  42 B is supplied with the low level voltage VL, while the negative input terminal is supplied with the medium level voltage VM, causing the difference BC to be negative. Accordingly, the amplifier  42 B outputs “0”. Moreover, the positive input terminal of the amplifier  42 C is supplied with the medium level voltage VM, while the negative input terminal is supplied with the high level voltage VH, causing the difference CA to be negative. Accordingly, the amplifier  42 C outputs “0”. 
     The clock generator unit  43  generates the clock signal RxCK on the basis of the output signals of the amplifiers  42 A,  42 B, and  42 C. 
     The flip flop  44  allows the output signals of the amplifiers  42 A,  42 B, and  42 C to be delayed by a term of one clock of the clock signal RxCK, and outputs the respective resultant signals. The flip flop  45  allows the three output signals of the flip flop  44  to be delayed by the term of one clock of the clock signal RxCK, and outputs the respective resultant signals. 
     The signal generator unit  46  generates transition signals RxF, RxR, and RxP, on the basis of the output signals of the flip flops  44  and  45  and on the basis of the clock signal RxCK. The transition signals RxF, RxR, and RxP respectively correspond to the transition signals TxF 9 , TxR 9 , and TxP 9  in the transmission device  10 , and indicate the transitions of the symbol. The signal generator unit  46  identifies the transition of the symbol ( FIG. 3 ) on the basis of the symbol indicated by the output signals of the flip flop  44  and on the basis of the symbol indicated by the output signals of the flip flop  45 , to generate the transition signals RxF, RxR, and RxP. 
     The processor unit  32  ( FIG. 1 ) performs predetermined processing on the basis of the transition signals RxF, RxR, and RxP and on the basis of the clock signal RxCK. 
     [Transmission Line  100 ] 
       FIG. 11  illustrates one configuration example of the transmission line  100 . The transmission line  100  is constituted by a so-called strip line. The transmission line  100  includes a conductive layer  101 , a dielectric layer  102 , and a conductive layer  103 , in addition to the line  110 A to  110 C. 
     The conductive layers  101  and  103  are made of, for example, metal, and are grounded in this example. The dielectric layer  102  is made of a dielectric. Relative permittivity Er of the dielectric layer  102  is “4.3” in this example. In the transmission line  100 , the conductive layer  101 , the dielectric layer  102 , and the conductive layer  103  are stacked in this order. 
     The lines  110 A,  110 B, and  110 C are provided, inside the dielectric layer  102 , at a position at a distance d 1  from the conductive layer  101 , and at a position at a distance d 2  from the conductive layer  103 . The distance d 1  is 0.06 [mm] in this example. The distance d 2  is 0.1 [mm] in this example. The lines  110 A,  110 B, and  110 C are formed with widths WA, WB, and WC, and are disposed side by side in this order at a pitch P. The widths WA, WB, and WC are each 0.05 [mm] in this example. The pitch P is 0.075 [mm] in this example. 
     In the transmission line  100 , the characteristic impedance of the line  110 B is higher than the characteristic impedance of the lines  110 A and  110 C. Specifically, for example, a resistance value of the line  110 B may be higher than resistance values of the lines  110 A and  110 C. At this occasion, for example, the line  110 B may be formed with the use of a material having high resistivity. Moreover, the resistance value of the line  110 B may be increased by mixing an impurity with the line  110 B. That way, in the communication system  1 , it is possible to enhance the communication quality, as described later. 
     Here, the line  110 A corresponds to one specific example of a “first line” of a first transmission line in the disclosure. The line  110 B corresponds to one specific example of a “second line” of the first transmission line in the disclosure. The line  110 C corresponds to one specific example of a “third line” of the first transmission line in the disclosure. 
     [Operation and Workings] 
     Description is given next of operation and workings of the communication system  1  according to this embodiment. 
     [Outline of Overall Operation] 
     First, description is given on an outline of overall operation of the communication system  1  with reference to  FIG. 1 . The clock generator unit  11  of the transmission device  10  generates the clock signal TxCK. The processor unit  12  performs the predetermined processing, to generate the transition signals TxF 0  to TxF 6 , TxR 0  to TxR 6 , and TxP 0  to TxP 6 . The transmission unit  20  generates the signals SIGA, SIGB, and SIGC on the basis of the transition signals TxF 0  to TxF 6 , TxR 0  to TxR 6 , and TxP 0  to TxP 6 . The transmission line  100  transmits the signals SIGA, SIGB, and SIGC. The reception unit  40  of the reception device  30  receives the signals SIGA, SIGB, and SIGC, and generates the transition signals RxF, RxR, and RxP and the clock signal RxCK, on the basis of the signals SIGA, SIGB, and SIGC. The processor unit  32  performs the predetermined processing, on the basis of the transition signals RxF, RxR, and RxP and on the basis of the clock signal RxCK. 
     [Regarding Transmission Line  100 ] 
     In the transmission line  100 , the characteristic impedance of the line  110 B is higher than the characteristic impedance of the lines  110 A and  110 C. Hence, in the communication system  1 , it is possible to enhance the communication quality. Detailed description is given below. 
       FIG. 12  illustrates one example of differential insertion loss characteristics in the transmission line  100 . In  FIG. 12 , a horizontal axis denotes a frequency, whereas a vertical axis denotes an S parameter Sdd 21 . A characteristic WAC 1  indicates a differential insertion loss characteristic of the lines  110 A and  110 C. A characteristic WAB 1  indicates a differential insertion loss characteristic of the lines  110 A and  110 B. It is to be noted that a differential insertion loss characteristic of the lines  110 B and  110 C is substantially equal to the differential insertion loss characteristic of the lines  110 A and  110 B (the characteristic WAB 1 ). In the transmission line  100 , it is possible to allow the differential insertion loss characteristic of the lines  110 A and  110 B (the characteristic WAB 1 ) and the differential insertion loss characteristic of the lines  110 A and  110 C (the characteristic WAC 1 ) to approximate to each other, and to reduce a differential insertion loss, as compared to a case of a comparative example described later. 
       FIG. 13  illustrates one example of differential reflection characteristics in the transmission line  100 . In  FIG. 13 , a horizontal axis denotes a frequency, whereas a vertical axis denotes an S parameter Sdd 11 . A characteristic WAC 2  indicates a differential reflection characteristic of the lines  110 A and  110 C. A characteristic WAB 2  indicates a differential reflection characteristic of the lines  110 A and  110 B. It is to be noted that a differential reflection characteristic of the lines  110 B and  110 C is substantially equal to the differential reflection characteristic of the lines  110 A and  110 B. In the transmission line  100 , it is possible to improve the differential reflection characteristic of the lines  110 A and  110 B (the characteristic WAB 2 ), as compared to the case of the comparative example described later. 
     Comparative Example 
     Description is given next of a transmission line  100 R according to the comparative example. The transmission line  100 R includes lines  110 RA,  110 RB, and  110 RC, as with the case of this embodiment ( FIG. 11 ). Unlike the transmission line  100  according to this embodiment, in the transmission line  100 R, the lines  100 RA,  100 RB, and  100 RC are equal in characteristic impedance to each other. 
       FIG. 14  illustrates one example of differential insertion loss characteristics in the transmission line  100 R. A characteristic WAC 3  indicates a differential insertion loss characteristic of the lines  110 RA and  110 RC. A characteristic WAB 3  indicates a differential insertion loss characteristic of the lines  110 RA and  110 RB. In the transmission line  100 R, a difference between the differential insertion loss characteristic (the characteristic WAB 3 ) of the lines  110 RA and  110 RB and the differential insertion loss characteristic (the characteristic WAC 3 ) of the lines  110 RA and  110 RC is slightly larger, as compared to the case of the transmission line  100  according to this embodiment ( FIG. 12 ). Moreover, in the transmission line  100 R, a value of the S parameter Sdd 21  is lower, as compared to the case of the transmission line  100  ( FIG. 12 ). In other words, in the transmission line  100 R, the differential insertion loss becomes larger, as compared to that of the transmission line  100 . 
       FIG. 15  illustrates one example of differential reflection characteristics in the transmission line  100 R. A characteristic WAC 4  indicates a differential reflection characteristic of the lines  110 RA and  110 RC. A characteristic WAB 4  indicates a differential reflection characteristic of the lines  110 RA and  110 RB. In the transmission line  100 R, a value of the S parameter Sdd 11  of the lines  110 RA and  110 RB is higher, as compared to the case of the transmission line  100  according to this embodiment ( FIG. 13 ). In other words, in the transmission line  100 R, the differential reflection characteristic of the lines  100 RA and  100 RB (the characteristic WAB 4 ) is lowered. 
     As described, in the transmission line  100  and the transmission line  100 R, the three lines are disposed side by side, causing inter-line distances to differ from one another. Specifically, for example, in the transmission line  100 R, a distance from the line  110 RA to the line  110 RB is shorter than a distance from the line  110 RA to the line  110 RC. Likewise, a distance from the line  110 RB to the line  110 RC is shorter than a distance from the line  110 RA to the line  110 RC. Accordingly, in a case where the lines  110 RA,  110 RB, and  110 RC are equal in the characteristic impedance to one another, a difference between the characteristic impedance of the lines  110 RA and  110 RB, and the characteristic impedance of the lines  110 RA and  110 RB becomes large. Likewise, a difference between the characteristic impedance of the lines  110 RA and  110 RC, and the characteristic impedance of the lines  110 RB and  110 RC becomes large. As a result, in the transmission line  100 R, as illustrated in  FIG. 14 , the differential insertion loss characteristic of the lines  110 RA and  110 RB (the characteristic WAB 3 ) is lowered, as compared to the differential insertion loss characteristic of the lines  110 RA and  110 RC (the characteristic WAC 3 ). In addition, as illustrated in  FIG. 15 , the differential reflection characteristic of the lines  110 RA and  110 RB (the characteristic WAB 4 ) is lowered. In such a case, an eye opening of a differential signal is narrowed, causing possibility of lowered communication quality. 
     Meanwhile, in the transmission line  100  according to this embodiment, the characteristic impedance of the line  110 B is higher than the characteristic impedance of the lines  110 A and  110 C. This makes it possible to allow the differential impedance of the lines  110 A and  110 B, the differential impedance of the lines  110 B and  110 C, and the differential impedance of the lines  110 A and  110 C to approximate to one another. In the transmission line  100 , as described, it is possible to enhance symmetry of the differential impedance. Hence, it is possible to reduce the differential insertion loss, as illustrated in  FIG. 12 , and to improve the differential reflection characteristic of the lines  110 A and  110 B (the characteristic WAB 2 ), as illustrated in  FIG. 13 . As a result, it is possible to enlarge the eye opening of the differential signal, leading to enhancement in the communication quality. 
     Effects 
     As described, in this embodiment, the characteristic impedance of the middle line of the three lines is higher than the characteristic impedance of the other lines. Hence, it is possible to enhance the communication quality. 
     Modification Example 1-1 
     In the forgoing embodiment, the resistance value of the line  110 B is higher than the resistance values of the lines  110 A and  110 C. At this occasion, for example, instead of forming the line  110 B with the use of the material having high resistivity, the lines  110 A and  110 C may be formed with the use of a material having low resistivity. This also makes it possible to allow the characteristic impedance of the line  110 B to be higher than the characteristic impedance of the lines  110 A and  110 C. Hence, it is possible to enhance the communication quality. 
     Modification Example 1-2 
     In the forgoing embodiment, the resistance value of the line  110 B is higher than the resistance values of the lines  110 A and  110 C, but this is non-limiting. In one alternative, for example, as in a transmission line  100 B as illustrated in  FIG. 16 , permittivity of a part WP near the line  110 B in the dielectric layer  102  may be lowered. In another alternative, for example, permittivity of a part near the line  110 A in the dielectric layer  102  may be increased, while permittivity of a part near the line  110 C in the dielectric layer  102  may be increased. This also makes it possible to allow the characteristic impedance of the line  110 B to be higher than the characteristic impedance of the lines  110 A and  110 C. Hence, it is possible to enhance the communication quality. 
     Modification Example 1-3 
     In the forgoing embodiment, the width WA of the line  110 A, the width WB of the line  110 B, and the width WC of the line  110 C are equal to one another. However, this is non-limiting. Alternatively, for example, as in a transmission line  100 C as illustrated in  FIG. 17 , the width WB of the middle line of the three lines (a line  110 B 2 ) may be smaller than the widths WA and WC of the other lines. In this example, the width WB is 0.03 [mm]. The widths WA and WC are each 0.05 [mm]. Moreover, the pitch P is 0.085 [mm]. In this way as well, it is possible to allow the characteristic impedance of the line  110 B 2  to be higher than the characteristic impedance of the lines  110 A and  110 C. Hence, it is possible to enhance the communication quality. 
     Modification Example 1-4 
     In the forgoing embodiment, the conductive layers  101  and  103  cover the three lines  110 A,  110 B, and  110 C. However, this is non-limiting. In one alternative, for example, as in a transmission line  100 D as illustrated in  FIG. 18 , the conductive layer  103  may be formed in a region confronted with the line  110 A and in a region confronted with the line  110 C, with no conductive layer formed in a region confronted with the line  110 B. A width WO of the region confronted with the line  110 B where no conductive layer is formed may be, for example, 0.1 [mm]. 
     Modification Example 1-5 
     In the forgoing embodiment, the transmission device  10  includes the single transmission unit  20 , whereas the reception device  30  includes the single reception unit  40 . However, this is non-limiting. The transmission device may include a plurality of transmission units, whereas the reception device may include a plurality of reception units. In what follows, a communication system  1 E according to this modification example is described in detail. 
       FIG. 19  illustrates one configuration example of the communication system  1 E. The communication system  1 E includes a transmission device  10 E, a transmission line  100 E, and a reception device  30 E. 
     The transmission device  10 E includes a processor unit  12 E and three transmission units  201  to  203 . The processor unit  12 E performs predetermined processing, to generate transition signals TxF 10  to TxF 16 , TxR 10  to TxR 16 , TxP 10  to TxP 16 , TxF 20  to TxF 26 , TxR 20  to TxR 26 , TxP 20  to TxP 26 , TxF 30  to TxF 36 , TxR 30  to TxR 36 , and TxP 30  to TxP 36 . The transmission unit  201  generates signals SIGA 1 , SIGB 1 , and SIGC 1 , on the basis of the transition signals TxF 10  to TxF 16 , TxR 10  to TxR 16 , and TxP 10  to TxP 16 , as with the transmission unit  20  according to the forgoing embodiment. The transmission unit  202  generates signals SIGA 2 , SIGB 2 , and SIGC 2 , on the basis of the transition signals TxF 20  to TxF 26 , TxR 20  to TxR 26 , and TxP 20  to TxP 26 , as with the transmission unit  20  according to the forgoing embodiment. The transmission unit  203  generates signals SIGA 3 , SIGB 3 , and SIGC 3 , on the basis of the transition signals TxF 30  to TxF 36 , TxR 30  to TxR 36 , and TxP 30  to TxP 36 , as with the transmission unit  20  according to the forgoing embodiment. 
     The transmission line  100 E includes lines  110 A 1 ,  110 B 1 , and  110 C 1 , lines  110 A 2 ,  110 B 2 , and  110 C 2 , and lines  110 A 3 ,  110 B 3 , and  110 C 3 . The line  110 A 1  transmits the signal SIGA 1 . The line  110 B 1  transmits the signal SIGB 1 . The line  110 C 1  transmits the signal SIGC 1 . In other words, the lines  110 A 1 ,  110 B 1 , and  110 C 1  constitute a single trio line. Likewise, the line  110 A 2  transmits the signal SIGA 2 . The line  110 B 2  transmits the signal SIGB 2 . The line  110 C 2  transmits the signal SIGC 2 . In other words, the lines  110 A 2 ,  110 B 2 , and  110 C 2  constitute a single trio line. The line  110 A 3  transmits the signal SIGA 3 . The line  110 B 3  transmits the signal SIGB 3 . The line  110 C 3  transmits the signal SIGC 3 . In other words, the lines  110 A 3 ,  110 B 3 , and  110 C 3  constitute a single trio line. 
     The reception device  30 E includes three reception units  401  to  403 , and a processor unit  32 E. The reception unit  401  generates transition signals RxF 1 , RxR 1 , and RxP 1 , and a clock signal RxCK 1 , on the basis of the signals SIGA 1 , SIGB 1 , and SIGC 1 , as with the reception unit  40  according to the forgoing embodiment. Likewise, the reception unit  402  generates transition signals RxF 2 , RxR 2 , and RxP 2 , and a clock signal RxCK 2 , on the basis of the signals SIGA 2 , SIGB 2 , and SIGC 2 . The reception unit  403  generates transition signals RxF 3 , RxR 3 , and RxP 3 , and a clock signal RxCK 3 , on the basis of the signals SIGA 3 , SIGB 3 , and SIGC 3 . The processor unit  32 E performs predetermined processing, on the basis of the transition signals RxF 1 , RxR 1 , and RxP 1 , and the clock signal RxCK 1 , on the basis of the transition signals RxF 2 , RxR 2 , and RxP 2 , and the clock signal RxCK 2 , and on the basis of the transition signals RxF 3 , RxR 3 , and RxP 3 , and the clock signal RxCK 3 . 
       FIG. 20  illustrates one configuration example of the transmission line  100 E. In the transmission line  100 E, as with the transmission line  100  according to the forgoing embodiment ( FIG. 11 ), in the dielectric layer  102 , the lines  110 A 1 ,  110 B 1 ,  110 C 1 ,  110 A 2 ,  110 B 2 ,  110 C 2 ,  110 A 3 ,  110 B 3 , and  110 C 3  are disposed side by side in this order at the pitch P. A width WA 1  of the line  110 A 1 , a width WC 1  of the line  110 C 1 , a width WB 2  of the line  110 B 2 , a width WA 3  of the line  110 A 3 , and a width WC 3  of the line  110 C 3  are each, for example, 0.03 [mm]. Moreover, a width WB 1  of the line  110 B 1 , a width WA 2  of the line  110 A 2 , a width WC 2  of the line  110 C 2 , and a width WB 3  of the line  110 B 3  are each, for example, 0.05 [mm]. The pitch P is, for example, 0.085 [mm]. Thus, in the transmission line  100 E, wide lines and narrow lines are alternatively disposed. 
       FIG. 21  illustrates one example of differential insertion loss characteristics regarding the lines  110 A 1 ,  110 B 1 , and  110 C 1 . Each numerical value in a row “the line  110 A 1 /the line  110 B 1 ” indicates a value of the S parameter Sdd 21  of the lines  110 A 1  and  110 B 1 . Likewise, each numerical vale in a row “the line  110 B 1 /the line  110 C 1 ” indicates a value of the S parameter Sdd 21  of the lines  110 B 1  and  110 C 1 . Each numerical value in a row “the line  110 A 1 /the line  110 C 1 ” indicates a value of the S parameters Sdd 21  of the lines  110 A 1  and  110 C 1 . Values in parentheses indicate one example of the differential insertion loss characteristics in a case where the widths of all the lines are equal to each other. In this comparative example, the widths of all the lines are each 0.05 [mm], and a pitch is 0.075 [mm]. 
     It is to be noted that  FIG. 21  illustrates the example of the differential insertion loss characteristics regarding the lines  110 A 1 ,  110 B 1 , and  110 C 1 , but the same applies to differential insertion loss characteristics regarding the lines  110 A 3 ,  110 B 3 , and  110 C 3 . 
       FIG. 22  illustrates one example of differential insertion loss characteristics regarding the lines  110 A 2 ,  110 B 2 , and  110 C 2 . Each numerical value in a row “the line  110 A 2 /the line  110 B 2 ” indicates a value of the S parameter Sdd 21  of the lines  110 A 2  and  110 B 2 . Likewise, each numerical value in a row “the line  110 B 2 /the line  110 C 2 ” indicates a value of the S parameter Sdd 21  of the lines  110 B 2  and  110 C 2 . Each numerical value in a row “the line  110 A 2 /the line  110 C 2 ” indicates a value of the S parameter Sdd 21  of the lines  110 A 2  and  110 C 2 . Values in parentheses indicate, as with  FIG. 21 , one example of the differential insertion loss characteristics in the case where the widths of all the lines are equal to one another. 
     As illustrated in  FIGS. 21 and 22 , in this modification example, it is possible to improve the differential insertion loss, for example, by about 0.3 to 0.5 dB both inclusive at 2.5 [GHz]. Hence, in the communication system  1 E, it is possible to enhance the communication quality. 
     Modification Example 1-6 
     In the forgoing embodiment, the three lines included in the trio line are formed at the same level, but this is non-limiting. In the following, a communication system  1 F according to this modification example is described in detail. 
     As illustrated in  FIG. 19 , the communication system  1 F includes a transmission line  160 F. The transmission line  160 F includes lines  170 A 1 ,  170 B 1 , and  170 C 1 , the lines  170 A 2 ,  170 B 2 , and  170 C 2 , and the lines  170 A 3 ,  170 B 3 , and  170 C 3 . 
       FIG. 23  illustrates one configuration example of the transmission line  160 F. In the transmission line  160 F, inside the dielectric layer  102 , the lines  170 A 1 ,  170 B 1 , and  170 C 1  are formed in this order at different levels from one another. The line  170 A 1  and the line  170 B 1  are disposed in confronted relation, while the line  170 B 1  and the line  170 C 1  are disposed in the confronted relation. Likewise, in the transmission line  160 F, inside the dielectric layer  102 , the lines  170 A 2 ,  170 B 2 , and  170 C 2  are formed in this order at different levels from one another. The line  170 A 2  and the line  170 B 2  are disposed in the confronted relation, while the line  170 B and the line  170 C 2  are disposed in the confronted relation. Likewise in the transmission line  160 F, inside the dielectric layer  102 , the lines  170 A 3 ,  170 B 3 , and  170 C 3  are formed in this order at different levels from one another. The line  170 A 3  and the line  170 B 3  are disposed in the confronted relation, while the line  170 B 3  and the line  170 C 3  are disposed in the confronted relation. The lines  170 A 1 ,  170 A 2 , and  170 A 3  are formed at the same level. The lines  170 B 1 ,  170 B 2 , and  170 B 3  are formed at the same level. The lines  170 C 1 ,  170 C 2 , and  170 C 3  are formed at the same level. 
     In the transmission line  160 F, the three lines that constitute the single trio line are disposed at the different levels from one another. In this way, for example, the line  170 B 1  is disposed at a position that is different from those of the lines  170 A 1  and C 1  and is away from both the conductive layer  101  and the conductive layer  103 . Likewise, the line  170 B 2  is disposed at a position that is different from those of the lines  170 A 2  and C 2  and is away from both the conductive layer  101  and the conductive layer  103 . The line  170 B 3  is disposed at a position that is different from those of the lines  170 A 3  and C 3  and is away from both the conductive layer  101  and the conductive layer  103 . Thus, it is possible to allow the characteristic impedance of the line  170 B 1  to be higher than the characteristic impedance of the lines  170 A 1  and  170 C 1 . It is possible to allow the characteristic impedance of the line  170 B 2  to be higher than the characteristic impedance of the lines  170 A 2  and  170 C 2 . It is possible to allow the characteristic impedance of the line  170 B 3  to be higher than the characteristic impedance of the lines  170 A 3  and  170 C 3 . As a result, in the communication system  1 F, it is possible to enhance the communication quality. 
     Other Modification Examples 
     Moreover, two or more of these modification examples may be combined. 
     2. Second Embodiment 
     Description is given next of a communication system  2  according to a second embodiment. This embodiment includes forming a plurality of lines at two levels. It is to be noted that constituent parts that are substantially the same as those of the communication system  1  according to the forgoing first embodiment are denoted by the same reference characters, and description thereof is omitted as appropriate. 
     As illustrated in  FIG. 1 , the communication system  2  includes a transmission line  120 . The transmission line  120  includes lines  130 A,  130 B, and  130 C. In the transmission line  120 , unlike the transmission line  100  according to the forgoing first embodiment, the lines  130 A,  130 B and  130 C are substantially equal in the characteristic impedance to one another. 
       FIG. 24  illustrates one configuration example of the transmission line  120 . The lines  130 A and  130 C are provided, inside the dielectric layer  102 , at a position at a distance d 11  from the conductive layer  101 . The line  130 B is disposed, inside the dielectric layer  102 , at a position at a distance d 13  from the conductive layer  103 . A level at which the lines  130 A and  130 C are formed and a level at which the line  130 B is formed are spaced away by a distance d 12 . In other words, the layer  110 B is provided at a different level from those of the lines  110 A and  110 C. In this example, the distance d 11  is 0.06 [mm]. The distance d 12  is 0.06 [mm]. The distance d 13  is 0.06 [mm]. Moreover, in this example, the width WA of the line  130 A, the width WB of the line  130 B, and the width WC of the line  130 C are each 0.05 [mm]. The pitch P between the lines  130 A,  130 B, and  130 C in an in-plane direction of the transmission line  120  is 0.025 [mm] in this example. 
     Here, the line  130 A corresponds to one specific example of a “first line” of a second transmission line in the disclosure. The line  130 B corresponds to one specific example of a “second line” of the second transmission line in the disclosure. The line  130 C correspond to one specific example of a “third line” of the second transmission line in the disclosure. 
     As described, in the transmission line  120 , the adjacent lines are formed at the different levels from one another, making it possible to increase the inter-line distances. This makes it possible to allow differential impedance of the lines  130 A and  130 B, differential impedance of the lines  130 B and  130 C, and differential impedance of the lines  130 A and  130 C to approximate to one another. Thus, in the transmission line  120 , it is possible to enhance the symmetry of the differential impedance, leading to the enhancement in the communication quality. 
     Moreover, in the transmission line  120 , as described, the adjacent lines are formed at the different levels from one another. This makes it possible to reduce the pitch P between the lines  130 A,  130 B, and  130 C in the in-plane direction of the transmission line  120 . As a result, in the transmission line  120 , it is possible to reduce wiring area. 
     As described, in this embodiment, the adjacent lines are formed at the different levels from one another. Hence, it is possible to enhance the communication quality, and to reduce the wiring area. 
     Modification Example 2-1 
     In the forgoing embodiment, the transmission device  10  includes the single transmission unit  20 , whereas the reception device  30  includes the single reception unit  40 . However, this is non-limiting. As with the case of the modification example 1-5 ( FIG. 19 ), the transmission device may include the plurality of the transmission units, whereas the reception device may include the plurality of the reception units. In what follows, a communication system  2 A according to this modification example is described in detail. 
       FIG. 25  illustrates one configuration example of the communication system  2 A. The communication system  2 A includes the transmission device  10 E, a transmission line  120 A, and the reception device  30 E. The transmission line  120 A includes lines  130 A 1 ,  130 B 1 , and  130 C 1 , lines  130 A 2 ,  130 B 2 , and  130 C 2 , lines  130 A 3 ,  130 B 3 , and  130 C 3 , and a line GL. The line  130 A 1  transmits the signal SIGA 1 . The line  130 B 1  transmits the signal SIGB 1 . The line  130 C 1  transmits the signal SIGC 1 . In other words, the lines  130 A 1 ,  130 B 1 , and  130 C 1  constitute a single trio line. Likewise, the line  130 A 2  transmits the signal SIGA 2 . The line  130 B 2  transmits the signal SIGB 2 . The line  130 C 2  transmits the signal SIGC 2 . In other words, the lines  130 A 2 ,  130 B 2 , and  130 C 2  constitute a single trio line. The line  130 A 3  transmits the signal SIGA 3 . The line  130 B 3  transmits the signal SIGB 3 . The line  130 C 3  transmits the signal SIGC 3 . In other words, the lines  130 A 3 ,  130 B 3 , and  130 C 3  constitute a single trio line. The line GL is grounded. 
       FIG. 26  illustrates one configuration example of the transmission line  120 A. The lines  130 A 1 ,  130 B 2 ,  130 B 3 ,  130 C 1  and GL are disposed side by side in this order, at a first level inside the dielectric layer  102 . Moreover, the lines  130 A 2 ,  130 A 3 ,  130 B 1 ,  130 C 2 , and  130 C 3  are disposed side by side in this order, at a second level inside the dielectric layer  102 . The line  130 A 1  and the line  130 A 2  are disposed in the confronted relation. The line  130 B 2  and the line  130 A 3  are disposed in the confronted relation. The line  130 B 3  and the line  130 B 1  are disposed in the confronted relation. The line  130 C 1  and the line  130 C 2  are disposed in the confronted relation. The line GL and the line  130 C 3  are disposed in the confronted relation. 
     In the transmission line  120 A, the three lines that constitute the single trio line are disposed out of adjacency to one another at the same level. For example, in the transmission line  120 A, the lines  130 A 1 ,  130 B 1 , and  130 C 1  are disposed out of adjacency to one another at the same level. Specifically, in the in-plane direction of the transmission line  120 A, the lines  130 A 1 ,  130 B 1 , and  130 C 1  are disposed in this order, while the line  130 B 1  is disposed at a different level from those of the lines  130 A 1  and  130 C 1 . Likewise, the lines  130 A 2 ,  130 B 2 , and  130 C 2  are disposed out of adjacency to one another at the same level. The lines  130 A 3 ,  130 B 3 , and  130 C 3  are disposed out of adjacency to one another at the same level. 
     As described, in the transmission line  120 A, for example, the lines  130 A 1 ,  130 B 1 , and  130 C 1  are disposed out of adjacency to one another at the same level. This makes it possible to allow differential impedance of the lines  130 A 1  and  130 B 1 , differential impedance of the lines  130 B 1  and  130 C 1 , and differential impedance of the lines  130 A 1  and  130 C 1  to approximate to one another. Hence, in the transmission line  120 A, it is possible to enhance the symmetry of the differential impedance, leading to the enhancement in the communication quality. The same applies to the lines  130 A 2 ,  130 B 2 , and  130 C 2 , and the same applies to the lines  130 A 3 ,  130 B 3 , and  130 C 3 . 
     It is to be noted that disposition of the lines is not limited to the example of the transmission line  120 A. In one alternative, for example, the lines may be disposed as in a transmission line  120 B as illustrated in  FIG. 27 . In the transmission line  120 B, for example, the line  130 C 1  is disposed at a different level from those of the lines  130 A 1  and  130 B 1 . Moreover, for example, in an in-plane direction of the transmission line  120 B, the lines  130 B 2 ,  130 A 2 , and  130 C 2  are disposed in this order. With this disposition as well, it is possible to keep the lines  130 A 1 ,  130 B 1 , and  130 C 1  from being in adjacency to one another at the same level. It is possible to keep the lines  130 A 2 ,  130 B 2 , and  130 C 2  from being in adjacency to one another at the same level. It is possible to keep the lines  130 A 3 ,  130 B 3 , and  130 C 3  from being in adjacency to one another at the same level. Hence, in the transmission line  120 B, it is possible to enhance the symmetry of the differential impedance, leading to the enhancement in the communication quality. 
     Third Embodiment 
     Description is given next of a communication system  3  according to a third embodiment. This embodiment includes providing three transmission units and three reception units. It is to be noted that constituent parts that are substantially the same as those of the communication system  1  according to the forgoing first embodiment are denoted by the same reference characters, and description thereof is omitted as appropriate. 
       FIG. 28  illustrates one configuration example of the communication system  3 . The communication system  3  includes a transmission device  50  and a transmission line  140 . 
     The transmission device  50  includes delay units  51 ,  52 , and  53 . The delay unit  51  delays the clock signal TxCK, and supplies a delayed clock signal to the transmission unit  201 . The delay unit  52  delays the clock signal TxCK, and supplies a delayed clock signal to the transmission unit  202 . The delay unit  53  delays the clock signal TxCK, and supplies a delayed clock signal to the transmission unit  203 . In this example, an amount of delay in the delay units  51  and  53  is set at a larger value than an amount of delay in the delay unit  52 . 
     The transmission line  140  includes lines  150 A 1 ,  150 B 1 , and  150 C 1 , lines  150 A 2 ,  150 B 2 , and  150 C 2 , lines  150 A 3 ,  150 B 3 , and  1150 C 3 , and lines GL 1  and GL 2 . The line  150 A 1  transmits the signal SIGA 1 . The line  150 B 1  transmits the signal SIGB 1 . The line  150 C 1  transmits the signal SIGC 1 . In other words, the lines  150 A 1 ,  150 B 1 , and  150 C 1  constitute a single trio line. Likewise, the line  150 A 2  transmits the signal SIGA 2 . The line  150 B 2  transmits the signal SIGB 2 . The line  150 C 2  transmits the signal SIGC 2 . In other words, the lines  150 A 2 ,  150 B 2 , and  150 C 2  constitute a single trio line. The line  150 A 3  transmits the signal SIGA 3 . The line  150 B 3  transmits the signal SIGB 3 . The line  150 C 3  transmits the signal SIGC 3 . In other words, the lines  150 A 3 ,  150 B 3 , and  150 C 3  constitute a single trio line. The lines GL 1  and GL 2  are grounded. These lines are formed at the same level as one another, as with the transmission line  100  according to the first embodiment ( FIG. 11 ). In the transmission line  140 , the lines  150 A 1 , GL 1 ,  150 A 3 ,  150 A 2 ,  150 B 3 ,  150 B 2 ,  150 B 1 ,  150 C 2 ,  150 C 1 , GL 2 , and  150 C 3  are disposed side by side in this order. 
     Here, the lines  150 A 1 ,  150 B 1 , and  150 C 1  correspond to one specific example of a “first trio line” of a third transmission line in the disclosure. The lines  150 A 2 ,  150 B 2 , and  150 C 2  correspond to one specific example of a “second trio line” of the third transmission line in the disclosure. 
     In the transmission line  140 , the three lines that constitute the single trio line are kept from being in adjacency to one another. Specifically, in the transmission line  140 , the lines  150 A 1 ,  150 B 1 , and  150 C 1  are kept from being in adjacency to one another. This makes it possible to allow the differential impedance of the lines  150 A 1  and  150 B 1 , the differential impedance of the lines  150 B 1  and  150 C 1 , and the differential impedance of the lines  150 A 1  and  150 C 1  to approximate to one another. Likewise, in the transmission line  140 , the lines  150 A 2 ,  150 B 2 , and  150 C 2  are kept from being in adjacency to one another. This makes it possible to allow the differential impedance of the lines  150 A 2  and  150 B 2 , the differential impedance of the lines  150 B 2  and  150 C 2 , and the differential impedance of the lines  150 A 2  and  150 C 2  to approximate to one another. Likewise, in the transmission line  140 , the lines  150 A 3 ,  150 B 3 , and  150 C 3  are kept from being in adjacency to one another. This makes it possible to allow the differential impedance of the lines  150 A 3  and  150 B 3 , the differential impedance of the lines  150 B 3  and  150 C 3 , and the differential impedance of the lines  150 A 3  and  150 C 3  to approximate to one another. As a result, in the transmission line  140 , it is possible to enhance the symmetry of the differential impedance, leading to the enhancement in the communication quality. 
     As illustrated in  FIG. 28 , for example, out of the lines  150 A 1 ,  150 B 1 , and  150 C 1 , the line  150 B 1  is adjacent to the lines  150 B 2  and  150 C 2 . The line  150 C 1  is adjacent to the line  150 C 2 . Moreover, out of the lines  150 A 2 ,  150 B 2 , and  150 C 2 , the line  150 A 2  is adjacent to the lines  150 A 3  and  150 B 3 . The line  150 B 2  is adjacent to the lines  150 B 3  and  150 B 1 . The line  150 C 2  is adjacent to the lines  150 B 1  and  150 C 1 . Moreover, out of the lines  150 A 3 ,  150 B 3 , and  150 C 3 , the line  150 A 3  is adjacent to the line  150 A 2 . The line  150 B 3  is adjacent to the lines  150 A 2  and  150 B 2 . Between the adjacent lines as mentioned above, there is possibility of occurrence of crosstalk. Therefore, in the communication system  3 , the delay units  51  to  53  are provided, to shift transition timing of the signals, between the trio lines. Hence, in the communication system  3 , it is possible to restrain degradation in the communication quality caused by the crosstalk between the adjacent lines. 
       FIG. 29  schematically illustrates phases of the signals. In this example, the phases of the signals SIGA 1 , SIGB 1 , SIGC 1 , SIGA 3 , SIGB 3 , and SIGC 3  are each delayed by a phase difference PD, with respect to the phases of the signals SIGA 2 , SIGB 2 , and SIGC 2 . Here, the phase difference PD may be set at, for example, about a half of a unit interval U 1 . 
     In the communication system  3 , as illustrated in  FIG. 29 , it is possible to shift the transition timing of the signals, between the trio lines. As a result, in the communication system  3 , thanks to the shift of the transition timing, it is possible to reduce possibility that the eye opening is narrowed, even if the crosstalk occurs between the adjacent lines. This makes it possible to restrain the degradation in the communication quality. 
     As described, in this embodiment, the three lines that constitute the single trio line are kept from being in adjacency to one another. Hence, it is possible to enhance the communication quality. 
     In this embodiment, the transition timing of the signals are shifted, between the trio lines. Hence, it is possible to restrain the degradation in the communication quality. 
     Modification Example 3-1 
     In the forgoing embodiment, the grounded lines GL 1  and GL 2  are provided. However, this is non-limiting. In the following, this modification example is described in detail by giving some examples. 
       FIG. 30  illustrates one configuration example of a communication system  3 A according to this modification example. The communication system  3 A includes a transmission line  140 A. The transmission line  140 A includes the lines  150 A 1 ,  150 B 1 , and  150 C 1 , the lines  150 A 2 ,  150 B 2 , and  150 C 2 , and the lines  150 A 3 ,  150 B 3 , and  150 C 3 . In other words, the transmission line  140 A is devoid of grounded lines, unlike the transmission line  140  according to the forgoing embodiment ( FIG. 28 ). In the transmission line  140 A, the lines  150 A 1 ,  150 A 2 ,  150 A 3 ,  150 B 1 ,  150 B 2 ,  150 B 3 ,  150 C 1 ,  150 C 2 , and  150 C 3  are disposed side by side in this order. Thus, in the transmission line  140 A, the three lines that constitute the single trio line are kept from being in adjacency to one another. 
       FIG. 31  schematically illustrates phases of the signals in the communication system  3 A. In this example, the phases of the signals SIGA 2 , SIGB 2 , and SIGC 2  are each delayed by a phase difference PD 1  from the phases of the signals SIGA 1 , SIGB 1  and SIGC 1 . The phases of the signals SIGA 3 , SIGB 3 , and SIGC 3  are each delayed by a phase difference PD 2  from the phases of the signals SIGA 2 , SIGB 2 , and SIGC 2 . Here, the phase differences PD 1  and PD 2  may each be set at, for example, about ⅓ of the unit interval U 1 . 
       FIG. 32  illustrates one configuration example of another communication system  3 B according to this modification example.  FIG. 33  schematically illustrates phases of the signals in the communication system  3 B. The communication system  3 B includes a transmission line  140 B. In the transmission line  140 B, the lines  150 A 1 ,  150 A 3 ,  150 A 2 ,  150 B 1 ,  150 B 2 ,  150 B 3 ,  150 C 2 ,  150 C 1 , and  150 C 3  are disposed side by side in this order. In this example, in this example, as illustrated in  FIG. 33 , the phases of the signals SIGA 3 , SIGB 3 , and SIGC 3  are each delayed by the phase difference PD 1  from the phases of the signals SIGA 2 , SIGB 2 , and SIGC 2 . The phases of the signals SIGA 1 , SIGB 1 , and SIGC 1  are each delayed by the phase difference PD 2  from the phases of the signals SIGA 3 , SIGB 3 , and SIGC 3 . 
       FIG. 34  illustrates one configuration example of another communication system  3 C according to this modification example.  FIG. 35  schematically illustrates phases of the signals in the communication system  3 C. The communication system  3 C includes a transmission line  140 C. In the transmission line  140 C, the lines  150 A 1 ,  150 A 3 ,  150 A 2 ,  150 B 3 ,  150 B 2 ,  150 B 1 ,  150 C 2 ,  150 C 1 , and  150 C 3  are disposed side by side in this order. In this example, as illustrated in  FIG. 35 , the phases of the signals SIGA 3 , SIGB 3 , and SIGC 3  are each delayed by the phase difference PD 1  from the phases of the signals SIGA 2 , SIGB 2 , and SIGC 2 . The phases of the signals SIGA 1 , SIGB 1 , and SIGC 1  are each delayed by the phase difference PD 2  from the phases of the signals SIGA 3 , SIGB 3 , and SIGC 3 . 
     Modification Example 3-2 
     In the forgoing embodiment, the delay units  51  to  53  delay the clock signal TxCK by the predetermined amount, but this is non-limiting. The amount of the delay in the delay units  51  to  53  may be adjusted. In what follows, a communication system  3 D according to this modification example is described in detail. 
       FIG. 36  illustrates one configuration example of the communication system  3 D according to this modification example. The communication system  3 D includes a transmission device  50 D, the transmission line  140 , and a reception device  60 D. 
     The transmission device  50 D includes a processor unit  54 D, a reception unit  56 D, a control unit  57 D, and delay units MD to  53 D. 
     The processor unit  54 D performs predetermined processing to generate the transition signals TxF 10  to TxF 16 , TxR 10  to TxR 16 , TxP 10  to TxP 16 , TxF 20  to TxF 26 , TxR 20  to TxR 26 , TxP 20  to TxP 26 , TxF 30  to TxF 36 , TxR 30  to TxR 36 , and TxP 30  to TxP 36 , as with the processor  12 E. The processor unit  54 D includes a data generator unit  55 D. The data generator unit  55 D generates, in a calibration mode, data (the transition signal) for calibration. With this configuration, the processor unit  54 D outputs, in the calibration mode, the data generated by the data generator unit  55 D, as the transition signals TxF 10  to TxF 16 , TxR 10  to TxR 16 , TxP 10  to TxP 16 , TxF 20  to TxF 26 , TxR 20  to TxR 26 , TxP 20  to TxP 26 , TxF 30  to TxF 36 , TxR 30  to TxR 36 , and TxP 30  to TxP 36 . 
     The reception unit  56 D receives a control signal CTL supplied from the reception device  60 D, and supplies the control unit  57 D with comparison result information INF (described later) included in the control signal CTL. The control unit  57 D sets the amount of the delay in each of the delay units MD to  53 D, on the basis of the comparison result information INF. The delay units  51 D to  53 D each delay the clock signal TxCK by the amount of the delay in accordance with an instruction from the control unit  57 D. 
     The reception device  60 D includes a processor unit  64 D and a transmission unit  66 D. The processor unit  64 D performs predetermined processing, on the basis of the transition signals RxF 1 , RxR 1 , and RxP 1 , and the clock signal RxCK 1 , on the basis of the transition signals RxF 2 , RxR 2 , and RxP 2 , and the clock signal RxCK 2 , and on the basis of the transition signals RxF 3 , RxR 3 , and RxP 3 , and the clock signal RxCK 3 , as with the processor unit  32 E. The processor unit  64 D includes a data comparison unit  65 D. The data comparison unit  65 D compares, in the calibration mode, the transition signals RxF 1 , RxR 1 , RxP 1 , RxF 2 , RxR 2 , RxP 2 , RxF 3 , RxR 3 , and RxP 3  with predetermined data (the transition signal), to generate the comparison result information INF. The predetermined data corresponds to the data for the calibration generated by the data generator unit  55 D. The transmission unit  66 D generates the control signal CTL on the basis of the comparison result information INF, and transmits the control signal CTL to the transmission device  50 D. 
     In the communication system  3 D, in the calibration mode, first, the data generator unit  55 D of the transmission device  50 D generates the data (the transition signal) for the calibration. The control unit  57 D sets the amount of the delay in the delay units  51 D to  53 D. The delay units  51 D to  53 D each delay the clock signal TxCK by the amount of the delay in accordance with the instruction from the control unit  57 D. The transmission unit  201  generates the signals SIGA 1 , SIGB 1 , and SIGC 1  on the basis of the clock signal supplied from the delay unit  51 D. The transmission unit  202  generates the signals SIGA 2 , SIGB 2 , and SIGC 2  on the basis of the clock signal supplied from the delay unit  52 D, and generates the signals SIGA 3 , SIGB 3 , and SIGC 3  on the basis of the clock signal supplied from the delay unit  53 D. 
     Moreover, the reception unit  401  of the reception device  60 D generates the transition signals RxF 1 , RxR 1 , and RxP 1 , and the clock signal RxCK 1 , on the basis of the signals SIGA 1 , SIGB 1 , and SIGC 1 . The reception unit  402  generates the transition signals RxF 2 , RxR 2 , and RxP 2 , and the clock signal RxCK 2 , on the basis of the signals SIGA 2 , SIGB 2 , and SIGC 2 . The reception unit  403  generates the transition signals RxF 3 , RxR 3 , and RxP 3 , and the clock signal RxCK 3 , on the basis of the signals SIGA 3 , SIGB 3 , and SIGC 3 . The data comparison unit  65 D compares the transition signals RxF 1 , RxR 1 , RxP 1 , RxF 2 , RxR 2 , RxP 2 , RxF 3 , RxR 3 , and RxP 3  with the predetermined data (the transition signal), to generate the comparison result information INF. The transmission unit  66 D generates the control signal CTL on the basis of the comparison result information INF, and transmits the control signal CTL to the transmission device  50 D. 
     Moreover, the reception unit  56 D of the transmission device  50 D receives the control signal CTL supplied from the reception device  60 D, and supplies the control unit  57 D with the comparison result information INF included in the control signal CTL. The control unit  57 D sequentially changes the amount of the delay in the delay units MD to  53 D, and sequentially acquires the comparison result information INF. Moreover, the control unit  57 D acquires a range of the amount of the delay in each of the delay units MD to  53 D that makes it possible to perform communication normally. Specifically, for example, with two of the delay units  51 D to  53 D being focused, setting of the amount of the delay is sequentially changed from setting that maximizes a skew to setting that minimizes the skew, to acquire the range of the amount of delay that makes it possible to perform communication normally. Moreover, the control unit  57 D determines, on the basis of the range of the amount of the delay thus acquired, the amount of the delay, to increase an operation margin. Specifically, for example, the amount of the delay may be determined on the basis of a center value of the range of the amount of the delay that makes it possible to perform communication normally. 
     Modification Example 3-3 
     In the forgoing embodiment, the transition timing of the signals in the adjacent lines are shifted from one another, but this is non-limiting. For example, even if the transition timing of the signals in the adjacent lines substantially coincide, it is unnecessary to shift the transition timing in a case where the communication quality does not lower so significantly. 
     4. Application Example 
     Description is given next of an application example of the communication systems described in the forgoing embodiments and the modification examples. 
       FIG. 37  illustrates an external appearance of a smartphone  300  (a multifunction mobile phone) to which the communication systems according to the forgoing example embodiments are applied. The smartphone  300  may be equipped with various devices. The communication systems according to the forgoing example embodiments are applied to a communication system that performs data exchange among the devices. 
       FIG. 38  illustrates one configuration example of an application processor  310  utilized in the smartphone  300 . The application processor  310  includes a CPU (Central Processing Unit)  311 , a memory control unit  312 , a power supply control unit  313 , an external interface  314 , a GPU (Graphics Processing Unit)  315 , a media processor unit  316 , a display control unit  317 , and an MIPI (Mobile Industry Processor Interface) interface  318 . The CPU  311 , the memory control unit  312 , the power supply control unit  313 , the external interface  314 , the GPU  315 , the media processor unit  316 , and the display control unit  317  are coupled to a system bus  319  in this example, and are able to perform the data exchange with one another through the system bus  319 . 
     The CPU  311  processes various pieces of information handled by the smartphone  300  in accordance with a program. The memory control unit  312  controls a memory  501  which the CPU  311  uses in performing information processing. The power supply control unit  313  controls a power supply of the smartphone  300 . 
     The external interface  314  is an interface provided for communication with an external device. In this example, the external interface  314  is coupled to a wireless communication unit  502  and an image sensor  410 . The wireless communication unit  502  performs wireless communication with a base station of mobile phones. The wireless communication unit  502  is so constituted that the wireless communication unit  502  includes a baseband unit and an RF (Radio Frequency) front end unit, without limitation. The image sensor  410  acquires an image. The image sensor  410  is so constituted that the image sensor  410  includes a CMOS sensor, without limitation. 
     The GPU  315  performs image processing. The media processor unit  316  processes information such as sound, characters, and figures. The display control unit  317  controls a display  504  through the MIPI interface  318 . The MIPI interface  318  transmits an image signal to the display  504 . As the image signal, for example, a signal of a YUV system, an RGB system, or other systems may be used. For example, the communication systems according to the forgoing example embodiments are applied to a communication system between the MIPI interface  318  and the display  504 . 
       FIG. 39  illustrates one configuration example of the image sensor  410 . The image sensor  410  includes a sensor unit  411 , an ISP (Image Signal Processor)  412 , a JPEG (Joint Photographic Experts Group) encoder  413 , a CPU  414 , a RAM (Random Access Memory)  415 , a ROM (Read Only Memory)  416 , a power supply control unit  417 , an I 2 C (Inter-Integrated Circuit) interface  418 , and an MIPI interface  419 . These blocks are each coupled to a system bus  420  in this example, and are able to perform the data exchange with one another through the system bus  420 . 
     The sensor unit  411  acquires the image, and is constituted by, for example, the CMOS sensor. The ISP  412  performs predetermined processing on the image acquired by the sensor unit  411 . The JPEG encoder  413  encodes the image processed by the ISP  412 , to generate an image of a JPEG form. The CPU  414  controls each block of the image sensor  410  in accordance with a program. The RAM  415  is a memory which the CPU  414  uses in performing image processing. The ROM  416  stores the program to be executed in the CPU  414 . The power supply control unit  417  controls a power supply of the image sensor  410 . The I 2 C interface  418  receives a control signal from the application processor  310 . Moreover, although not illustrated, the image sensor  410  receives, from the application processor  310 , a clock signal in addition to the control signal. Specifically, the image sensor  410  is configured to be able to operate on the basis of the clock signals having various frequencies. The MIPI interface  419  transmits the image signal to the application processor  310 . As the image signal, for example, the signal of the YUV system, the RGB system, or other systems may be used. For example, the communication systems according to the forgoing example embodiments are applied to a communication system between the MIPI interface  419  and the application processor  310 . 
     Although description has been made by giving the embodiments and the modification examples, and the application example to the electronic apparatus as mentioned above, the contents of the technology are not limited to the above-mentioned example embodiments and may be modified in a variety of ways. 
     For example, in the forgoing example embodiments, the transmission line is constituted by a strip line. However, this is non-limiting. Alternatively, for example, as illustrated in  FIGS. 40 and 41 , the transmission line may be constituted by a micro strip line.  FIG. 40  illustrates one configuration example of a transmission line  800  according to this modification example, and corresponds to the transmission line  100  ( FIG. 11 ). The transmission line  800  includes a dielectric layer  802 , a conductive layer  803 , and lines  810 A,  810 B, and  810 C. The dielectric layer  802  and the conductive layer  803  are stacked. Moreover, the lines  810 A,  810 B, and  810 C are formed on a surface of the dielectric layer  802 .  FIG. 41  illustrates one configuration example of another transmission line  820  according to this modification example, and corresponds to the transmission line  120  ( FIG. 24 ). The transmission line  820  includes lines  830 A,  830 B, and  830 C. The lines  830 A and  830 C are formed on the surface of the dielectric layer  802 , whereas the line  830 B is formed inside the dielectric layer  802 . 
     Moreover, for example, in the forgoing example embodiments, in the case where the voltage of the output terminal ToutA is set at the medium level voltage VM, for example, two out of the control signals PU 1 A to PU 5 A are brought to “1”, with the remaining ones being brought to “0”, whereas for example, two out of the control signals PD 1 A to PD 5 A are brought to “1”, with the remaining ones being brought to“0”. However, this is non-limiting. Alternatively, for example, all the control signals PU 1 A to PU 5 A and PD 1 A to PD 5 A may be brought to “0”. Thus, in the driver unit  29 A, the five transistors  91  and the five transistors  94  are turned off, causing output impedance to be high impedance. At this occasion, the voltage of the output terminal ToutA is set at the medium level voltage VM by the resistors  41 A to  41 C of the reception unit  40 . 
     It is to be noted that the technology may have the following configurations. 
     (1) A transmission line, including: 
     a first line; 
     a second line having characteristic impedance higher than characteristic impedance of the first line; and 
     a third line, 
     the transmission line transmitting a symbol that corresponds to a combination of signals in the first line, the second line, and the third line. 
     (2) The transmission line according to (1), in which 
     the first line, the second line, and the third line are disposed in order at a same level as one another, and 
     the characteristic impedance of the second line is higher than characteristic impedance of the third line. 
     (3) The transmission line according to (1) or (2), in which the second line is formed with use of a material having higher resistivity than the first line. 
     (4) The transmission line according to any one of (1) to (3), in which the second line includes more impurity than the first line. 
     (5) The transmission line according to any one of (1) to (4), in which a width of the second line is smaller than a width of the first line. 
     (6) The transmission line according to any one of (1) to (5), further including: 
     a dielectric layer; 
     a first conductive layer; and 
     a second conductive layer, 
     the first conductive layer, the dielectric layer, and the second conductive layer being stacked in order, and 
     the first line, the second line, and the third line being formed inside the dielectric layer. 
     (7) The transmission line according to (6), in which 
     the first conductive layer is formed in a region corresponding to the first line, and in a region corresponding to the third line. 
     (8) The transmission line according to any one of (1) to (5), further including: 
     a dielectric layer; and 
     a conductive layer, 
     the dielectric layer and the conductive layer being stacked, and 
     the first line, the second line, and the third line being formed on an opposite surface of the dielectric layer to a surface on which the conductive layer is provided. 
     (9) The transmission line according to any one of (6) to (8), in which 
     permittivity of the dielectric layer in vicinity of the second line is lower than permittivity of the dielectric layer in vicinity of the first line. 
     (10) The transmission line according to (1), further including: 
     a dielectric layer; 
     a first conductive layer; and 
     a second conductive layer, 
     the first conductive layer, the dielectric layer, and the second conductive layer being stacked in order, 
     the first line, the second line, and the third line being stacked in order at different levels from one another inside the dielectric layer, and 
     the characteristic impedance of the second line being higher than characteristic impedance of the third line. 
     (11) A transmission line, including: 
     a first line formed in a first layer; 
     a second line formed in a second layer; and 
     a third line formed in the first layer, 
     the transmission line transmitting a symbol that corresponds to a combination of signals in the first line, the second line, and the third line. 
     (12) The transmission line according to (11), in which 
     the second line is disposed between the first line and the third line, in a plane that crosses a direction in which the layers are stacked. 
     (13) The transmission line according to (12), further including a fourth line, a fifth line, and a sixth line, in which 
     the fifth line is formed in the first layer, 
     the fourth line and the sixth line are formed in the second layer, 
     the first line is confronted with the fourth line, 
     the second line is confronted with the fifth line, and 
     the third line is confronted with the sixth line. 
     (14) A transmission line, including: 
     a first trio line that includes three lines and transmits a first symbol corresponding to a combination of signals; and 
     a second trio line that includes three lines and transmits a second symbol corresponding to a combination of signals, 
     one of the three lines in the second trio line being disposed between two of the three lines in the first trio line. 
     (15) The transmission line according to (14), in which 
     the three lines in the first trio line are disposed out of adjacency to one another. 
     (16) The transmission device according to (14) or (15), in which 
     first transition timing of the signal transmitted by the first trio line and second transition timing of the signal transmitted by the second trio line are different from each other. 
     (17) A communication system, including: 
     a transmission device; 
     a reception device; and 
     a transmission line that transmits a signal from the transmission device to the reception device, 
     the transmission line including
         a first line,   a second line having characteristic impedance higher than characteristic impedance of the first line, and   a third line, and   the transmission line transmitting a symbol that corresponds to a combination of signals in the first line, the second line, and the third line.       

     (18) A communication system, including: 
     a transmission device; 
     a reception device; and 
     a transmission line that transmits a signal from the transmission device to the reception device, 
     the transmission line including
         a first line formed in a first layer,   a second line formed in a second layer, and   a third line formed in the first layer, and   the transmission line transmitting a symbol that corresponds to a combination of signals in the first line, the second line, and the third line.       

     (19) A communication system, including: 
     a transmission device; 
     a reception device; and 
     a transmission line that transmits a signal from the transmission device to the reception device, 
     the transmission line including
         a first trio line that includes three lines and transmits a first symbol corresponding to a combination of signals, and   a second trio line that includes three lines and transmits a second symbol corresponding to a combination of signals,   one of the three lines in the second trio line being disposed between two of the three lines in the first trio line.       

     (20) The communication system according to (19), in which 
     the transmission device includes
         a first phase adjuster unit that adjusts phases of signals in the three lines of the first trio line; and   a second phase adjuster unit that adjusts phases of signals in the three lines of the second trio line.       

     (21) The communication system according to (20), in which 
     the transmission device further includes a control unit, the control unit transmitting a signal that includes a predetermined data pattern, and controlling the first phase adjuster unit and the second phase adjuster unit on the basis of a result of reception in the reception device of the signal that includes the predetermined data pattern. 
     This application claims the benefit of Japanese Priority Patent Application JP2015-97930 filed on May 13, 2015, the entire contents of which are incorporated herein by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof