Abstract:
A digital signal transmission apparatus that transmits a digital signal to another apparatus via a coaxial cable connected therebetween includes a signal divider configured to divide a digital signal to be transmitted into a plurality of signal groups; a plurality of modulators configured to modulate each of the plurality of signal groups obtained by the signal divider by using an orthogonal frequency division multiplexing method; a plurality of frequency shifters configured to shift each of the frequency bands of a plurality of modulation signals generated by the plurality of modulators from a baseband to a mutually different frequency band; and a signal sender configured to combine a plurality of modulation signals whose frequency bands are shifted by the plurality of frequency shifters and to transmit the signals to the coaxial cable.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2006-1207341 filed in the Japanese Patent Office on Apr. 25, 2006, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a digital signal transmission apparatus and a method for transmitting a digital signal, which are suitably used to transmit a video signal between, for example, a camera and a camera control unit (CCU), to a digital signal receiving apparatus and a method for receiving a digital signal, and to a digital signal transmission system. 
     2. Description of the Related Art 
     In the related art, when a plurality of cameras are used, for example, in drama shooting in a studio or in shooting a baseball game, a plurality of cameras are placed at different places in order to capture each necessary scene. The plurality of cameras are usually centrally controlled at one place. 
     Each camera is connected to an individual camera control unit (CCU) that controls the camera and that supplies electrical power to the camera. The reason for this is that, if a power supply is secured at one place, it is possible to supply electrical power to each camera. Another reason is that centralized control is more convenient because an input to a switcher that receives signals from a plurality of CCUs and that selectively outputs a predetermined signal becomes easier by applying synchronization without phase difference to individual cameras. 
     A video signal is transmitted mutually between a camera and a CCU. That is, a captured-image video signal is sent as a main line signal from the camera to the CCU, and conversely, a return video signal for confirming a final main line video that is recorded by a cameraman or that is on-air is sent from the CCU to the camera. 
     In the related art, for bidirectional video signal transmission between a camera and a CCU, when broadly classified, there are three types of method described below. 
     (1) There is bidirectional video signal transmission that involves transmission using an optical fiber cable. Since video transmission between a camera and a CCU is bidirectional transmission, it is common practice that two optical fiber cables, one for the upstream and one for the downstream, are used to perform digital transmission. That is, one optical fiber cable is used to send a main line signal from the camera to the CCU, and the other optical fiber cable is used to send a return video signal from the CCU to the camera. 
       FIG. 6  shows a transmission system  200 A using an optical fiber cable. The transmission system  200 A is configured in such a manner that a camera  201  and a CCU  202  are connected to each other with an optical fiber cable  203 . A remote controller  204  and monitors  205  to  207  are connected to the CCU  202 . 
     (2) There is bidirectional video signal transmission that involves transmission using a type of coaxial cable called a triaxial cable. This is a method in which a double-shielded wire of a triaxial cable is used, signals in both directions, including that for the power supply, are modulated, and frequency multiplexing transmission is performed. A transmission system using a triaxial cable in this manner is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 1998-341357. 
       FIG. 7  shows an analog transmission system  200 B using a triaxial cable. The transmission system  200 B is configured in such a manner that a camera  201  and a CCU  202  are connected to each other with a triaxial cable  208 . 
     In this case, a digital video signal as a main line signal that is output from the camera  201  or a return video signal output from the CCU  202  are each converted into an analog video signal by using a D/A converter, thereafter FM modulation or AM modulation is performed thereon, and bidirectional transmission is performed by means of a frequency multiplexing method using a multiplex filter. On the reception side, the analog video signal that is obtained after FM demodulation or AM demodulation is performed once thereon is A/D-converted into a digital video signal and is output. 
       FIG. 8  shows an example of the circuit configuration on the camera  201  side and on the CCU  202  side. On the camera  201  side, digital video signals as main line signals, that is, a luminance signal Y that is 10-bit parallel data and color difference signals Cb/Cr that are 10-bit parallel data, are each converted from a digital signal into an analog signal by D/A converters  210  and  211 . Thereafter, the signals are AM-modulated by AM modulators  212  and  213 , and the signals are combined by a multiplex filter  214  and are sent to a triaxial cable  208 . The color difference signals Cb/Cr indicate sequence signals of a blue color difference signal Cb and a red color difference signal Cr. 
     In connection with the above, on the CCU  202  side, the AM-modulated signals of the luminance signal Y and the color difference signals Cb/Cr are separated and extracted from the triaxial cable  208  by a multiplex filter  215 . Thereafter, the AM-modulated signals are demodulated by AM demodulators  216  and  217 , the AM-modulated signals are converted from analog signals into digital signals by A/D converters  218  and  219  in order to obtain a luminance signal Y and color difference signals Cb/Cr, which are 10-bit parallel data. 
     On the CCU  202  side, a return video signal Ret. Video (luminance signal Y) that is 10-bit parallel data is converted from a digital signal into an analog signal by a D/A converter  220 . Thereafter, the signal is FM-modulated by an FM modulator  221 , and the signal is sent to the triaxial cable  208  through the multiplex filter  215 . 
     In connection with the above, on the camera  201  side, an FM-modulated signal of the return video signal Ret. Video is extracted from the triaxial cable  208  by the multiplex filter  214 . Thereafter, the FM-modulated signal is demodulated by an FM demodulator  222 , the signal is converted from an analog signal into a digital signal by the conversion, and a return video signal Ret. Video (Y) is obtained by an A/D converter  223 . 
       FIG. 9  shows an example of the arrangement of frequencies of an analog transmission system using a triaxial cable. The center frequency of the AM-modulated signals (the color difference signals Cb/Cr) transmitted from the camera  201  to the CCU  202  is set at 21.6 MHz. The center frequency of the AM-modulated signal (the luminance signal Y) transmitted from the camera  201  to the CCU  202  is set at 64.8 MHz. The center frequency of the FM-modulated signal (the luminance signal Y) transmitted from the CCU  202  to the camera  201  is set at 126 MH. As a result, the frequency bands are made different. 
     Although not shown in the example of the circuit configuration of  FIG. 8 , an audio signal and a control signal, which are frequency-multiplexed, are transmitted between the camera  201  and the CCU  202 .  FIG. 9  also shows an audio signal SA 1  transmitted from the camera  201  to the CCU  202 , and an audio signal SA 2  transmitted from the CCU  202  to the camera  201 . 
     (3) As a system for bidirectionally transmitting digital video by using a triaxial cable, there is a transmission system using a time-domain multiplexing method. That is, a time period is divided so that a time period during which a main-line video signal is sent and a time period during which a return video signal is sent are alternately determined, and by repeating this operation, bidirectional transmission is performed. In this system, management of time is difficult. Furthermore, as the transmission distance becomes longer, the wait time for the sent video signal to be reached becomes longer, and the time period for sending a video signal becomes shorter. 
     SUMMARY OF THE INVENTION 
     In the case of (1), the optical fiber cable enables a signal of a very wide band to be transmitted and also, since transmission loss in optical power is small, long distance transmission is easily possible. For this reason, this transmission is in dominant use in the case of an HD (High-Definition) camera system. However, since optical fiber cables are expensive, their use outdoors where there is much dust and dirt is a matter of concern, and cleaning the end surfaces of connectors is troublesome. Another problem is that, if optical fiber cables are handled roughly, they can be broken, and once broken, repairs are difficult. The bidirectional transmission method which employs one optical fiber cable and uses wavelength multiplexing is expensive because the divider/combiner necessary for wavelength multiplexing is expensive and a laser diode whose wavelength is managed is necessary and is also expensive. 
     In the case of (2), a digital video signal output from the camera or the CCU is returned once to an analog signal and the signal is returned again to a digital video signal on the output side and therefore, it is difficult to avoid the deterioration of the signal. Furthermore, since FM modulation/demodulation or AM modulation/demodulation of an analog signal is performed for the transmission, a deterioration in S/N also occurs. Furthermore, as the transmission distance becomes longer, since the C/N of the transmission line decreases, the S/N of a video becomes poor. 
     In the case of (3), management of time is difficult, and as the transmission distance becomes longer, the waiting time for the sent video signal to be reached becomes shorter, presenting a problem in that it is difficult to transmit a signal of a wide band like an HD video signal. 
     It is desirable to enable a digital signal to be satisfactorily transmitted through a coaxial cable. 
     According to an embodiment of the present invention, there is provided a digital signal transmission apparatus that transmits a digital signal to another apparatus via a coaxial cable connected therebetween, the digital signal transmission apparatus including: a signal divider configured to divide a digital signal to be transmitted into a plurality of signal groups; a plurality of modulators configured to modulate each of the plurality of signal groups obtained by the signal divider by using an orthogonal frequency division multiplexing method; a plurality of frequency shifters configured to shift each of the frequency bands of a plurality of modulation signals generated by the plurality of modulators from a baseband to a mutually different frequency band; and a signal sender configured to combine a plurality of modulation signals whose frequency bands are shifted by the plurality of frequency shifters and to transmit the signals to the coaxial cable. 
     In an embodiment of the present invention, a digital signal to be transmitted is divided into a plurality of signal groups, and the signal groups are modulated in accordance with an orthogonal frequency division multiplexing method. Even when the bit rate of a digital signal to be transmitted is high, the bit rate of each of the signal groups obtained by the division becomes low. Therefore, digital modulation for each signal group becomes possible. In this case, a digital signal may also be divided into a plurality of signal groups after a data compression process is performed on the digital signal to be transmitted, and in this case, the bit rate can be decreased further. 
     For example, when digital video signals (e.g., a luminance signal Y and color difference signals Cb/Cr) as main line signals (captured-image video signals) are to be sent from the camera to the CCU via a coaxial cable, the digital video signals as the main line signals become digital signals to be transmitted. Furthermore, for example, when a digital video signal (for example, a luminance signal Y) as a return video signal is to be sent from the CCU to the camera via a coaxial cable, the digital video signal as a return video signal becomes a digital signal to be transmitted. 
     A plurality of modulation signals related to a plurality of signal groups are combined after the frequency bands of the modulation signals are each shifted from the baseband to a mutually different frequency band and are sent to a coaxial cable. Since the modulation signals are shifted so that the frequency bands of the plurality of modulation signals related to the plurality of signal groups are mutually different, it is possible to satisfactorily transmit them in such a manner that the plurality of signal groups do not interfere with one another. In this case, when a plurality of modulation signals are similarly sent from a transmission location, the plurality of modulation signals are arranged so that the above-described frequency bands of the plurality of modulation signals do not overlap the frequency bands of the plurality of modulation signals from the transmission location. As a result, bidirectional transmission via one coaxial cable becomes possible. 
     As described above, a plurality of modulation signals related to a plurality of signal groups sent to a coaxial cable are supplied to another apparatus via the coaxial cable. In the other apparatus, first, the plurality of modulation signals are separated and obtained from the coaxial cable. After the frequency bands of the plurality of the obtained modulation signals are returned to a baseband, the modulation signals are demodulated, and a plurality of signal groups are generated. Then, the plurality of signal groups are combined to obtain a digital signal to be received. 
     The plurality of signal groups obtained by dividing a digital signal are sent to a coaxial cable after each of them is modulated by an orthogonal frequency division multiplexing method, and the symbol rate can be decreased so as to be lower than the bit rate of the digital signal. Therefore, even if reflection occurs in the connector portion or the like, the influence thereof is not easily received, and the digital signal can be satisfactorily transmitted via the coaxial cable. Furthermore, the digital signal is not converted into an analog signal, AM modulation or FM modulation is not performed thereon, and is not transmitted via the coaxial cable. Consequently, the deterioration in S/N does not occur, and the digital signal can be transmitted at a distance longer than that in an analog transmission system. 
     According to embodiments of the present invention, after a digital signal is divided into a plurality of signal groups, each of the signal groups is modulated by an orthogonal frequency division multiplexing method and is sent to a coaxial cable. It becomes possible to modulate even a digital signal with a high bit rate by an orthogonal frequency division multiplexing method. Furthermore, as a result of the modulation, the symbol rate can be decreased so as to be lower than the bit rate of the digital signal. Therefore, even if reflection occurs in the connector portion or the like, the influence thereof is not easily received, and the digital signal can be satisfactorily transmitted through the coaxial cable. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the configuration of a transmission system using a triaxial cable according to an embodiment of the present invention; 
         FIG. 2  shows an example of the arrangement of frequencies of modulation signals related to main line signals and a return video signal, which are transmitted via a triaxial cable; 
         FIG. 3  is a block diagram showing a detailed configuration on the camera side; 
         FIG. 4  is a block diagram showing the configuration of a modulation processor for performing a modulation process by an OFDM method; 
         FIG. 5  is a block diagram showing a detailed configuration of a conversion unit on the CCU side; 
         FIG. 6  is a block diagram showing the configuration of a transmission system using an optical fiber cable in the related art; 
         FIG. 7  is a block diagram showing the configuration of an analog transmission system using a triaxial cable in the related art; 
         FIG. 8  is a block diagram showing an example of a circuit configuration on the camera side and on the CCU side in an analog transmission system using a triaxial cable in the related art; and 
         FIG. 9  shows an example of the arrangement of frequencies of an analog transmission system using a triaxial cable in the related art. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will be described below with reference to the drawings.  FIG. 1  shows the configuration of a transmission system  100  using a triaxial cable according to an embodiment of the present invention. The transmission system  100  includes a camera  101 , a conversion unit  102 , a triaxial cable  103  as a coaxial cable, a conversion unit  104 , and a CCU  105 . 
     The camera  101  captures an image of a subject. A captured-image video signal is transmitted as a main line signal from the camera  101  to the CCU  105 . The CCU  105  controls the camera  101  and also supplies electrical power to the camera  101 . A return video signal is transmitted from the CCU  105  to the camera  101 . A remote controller  106  and monitors  107  to  109  are connected to the CCU  105 . 
     The conversion unit  102  is provided between the camera  101  and the triaxial cable  103 . The conversion unit  102  divides a luminance signal Y and color difference signals Cb/Cr, which are digital video signals constituting the main line signals output from the camera  101 , into a plurality of signal groups, modulates each of the signal groups by an orthogonal frequency division multiplexing (OFDM) method, shifts the frequency band of each of the modulation signals from the baseband to a mutually different frequency band, and sends the signals to the triaxial cable  103 . 
     Furthermore, the conversion unit  102  demodulates the signal groups by shifting the frequency bands of the plurality of modulation signals obtained by an OFDM method to the baseband, which are related to the plurality of signal groups obtained by dividing a return video signal Ret. Video sent from the CCU  105  side via the triaxial cable  103 , combines the plurality of obtained signal groups in order to obtain a return video signal Ret. Video, and supplies the return video signal Ret. Video (the luminance signal Y) to the camera  101 . 
       FIG. 2  shows an example of the arrangement of frequencies of modulation signals related to main line signals and a return video signal, which are transmitted via the triaxial cable  103 . In this embodiment, for the sake of simplicity of description, it is assumed that the luminance signal Y and the color difference signals Cb/Cr constituting the main line signals transmitted from the camera  101  to the CCU  105  are each divided into three signal groups, and the return video signal Ret. Video (Y) transmitted from the CCU  105  to the camera  101  is divided into three signal groups. 
     Modulation signals My 1 , My 2 , and My 3  related to the three signal groups obtained by dividing the luminance signal Y are shifted in frequency so that their respective center frequencies become F 1 , F 2 , and F 3 . The modulation signals Mc 1 , Mc 2 , and Mc 3  related to the three signal groups obtained by dividing the color difference signals Cb/Cr are shifted in frequency so that their respective center frequencies become F 4 , F 5 , and F 6 . The modulation signals Mr 1 , Mr 2 , and Mr 3  related to the three signal groups obtained by dividing the return video signal Ret. Video are shifted in frequency so that their respective center frequencies become F 7 , F 8 , and F 9 . Since the signals are shifted in frequency so that the modulation signals do not overlap one another in the manner described above, it is possible to transmit the modulation signals through the triaxial cable  103  in such a way that the luminance signal Y, the color difference signals Cb/Cr, and the return video signal Ret. Video do not interfere with one another. 
     The conversion unit  104  is provided between the CCU  105  and the triaxial cable  103 . The conversion unit  104  divides the return video signal Ret. Video (Y) that is a digital video signal output from the CCU  102  into a plurality of signal groups, modulates each signal group by an OFDM method, shifts the frequency band of each modulation signal from a baseband to a mutually different frequency band, and sends the signal group to the triaxial cable  103 . Furthermore, the conversion unit  104  demodulates the signal groups by shifting the frequency bands of a plurality of modulation signals obtained by modulating the signal group by the OFDM method to the baseband, which are related to a plurality of signal groups obtained by dividing the luminance signal Y and the color difference signals Cb/Cr that constitute the main line signals, which are sent from the camera  101  via the triaxial cable  103 , combines the plurality of obtained signal groups in order to obtain a luminance signal Y and color difference signals Cb/Cr, and supplies the signals to the CCU  105 . 
     A description will be given of the operation of the transmission system  100  using a triaxial cable, which is shown in  FIG. 1 . 
     The luminance signal Y and the color difference signals Cb/Cr constituting the main line signals, which are output from the camera  101 , are supplied to the conversion unit  102  on the camera  101  side. In the conversion unit  102 , each of the luminance signal Y and the color difference signals Cb/Cr is divided into three signal groups, and each signal group is modulated by the OFDM method. 
     Then, the modulation signals My 1 , My 2 , and My 3  related to the luminance signal Y are each shifted in frequency so that their respective center frequencies become F 1 , F 2 , and F 3 , as shown in  FIG. 2 , and thereafter are sent to the triaxial cable  103 . The modulation signals Mc 1 , Mc 2 , and Mc 3  related to the color difference signals Cb/Cr are each shifted in frequency so that their respective center frequencies become F 4 , F 5 , and F 6 , as shown in  FIG. 2 , and thereafter are sent to the triaxial cable  103 . 
     The modulation signals My 1 , My 2 , My 3 , Mc 1 , Mc 2 , and Mc 3  sent to the triaxial cable  103  in the manner described above are supplied to the conversion unit  104  on the CCU  105  side via the triaxial cable  103 . In the conversion unit  104 , after the frequency bands of the modulation signals My 1 , My 2 , My 3 , Mc 1 , Mc 2 , and Mc 3  are each shifted to the baseband, the modulation signals My 1 , My 2 , My 3 , Mc 1 , Mc 2 , and Mc 3  are demodulated, six signal groups related to the luminance signal Y and the color difference signals Cb/Cr are obtained, the signal groups are combined, and the luminance signal Y and the color difference signals Cb/Cr are obtained. Then, the luminance signal Y and the color difference signals Cb/Cr are supplied from the conversion unit  104  to the CCU  105 . 
     On the other hand, the return video signal Ret. Video (Y) output from the CCU  105  is supplied to the conversion unit  104  on the CCU  105  side. In the conversion unit  104 , the return video signal Ret. Video is divided into three signal groups, and each signal group is modulated by the OFDM method. Then, the modulation signals Mr 1 , Mr 2 , and Mr 3  related to the return video signal Ret. Video are shifted in frequency so that their respective center frequencies become F 7 , F 8 , and F 9 , as shown in  FIG. 2 , and thereafter are sent to the triaxial cable  103 . 
     The modulation signals Mr 1 , Mr 2 , and Mr 3  sent to the triaxial cable  103  in the manner described above are supplied to the conversion unit  102  on the camera  101  side via the triaxial cable  103 . In the conversion unit  102 , the modulation signals Mr 1 , Mr 2 , and Mr 3  are demodulated after the frequency bands of the modulation signals Mr 1 , Mr 2 , and Mr 3  are each shifted to the baseband, three signal groups related to the return video signal Ret. Video are obtained, the signal groups are combined, and the return video signal Ret. Video is obtained. Then, the return video signal Ret. Video is supplied from the conversion unit  102  to the camera  101 . 
     Next, a description will be given of the details of the configuration of the camera  101  side, that is, the configuration of the camera  101  and the conversion unit  102 .  FIG. 3  shows the configuration thereof. 
     The camera  101  includes an image-capturing optical system  111  formed image-capturing lenses and the like, CCD (Charge Coupled Devices) solid-state image-capturing elements  112 R,  112 G, and  112 B for obtaining red, green, blue image-captured signals, respectively, an analog signal processor  113 , a digital signal processor  117 , a converter  118  for performing conversion from a serial signal to a parallel signal and vice versa, and a viewfinder  119 . 
     The analog signal processor  113  performs a process for sampling and holding a signal of each pixel for each of the red, green, blue image-capturing signals R, G, and B obtained from the CCD solid-state image-capturing elements  112 R,  112 G, and  112 B, respectively, a process for amplifying the sampled-and-held signal of each pixel, and an A/D conversion process for converting the amplified signal of each pixel from an analog signal to a digital signal and for obtaining red, green, blue digital color signals Rd, Gd, and Bd. 
     For this purpose, the analog signal processor  113  is formed of sample-and-hold circuits (S/H)  114 R,  114 G, and  114 B, video amplifiers (VA)  115 R,  115 G, and  115 B, and A/D converters  116 R,  116 G, and  116 B, which correspond to the red, green, and blue color signals, respectively. 
     The digital signal processor  117  performs processing such as color correction, high-frequency correction, gamma correction, and high-luminance compression, and signal processing for enabling an image to be viewed beautifully, on each of the digital color signals Rd, Gd, and Bd of red, green, and blue, which are obtained by the analog signal processor  113 . Furthermore, on the basis of the processed digital color signals Rd, Gd, and Bd, the digital signal processor  117  performs a matrix process in order to generate a luminance signal Y and color difference signals Cb/Cr, which are digital video signals as the main line signals. The luminance signal Y and the color difference signals Cb/Cr are each, for example, 10-bit parallel data. 
     The converter  118  converts the luminance signal Y and the color difference signals Cb/Cr as the main line signals obtained by the digital signal processor  117  from the parallel data to serial data by using HD-SDI (Serial Digital Interface) that is a digital serial transmission standard, and supplies the converted serial data to the conversion unit  102 . Furthermore, the converter  118  converts the serial data of the HD-SDI, which is related to the return video signal Ret. Video supplied from the conversion unit  102 , into parallel data and supplies the data to the viewfinder  119 . 
     The viewfinder  119  selectively displays, for example, a captured-image video signal, that is, an image by the main line signal, or an image by the return video signal Ret. Video. This selection operation can be performed using a button (not shown) by, for example, a cameraman. 
     The conversion unit  102  includes a converter  121  for converting serial data into parallel data, a signal divider  122 , modulation processors  123   a  to  123   f , frequency shifters  124   a  to  124   f , a multiplex filter (MPX filter)  125 , and frequency shifters  126   a  to  126   c , demodulation processors  127   a  to  127   c , a signal combiner  128 , and a converter  129  for converting parallel data into serial data. 
     The converter  121  converts serial data of HD-SDI, which is related to the main line signals (the luminance signal Y and the color difference signals Cb/Cr) supplied from the camera  101 , into parallel data of the luminance signal Y and the color difference signals Cb/Cr. 
     The signal divider  122  divides the luminance signal Y and the color difference signals Cb/Cr, which are obtained by the converter  121 , into three signal groups. In this case, the bit rate of each signal group is ⅓ the bit rate of the luminance signal Y or the color difference signals Cb/Cr. The modulation processors  123   a  to  123   f  perform a modulation process of the OFDM method on the six signal groups obtained by the signal divider  122  and generate modulation signals My 1 , My 2 , My 3 , Mc 1 , Mc 2 , and Mc 3  correspondingly, which are related to the luminance signal Y and the color difference signals Cb/Cr. 
     A description will now be given of the configuration of the modulation processors  123   a  to  123   f . Here, a description is given by using the modulation processor  123   a  as an example. Although detailed description has been omitted, the modulation processors  123   b  to  123   f  are configured identically to the modulation processor  123   a .  FIG. 4  shows a specific configuration of the modulation processor  123   a.    
     The energy of the signal group that is 8-bit parallel data is diffused by an energy diffuser  131 , and signal processing with which error correction is possible is performed by an outer coder  132 . Thereafter, an outer interleaver  133  interleaves a signal for the purpose of improving robustness against errors during transmission. In the outer coder  132 , usually, error correction code like a Reed-Solomon code is attached. 
     Convolution coding is performed on a signal output from the outer interleaver  133  in order to correct errors by an inner coder  134 , and the signal is interleaved by an inner interleaver  135 . Thereafter, primary modulation, such as QAM (Quadrature Amplitude Modulation) or PSK (Phase Shift Keying) is performed by a primary modulator  136 . 
     Thereafter, in a signal attacher  137 , a pilot signal and a transmission parameter signal (TPS) that are necessary to perform demodulation on the reception side are attached. Furthermore, an OFDM section  138  performs orthogonal frequency division multiplexing modulation, and thereafter a guard interval inserter  139  attaches a signal for reflection on the transmission line, which is called a guard interval, and a modulation signal My 1  related to the above-described input signal group is obtained. 
     Referring back to  FIG. 3 , the frequency shifters  124   a  to  124   f  shift in frequency the frequency bands of the six modulation signals My 1  to Mc 3  related to the luminance signal Y and the color difference signals Cb/Cr, which are obtained by the modulation processors  123   a  to  123   f  from a baseband so that their respective center frequencies become F 1  to F 6  (See  FIG. 2 ). The frequency shifters  124   a  to  124   f  are each formed of a multiplier and perform a shift process by applying the frequency signals (carrier wave signals) of the frequencies F 1  to F 6  to the modulation signals My 1  to Mc 3 , respectively. 
     The multiplex filter  125  combines the modulation signals My 1  to Mc 3  that are shifted by the frequency shifters  124   a  to  124   f  and sends the signals to the triaxial cable  103 , and separates and extracts from the triaxial cable  103 , three modulation signals Mr 1 , Mr 2 , and Mr 3  related to the return video signal Ret. Video (Y) sent from the CCU  105 , in which their respective center frequencies are F 7  to F 9  (See  FIG. 2 ). 
     The frequency shifters  126   a  to  126   c  shift the frequency bands of the modulation signals Mr 1  to Mr 3  extracted by the multiplex filter  125  to the baseband. The frequency shifters  126   a  to  126   c  are each formed of a multiplier and perform a shift process by applying frequency signals of frequencies F 7  to F 9  to the modulation signals Mr 1  to Mr 3 , respectively. 
     The demodulation processors  127   a  to  127   c  perform a demodulation process of the OFDM method on the modulation signals Mr 1  to Mr 3  that are shifted by the shift process by the frequency shifters  126   a  to  126   c , and generate three signal groups related to the return video signal Ret. Video, each of which signal groups is 8-bit parallel data. Although detailed description has been omitted, the demodulation processors  127   a  to  127   c  are configured to perform processing reverse to that of the modulation processors  123   a  to  123   f.    
     The signal combiner  128  combines the three signal groups that are generated by the demodulation processors  127   a  to  127   c  in order to generate a return video signal Ret. Video that is 10-bit parallel data. The converter  129  converts the return video signal Ret. Video generated by the signal combiner  128  from parallel data to serial data of HD-SDI and supplies the converted serial data to the camera  101 . 
     A description will be given of the operation on the side of the camera  101  shown in  FIG. 3 . 
     On the image-capturing plane of the image-capturing elements  112 R,  112 G, and  112 B, optical signals of red, green, and blue are formed by the image-capturing optical system  111 . Then, photoelectrical conversion is performed by the image-capturing elements  112 R,  112 G, and  112 B, and image-capturing signals R, G, and B of red, green, and blue are obtained from the image-capturing elements  112 R,  112 G, and  112 B, respectively. The image-capturing signals R, G, and B are supplied to the analog signal processor  113 , where a sample-and-hold process, an amplification process, an A/D conversion process, and the like are performed to obtain digital color signals Rd, Gd, and Bd of red, green, and blue. 
     The color signals Rd, Gd, and Bd are supplied to the digital signal processor  117 , where processing, such as color correction, high-frequency correction, gamma correction, and high-luminance compression, is performed, and signal processing for enabling an image to be viewed beautifully are performed. On the basis of the processed digital color signals Rd, Gd, and Bd, the digital signal processor  117  performs a matrix process and generates digital video signals (the luminance signal Y and the color difference signals Cb/Cr) as main line signals to be supplied to the CCU  105 . 
     The luminance signal Y and the color difference signals Cb/Cr are supplied to the converter  118 , where they are converted from the parallel data to serial data of HD-SDI. Then, the converted serial data is supplied to the converter  121  of the conversion unit  102  as an output from the camera  101 . In the converter  121 , the serial data of HD-SDI, which is related to the main line signals (the luminance signal Y and the color difference signals Cb/Cr), which are supplied from the camera  101 , is converted into parallel data of the luminance signal Y and the color difference signals Cb/Cr. 
     The luminance signal Y and the color difference signals Cb/Cr, which are 10-bit parallel data output from the converter  121 , are supplied to the signal divider  122 . In the signal divider  122 , the luminance signal Y is divided into, for example, three signal groups, each of which is 8-bit parallel data, and the color difference signals Cb/Cr are each divided into three signal groups, each of which is 8-bit parallel data. 
     The six signal groups obtained by being divided by the signal divider  122  are supplied to the modulation processors  123   a  to  123   f , correspondingly. In the modulation processors  123   a  to  123   f , the signal groups, each of which is 8-bit parallel data, are modulated by the OFDM method, and modulation signals My 1  to My 3  related to the luminance signal Y and modulation signals Mc 1  to Mc 3  related to the color difference signals Cb/Cr are generated. 
     The modulation signals My 1  to Mc 3  generated by the modulation processors  123   a  to  123   f  are supplied to the frequency shifters  124   a  to  124   f , respectively. In the frequency shifters  124   a  to  124   f , the frequency bands of the modulation signals My 1  to Mc 3  are shifted in frequency from a baseband so that their respective center frequencies become F 1  to F 6 . 
     The modulation signals My 1  to Mc 3  that are shifted by the frequency shifters  124   a  to  124   f  are combined by the multiplex filter  125  and are sent to the triaxial cable  103 . As a result, the modulation signals My 1  to Mc 3  related to the main line signals (the luminance signal Y and the color difference signals Cb/Cr) are sent to the CCU  105  via the triaxial cable  103 . 
     The multiplex filter  125  separates and extracts from the from the triaxial cable  103 , the modulation signals Mr 1  to Mr 3 , in which their respective center frequencies are F 7  to F 9 , which are related to the return video signal Ret. Video (Y) sent from the CCU  105 . 
     The modulation signals Mr 1  to Mr 3  are supplied to the frequency shifters  126   a  to  126   c , where their respective frequency bands are shifted to the baseband. The shifted modulation signals Mr 1  to Mr 3  are supplied to the demodulation processors  127   a  to  127   c , respectively. In the demodulation processors  127   a  to  127   c , the modulation signals Mr 1  to Mr 3  are demodulated by the OFDM method, respectively, and three signal groups related to the return video signal Ret. Video, each of which signal groups is, for example, 8-bit parallel data, are generated. 
     The three signal groups are combined by the signal combiner  128 , and a return video signal Ret. Video that is 10-bit parallel data is obtained. The return video signal Ret. Video is supplied to the converter  129 , where the parallel data is converted into serial data of HD-SDI. Then, the converted serial data is supplied as an output from the conversion unit  102  to the converter  118  of the camera  101 . 
     In the converter  118 , the serial data of HD-SDI, which is related to the return video signal Ret. Video supplied from the conversion unit  102 , is converted into parallel data and is supplied to the viewfinder  119 . For example, a captured-image video signal is also supplied from the digital signal processor  117  to the viewfinder  119 . In the viewfinder  119 , an image by the main line signal or an image by the return video signal Ret. Video is selectively displayed in response to a button operation by the cameraman. 
     Next, a description of the details of the configuration of the conversion unit  104  on the CCU  105  side will be given.  FIG. 5  shows the configuration of the conversion unit  104 . 
     The conversion unit  104  includes a multiplex filter  141 , frequency shifters  142   a  to  142   f , demodulation processors  143   a  to  143   f , a signal combiner  144 , a converter  145  for converting parallel data into serial data, a converter  146  for converting serial data into parallel data, a signal divider  147 , modulation processors  148   a  to  148   c , and frequency shifters  149   a  to  149   c.    
     The multiplex filter  141  separates and extracts from the triaxial cable  103 , modulation signals My 1  to Mc 3  related to the luminance signal Y and the color difference signals Cb/Cr as the main line signals, the modulation signals My 1  to Mc 3  being sent from the camera  101 . Here, the modulation signals My 1  to Mc 3  have been shifted so that their respective center frequencies are F 1  to F 6  (See  FIG. 2 ). 
     Furthermore, the multiplex filter  141  combines the modulation signals Mr 1  to Mr 3  related to the return video signal Ret. Video (Y), which have been shifted by the frequency shifters  149   a  to  149   c , and sends the signals to the triaxial cable  103 . The frequency shifters  142   a  to  142   f  shift the frequency bands of the modulation signals My 1  to Mc 3  extracted by the multiplex filter  141  to the baseband, respectively. The frequency shifters  142   a  to  142   f  are each formed of a multiplier and perform a shift process by applying frequency signals of frequencies F 1  to F 6  to the modulation signals My 1  to Mc 3 , respectively. 
     The demodulation processors  143   a  to  143   f  perform a demodulation process of the OFDM method on the modulation signals My 1  to Mc 3  that have been shifted by the frequency shifters  142   a  to  142   f , respectively, and generate signal groups related to the luminance signal Y and the color difference signals Cb/Cr, each of which signal groups is 8-bit parallel data. Although detailed description has been omitted, the demodulation processors  143   a  to  143   f  are configured to perform processing reverse to that of the modulation processors  123   a  to  123   f.    
     The signal combiner  144  combines the six signal groups generated by the demodulation processors  143   a  to  143   f  and generates a luminance signal Y and color difference signals Cb/Cr that are 10-bit parallel data. The converter  145  converts the luminance signal Y and the color difference signals Cb/Cr generated by the signal combiner  144  from parallel data to serial data of HD-SDI, and supplies the converted serial data to the CCU  105 . 
     The converter  146  converts the serial data of HD-SDI related to the return video signal Ret. Video (Y) supplied from the CCU  105  into parallel data. The signal divider  147  divides the return video signal Ret. Video obtained by the converter  146  into three signal groups. In this case, the bit rate of each signal group is ⅓ the bit rate of the return video signal Ret. Video. 
     The modulation processors  148   a  to  148   c  perform a modulation process of the OFDM method on the three signal groups obtained by the signal divider  147 , and generate three modulation signals Mr 1  to Mr 3  related to the return video signal Ret. Video, respectively. Although detailed description has been omitted, the modulation processors  148   a  to  148   c  are configured identically to the above-described modulation processors  123   a  to  123   f.    
     The frequency shifters  149   a  to  149   c  shift the frequency bands of the three modulation signals Mr 1  to Mr 3  related to the return video signal Ret. Video, which are obtained by the modulation processors  148   a  to  148   c , from the baseband so that their respective center frequencies become F 7  to F 9  (See  FIG. 2 ). The frequency shifters  149   a  to  149   c  are each formed of a multiplier and perform a shift process by applying frequency signals (carrier wave signals) of frequencies F 7  to F 9  to the modulation signals Mr 1  to Mr 3 , respectively. 
     The operation of the conversion unit  104  shown in  FIG. 5  on the CCU  105  side will be described below. 
     The multiplex filter  141  extracts, from the triaxial cable  103 , six modulation signals My 1  to Mc 3 , in which their respective center frequencies are F 1  to F 6 , which are related to the main line signals (the luminance signal Y and the color difference signals Cb/Cr) sent from the camera  101 . 
     The modulation signals My 1  to Mc 3  are supplied to the frequency shifters  142   a  to  142   f , respectively, where the frequency bands are shifted to the baseband. The modulation signals My 1  to Mc 3  that have been shifted by the shift process are supplied to the demodulation processors  143   a  to  143   f , respectively. In the demodulation processors  143   a  to  143   f , the modulation signals My 1  to Mc 3  are demodulated, respectively, by the OFDM method, and six signal groups, each of which is 8-bit parallel data, which are related to the luminance signal Y and the color difference signals Cb/Cr, are generated. 
     The six signal groups are combined by the signal combiner  144 , thereby obtaining a luminance signal Y and color difference signals Cb/Cr, each of which is 10-bit parallel data. The luminance signal Y and the color difference signals Cb/Cr are supplied to the converter  145 , where the parallel data is converted into serial data of HD-SDI. Then, the converted serial data is supplied as an output from the conversion unit  104  to the CCU  105 . 
     Furthermore, the serial data of HD-SDI, which is related to the return video signal Ret. Video (Y) supplied from the CCU  105 , is supplied to the converter  146 . In the converter  146 , the serial data is converted into parallel data of the return video signal Ret. Video. 
     The return video signal Ret. Video is supplied to the signal divider  147 . In the signal divider  147 , the return video signal Ret. Video is divided into three signal groups, each of which is 8-bit parallel data. The three signal groups are correspondingly supplied to the modulation processors  148   a  to  148   c . In the modulation processors  148   a  to  148   c , signal groups, each of which is 8-bit parallel data, are modulated by the OFDM method, and modulation signals Mr 1  to Mr 3  related to the return video signal Ret. Video are generated. 
     The modulation signals Mr 1  to Mr 3  generated by the modulation processors  148   a  to  148   c  are supplied to the frequency shifters  149   a  to  149   c , respectively. In the frequency shifters  149   a  to  149   c , the frequency bands of the modulation signals Mr 1  to Mr 3  are shifted in frequency from the baseband so that their respective center frequencies become F 7  to F 9 . 
     The modulation signals Mr 1  to Mr 3  that have been shifted by the frequency shifters  149   a  to  149   c , respectively, are combined by the multiplex filter  141  and are sent to the triaxial cable  103 . As a result, the modulation signals Mr 1  to Mr 3  related to the return video signal Ret. Video (Y) are sent to the camera  101  via the triaxial cable  103 . 
     As described above, according to the transmission system  100  shown in  FIG. 1 , the main line signals (the luminance signal Y and the color difference signals Cb/Cr) to be transmitted from the camera  101  to the CCU  105  is divided into a plurality of signal groups by the conversion unit  102 , thereafter are modulated by the OFDM method, and are sent to the triaxial cable  103 . On the other hand, the return video signal Ret. Video (Y) to be transmitted from the CCU  105  to the camera  101  is divided into a plurality of signal groups by the conversion unit  104 , thereafter is modulated by the OFDM method, and is sent to the triaxial cable  103 . 
     As described above, since each of the digital video signals (the luminance signal Y, the color difference signals Cb/Cr, and the return video signal Ret. Video) is divided into a plurality of signal groups and the bit rate is decreased, even if the bit rate of the video signals is high like an HD video signal, it is possible to perform digital modulation (modulation of the OFDM method). 
     Furthermore, since modulation of the OFDM method is performed as digital modulation, the symbol rate can be decreased so as to be lower than the bit rate of the luminance signal Y, the color difference signals Cb/Cr, or the return video signal Ret. Video (Y). Therefore, even if reflection occurs in the connector portion or the like, the influence thereof is not easily received, and the digital signal can be satisfactorily transmitted via the coaxial cable. 
     Since transmission is performed using many carrier waves as a result of the modulation of the OFDM method, several waves among the many waves can be assigned to a pilot signal, a transmission parameter signal, and the like. Therefore, this is advantageous when AGC (automatic gain control) and cable equalization are performed. 
     According to the transmission system  100  shown in  FIG. 1 , a frequency shift process is performed so that the frequency bands of the modulation signals My 1  to Mc 3  related to the main line signals (the luminance signal Y and the color difference signals Cb/Cr) to be transmitted from the camera  101  to the CCU  105  and the frequency bands of the modulation signals Mr 1  to Mr 3  related to the return video signal Ret. Video (Y) to be transmitted from the CCU  105  to the camera  101  do not overlap one another. Therefore, it is possible to satisfactorily transmit the modulation signals My 1  to Mc 3  and Mr 1  to Mr 3  by using one triaxial cable  103  in such a manner that the luminance signal Y, the color difference signals Cb/Cr, and the return video signal Ret. Video do not interfere with one another. 
     According to the transmission system  100  shown in  FIG. 1 , the camera  101  and the CCU  105  are connected to each other with one triaxial cable  103 , and since the connection between the camera  101  and the CCU  105  is very simple, the transmission system  100  can be easily moved. Furthermore, the triaxial cable has high reliability and is inexpensive compared with an optical fiber cable. This is in contrast with that two optical fiber cables are used to perform bidirectional transmission. 
     In the transmission system  100  shown in  FIG. 1 , the camera  101  and the CCU  105  are connected to each other with the triaxial cable  103  that is a coaxial cable. Connectors can be easily connected anytime and in any environment or place and also, multi-stage connection of coaxial cables can be easily performed. Even if a coaxial cable is broken, repair can be easily performed. There is no need to be concerned with the accumulation of dirt on connectors unlike in the case of optical fiber cables, and connector connection is possible in an environment in which there is much rain and dust. 
     According to the transmission system  100  shown in  FIG. 1 , the luminance signal Y, the color difference signals Cb/Cr, and the return video signal Ret. Video (Y) that are digital signals are modulated by the OFDM method and transmitted. The digital signals are not converted into analog signals, are not AM- or FM-modulated, and are not transmitted. The deterioration of the S/N does not occur, and transmission at a distance longer than that of an analog transmission system becomes possible. A relay may be provided halfway as necessary so that transmission over a longer distance can be performed. 
     In the above-described embodiments, the luminance signal Y and the color difference signals Cb/Cr as the main line signals output from the camera  101  are supplied to the conversion unit  102  and are modulated by the OFDM method. Alternatively, the camera  101  may have the same function as that of the conversion unit  102 . In this case, the converter  118  in the camera  101  of  FIG. 3  and the converters  121  and  129  are not necessary. 
     In the above-described embodiments, the return video signal Ret. Video (Y) output from the CCU  105  is supplied to the conversion unit  104 , where modulation is performed by means of the OFDM method. Alternatively, the CCU  105  may have the same function as that of the conversion unit  104 . In this case, the converters  145  and  146  in the conversion unit  104  of  FIG. 5  and conversion (not shown) in the CCU  105  are not necessary. 
     In the above embodiments, a case has been described in which the luminance signal Y, the color difference signals Cb/Cr, and the return video signal Ret. Video, which are 10-bit parallel data, are directly divided into a plurality of signal groups by the signal dividers  122  and  147 . Alternatively, after a data compression process is performed on these signals, the signals may be divided into a plurality of signal groups. As a result, the bit rate can be decreased further, and digital modulation becomes easier. In this case, in the signal combiners  128  and  144  on the reception side, a data expansion process is performed. 
     In the above embodiments, the present invention is applied to the transmission system for transmitting a digital video signal. The present invention can similarly be applied to a case in which other digital signals are transmitted. 
     The present invention is capable of satisfactorily transmitting a digital signal via a coaxial cable and can be applied to, for example, transmission of a video signal between a camera and a CCU. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.