Abstract:
An image-transmitting-side device comprises: a one-phase to two-phase converter circuit for separating parallel image data, which are to be transmitted, into even and odd data; a first parallel-serial converting circuit; a second parallel-serial converting circuit; means for allowing a user to select, as the resolution mode for the image data to be transmitted, one of a first resolution mode and a second resolution mode that is higher in resolution than the first resolution mode; and switch means for applying the parallel image data, which are to be transmitted, to the first parallel-serial converting circuit when the first resolution mode is selected, and for applying the parallel image data, which are to be transmitted, to the one-phase to two-phase converter circuit when the second resolution mode is selected.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an image signal transmission apparatus for transmitting image signals produced in a personal computer to a display device, such as a liquid crystal projector, plasma display panel (PDP) or the like, which is relatively remote from the personal computer. 
     2. Description of the Prior Art 
     [1] In a case of transmitting image signals produced in a personal computer to a display device via an analog transmission cable, if the analog transmission cable is long, it is apt to cause image deterioration. Such image deterioration will be noticeable especially on the display device having a high resolution of 1024.times.768 pixels (XGA), 1280.times.1024 pixels (SXGA) or the like. 
     Some image signal transmission apparatus have already been developed that cause no image deterioration even when the transmission cables are long. An example of those apparatus is “PanelLink” by Silicon Image, Inc. in the United States, which has been developed based on a signal transmission technology called TMDS (Transition Minimized Differential Signaling). 
     According to this signal transmission technology of TMDS, red, blue and green signals (RGB) and a clock signal are serially transmitted in a differential method. This differential method, which is a method for transmitting a single signal by use of two transmission lines, realizes noise immunity and a stable signal transmission and further achieves a high transmission speed and a long-distance cable transmission. It becomes, however, difficult for this method to provide such transmissions if the resolution of image data is further raised up to an ultrahigh resolution of 1600.times.1200 pixels (UXGA), 2048.times.1536 pixels (QXGA) or the like which causes the transmission cable to reach its own physical limit. As a solution to this problem, there has been proposed “Dual Link Method” by the DDWG (Digital Display Working Group) in its DVI (Digital Visual Interface) specification. Unlike the conventional “PanelLink” (which will be referred to as “Single Link” method in contrast to the “Dual Link” method hereinafter), the “Dual Link” method transmits the R, G, and B signals, by use of not a respective single channel for each signal, that is, three channels but two channels for each signal, that is, six channels, after performing a one-phase to two-phase conversion of each signal. In this way, the “Dual Link” method can ensure a bandwidth that is twice as wide as that of the “Single Link” method, allowing the image transmission of an ultrahigh resolution, such as UXGA, QXGA or the like. Additionally, the one-phase to two-phase conversion of each signal can reduce the required transmission rate and hence realizes the signal transmission through a longer cable. 
     FIGS. 1 and 2 show the structures of signal transmission apparatus employing the “Single Link” and “Dual Link” methods, respectively. 
     In the signal transmission apparatus employing the “Single Link” method, a PanelLink transmitter  301  receives image data represented by parallel signals and performs a parallel-serial conversion of the image data from the parallel signals to serial signals. The image data of serial-converted signals are transmitted through a cable  302  to a PanelLink receiver  303 . The cable  302  comprises three pairs of signal lines for transmitting the image data and one pair of signal lines for transmitting a clock signal. The PanelLink receiver  303  performs a serial-parallel conversion of the received serial signals to the parallel signals. 
     In the signal transmission apparatus employing the “Dual Link” method, the even data of image data represented by parallel signals are applied to a PanelLink transmitter  401 , while the odd data of the image data are applied to a PanelLink transmitter  402 . The PanelLink transmitters  401  and  402  each perform a parallel-serial conversion of the received image data from the parallel signals to serial signals. 
     The even data of the image data that have been converted into the serial signals by the PanelLink transmitter  401  are transmitted through a cable  403  to a PanelLink receiver  404 . The odd data of the image data that have been converted into the serial signals by the PanelLink transmitter  402  are transmitted through the cable  403  to a PanelLink receiver  405 . The cable  403  comprises six pairs of signal lines for transmitting the image data and one pair of signal lines for transmitting a clock signal. The PanelLink receivers  404  and  405  each perform a serial-parallel conversion of the received serial signals to the parallel signals. 
     The object of the present invention is to provide an image signal transmission apparatus that allows the transmission of image signals to be done by use of a transmission method suitable for the resolution of the image signals. 
     [2] If, in the “Dual Link” method of FIG. 2, the RGB image data represented by the parallel signals are not separated into even and odd data but are applied to the PanelLink transmitter  401  as they are, and if the PanelLink receiver  404  only is activated, while the PanelLink transmitter  402  and PanelLink receiver  405  not being activated, then it would correspond to the “Single Link” method of FIG.  1 . 
     It is difficult for the “PanelLink” method to effect the transmission of an ultrahigh resolution, such as UXGA, QXGA or the like, and/or effect a long-distance transmission. Thus, in the case of effecting such an ultrahigh resolution transmission and/or a long-distance transmission, the “Dual Link” method is effective. 
     On the other hand, in the case of transmitting the signal of a low-resolution, such as VGA or the like, the “Single Link” method works to a sufficient degree. In addition, because of an increasing demand for portability of the apparatus on the image transmitting side, such as a personal computer or the like, and that on the image receiving side, such as a liquid crystal projector or the like, the need to reduce the power consumption has become important. Thus, when a low-resolution signal is transmitted or when no long-distance transmission is required, it is more desirable to effect the signal transmission by use of the “Single Link” method. 
     The object of the present invention is to provide an image signal transmission apparatus that allows the transmission of image signals to be done by use of a transmission method suitable for the resolution of the image signals and for the cable length. [3] In a case of transmitting image signals produced in a personal computer to a liquid crystal projector via an analog transmission cable, if the analog transmission cable is long, it is apt to cause image deterioration. Such image deterioration will be noticeable especially on the liquid crystal projector having a high resolution of 1024.times.768 pixels (XGA), 1280.times.1024 pixels (SXGA) or the like. 
     There have already been developed some image signal transmission apparatus that can avoid image deterioration even when the transmission cable is long. An example thereof is “FPDLink” developed by National Semiconductor Corp. in the United States. This image signal transmission apparatus is also called “LVDS.” 
     Another example is “PanelLink” by Silicon Image, Inc. in the United States. 
     FIG. 3 shows the structure of “PanelLink,” which comprises a graphics controller (graphics board)  151  built in a personal computer, a transmitting-side unit  152 , a receiving-side unit  153 , and a cable  154  for connecting the transmitting-side unit  152  and the receiving-side unit  153 . 
     The transmitting-side unit  152  includes an encoding/parallel-serial converting circuit  161 , a PLL circuit  162 , and an amplitude control circuit  163 . The encoding/parallel-serial converting circuit  161  receives image data, DE (i.e., a display enable signal for discriminating between a display mode and a standby mode), and a control signal from the graphics controller  151 . 
     In the encoding/parallel-serial converting circuit  161 , a parallel-serial conversion of the 24-bit parallel image data is performed, whereby the number of signal lines can be reduced and hence the signal transmission can be effected through a thin cable. Then, the signal amplitude is reduced so as to reduce EMI noise. Additionally, the encoding is performed at the time of the parallel-serial conversion. When the encoding is performed, the variation of the level of the signals to be transmitted is reduced so as to further reduce the EMI noise. 
     The PLL circuit  162  generates a clock signal for the encoding/parallel-serial converting circuit  161  on the basis of a clock signal applied from the graphics controller  151 . 
     The cable  154  comprises three pairs of signal lines for transmitting the codes including the image data and the control signal, and one pair of signal lines for transmitting the clock signal generated by the PLL circuit  162 . 
     The amplitude control circuit  163  adjusts, in accordance with the resistance value of a single external resistor  165 , the amplitude of the signals (i.e., the codes including the image data and the control signal, and the clock signal) to be applied from the transmitting-side unit  152  to the cable  154 . Specifically, the signal amplitude can be adjusted within a range between 0.5 V and 2.5 V by establishing the resistance value of the single external resistor  165 . 
     The receiving-side unit  153  includes a data extracting/serial-parallel converting/decoding circuit  181  and a PLL circuit  182 . The data extracting/serial-parallel converting/decoding circuit  181  performs a data extraction, a serial-parallel conversion and a decoding with respect to the codes applied from the transmitting-side unit  152  to produce the image data, DE and the control signal. 
     The PLL circuit  182  produces a clock signal for the data extracting/serial-parallel converting/decoding circuit  181  on the basis of the clock signal applied from the transmitting-side unit  152 . 
     The image data, DE and control signal produced by the data extracting/serial-parallel converting/decoding circuit  181  and the clock signal produced by the PLL circuit  182  are applied to a liquid crystal panel  155  of digital drive type. 
     The “LVDS” method is similar to the “PanelLink” method but different therefrom in that the total number of the signal lines in the cable is five because of performing neither encoding nor decoding. 
     FIG. 4 shows the structure of an image signal transmission apparatus that has already been developed by the Applicant of the subject application (See Japanese Official Gazette of Laid Open Patent Application, TOKUKAI, No. 2000-341177.) In FIG. 4, elements corresponding to the same elements in FIG. 3 are identified by the same reference designations, and the explanation of those elements is omitted. 
     The image signal transmission apparatus of FIG. 4, which utilizes the conventional “PanelLink”, comprises a transmission unit  10  set in a personal computer (which is not shown and which will be referred to as “PC” hereinafter)  1 , a receiving-side unit  153  set in a liquid crystal projector  20 , and a cable  154  for connecting the transmission unit  10  and the receiving-side unit  153 . 
     The transmission unit  10  comprises a graphics controller (graphics board)  151  and a transmitting-side unit  152  connected thereto. The graphics controller  151  is connected to a main CPU  2  in the PC  1  via a bus  3  also therein. The transmitting-side unit  152  in the transmission unit  10  is connected to the receiving-side unit  153  via the cable  154 . 
     The receiving-side unit  153  in the liquid crystal projector  20  is connected to a liquid crystal panel  155  of digital drive type also in the liquid crystal projector  20 . 
     An amplitude control circuit  163  adjusts, in accordance with the resistance value of an external variable resistor circuit  164 , the amplitude of the signals (i.e., the codes including the image data and the control signal, and the clock signal) to be provided from the transmitting-side unit  152  to the cable  154 . The variable resistor circuit  164  can be switched, for example, between two resistance values to thereby switch, between two values, the amplitude of the signals to be provided from the transmitting-side unit  152  to the cable  154 . 
     An application software for setting the cable length has been installed on the PC  1 . When setting the cable length, the user initiates this application software. When this application software is initiated, the PC  1  produces, on its display device, an on-screen selection guide for instructing the user to designate the length of the cable  154  actually being used. The user selects a cable length, following the on-screen selection guide. 
     When the user selects the cable length, the PC  1  sends a command signal (amplitude command signal) responsive to the cable length selected by the user to the graphics controller  151  via the bus  3 . Receiving this command signal, the graphics controller  151  sends an amplitude control signal responsive to the command signal to the variable resistor circuit  164  to change the resistance value thereof. 
     According to the image signal transmission apparatus of FIG. 4, the user can adjust, in accordance with the cable length, the amplitude of the signals to be provided from the transmitting-side unit  152  to the cable  154  by operating the PC  1  in which the graphics controller  151  has been disposed. 
     Meanwhile, in the image signal transmission apparatus of the “LVDS” or “PanelLink” method, if the amplitude of signals to be transmitted is enlarged, it increases the possible distance of transmission by the cable but also increases unwanted radiation signals. In contrast to this, if the amplitude of signals to be transmitted is reduced, it reduces the unwanted radiation signals but also reduces the possible transmission distance. 
     Thus, it is desirable to optimize the signal amplitude according to the cable length. The “LVDS” method, however, has no function to adjust the signal amplitude. As described above, the “PanelLink” method indeed has the function to adjust the signal amplitude by use of the single external resistor  165 . However, since the external resistor  165  is disposed at the stage of design, it is difficult for the user to adjust the signal amplitude according to the cable length. 
     Additionally, in the case of the image signal transmission apparatus of FIG. 4, it was necessary for the user to enter into the PC  1  the amplitude control command for changing the amplitudes of the signals to be provided from the transmitting-side unit to the cable, according to the cable length. 
     It was, therefore, necessary to preinstall the cable length setting application software onto the PC  1 . Additionally, at the time of setting the cable length, it was necessary for the user to initiate this cable length setting application software to cause the on-screen selection guide for setting the cable length to be displayed on the PC  1 , and then select the cable length, following this on-screen selection guide. 
     Moreover, each time changing a cable for another having a different length, the user had to enter an amplitude control command into the computer. 
     The object of the present invention is to provide an image signal transmission apparatus that does not necessitate the user&#39;s setting of the cable length but automatically adjusts the amplitude of signals to be applied from the image transmitting-side unit in response to the level of the received signals that varies with the cable length. 
     SUMMARY OF THE INVENTION 
     A first image signal transmission apparatus according to the present invention performs a parallel-serial conversion of parallel image data by use of an image-transmitting-side device, thereafter transmits the image data to an image-receiving-side device via a cable, and then performs a serial-parallel conversion of the received image data by use of the image-receiving-side device, said image-transmitting-side device comprising: a one-phase to two-phase converter circuit for separating the parallel image data, which are to be transmitted, into even and odd data; a first parallel-serial converting circuit; a second parallel-serial converting circuit; means for allowing a user to select, as the resolution mode for the image data to be transmitted, one of a first resolution mode and a second resolution mode that is higher in resolution than the first resolution mode; and switch means for applying the parallel image data, which are to be transmitted, to the first parallel-serial converting circuit when the first resolution mode is selected, and for applying the parallel image data, which are to be transmitted, to the one-phase to two-phase converter circuit when the second resolution mode is selected; wherein when the second resolution mode is selected, the even data obtained by the one-phase to two-phase converter circuit are applied to one of the first and second parallel-serial converting circuits, and the odd data obtained by the one-phase to two-phase converter circuit are applied to the other parallel-serial converting circuit. 
     A second image signal transmission apparatus according to the present invention performs a parallel-serial conversion of parallel image data by use of an image-transmitting-side device, thereafter transmits the image data to an image-receiving-side device via a cable, and then performs a serial-parallel conversion of the received image data by use of the image-receiving-side device, said image-transmitting-side device comprising: a one-phase to two-phase converter circuit for separating the parallel image data, which are to be transmitted, into even and odd data; a first parallel-serial converting circuit; a second parallel-serial converting circuit; means for automatically determining the resolution of the image data to be transmitted, and then automatically selecting, as the resolution mode for the image data to be transmitted, one of a first resolution mode and a second resolution mode that is higher in resolution than the first resolution mode; and switch means for applying the parallel image data, which are to be transmitted, to the first parallel-serial converting circuit when the first resolution mode is selected, and for applying the parallel image data, which are to be transmitted, to the one-phase to two-phase converter circuit when the second resolution mode is selected; wherein when the second resolution mode is selected, the even data obtained by the one-phase to two-phase converter circuit are applied to one of the first and second parallel-serial converting circuits, and the odd data obtained by the one-phase to two-phase converter circuit are applied to the other parallel-serial converting circuit. 
     In the above-described first or second image signal transmission apparatus, said image-receiving-side device comprises: a first serial-parallel converting circuit for converting the serial data, transmitted from the first parallel-serial converting circuit via the cable, to the parallel data; and a second serial-parallel converting circuit for converting the serial data, transmitted from the second parallel-serial converting circuit via the cable, to the parallel data. 
     The above-described first serial-parallel converting circuit includes means for performing a one-phase to two-phase conversion of the parallel data obtained by the serial-parallel conversion to provide separated even and odd data in the case when the first resolution mode is selected. 
     A third image signal transmission apparatus according to the present invention performs a parallel-serial conversion of parallel image data by use of an image-transmitting-side device, thereafter transmits the image data to an image-receiving-side device via a cable, and then performs a serial-parallel conversion of the received image data by use of the image-receiving-side device, 
     said image-receiving-side device comprising: means for detecting the level of the signals transmitted from the image-transmitting-side device through the cable to the image-receiving-side device and for generating an operation mode control signal for designating a first operation mode when the detected signal level is higher than a predetermined value and for designating a second operation mode when the detected signal level is equal to or lower than the predetermined value; and means for transmitting the operation mode control signal to the image-transmitting-side device, 
     said image-transmitting-side device comprising: a one-phase to two-phase converter circuit for separating the parallel image data, which are to be transmitted, into even and odd data; a first parallel-serial converting circuit; a second parallel-serial converting circuit; and switch means for applying the parallel image data, which are to be transmitted, to the first parallel-serial converting circuit when the operation mode control signal from the image-receiving-side device designates the first operation mode, and for applying the parallel image data, which are to be transmitted, to the one-phase to two-phase converter circuit when the operation mode control signal from the image-receiving-side device designates the second operation mode, 
     wherein when the operation mode control signal from the image-receiving-side device designates the second operation mode, the even data obtained by the one-phase to two-phase converter circuit are applied to one of the first and second parallel-serial converting circuits, and the odd data obtained by the one-phase to two-phase converter circuit are applied to the other parallel-serial converting circuit. 
     In the above-described third image signal transmission apparatus, said image-receiving-side device comprises: a first serial-parallel converting circuit for converting the serial data, transmitted from the first parallel-serial converting circuit via the cable, to the parallel data; and a second serial-parallel converting circuit for converting the serial data, transmitted from the second parallel-serial converting circuit via the cable, to the parallel data. 
     The above-described first serial-parallel converting circuit includes means for performing a one-phase to two-phase conversion of the parallel data obtained by the serial-parallel conversion to provide separated even and odd data in the case when the first operation mode is selected. 
     The above-described means for transmitting the operation mode control signal to the image-transmitting-side device may transmit the operation mode control signal by wire or by wireless. 
     A fourth image signal transmission apparatus according to the present invention comprises comprising an image-transmitting-side device, an image-receiving-side device, and a cable connecting the image-transmitting-side device with the image-receiving-side device, 
     said image-receiving-side device comprising: means for detecting the level of the signals transmitted from the image-transmitting-side device through the cable to the image-receiving-side device and for generating, in accordance with the detected signal level, a control signal for controlling the amplitude of the signals to be applied from the image-transmitting-side device to the cable; and means for transmitting the control signal to the image-transmitting-side device, 
     said image-transmitting-side device having amplitude control means for controlling, in accordance with the control signal from the image-receiving-side device, the amplitude of the signals to be applied from the image-transmitting-side device to the cable. 
     In the above-described fourth image signal transmission apparatus, said amplitude control means may comprise: for example, an amplitude control circuit for changing, in accordance with the resistance value of an external amplitude control resistor, the amplitude of the signals to be applied from the image-transmitting-side device to the cable; a variable resistor circuit disposed, as the amplitude control resistor, externally to the amplitude control circuit; and means for controlling the resistance value of the variable resistor circuit in accordance with the control signal from the image-receiving-side device. 
     The above-described means for transmitting the control signal to the image-transmitting-side device may transmit the control signal by wire or by wireless. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the structure of an image signal transmission apparatus employing the “Single Link” method. 
     FIG. 2 is a block diagram showing the structure of an image signal transmission apparatus employing the “Dual Link” method. 
     FIG. 3 is a block diagram showing the structure of “PanelLink.” 
     FIG. 4 is a block diagram showing the structure of an image transmitting system that has already been developed by the Applicant of the subject application. 
     FIG. 5, showing a first embodiment, is a block diagram showing the structure of a digital image signal transmission apparatus. 
     FIG. 6 is a block diagram showing the data flow in a case when CPU  13  detects that the image signals to be transmitted are of a high resolution (equal to or lower than the resolution of SXGA). 
     FIG. 7 is a block diagram showing the data flow in a case when CPU  13  detects that the image signals to be transmitted are of an ultrahigh resolution (equal to or higher than the resolution of UXGA). 
     FIG. 8, showing a second embodiment, is a block diagram showing the structure of a digital image signal transmission apparatus. 
     FIG. 9 is a block diagram showing the structure of a signal level detection. 
     FIG. 10 is a block diagram showing an example of modification of the second embodiment. 
     FIG. 11, showing a third embodiment, is a block diagram showing the structure of an image transmitting system. 
     FIG. 12 is a block diagram showing the structure of a transmission apparatus. 
     FIG. 13 is a block diagram showing the structure of a signal level detection. 
     FIG. 14 is a block diagram showing an example of modification of the third embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     [1] Description of First Embodiment 
     A first embodiment of the present invention will be described below with reference to FIGS. 5 to  7 . 
     FIG. 5 shows the structure of a digital image signal transmission apparatus. This image signal transmission apparatus comprises a transmission unit  10  set in a personal computer, a liquid crystal projector  20  and a cable  30  connecting them. 
     The transmission unit  10  includes a VGA controller (graphic chip)  11 , a switch circuit  12 , a CPU  13 , a one-phase to two-phase converter circuit  14 , a first PanelLink transmitter  15  and a second PanelLink transmitter  16 . 
     The liquid crystal projector  20  includes a first PanelLink receiver  21 , a second PanelLink receiver  22  and a liquid crystal panel  23  of digital drive type. The first PanelLink receiver  21  incorporates a one-phase to two-phase converter circuit  21   a.    
     The cable  30  comprises six pairs of signal lines for transmitting image data and one pair of signal lines for transmitting a clock signal. 
     The CPU  13  detects which the image signals to be transmitted are of, a high resolution (equal to or lower than the resolution of SXGA) or an ultrahigh resolution (equal to or higher than the resolution of UXGA), on the basis of a control signal (or both of the control signal and image data) applied from the VGA controller  11 . The CPU  13  controls the switch circuit  12  in accordance with the result of this detection. 
     FIG. 6 shows the data flow of a case when the CPU  13  detects that the image signals to be transmitted is of a high resolution (equal to or lower than the resolution of SXGA). 
     Parallel image data (R, G, B) output by the VGA controller  11  are input to the first PanelLink transmitter  15  via the switch circuit  12 . The PanelLink transmitter  15  converts the image data from the parallel signals to serial ones. The resultant serial R, G, and B signals each for a respective channel are transmitted through the cable  30  to the first PanelLink receiver  21 , which converts the received serial signals into the parallel ones. 
     When the second PanelLink receiver  22  receives no image signals, the first PanelLink receiver  21  uses the one-phase to two-phase converter circuit  21   a  to perform a one-phase to two-phase conversion of the obtained parallel signals, thereby obtaining the RGB even and odd data, which are then applied to the liquid crystal panel  23  of digital drive type. 
     The information as to whether or not the second PanelLink receiver  22  receives any image signals is sent therefrom to the first PanelLink receiver  21 . 
     In a case when the second PanelLink receiver  22  does receive the image signals, the first PanelLink receiver  21  applies the obtained parallel signals, as they are, to the liquid crystal panel  23 . 
     FIG. 7 shows the data flow of a case when the CPU  13  detects that the image signals to be transmitted is of an ultrahigh resolution (equal to or higher than the resolution of UXGA). 
     Parallel image data (R, G, B) output by the VGA controller  11  are applied through the switch circuit  12  to the one-phase to two-phase converter circuit  14 , being separated into even and odd data. The even data are applied to the first PanelLink transmitter  15 , while the odd data are applied to the second PanelLink transmitter  16 . 
     The first PanelLink transmitter  15  converts the even data from the parallel signals to serial ones. The second PanelLink transmitter  16  converts the odd data from the parallel signals to serial ones. 
     The serial R, G and B signals, each for two channels, obtained by PanelLink transmitters  15  and  16  are transmitted through the cable  30  to the first and second PanelLink receivers  21  and  22 . 
     The first PanelLink receiver  21  converts the received even data from the serial signals to the parallel signals, while the second PanelLink receiver  22  converts the received odd data from the serial signals to the parallel signals. 
     The parallel signals, RGB even and odd data, thus obtained by the first and second PanelLink receivers  21  and  22  are applied to the liquid crystal panel  23 . 
     In the above-described embodiment, when it is detected that the image data to be transmitted are of a high resolution, the image data are transmitted by use of the “Single Link” method. When it is detected that the image data to be transmitted are of an ultrahigh resolution, the image data are transmitted by use of the “Dual Link” method. 
     In the above-described first embodiment, the CPU  13  detected which the image signals to be transmitted were of, a high resolution (equal to or lower than the resolution of SXGA) or an ultrahigh resolution (equal to or higher than the resolution of UXGA), and the result of this detection was used to control the switch circuit  12 . Instead, it may be arranged that the user selects, according to the resolution of the image signals to be transmitted, the high or ultrahigh resolution and that the switch circuit  12  is controlled based on his selected resolution. 
     [2] Description of Second Embodiment 
     A second embodiment will be described below with reference to FIGS. 8 to  10 . 
     FIG. 8 shows the structure of a digital image signal transmission apparatus. 
     This image signal transmission apparatus comprises a transmission unit  10  set in a personal computer (PC)  1 , a receiving-side unit  105  set in a liquid crystal projector  20 , and a cable  30  connecting the transmission unit  10  and the receiving-side unit  105 . 
     The transmission unit  10  includes a graphics controller (graphic board)  101 , a switch circuit  102 , a one-phase to two-phase converter circuit  103  and a transmitting-side unit  104 . The graphics controller  101  is connected to a main CPU  2  in the PC  1  via a bus  3  also therein. The main CPU  2  is connected to a line receiver  146 . The transmitting-side unit  104  includes first and second PanelLink transmitters  111  and  112 , respectively. 
     The receiving-side unit  105  in the liquid crystal projector  20  is connected to a liquid crystal panel  106  of digital drive type in the liquid crystal projector  20 . The receiving-side unit  105  includes a first PanelLink receiver  131 , a second PanelLink receiver  132  and a coupler  141 . The first PanelLink receiver  131  incorporates a one-phase to two-phase converter circuit  131   a . The liquid crystal projector  20  also includes a detector circuit  142 , an A/D converter  143 , a CPU  144  and a line driver  145 . 
     The transmitting-side unit  104  in the transmission unit  10  is connected through the cable  30  to the receiving-side unit  105  in the liquid crystal projector  20 . The cable  30  comprises six pairs of signal lines for transmitting the image data and one pair of signal lines for transmitting the clock signal. 
     This image signal transmission apparatus has, as its operation modes, a DualLink mode for performing the signal transmission by use of the Dual Link method, and a SingleLink mode for performing the signal transmission by use of the Single Link method. 
     When the SingleLink mode is selected as the operation mode, the parallel image data (R, G, B), the clock signal and the control signals (H, V, DE (Display Enable)) from the graphics controller  101  are applied through the switch circuit  102  directly to the first PanelLink transmitter  111  without being applied to the one-phase to two-phase converter circuit  103 . In that case, the second PanelLink transmitter  112  is inactive in a power down mode. 
     The first PanelLink transmitter  111  encodes the image data and clock signal, and performs a parallel-serial conversion that converts the image data from the parallel signals to serial ones. The thus obtained serial R, G and B signals each for a respective channel are transmitted through the cable  30  to the first PanelLink receiver  131  in the receiving-side unit  105 . In that case, the second PanelLink receiver  132  is inactive, entering the power down mode. The first PanelLink receiver  131  performs a data extraction, a serial-parallel conversion and a decoding with respect to the codes applied from the first PanelLink transmitter  111  in the transmitting-side unit  104 , thereby producing the parallel image data, H, V and DE. 
     When the second PanelLink receiver  132  receives no image signals, the first PanelLink receiver  131  uses the one-phase to two-phase converter circuit  131   a  to perform a one-phase to two-phase conversion of the produced parallel image signals, and then applies the thus obtained RGB even and odd data to the liquid crystal panel  106 . The first PanelLink receiver  131  also uses the one-phase to two-phase converter circuit  131   a  to perform one-half frequency divisions of the produced H, V and DE and of the received clock signal, and then applies them to the liquid crystal panel  106 . 
     The information as to whether or not the second PanelLink receiver  132  receives any image signals is sent therefrom to the first PanelLink receiver  131 . 
     In a case when the second PanelLink receiver  132  does receive the image signals, the first PanelLink receiver  131  applies its obtained parallel signals, as they are, to the liquid crystal panel  106 . 
     When the DualLink mode is selected as the operation mode, the parallel image data (R, G, B) output by the graphics controller  101  are applied through the switch circuit  102  to the one-phase to two-phase converter circuit  103 , being separated thereby into even and odd data. The even data are applied to the first PanelLink transmitter  111  in the transmitting-side unit  104 , while the odd data are applied to the second PanelLink transmitter  112  also in the transmitting-side unit  104 . 
     In the meantime, the clock signal and control signals (H, V, DE (Display Enable)) output by the graphics controller  101  are applied to the one-phase to two-phase converter circuit  103 , being one-half frequency divided thereby, and then being applied to the first and second PanelLink transmitters  111  and  112  in the transmitting-side unit  104 . 
     The first PanelLink transmitter  111  encodes the even data and clock signal, and performs a parallel-serial conversion that converts the even data from the parallel signals to serial ones. The second PanelLink transmitter  112  encodes the odd data and clock signal, and performs a parallel-serial conversion that converts the odd data from the parallel signals to serial ones. 
     The R, G and B serial signals each for two channels, obtained by the first and second PanelLink transmitters  111  and  112 , are transmitted through the cable  30  to the first and second PanelLink receivers  131  and  132  in the receiving-side unit  105 . 
     The first PanelLink receiver  131  performs a data extraction, a serial-parallel conversion and a decoding with respect to the codes applied from the first PanelLink transmitter  111 , thereby producing the parallel signals with respect to the even data and also producing H, V and DE. The second PanelLink receiver  132  performs a data extraction, a serial-parallel conversion and a decoding with respect to the codes applied from the second PanelLink transmitter  112 , thereby producing the parallel signals with respect to the odd data. 
     The parallel signals (RGB even and odd data) obtained by the first and second PanelLink receivers  131  and  132  are applied to the liquid crystal panel  106 . The control signals (H, V and DE) produced by the first PanelLink receiver  131  and the clock signal received thereby are also applied to the liquid crystal panel  106 . 
     Incidentally, the three pairs of image data of the R, G and B signals and one pair of clock signals each are transmitted as a pair of differential signals from the transmitting-side unit  104  to the receiving-side unit  105 , and hence are almost at the same signal level. The transmission speed of the digital image data (the serial image data) along the cable  30  is, for example in the case of UXGA, 1.65 Gbps in the SingleLink mode, and half the same, 825 Mbps, in the DualLink mode. 
     In this embodiment, the coupler  141  is located on one of two signal lines for the pair of differential signals, CLK+ and CLK−, received as the clock signal, specifically, on the signal line of signal CLK−, as shown in FIG. 9, and its coupling output is applied to the detector circuit  142 . The CLK+ and CLK−signal lines each exhibit a characteristic impedance of 50 ohms at the single end, and a terminating resistor  141   a  of the coupler  141  exhibits, for example, 50 ohms. 
     The detector circuit  142  converts the CLK−coupling output signal into an analog signal of a DC voltage proportional to the amplitude level of the CLK−coupling output signal. The detector circuit  142  may comprise, for example, an IC of AD8313 available from Analog Devices Inc. 
     The detection signal output by the detection circuit  142  is converted into a digital signal by the A/D converter circuit  143  and then applied to an input port of the CPU  144 . The CPU  144  compares the level of the received CLK−signal with a predetermined threshold value and provides a control signal for switching the operation mode (from the SingleLink mode to the DualLink mode or vice versa). This control signal is transmitted through the line driver  145  to the line receiver  146  in the PC  1  on the transmitting side as a feedback signal. 
     As to the transmission of the control signal responsive to the level of the received signals to the line receiver  146  in the PC  1 , in a case of using, for example, a 24-pin connector of Digital Visual Interface (DVI) standard specified as a digital interface between PCs and liquid crystal projectors or the like in the United States, its unused pin terminal  8  (NC) can be utilized, without any additional wiring, to effect a serial transmission of the one-bit control signal. Instead, however, an additional signal wiring may be used as the interface for transmitting the control signal. 
     The control signal received by the line receiver  146  in the PC  1  is applied to the main CPU  2 . On the basis of the control signal applied to the main CPU 2 , the PC  1  sends an operation mode switch signal for switching between the SingleLink mode and the DualLink mode to the graphics controller  101  via the bus  3 . When receiving the operation mode switch signal, the graphics controller  101  sends a control signal responsive to this operation mode switch signal to the switch circuit  102 . 
     In this embodiment, the initial operation mode of the transmission unit  10  has been set to the SingleLink mode. When the transmission of image data to the liquid crystal projector  20  is started for the first time after turn-on of the transmission unit  10 , it is decided which mode should be used as the operation mode. 
     In a case of using a short cable of three meters or less as the cable  30 , even when the image signals to be transmitted are of a high resolution of UXGA that exhibits a transmission speed of 1.65 Gbps in the Single Link method, the attenuation amount of the signals transmitted through the cable  30  is less than five or six dB. Thus, the SingleLink mode can be used to provide a sufficient C/N ratio to reproduce the received signals without any errors. 
     In such a case, the amplitude of a CLK−coupling output signal developed by the coupler  141  is large, and the amplitude level of a signal output by the detector circuit  142  is high. Thus, the CPU  144  develops a control signal for selecting the SingleLink mode as the operation mode. This control signal is conveyed through the line driver  145  to the transmitting side. When receiving this control signal, the main CPU  2  in the PC  1  selects the SingleLink mode as the operation mode and applies an operation mode switch signal responsive to this mode selection to the graphics controller  101 . The graphics controller  101  then applies a switch control signal for the SingleLink mode to the switch circuit  102 . As a result, the initial operation mode (SingleLink mode) remains unchanged. 
     In a case of using a long cable of ten meters or more as the cable  30 , when the image signals to be transmitted are of a high resolution of UXGA that exhibits a transmission speed of 1.65 Gbps in the Single Link method, the attenuation amount of the signals transmitted through the cable  30  is 20 dB or so. Thus, the SingleLink mode cannot be used to provide any sufficient C/N ratio to reproduce the received signals to a normal degree. 
     In such a case, the amplitude of a CLK−coupling output signal developed by the coupler  141  is small, and the amplitude level of a signal output by the detector circuit  142  is low. Thus, the CPU  144  develops a control signal for selecting the DualLink mode as the operation mode. This control signal is conveyed through the line driver  145  to the transmitting side. When receiving this control signal, the main CPU  2  in the PC  1  selects the DualLink mode as the operation mode and applies an operation mode switch signal responsive to this mode selection to the graphics controller  101 . The graphics controller  101  then applies a switch control signal for the DualLink mode to the switch circuit  102 . As a result, the initial operation mode (SingleLink mode) is switched to the DualLink mode. 
     When the operation mode is thus switched to the DualLink mode, the transmission speed of the signals through the cable  30  is reduced to 825 Mbps, and the attenuation amount of the signals through the cable  30  is reduced to, for example, 10 dB or so, resulting in a sufficient C/N ratio to reproduce the received signals. 
     In a case of using a cable of five meters as the cable  30 , it is possible for the Single Link method to transmit UXGA resolution signals. However, when QXGA resolution signals, the transmission speed of which is higher, are transmitted, the amplitude of a CLK−coupling output signal developed by the coupler  141  is small and hence the amplitude level of a signal output by the detector circuit  142  is low. Consequently, the operation mode is automatically switched from the SingleLink mode to the DualLink mode in the same manner as stated above, resulting in a reproduction of the received signals without any errors. 
     According to the second embodiment described above, an appropriate transmission method, either the Single Link method or the Dual Link method, can automatically be selected, as the method for transmitting the signals from the transmitting-side unit, in accordance with the resolution of the image data to be transmitted and the length of the cable actually used. 
     In the second embodiment described above, the coupler  141  was located on the CLK− signal line, but it may be located on the CLK+signal line, or on any one of the other RXR+, RXR−, RXG+, RXG−, RXB+, and RXB−image signal lines. 
     Additionally, as shown in FIG. 10, the transmission of the control signal from the liquid crystal projector  20  on the receiving side to the PC  1  on the transmitting side may be effected by use of a wireless interface such that the control signal output by the CPU  144  is transmitted from a wireless transmitter  147  and received by a wireless receiver  148 . 
     [3] Description of Third Embodiment 
     A third embodiment will be described blow with reference to FIGS. 11 to  14 . 
     FIG. 11 shows the structure of an image signal transmission apparatus. In FIG. 11, elements corresponding to the same elements in FIGS. 3 and 4 are identified by the same reference designations. 
     This image signal transmission apparatus comprises a transmission unit  10  set in a personal computer PC  1 , a receiving-side unit  153  set in a liquid crystal projector  20 , and a cable  154  connecting the transmission unit  10  and the receiving-side unit  153 . 
     The transmission unit  10  comprises a graphics controller (graphics board)  151  and a transmitting-side unit  152  connected thereto. The graphics controller  151  is connected to a main CPU  2  in the PC  1  via a bus  3  also therein. The transmitting-side unit  152  in the transmission unit  10  is connected to the receiving-side unit  153  via the cable  154 . The main CPU  2  is connected to a line receiver  196 . 
     The receiving-side unit  153  in the liquid crystal projector  20  is connected to a liquid crystal panel  155  of digital drive type in the liquid crystal projector  20 . The liquid crystal projector  20  also includes a detector circuit  192 , an A/D converter  193 , a CPU  194  and a line driver  195 . 
     The transmitting-side unit  152  includes an encoding/parallel-serial converting circuit  161 , a PLL circuit  162 , and an amplitude control circuit  163 . The encoding/parallel-serial converting circuit  161  receives image data, DE (a display enable signal) and a control signal from the graphics controller  151 . In the encoding/parallel-serial converting circuit  161 , a parallel-serial conversion of the 24-bit parallel image data is performed. Then, the signal amplitude is reduced so as to effect a reduction of EMI noise. Additionally, an encoding is performed at the time of the parallel-serial conversion. When this encoding is performed, the variation of the level of the signals to be transmitted is reduced so as to further reduce the EMI noise. 
     The PLL circuit  162  generates a clock signal for the encoding/parallel-serial converting circuit  161  on the basis of a clock signal applied from the graphics controller  151 . 
     The cable  154  comprises three pairs of signal lines for transmitting the codes including both the image data and the control signal, and one pair of signal lines for transmitting the clock signal generated by the PLL circuit  162 . 
     The amplitude control circuit  163  adjusts, in accordance with the resistance value of an external variable resistor circuit  164 , the amplitude of the signals (i.e., the codes including both the image data and the control signal, and the clock signal) to be applied from the transmitting-side unit  152  to the cable  154 . As shown in FIG. 12, the variable resistor circuit  164  comprises a parallel combination of a series circuit of a first resistor  201  and a first switch  205 , a series circuit of a second resistor  202  and a second switch  206 , a series circuit of a third resistor  203  and a third switch  207 , and a series circuit of a fourth resistor  204  and a fourth switch  208 . 
     For example, the resistance value of the first resistor  201  is 820 ohms, that of the second resistor  202  is 620 ohms, that of the third resistor  203  is 390 ohms, and that of the fourth resistor  204  is 180 ohms. 
     The switches  205  to  208  are controlled by a 2-bit amplitude control signal (00, 01, 10, 11). When the amplitude control signal is, for example, “00”, the first switch  205  only is turned on, the other switches  206 ,  207  and  208  being turned off. In that case, the variable resistor circuit  164  exhibits the resistance value of 820 ohms. 
     When the amplitude control signal is, for example, “01”, the second switch  206  only is turned on, the other switches  205 ,  207  and  208  being turned off. In that case, the variable resistor circuit  164  exhibits the resistance value of 620 ohms. 
     When the amplitude control signal is, for example, “10”, the third switch  207  only is turned on, the other switches  205 ,  206  and  208  being turned off. In that case, the variable resistor circuit  164  exhibits the resistance value of 390 ohms. 
     When the amplitude control signal is, for example, “11”, the fourth switch  208  only is turned on, the other switches  205 ,  206  and  207  being turned off. In that case, the variable resistor circuit  164  exhibits the resistance value of 180 ohms. 
     In this way, the resistance value of the variable resistor circuit  164  can be switched among the four values. In other words, the on/off control of the switches  205 ,  206 ,  207  and  208  can switch, among four values, the amplitude of the signals to be applied from the transmitting-side unit  152  to the cable  154 . 
     The receiving-side unit  153  includes a data extracting/serial-parallel converting/decoding circuit  181 , a PLL circuit  182  and a coupler  191 . The data extracting/serial-parallel converting/decoding circuit  181  performs a data extraction, a serial-parallel conversion and a decoding with respect to the codes applied from the transmitting-side unit  152  to produce the image data, DE and the control signal. 
     The PLL circuit  182  produces a clock signal for the data extracting/serial-parallel converting/decoding circuit  181  on the basis of the clock signal applied from the transmitting-side unit  152 . The image data, DE and control signal produced by the data extracting/serial-parallel converting/decoding circuit  181  and the clock signal produced by the PLL circuit  182  are applied to the liquid crystal panel  155 . 
     The three pairs of image data of R, G and B signals each pair are transmitted as a pair of differential signals from the transmitting-side unit  152  to the receiving-side unit  153 , and hence are almost at the same signal level. The transmission speed of the digital image data (the serial image data) along the cable is, for example in a case of SXGA signals, 1.08 Gbps. 
     In this embodiment, the coupler  191  is located on one of two signal lines for the pair of differential signals, RXR+and RXR−, received as the R signal, specifically, on the line for the signal RXR−, as shown in FIG. 13, and its coupling output is applied to the detector circuit  192 . The RXR+and RXR−signal lines each exhibit a characteristic impedance of 50 ohms at the single end, and a terminating resistor  501  of the coupler  191  exhibits, for example, 50 ohms. 
     The detector circuit  192  converts the RXR−coupling output signal into an analog signal of a DC voltage proportional to the amplitude level of the RXR−coupling output signal. The detector circuit  192  may comprise, for example, an IC of AD8313 available from Analog Devices Inc. 
     A detection signal output by the detection circuit  192  is converted into a digital signal by the AID converter circuit  193  and then applied to an input port of the CPU  194 . The CPU  194  provides, in response to the level of the RXR−received signal, a control signal to be conveyed so as to change the level of the signals to be transmitted from the transmitting side. This control signal is conveyed through the line driver  195  to the line receiver  196  in the PC  1  on the transmitting side as a feedback signal. 
     As to the transmission of the control signal responsive to the level of the received signals to the line receiver  196  in the PC  1 , in a case of using, for example, a 24-pin connector of Digital Visual Interface (DVI) standard specified as a digital interface between PCs and liquid crystal projectors or the like in the United States, its unused pin terminal  8  (NC) can be utilized, without any additional wiring, to effect a serial transmission of the one-bit control signal. Instead, however, an additional signal wiring may be used as the interface for transmitting the control signal. 
     The control signal received by the line receiver  196  in the PC  1  is applied to the main CPU  2 . On the basis of the control signal applied to the main CPU  2 , the PC  1  sends a command signal (amplitude command signal) to the graphics controller  151  via the bus  3 . When receiving the command signal, the graphics controller  151  applies an amplitude control signal responsive to this command signal to the variable resistor circuit  164 . 
     In a case of using a short cable of one meter or less as the cable  154 , the amplitude of a RXR−coupling output signal developed by the coupler  191  is large, and the amplitude level of the signal output by the detector circuit  192  is high. Thus, the aforementioned control signal, developed by the CPU  194 , to be conveyed so as to change the level of the signals to be transmitted from the transmitting side is conveyed through the line driver  195  to the transmitting side so as to reduce the amplitude level of the signals to be transmitted from the transmitting side. 
     On the basis of the control signal conveyed from the receiving side, the PC  1  applies a command signal (amplitude command signal) through the bus  3  to the graphics controller  151 , which then provides a 2-bit amplitude control signal of “00”. In that case, the switch  205  is in the on-state, while the other switches  206  to  208  being in the off-state. Thus, the variable resistor circuit  164  exhibits the largest one of the four resistance values, 820 ohms, so that the amplitude of the signals to be applied from the transmitting-side unit  152  to the cable  154  becomes the smallest one of the four amplitude values. 
     In a case of using a long cable of ten meters or more as the cable  154 , the amplitude of a RXR− coupling output signal developed by the coupler  191  is small, and the amplitude level of a signal output by the detector circuit  192  is low. Thus, the aforementioned control signal, developed by the CPU  194 , to be conveyed so as to change the level of the signals to be transmitted from the transmitting side is conveyed through the line driver  195  to the transmitting side so as to raise the amplitude level of the signals to be transmitted from the transmitting side. 
     On the basis of the control signal conveyed from the receiving side, the PC  1  applies a command signal (amplitude command signal) through the bus  3  to the graphics controller  151 , which then provides a 2-bit amplitude control signal of “11”. In that case, the switch  208  is in the on-state, while the other switches  205  to  207  being in the off-state. Thus, the variable resistor circuit  164  exhibits the smallest one of the four resistance values, 180 ohms, so that the amplitude of the signals to be applied from the transmitting-side unit  152  to the cable  154  is the largest one of the four amplitude values. 
     According to the third embodiment described above, the PC  1 , in which the graphics controller  151  has been set, can automatically adjust the amplitude of the signals to be applied from the transmitting-side unit  152  to the cable  154  so that the amplitude level of the image data will be appropriate on the receiving side. 
     The third embodiment was described above with respect to the case of adjusting, among the four values, the amplitude of the signals to be applied from the transmitting-side unit  152  to the cable  154 . The present invention, however, is not limited only to this number of signal amplitude values but may be applied to any other number of signal amplitude values. 
     In the third embodiment described above, the coupler  191  was located on the RXR−signal line, but it may be located on the RXR+ signal line, or on any one of the other RXG+, RXG−, RXB+, and RXB− image signal lines. 
     Additionally, as shown in FIG. 14, the transmission of the control signal from the liquid crystal projector  20  on the receiving side to the PC  1  on the transmitting side may be effected by use of a wireless interface such that the control signal output by the CPU  194  is transmitted from a wireless transmitter  197  and received by a wireless receiver  198 .