Abstract:
The present invention provides a video signal transmission apparatus including: an optical transmitter that uni-directionally transmits video data from a video source; an optical receiver that receives the video data and outputs the received video data to a sink device; a first transmission medium that transmits the video data at a high speed; a second transmission medium that transmits identification information for identifying the sink device at a low speed; an identification information acquisition control section that acquires the identification information from the sink device; a storage section that stores general-purpose identification information used for plural types of sink devices; a acquisition possibility determination section that determines whether the identification information can be acquired from the sink device; and a general-purpose identification information reply control section that replies the stored general-purpose identification information to the video source device if determined that the identification information cannot be acquired.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2010-108161 filed on May 10, 2010. 
     BACKGROUND 
     1. Technical Field 
     The present invention relates to a video signal transmission apparatus, an identification information acquisition method for a video signal transmission system, and a computer readable medium storing an identification information acquisition program for a video signal transmission system. 
     2. Related Art 
     A serial data signal for a digital video so called a DVI (Digital Visual Interface) or HDMI (High Definition Multimedia Interface) requires high-speed signal which is equal to or higher than 1 Gbps. Therefore, such signal can be transmitted only up to about 10 m, when transmitted by an electric cable. Accordingly, when transmission of such signal for more than 10 m is required, the serial data signal needs to be converted into an optical signal and an optical fiber may be used to transfer such optical signal. In the case of using the optical fiber, an optical transmitter and an optical receiver, connected to both ends of the optical fiber, may be provided between a video source device such as a PC (including a video card) and a sink device such as a display. 
     The serial data signal includes a high-speed video signal, information of the display (hereinafter referred to as “EDID”), and a DDC (Display Data Channel) control system signal used to exchange an encryption key called an HDCP (High-bandwidth Digital Content Protection). Since this DDC control system signal is a DC signal or a low-speed signal of lower than 100 KHz, and is a bidirectional signal. The DDC control system signal may be transmitted through a metal cable such as a LAN (Local Area Network) cable. 
     Namely, when transmitting the serial data signal of digital video, different kinds of cables may be used to transmit the video signal and the DDC control system signal, respectively. 
     The HDCP is a type of digital copyright management technology that functions to prevent illegal copying by encrypting a digital type image or an output signal of video content. 
     Also, the DDC is a standard for exchanging various kinds of information between the display and the PC for realizing PnP (Plug and Play). According to the DDC, information representing permissible resolution of a display, color depth, a scanning frequency, and a model number of a product is exchanged between the PC (video source device) and the display (sink device). Through the exchange of the information, setting information of the display is transferred, and thus the setting is automatically performed to match the performance of the respective displays. 
     SUMMARY 
     According to a first aspect of the present invention, there is provided a video signal transmission apparatus including: an optical transmitter, connected to a video source device, that uni-directionally transmits video data input from the video source device; an optical receiver, connected to a sink device, that receives the video data transmitted from the optical transmitter and outputs the received video data to the sink device; a first transmission medium, connected to the optical transmitter and the optical receiver, that transmits the video data at a speed higher than a predetermined reference transmission speed; a second transmission medium, connected to the optical transmitter and the optical receiver independently from the first transmission medium, that transmits identification information for identifying the sink device at a speed lower than the predetermined reference transmission speed; an identification information acquisition control section, provided in the optical transmitter, that acquires the identification information from the sink device through a bidirectional communication using the second transmission medium in accordance with a request from the video source device; a storage section, provided in the optical receiver, that stores general-purpose identification information generally used for a plurality of types of sink devices that are connectable to the optical receiver; an acquisition possibility determination section that determines whether the identification information can be acquired from the sink device; and a general-purpose identification information reply control section that replies the general-purpose identification information stored in the storage section to the video source device if the acquisition possibility determination section determine that the identification information cannot be acquired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a structural diagram illustrating the configuration of a video signal transmission system according to an exemplary embodiment of the present invention; 
         FIG. 2  is a functional block diagram illustrating an electrical connection in a video signal transmission system according to an exemplary embodiment of the present invention; 
         FIG. 3  is a flowchart illustrating a flow of LAN cable connection state monitoring control executed by an optical transmitter cable connection circuit, an address setting circuit, and a delay circuit, according to an exemplary embodiment of the present invention; and 
         FIG. 4  is a functional block diagram illustrating an electrical connection in a video signal transmission system according to an alternative exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Herebelow, an example of an exemplary embodiment of the present invention will be described in detail with reference to the drawings. 
       FIG. 1  is a structural diagram illustrating the configuration of a video signal transmission system according to an exemplary embodiment of the present invention. 
     In a video signal transmission system, a host computer  10  and a display  12  are connected through a video signal transmission apparatus  14  for optical communication of a video signal. The host computer  10  is applicable as a video source device. The display  12  is applicable as a sink device. 
     The video signal transmission apparatus  14  includes an optical transmitter  16 , an optical receiver  18 , and an optical fiber cable  20 . The optical fiber cable  20  is provided between the optical transmitter  16  and the optical receiver  18 . 
     The optical fiber cable  20  includes an optical fiber harness  22  for respective colors (R, G, and B) and a clock (CLK) which corresponds to a DVI video signal. Both end portions of the optical fiber harness  22  are bundled and are connected to optical fiber connectors  24 , respectively. Each optical fiber connector  24  is connected to an optical transmission interface  26  of the optical transmitter  16 , and an optical reception interface  28  of the optical receiver  18 , respectively. 
     The optical transmitter  16  includes an interface  30  for receiving a serial data signal of a digital video, such as DVI or HDMI, from the host computer  10 . The host computer  10  includes an interface  32  for outputting a serial data signal of the digital video. Accordingly, the host computer  10  and the optical transmitter  16  are electrically connected when the connectors  36  installed at both ends of the DVI or HDMI dedicated connection cable  34  are connected to the interface  32  of the host computer  10  and the interface  30  of the optical transmitter  16 . 
     Further, the optical receiver  18  includes an interface  38  for outputting the serial data signal of the digital video, such as DVI or HDMI, to the display  12 . The display  12  includes an interface  40  for receiving the serial data signal of the digital video. Accordingly, the optical receiver  18  and the display  12  are electrically connected when the connectors  44  installed at both ends of the DVI or HDMI dedicated connection cable  42  are connected to the interface  38  of the optical receiver  18  and the interface  40  of the display  12 . 
     Here, the serial data signal of the digital video, such as DVI or HDMI, includes a DDC control system signal in addition to the video signal. 
     The DDC control system signal is a standard for transmitting/receiving information between the host computer  10  and the display  12  for realizing PnP (Plug and Play). In the communication of the DDC control system signal (hereinafter referred to as “DDC communication”), information representing a permissible resolution of the display  12 , color depths, a scanning frequency, and a model number of a product is exchanged between the host computer  10  and the display  12 . According to this information, the setting is automatically performed to match the performance and specification of the display  12  connected to the optical receiver  18 . 
     The optical transmitter  16  transmits only the video signal to the optical receiver  18  through the optical fiber cable  20  in uni-directional communication. Optical communication using the optical fiber is advantageous in transmitting a high-speed signal of equal to or higher than 1 Gbps over a transmission distance of equal to or longer than 10 m. In other words, the high-speed signal of equal to or higher than 1 Gbps has the limit of transmission distance of 10 m, when transmitted via a metal cable. In the present exemplary embodiment, the optical communication by the optical fiber is performed particularly in transmitting the video signal. 
     On the other hand, the DDC control signal is a low-speed signal (in comparison to the transmission speed of the video signal) of about 100 kHz, and also requires bidirectional communication. Accordingly, in the present exemplary embodiment, the DDC control signal is bidirectionally communicated by using a LAN cable  46  which is cheaper than the optical fiber cable  20  and can be applied as a metal cable. 
     Namely, as illustrated in  FIG. 1 , in the optical transmitter  16  and the optical receiver  18 , LAN interfaces  50  and  52  are provided, to which the connectors  48  connected to the end portions of the LAN cable  46  are connectable. 
       FIG. 2  is a functional block diagram illustrating an electrical connection in a video signal transmission system shown in  FIG. 1 . 
     [Video Signal Transmission System] 
     Four laser drivers  100  are connected to the interface  30  of the optical transmitter  16  respectively. The DVI video signals R, G, B, and CLK from the host computer  10  is input to the four laser drivers  100 . 
     Laser diodes  102  are connected to the four laser drivers  100 . The laser diodes  102  emit light or are turned OFF based on light-emitting control signals from the laser drivers  100 . Namely, the light emitting of the laser diodes is controlled based on the video signal input to the laser drivers  100 . 
     The laser diode  102  is connected to one end of each optical fiber  104 . The other end of the optical fiber  104  is connected to the optical transmission interface  26 . At one side of the optical fiber connector  24  of the optical fiber cable  20 , the optical transmission interface  26  is connected. The optical transmission interface  26  configures the optical fiber  104  connected to the laser diode  102  and the optical fiber constituting the optical fiber cable  20 , substantially coaxial with each other. Note that the term “substantially” means that the light emitted from the laser diode  102  is optically coaxial to the optical fiber cable  20 , and may not be physically coaxial. 
     The other-side of the optical fiber connector  24  of the optical fiber cable  20  is connected to the optical reception interface  28  of the optical receiver  18 . The optical reception interface  28  has a function that is equal to the optical transmission interface  26 . Namely, four photodiodes  106  are installed in the optical receiver  18 , and one end of each optical fiber  108  is connected to the optical receiver  18 , respectively. In the optical fiber cable  20 , the surface of one end portion of the four optical fibers are configured to be substantially coaxial with the surface of the other end portion of the optical fibers  108  connected to the photodiodes  106 . The surfaces of the end portions of the four optical fibers exchange optical communication information (namely, the optical converted video signal). Note that the term “substantially” means that the light emitted from the optical fiber on the side of the optical fiber cable  20  is optically coaxial to the optical fiber  108  on the side of the optical receiver  18 , and may not be physically coaxial. 
     The four photodiodes  106  are connected to amplifiers  110 , respectively. The amplifiers  110  amplify the converted electric signals received by the photodiodes  106 , convert the electric signals into a DVI video signal R, G, B, and CLK, and output the DVI video signal to the display  12  through the interface  38 . 
     [DDC Communication Control Process] 
     The DDC communication control process is executed by the host computer  10  when it is recognized that the display  12  is connected in an HPD determination control process, which will be described later. 
     A DDC-CLK/DATA buffer circuit (hereinafter simply referred to as “buffer circuit”)  112  is connected to the interface  30  of the optical receiver  14 . 
     The buffer circuit  112  is connected to a buffer circuit  114  of the optical receiver  18  through the LAN interfaces  50  and  52  and the LAN cable  46 . 
     When DDC communication control process, the host computer  10  outputs an address that specifies a storage area of the display  12  in order to acquire a display identification code (hereinafter referred to as “EDID”) stored in a storage area (not illustrated) within the display  12 . 
     The buffer circuit  112  at the optical transmitter  16  acquires the EDID by accessing the storage area of the display  12  through the buffer circuit  114  at the optical receiver  18 , based on the address. The EDID acquired by the buffer circuit  112  at the optical transmitter  16  is output to the host computer  10 . The host computer  10  executes processes such as correction of the video signal based on the acquired EDID. 
     In the present exemplary embodiment, a configuration that transmits the video signal is configured even when the LAN cable  46  is not connected. 
     When the LAN cable  46  is not connected, the DDC communication control process can not be executed. Therefore, in the present exemplary embodiment, a storage section  116  that stores a virtual EDID is installed at the optical transmitter  16 . When the LAN cable  46  is connected, a form that acquires the EDID from the display  12  actually connected (hereinafter referred to as “first form”) is selectively executed, while when the LAN cable  46  is not connected, a form that acquires the virtual EDID from the EDID storage section  116  (hereinafter referred to as “second form”) is selectively executed. 
     In order to select the first form or the second form, a cable connection detection circuit  118  is provided in the optical transmitter  16 . 
     The cable connection detection circuit  118  is connected to a cable connection signal output circuit  120  of the optical receiver  18  through the LAN interfaces  50  and  52  and the LAN cable  46 . 
     The cable connection signal output circuit  120 , for example, has a simple loop circuit formed therein, and the cable connection detection circuit  118  determines the connection state of the LAN cable  46  by detecting whether a voltage applied from the corresponding cable connection detection circuit  118  is maintained and returns thereto. 
     The cable connection signal detection circuit  118  is connected to an address setting circuit  122 . This address setting circuit  122  is connected to the EDID storage section  116 . The address setting circuit  122  serves to set an address that is equal the storage area of the display  12  with respect to the corresponding EDID in the storage section  116 . 
     Namely, the cable connection detection circuit  118  outputs H (high level) signal to the address setting circuit  122  when the LAN cable  46  is connected, and outputs L (low level) signal to the address setting circuit  122  when the LAN cable  46  is not connected. 
     The address setting circuit  122  does not set the address with respect to the EDID storage section  116  when H signal is received from the cable connection detection circuit  118  (execution of the first form). On the other hand, the address setting circuit  122  sets the address with respect to the EDID storage section  116  when L signal is received from the cable connection detection circuit  118  (execution of the second form). 
     When an address is set in the EDID storage section  116 , the LAN cable  46  is not connected. Accordingly, the host computer  10  acquires the virtual EDID from the EDID storage section  116  based on the address reported in the DDC communication control process. 
     [HPD Determination Control Process] 
     An HPD setting circuit  124  is connected to the interface  30  of the optical transmitter  16 . The HPD setting circuit  124  reports whether the display  12  is connected to the host computer  10 . More specifically, the HPD setting circuit  124  outputs a different two-value signal when the display  12  is connected or is not connected (for example, H signal when the display is connected, and L signal when the display is not connected). 
     When it is recognized that the display  12  is connected through the HPD signal, the host computer  10  executes the above-described DDC communication control process. 
     In the present exemplary embodiment, even in the case where the LAN cable  46  is not connected, the HPD setting circuit  124  operates control to falsely report that the display  12  is connected to the host computer  10 . Namely, in the present exemplary embodiment, the first form and the second form are used together. 
     Accordingly, the HPD setting circuit  124  is connected to the cable connection detection circuit  118  through the HPD detection circuit  126  and the delay circuit  128 . The details of the delay circuit  128  will be described later. 
     [First Form] 
     The HPD detection circuit  126  is connected to an HPD detection transmission circuit  130  of the optical receiver  18  via the LAN interfaces  50  and  52  and the LAN cable  46 . For example, in the case where the display  12  is connected, the HPD detection transmission circuit  130  outputs a detection signal of 5 V (H signal) to the HPD detection circuit  126 . On the other hand, in the case where the display  12  is not connected, the HPD detection transmission circuit  130  output a detection signal of 0 V (L signal) to the HPD detection circuit  126 . This signal is output to the HPD setting circuit  124 , and when the signal from the cable connection detection circuit  118  is a signal (“H signal” to be described later) that indicates the LAN cable in a connected state, the HPD setting circuit  124  outputs the signal which indicates that the display  12  is connected, to the host computer  10 . 
     As a result, the host computer  10  recognizes whether the display  12  is connected or not by the signal from the HPD setting circuit  124 , and executes the DDC communication control process accordingly. 
     [Second Form] 
     On the other hand, when the signal from the cable connection detection circuit  118  is the signal (“L signal”) that indicates the LAN cable in a disconnected state, the HPD setting circuit  124  converts the HPD signal into the H signal (false signal), and outputs the H signal to the host computer  10 . The host computer  10  recognizes whether the display  12  is connected or not by the signal from the HPD setting circuit  124 , and executes the DDC communication control process accordingly. Namely, according to the second form, even in the case where the LAN cable  46  is not connected, the false HPD signal is output as if the display  12  was connected, and thus the host computer  10  executes the DDC communication control process accordingly. 
     [Function of Delay Circuit] 
     Here, as described above, a delay circuit  128  is provided between the HPD setting circuit  124  and the cable connection detection circuit  118 . The delay circuit  128  delays the transmission of the signal from the cable connection detection circuit  118  for 150 msec. 
     As a result, the HPD setting circuit  124  converts the H signal into the L signal after 150 msec, starting from a time when the connected LAN cable  46  is disconnected (or starting from a time when the disconnected LAN cable is connected). 
     Namely, at an initial setting such as starting (power ON) of the host computer  10 , the host computer  10  executes the DDC communication control process regardless of the connection/disconnection of the LAN cable  46 . However, in the case where the LAN cable  46  is disconnected during the operation of the host computer  10  (for example, outputting of the video signal or the like), the host computer  10  instantaneously (for example, in 100 msec or shorter) performs conversion from a true HPD signal (H signal) into a false HPD signal (H signal) using the signal from the cable connection detection circuit  118 . 
     On the other hand, during the execution of the DDC communication control process, a detection period of the L signal of the HPD signal equal to or longer than 100 msec is required. Therefore, the host computer  10  is unable to execute (re-execute) the DDC communication control process when the LAN cable  46  is disconnected. 
     Accordingly, by intentionally generating a disconnected state of the LAN cable  46  for equal to or longer than 150 msec by the delay circuit  128 , the execution of the DDC communication control process can be secured. 
     In the above, a case in which the LAN cable  46  in a connection state is disconnected during the operation (outputting of the video signal) has been described, however, the reverse is also the same. Namely, when disconnected LAN cable  46  is connected during the operation (outputting of the video signal), the signal sent from the HPD setting circuit  124  to the host computer  10  is temporarily (150 msec) in an L signal state, in the same manner. 
     Table 1 shows the output of the cable connection detection circuit  128  (LAN cable detection), the output of the HPD detection circuit  126  (HPD detection), and the output of the HPD setting circuit  124  (HPD output) based on the connection state of the LAN cable  46  and the connection state of the display  12 . 
     In Table 1, “non-detection (L)” indicates that the communication system from the HPD transmission circuit  130  to the HPD detection circuit  126  is disconnected due to disconnection of the LAN cable  46 , and as a result, a non-detection signal (L signal) is produced. 
     Further, in Table 1, “false H” indicates that the original signal is the L signal, but in order to realize the second form, the H signal is falsely output from the HPD setting circuit  124  to the host computer  10 . 
     
       
         
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 A signal 
                 B signal 
                   
                   
               
               
                 LAN cable 
                 HPD 
                 C signal 
                   
               
               
                 detection 
                 detection 
                 HPD output 
                 State 
               
               
                   
               
             
             
               
                 H 
                 H 
                 H 
                 (a) LAN cable 
               
               
                   
                   
                 shifted from state (d) to H 
                 connected, 
               
               
                   
                   
                 after 150 msec (L) (*1) 
                 display 
               
               
                   
                   
                   
                 connected 
               
               
                 H 
                 L 
                 L 
                 (b) LAN cable 
               
               
                   
                   
                   
                 connected, 
               
               
                   
                   
                   
                 display 
               
               
                   
                   
                   
                 disconnected 
               
               
                 L 
                 Non- 
                 False H 
                 (c) LAN cable 
               
               
                   
                 detection (L) 
                   
                 disconnected, 
               
               
                   
                   
                   
                 display 
               
               
                   
                   
                   
                 disconnected 
               
               
                 L 
                 Non- 
                 False H 
                 (d) LAN cable 
               
               
                   
                 detection (L) 
                 shifted from state (a) to 
                 disconnected, 
               
               
                   
                   
                 false H after 150 msec (L) 
                 display 
               
               
                   
                   
                   
                 connected 
               
               
                   
               
               
                 * For each output signal, H denotes detection, and L denotes non-detection 
               
               
                 (*1) When A signal is L and B signal is H, C signal becomes L (in the case where A signal is delayed) 
               
             
          
         
       
     
     In Table 1, when shifting from state (a) to state (d), namely, in the case where the connected LAN cable  46  is disconnected, the HPD setting circuit  124  outputs a false H signal to the host computer  10  after temporarily (for a period of 150 msec) outputting a L signal. Accordingly, the DDC communication control process can be executed. 
     On the other hand, in Table 1, when shifting from state (d) to state (a), namely, in the case where the disconnected LAN cable  46  is connected, the HPD setting circuit  124  outputs a H signal to the host computer  10  after temporarily (for a period of 150 msec) outputting a L signal. Accordingly, the DDC communication control process can be executed. 
     Hereinafter, the operation in the present exemplary embodiment will be described. 
     Firstly, a flow of video signal transmission process when the optical fiber cable  20  and the LAN cable  46  are connected during power ON, will be described. 
     When the power is input to the host computer  10 , the optical transmitter  16 , the optical receiver  18 , and the display  12 , the host computer  10  receives an HPD detection signal from the HPD setting circuit  124  of the optical receiver  16 , and confirms the connection state of the display  12 . 
     When it is confirmed that the display  12  is connected, the host computer  10  executes the DDC communication control process for acquiring the EDID of the display  12  through a buffer circuit  112  of the optical transmitter  16 . 
     When a control signal form acquiring EDID information is received, the display  12  outputs a signal that indicates the EDID information, and the host computer  10  acquires the EDID information through a buffer circuit  114 , the LAN cable  46 , and the buffer circuit  112 . 
     Next, when the EDID is acquired, the host computer  12  recognizes a type of the display  12  and set values based on the corresponding EDID, generates and outputs a video signal that is in the specification of the display  12  based on the image information. This video signal is transmitted from the optical transmitter  16  to the optical receiver  18  through the optical fiber cable  20 . 
     Next, the optical receiver  18  converts the light signal received through the photodiodes  106  into electric signals, and outputs the electric signals to the display  12  to display an image. 
     Here, in the present exemplary embodiment, the video signal is transmitted via the optical fiber  20 , and the DDC control signal is transmitted via the LAN cable  46 . However, when the LAN cable  46  is not connected, the video signal can also be transmitted by the optical fiber  20 . 
       FIG. 3  is a flowchart illustrating a flow of LAN cable connection monitoring control in the cable connection circuit  118 , the address setting circuit  122 , and the delay circuit  128  of the optical transmitter  16 , that starts when the power of the optical transmitter is turned ON. 
     In step  150 , an initial resetting is performed, and in step  152 , the cable connection detection circuit  118  acquires the signal from the cable connection signal output circuit  120 . 
     In step  154 , it is determined whether the signal detected by the cable connection detection circuit  118  is H signal that indicates a connection state or L signal that indicates a disconnection state. The result of determination is reported to the address setting section  122 . 
     In step  156 , if the reported signal is L signal, the address setting section  122  sets an address of the EDID storage area of the display  12  in the EDID storage section  116  of the optical transmitter  16 , and proceeds to step  158 . Also, in step  154 , if the reported signal is H signal, the address setting unit  122  proceeds to step  158 . 
     Accordingly, the host computer  10  can acquire the EDID as described above, regardless of connection state of the LAN cable  46 . 
     In step  158 , by the monitoring performed by the cable connection detection circuit  118 , it is determined whether the connection state has changed or not. 
     Here, if the connection state has changed, the process proceeds from step  158  to step  160 , and is determined whether the change of the connection state is from H to L (the connected LAN cable  46  has been disconnected) or from L to H (the disconnected LAN cable  46  has been connected). 
     In step  160 , if it is determined that the change is from H to L, the process proceeds to step  162 , and the address of the EDID storage area of the display  12  is set in the EDID storage section  116 . Then, the process proceeds to step  166 . On the other hand, if it is determined that the change is from L to H in step  160 , the process proceeds to step  164 , and the address of the EDID storage area of the display  12  that is set in the EDID storage section  116  is canceled. Then the process proceeds to step  166 . 
     Next, in step  166 , the delay circuit  128  waits for the state that has been set in step  162  or  164  for 150 msec, and then proceeds to step  168  to report that the connection state of the HPD setting circuit  124  has been changed. 
     The host computer  10  executes the DDC communication control process again if the signal from the HPD setting circuit  124  becomes L signal for equal to or longer than 100 msec. 
     When the connected LAN cable  46  has been disconnected, the HPD setting circuit  124  is shifted from the state (a) to the state (d) in Table 1. In this case, since the output of a false H signal is delayed for  150  msec in which the L signal is maintained, the host computer  10  obtains the timing for executing the DDC communication control process. 
     Also, when the disconnected LAN cable  46  has been connected, the HPD setting circuit  124  is shifted from the state (d) to the state (a). In this case, since the connection is reported to the HPD setting circuit after a delay time of 150 msec, the HPD setting circuit  124  is in an actually non-existing combination state (which does not exist in Table 1) in which the LAN cable  46  is not connected (L signal) and the HPD detection circuit  126  detects the display (H signal). Accordingly, the output of the HPD setting circuit  124  becomes in a non-signal state (equal to the L signal), and after a delay time of 150 msec, the HPD setting circuit  124  is shifted to the state (a) in Table 1 to output a H signal, resulting in that the host computer  10  obtains the timing for executing the DDC communication control process. In this case, in order to cope with the case where the disconnected LAN cable  46  has been connected, a delay circuit may be separately installed between the HPD setting circuit  124  and the HPD detection circuit  126 . 
     In the present exemplary embodiment, a case in which the connection state of the LAN cable  46  is monitored and controlled by circuits has been described. However, the cable connection circuit  118 , the address setting circuit  122 , and the delay circuit  128  are electrical circuits, and thus are not operated by a software program. Note that the connection state monitoring control explained in the flowchart is to clarify the flow of process. 
     By contrast, instead of the circuit operation as described above, the connection state monitoring control of the LAN cable  46  may be executed by a software program under a hardware configuration of a computer including a CPU, a RAM, a ROM, and a bus. 
     In the above present exemplary embodiment, a case in which the delay circuit  128  is installed to cope with the case where the connection state of the LAN cable  46  is changed after power on (after the DDC communication control process is executed) has been described. However, if the configuration has been made such that the connection state of the LAN cable  46  does not change after power ON, the delay circuit  128  may be unnecessary. 
       FIG. 4  is a functional block diagram illustrating an electrical connection in a video signal transmission system without the delay circuit  128  according to an alternative exemplary embodiment of the present invention. Since the difference between the circuits in  FIG. 2  and  FIG. 4  is only the existence of the delay circuit  128 , the same reference numerals are used, and the explanation of the configuration will be omitted. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the present invention and its practical applications, thereby enabling others skilled in the art to understand the present invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the present invention be defined by the following claims and their equivalents.