Abstract:
A receiving apparatus includes: a signal-level detecting unit detecting a signal level of an input signal; a first signal-level converting unit including amplifiers, capturing the input signal and converting a signal level of the input signal; a switching unit switching an output of a converted level signal; a switching control unit selecting a specific amplifier and controlling the switching unit to select an output signal from the selected amplifier; a band-pass filter allowing only a predetermined frequency hand in the switched signal to pass; a second signal-level converting unit converting a signal level of an output signal from the amplifier into a predetermined signal level at which S/N of a mixer is maximized; the mixer mixing the converted signal passed through the band-pass filter, and an oscillation signal to generate an intermediate frequency signal; and a demodulating unit demodulating the intermediate frequency signal.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a receiving apparatus, an imaging apparatus, and a receiving method, and, more particularly to a technique for controlling a gain such that a signal level of an input signal transmitted through a communication cable is converted into a fixed level range. 
     2. Description of the Related Art 
     In the past, in a broadcasting station, plural cameras are connected to a camera control unit (hereinafter simply referred to as a control unit) via a cable for transmitting an analog signal. Video signals and sound signals captured by the cameras are transmitted to the control unit through the cable. Return signals for instruction and confirmation are transmitted from the control unit through the cable. 
     As the cable for connecting the cameras and the control unit, in general, a coaxial cable is used because the coaxial cable is relatively low in cost. For example, in a TRIAX system adopted in many broadcasting stations, power supply is performed and video signals, sound signals, command signals, and camera return signals are transmitted with a frequency division multiplexing wave by using one coaxial cable. 
     In recent years, signals of an HDTV (High Definition Television) system having a large image information amount compared with the system in the past have been spread. The coaxial cable is also used for transmission of such signals having a large image information amount. Originally, it is desirable to use an optical fiber for a signal transmission channel for transmitting an HD signal in a wide band to the control unit. However, large cost and time are necessary for installing the optical fiber. Therefore, under the present situation, replacement with the optical fiber is not advanced. 
     Further, in recent years, in order to improve mobility and usability, longer cable length is requested for the cable for connecting the control unit and the cameras. In this way, it is demanded to extend the distance of the existing coaxial cable and use the coaxial cable for transmission of signals having a large image information amount such as the HD signal. However, since the analog transmission system has the problems explained below, it is not easy to realize the demands. 
     As a first problem, as the distance of the cable is extended, a signal level falls because of an increase in a loss of the cable and an S/N value at a reception end may be deteriorated in proportion to the cable distance. As a second problem, there is a transmission loss characteristic that the signal level is attenuated as a frequency of an analog signal increases.  FIG. 14  is a graph of a relation between a frequency and an attenuation amount of a signal in the coaxial cable. The abscissa indicates the frequency (MHz) and the ordinate indicates the attenuation amount (dB). In  FIG. 14 , an example of an attenuation characteristic obtained when the cable distance is 1 km is shown. 
     As it is evident from  FIG. 14 , the attenuation amount increases as the frequency increases. For example, when a signal of 10 MHz and a signal exceeding 100 MHz are compared, it is seen that a difference between signal levels of the signals is equal to or more than 90 dB. Therefore, a √f (f is a frequency) cable equalizing circuit and a gain control circuit (automatic gain control (AGC)) circuit for outputting, at a fixed level, reception signals input at various levels are necessary according to the cable distance. 
     For example, JP-A-2008-029000 discloses a gain control circuit that switches to use plural gain amplifiers according to the level of an input signal to suppress fluctuation in a signal loss caused in a process of transmission through a communication cable and enable extension of communication cable length. 
     SUMMARY OF THE INVENTION 
       FIG. 15  is a diagram of a configuration example of a receiving apparatus having a function of switching plural gain amplifiers. A receiving apparatus  200  shown in  FIG. 15  receives an OFDM signal transmitted in a TRIAX system and modulated in an OFDM (Orthogonal Frequency-Division Multiplexing) system. The receiving apparatus  200  includes a low-noise amplifier (LNA)  201 , a band-pass filter (BPF)  202 , an amplifier  203 , a step AGC circuit  204 , a frequency converting unit  210 , an AGC  217 , and an OFDM demodulation unit  218 . 
     The LNA  201  selects and amplifies an input signal and outputs the input signal to the BPF  202 . The BPF  202  allows only a signal in a predetermined frequency band among signals input from the low-noise amplifier  201  to pass and outputs the signal to the amplifier  203 . The amplifier  203  amplifies the signal input from the BPF  202  at a predetermined amplification factor and supplies the signal to the step AGC circuit  204 . 
     The step AGC circuit  204  is a circuit for absorbing level fluctuation in an input signal and minimizing S/N deterioration. The step AGC circuit  204  includes plural amplifiers (not shown in the figure) provided to correspond to respective ranges of a signal level of the input signal. Different gains are set in the respective amplifiers. Control for selecting an output signal of an amplifier used for the output signal is performed according to the level of a signal input thereto. Consequently, a range of the level of the input signal is compressed in a predetermined range. 
     The frequency converting unit  210  performs processing for converting a frequency of an input signal into a predetermined intermediate frequency. The frequency converting unit  210  includes a PLL (phase-Locked Loop) unit  211 , a local oscillator  212 , a mixer  213 , a variable-gain amplifier  214 , a SAW (Surface Acoustic Wave) filer  215 , and a variable-gain amplifier  216 . The AGC  217  adjusts the level of a signal input thereto such that an intermediate frequency signal input to an OFDM demodulation unit  218  at a post-stage is at a fixed level and outputs the input signal to the OFDM demodulation unit  218 . Control in the AGC  217  is performed on the basis of a control signal input from the OFDM demodulation unit  218 . The OFDM demodulation unit  218  demodulates and outputs an input OFDM signal. 
     Since the step AGC circuit  204  is provided at a pre-stage of the frequency converting unit  210  in this way, even a reception OFDM signal having large fluctuation in a signal level, which is received when the cable length is large, is converted into a signal level within a fixed range. Consequently, since deterioration in S/N can be minimized, it is possible to extend the length of a coaxial cable, which is limited for the purpose of preventing deterioration in the S/N of a signal to be transmitted, to about 1 km. 
     However, even when such a configuration is adopted, it is difficult to prevent deterioration in S/N due to an image signal component in the mixer  213  (see  FIG. 15 ) during reception and an adverse effect due to fluctuation in S/N depending on an input signal level in the mixer  213 . 
       FIGS. 16A and 16B  are diagrams for explaining deterioration in S/N due to an image component. Frequency components of a signal before and after being input to the mixer  213  are shown in  FIGS. 16A and 16B . The abscissa indicates a frequency (MHz) and the ordinate indicates a signal level.  FIG. 16A  is a diagram of a frequency band of a signal input to the mixer  213 . In an example shown in  FIG. 16A , a desired wave signal desired to be received has a frequency of 100 MHz and a frequency of 150 MHz is output from the local oscillator  212  (see  FIG. 15 ) and converted into an intermediate frequency signal of 50 MHz. 
     As shown in  FIG. 16A , a desired wave in a 100 MHz band received by the mixer  213  and an oscillation signal of 150 MHz from the local oscillator  212  are mixed. As shown in  FIG. 16B , the desired wave is converted into an intermediate signal of 50 MHz. When such frequency conversion shown in  FIGS. 16A and 16B  is assumed, an input signal of 200 MHz shown in  FIG. 16A  is also included in the intermediate frequency signal of 50 MHz after the conversion as an image frequency component as shown in  FIG. 16B . When the frequency conversion by the mixer  213  is performed while an included image frequency is not removed, noise included in an output signal increases by about 3 dB. 
       FIGS. 17 and 18  are diagrams for explaining fluctuation in output S/N that depends on an input signal level in the mixer  213 . In order to facilitate explanation concerning the “fluctuation in output S/N that depends on an input signal level”, first, an input and output characteristic example in a general amplifier is explained with reference to  FIG. 17 . In  FIG. 17 , the abscissa indicates an input signal level (dBm) and the ordinate indicates an output signal level (dBm). A lower area indicated by dots in the figure indicates a noise floor generated by heat noise of the amplifier itself. 
     It is seen that, when the level of an input signal is low, the output S/N is deteriorated by the influence of the heat noise and, when the level of an input signal is high, the output S/N is deteriorated because distortion occurs in an output signal affected by the influence of third-order inter-modulation distortion. 
       FIG. 18  is a graph of an output S/N characteristic example in the general amplifier. The abscissa indicates a level (dBm) of an input signal and the ordinate indicates output S/N (dBc).  FIG. 18  indicates that, when the input signal level is near −21 dBm, a dynamic range is the maximum and, when the input signal level is higher than −21 dBm, the output S/N is suddenly deteriorated.  FIG. 19  is a graph of an output S/N characteristic in the mixer  213 .  FIG. 19  indicates that, in the mixer  213 , when an input signal level is near −15 dBm, a maximum dynamic range is obtained and that a curve indicating an output S/N characteristic is steeper than that in the amplifier. 
       FIG. 20  is a graph in which the output S/N characteristic in the amplifier and the output S/N characteristic in the mixer  213  are shown in a superimposed manner. In  FIG. 20 , to facilitate explanation, it is assumed that the gain of the amplifier and the gain of the mixer  213  are the same. Specifically, the output S/N characteristic of the amplifier indicated by a broken line in  FIG. 20  is obtained by translating an actual characteristic in the right direction on the graph.  FIG. 20  indicates that, in sections indicated by arrows in the figure, the output S/N characteristic of the mixer  213  is inferior to the output S/N characteristic of the amplifier. In other words, it is seen that the output S/N characteristic of the mixer  213  has a larger influence on the S/N of a signal. Therefore, even when level fluctuation in the signal is absorbed by the step AGC circuit  204  (see  FIG. 15 ), depending on the level of the signal, the S/N of the signal is deteriorated when the signal passes the mixer  213 . 
     Therefore, it is desirable to suppress fluctuation in a signal loss that occurs in a process of transmitting a signal through a communication cable and extend the length of the communication cable. 
     A receiving apparatus according to an embodiment of the present invention includes a signal-level detecting unit that detects a signal level of an input signal transmitted through a communication cable. The receiving apparatus also includes a first signal-level converting unit including plural amplifiers that are provided for respective predetermined signal level areas obtained by dividing a signal level range of the input signal and in which a gain for converting input signals in the signal level areas into a fixed level range is set. The receiving apparatus also includes a switching unit that is provided to correspond to the plural amplifiers and switches an output of a signal, a level of which is converted by the plural amplifiers, and a switching control unit that selects a specific amplifier out of the plural amplifiers on the basis of the signal level of the input signal detected by the signal-level detecting unit. The receiving apparatus also includes a band-pass filter that allows only a predetermined frequency band in the signal, the output of which is switched by the switching unit, to pass and a second signal-level converting unit that converts a signal level of an output signal from the amplifier into a predetermined signal level. Further, the receiving apparatus includes a mixer that mixes the signal having passed through the band-pass filter, the signal level of which is converted by the second signal-level converting unit, and a local oscillation signal generated by a local oscillator to generate an intermediate frequency signal and a demodulating unit that demodulates the intermediate frequency signal generated by the mixer. 
     Consequently, first, the level of the input signal is converted into the fixed level range by the first signal-level converting unit and then the signal level is further converted into the predetermined level by the second signal-level converting unit. The signal band-limited in this way and having passed through the band-pass filter is input to the mixer. The intermediate frequency signal generated by the mixer is demodulated. 
     According to the embodiment of the present invention, since the signal is input to the mixer after the signal level is converted into the predetermined level by the first signal-level converting unit and the second signal-level converting unit, the S/N of the signal demodulated by the demodulating unit is improved. Therefore, fluctuation in a signal loss that occurs in a process of transmitting the signal through the communication cable is suppressed and the length of the communication cable can be extended. 
     In this case, since a signal in an unnecessary frequency band is removed by the band-pass filter, the S/N of the signal input to the demodulating unit is also improved and a quality of a reception signal (a demodulated signal) as a signal after the demodulation is improved. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a configuration example of an image signal transmission system according to an embodiment of the present invention; 
         FIG. 2  is a graph of OFDM signals and cable loss characteristics in the embodiment; 
         FIG. 3  is a block diagram of a configuration example of a reception-signal processing unit according to the embodiment; 
         FIG. 4  is a graph of an example of gain setting for each amplifier in a step AGC circuit according to the embodiment; 
         FIG. 5  is a diagram of input and output characteristics of the step AGC circuit according to the embodiment; 
         FIG. 6  is a diagram of a configuration example of the step AGC circuit and an AGC according to the embodiment; 
         FIG. 7  is a graph of an example of an output S/N characteristic of a mixer according to the embodiment; 
         FIG. 8  is a block diagram of an internal configuration example of a frequency converting unit according to the embodiment; 
         FIG. 9  is a block diagram of an internal configuration example of the AGC according to the embodiment; 
         FIG. 10  is a graph of an example of a cut-off characteristic of a BPF according to the embodiment; 
         FIG. 11  is a graph of an example of output S/N characteristics of the AGC and the mixer according to the embodiment; 
         FIG. 12  is a graph of an example of the S/N of a reception signal with respect to fluctuation in the length of a cable; 
         FIG. 13  is a graph of an example of the S/N of the reception signal with respect to the fluctuation in the length of the cable; 
         FIG. 14  is a graph of a relation between a frequency and an attenuation amount of a signal in a coaxial cable in the past; 
         FIG. 15  is a block diagram of an internal configuration example of a TRIAX receiver in the past; 
         FIGS. 16A and 16B  are diagrams of an influence due to an image frequency in the past; 
         FIG. 17  is a graph of input and output characteristics in an amplifier in the past; 
         FIG. 18  is a graph of an output S/N characteristic in the amplifier in the past; 
         FIG. 19  is a graph of an output S/N characteristic in a mixer in the past; and 
         FIG. 20  is a graph of comparison of the output S/N characteristic in the amplifier and the output S/N characteristic in the mixer in the past. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A best mode for carrying of the present invention (hereinafter referred to as embodiment) is explained below. A receiving apparatus according to the embodiment of the present invention is applied to a receiving apparatus used in a broadcast signal transmission system. 
     The explanation is performed in the order described below. 
     1. Overall configuration example of the broadcast signal transmission system 
     2. Configuration example of a step AGC circuit 
     3. Configuration example of an AGC provided at a post-stage of the step AGC circuit 
     4. Effects of the embodiment 
     5. Modifications 
     Overall Configuration Example of the Broadcast Signal Transmission System 
       FIG. 1  is a diagram of the configuration of the broadcast signal transmission system according to this embodiment. In the broadcast signal transmission system according to this embodiment, an imaging apparatus (hereinafter referred to as a camera)  110  for broadcast and a control apparatus  120  are connected via a TRIAX cable  150 . 
     A camera HD signal or the like is transmitted from the camera  110 . A camera HD return signal or the like for checking with the camera  110 , an image photographed by the camera  110  is transmitted from the control apparatus  120 . The signal transmitted by one apparatus is input to the other apparatus through the TRIAX cable  150 . 
     The camera HD signal and the camera HD return signal are subjected to OFDM modulation of 64 QAM (Quadrature Amplitude Modulation) and allocated to an OFDM signal, a frequency band of one wave of which is 8 MHz. 
     The camera  110  includes an OFDM modulation unit  111 , a frequency converting unit  112 , an MPX filter  113 , a reception-signal (H) processing unit  114 , an OFDM demodulation unit  115 , an imaging unit  116 , and a monitor display unit  117 . 
     The OFDM modulation unit  111  applies the OFDM modulation to a camera HD signal generated by the imaging unit  116  and generates plural OFDM signals. As the OFDM signals, three groups of a group L, a group M, and a group H are generated in order from one having a lowest frequency band. The camera HD signal transmitted from the camera  110  is allocated to the OFDM signals of the group L and the group M among the three groups. 
     Details of the OFDM signals of the group L, the group M, and the group H are explained later with reference to  FIG. 2 . 
     The frequency converting unit  112  converts the OFDM signals generated by the OFDM modulation unit  111  into a predetermined transmission frequency. The frequency converting unit  112  outputs the transmission OFDM signals of the group L and the group M to the MPX filter  113 . The MPX filter  113  is a filter that separates an input signal into frequency bands of the group L, the group M, and the group H. The group L and the group M are set for transmission and the group H is set for reception. 
     Therefore, the MPX filter  113  transmits the OFDM signals of the group L and the group M separated by the MPX filter  113  to the TRIAX cable  150  and outputs the OFDM signal of the group H input through the TRIAX cable  150  to the reception-signal (H) processing unit  114 . Details of the reception-signal (H) processing unit  114  are explained later. 
     The OFDM demodulation unit  115  demodulates the camera HD return signal from the OFDM signals. The imaging unit  116  generates a camera HD signal based on a photographed video signal. The monitor display unit  117  reproduces and displays the camera HD return signal. 
     The control apparatus  120  includes an OFDM modulation unit  121 , a frequency converting unit  122 , an MPX filter  123 , a reception-signal (L) processing unit  124 L, a reception-signal (M) processing unit  124 M, an OFDM demodulation unit  125 L, an OFDM demodulation unit  125 M, and a control unit  127 . 
     The OFDM modulation unit  121  applies the OFDM modulation to the camera HD return signal generated by the control unit  127  and generates the OFDM signal of the group H. The frequency converting unit  122  converts the OFDM signal generated by the OFDM modulation unit  121  into a predetermined transmission frequency band and generates a transmission OFDM signal. 
     The MPX filter  123  is a filter that separates the group L, the group M, and the group H. The group L and the group M are set for reception and the group H is set for transmission. Therefore, the MPX filter  123  outputs the OFDM signals of the group L and the group M separated by the MPX filter  123  to the reception-signal (L) processing unit  124 L and the reception-signal (M) processing unit  124 M, respectively. The MPX filter  123  transmits the OFDM signal of the group H to the TRIAX cable  150 . 
     The OFDM demodulation unit  125 L and the OFDM demodulation unit  125 M demodulate the camera HD signal from the OFDM signals. The control unit  127  captures the camera HD signal and generates the camera HD return signal from the camera HD signal. 
     The OFDM signals are explained below. 
       FIG. 2  is a graph of OFDM signals and cable loss characteristics applied to this embodiment. 
     In this embodiment, three carriers arranged at a minimum interval in the OFDM modulation are collectively allocated to one group. Spaces are set among groups. The groups are represented as group L 51 , group M 52 , and group H 53  in order from one having a lowest frequency. A camera HD signal and a return signal are allocated to each of the groups. 
     Chain lines in the figure indicate frequency bands of one wave of the OFDM signals. Three OFDM signals having lower frequencies belong to the group L 51 . Similarly, three OFDM signals having intermediate frequencies belong to the group M 52  and three OFDM signals having higher frequencies belong to the group H 53 . 
     The camera  110  transmits the HD signal to the control apparatus  120  using frequency bands of the group L 51  and the group M 52 . The control apparatus  120  transmits the return signal to the camera  110  using a frequency band of the group H 53 . 
     Since the OFDM signals are used in this way, a reception side only has to apply simple level control to OFDM waves independent from one another. Therefore, the cable equalizing circuit necessary in the past is unnecessary and a reduction in cost of a transmitting apparatus can be realized. Since the OFDM signals are digital signals, there is an advantage that the same signal quality is obtained in both a maximum cable extension point (a demodulation limit point) and a near distance point as long as a received C/N value is within a demodulation range. 
     In  FIG. 2 , the abscissa indicates a frequency and the ordinate indicates a cable loss. A loss characteristic 54 obtained when cable length is 1 km, a loss characteristic 55 obtained when the cable length is 500 m, and a loss characteristic 56 obtained when the cable length is 10 m are shown in the figure. As it is evident from  FIG. 2 , as the frequency is higher, an attenuation amount of a signal is larger. A fluctuation amount of a signal level of the signal is larger as the cable length is larger. 
     In this embodiment, even a reception OFDM signal having large fluctuation in a signal level in this way, which is received when the cable length is large, is converted into a signal level within a fixed range by using the step AGC circuit. 
     The reception-signal (H) processing unit  114 , the reception-signal (M) processing unit  124 M, and the reception-signal (L) processing unit  124 L including such a step AGC circuit are explained below with reference to  FIG. 3 . In the following explanation, since the reception-signal (L) processing unit  124 L and the reception-signal (M) processing unit  124 M have the same structure, the reception-signal (L) processing unit  124 L and the reception-signal (M) processing unit  124 M are collectively referred to as reception-signal processing unit  124 . Similarly, the OFDM demodulation unit  125 L and the OFDM demodulation unit  125 M are collectively referred to as OFDM demodulation unit  125 . 
       FIG. 3  is a block diagram of a configuration example of a reception-signal processing unit in which a signal area for a reception signal is divided into four. An example on the control apparatus  120  side is shown in  FIG. 3 . 
     The reception-signal processing unit  124  includes a BPF  410 , a step AGC circuit  430 , an AGC  450 , and a frequency converting unit  460 . 
     The BPF  410  further separates, wave by wave, one group of the OFDM signals separated from the MPX filter  123  (see  FIG. 1 ). The AMP  420  performs adjustment of gains of the OFDM signals separated by the BPF  410 . The step AGC circuit  430  subjects a reception signal output from the BPF  410  to level conversion according to a signal level and compresses level fluctuation in the reception signal. 
     The AGC  450  converts the level of the signal output from the step AGC circuit  430  into −15 dBm at which best output S/N of the mixer in the frequency converting unit  460  is obtained and outputs the signal to the mixer. In this embodiment, it is assumed that the gain of the mixer is fixed to 0 dB. 
     The frequency converting unit  460  frequency-converts the output signal of the step AGC circuit  430  into a demodulated band. 
     Configuration Example of the Step AGC Circuit 
     The step AGC circuit  430  is explained below. The step AGC circuit  430  includes an amplifier (hereinafter referred to as an AMP)  431 , an AMP  432 , an AMP  433 , and an AMP  434  and a switch (hereinafter referred to as an SW)  435 , an SW  436 , an SW  437 , and an SW  438 . The amplifiers include an amplifier that operates as an attenuator, the gain of which is minus. 
     The step AGC circuit  430  includes an RSSI (Received Signal Strength Indicator) detection circuit  439  as a signal-level detecting circuit, an SW control unit  440  as a switching control circuit, and a hold circuit  441 . 
     The AMP  431 , the AMP  432 , the AMP  433 , and the AMP  434  are connected in parallel with respect to an input signal input from the BPF  410 . The AMP  431 , the AMP  432 , the AMP  433 , and the AMP  434  convert the input signal with gains respectively set therein and output the input signal. In this embodiment, as explained above, signal level fluctuation of the input signal that occurs when the coaxial cable is extended to about 1 km is compressed to the fixed level range. 
     Gain setting of the step AGC circuit  430  is explained below.  FIG. 4  is a graph of an example of gain setting performed when the signal level area is divided into four. 
     An example of gains respectively set in the AMP  431 , the AMP  432 , and the AMP  433  is shown in  FIG. 4 . The step AGC circuit  430  in this example divides the signal level area into four according to an attenuation amount of a signal level of an input signal. 
     The divided signal level areas include a shortest distance level  4311  in which a distance of a cable is extremely short and an attenuation amount is the smallest, a short distance level  4321  in which the distance of the cable is longer than the shortest distance and the attenuation amount is larger, an intermediate distance level  4331  in which the distance of the cable is an intermediate distance and the attenuation amount is still larger, and a long distance level  4341  in which the distance of the cable is long and the attenuation mount is the largest. 
     The AMP  431  is allocated to the shortest distance level  4311 , the AMP  432  is allocated to the short distance level  4321 , the AMP  433  is allocated to the intermediate distance level  4331 , and the AMP  434  is allocated to the long distance level  4341 . The gain of the AMP  431  is set to −12 dB. The AMP  431  reduces an input signal level of the shortest distance level  4311  (an attenuation of a signal level is about −5 dBm to −15 dBm) by 12 dB. 
     The gain of the AMP  432  is set to −4 dB. The AMP  432  reduces an input signal level of the short distance level  4321  (an attenuation amount of a signal level is about −15 dBm to −35 dBm) by 4 dB. The gain of the AMP  433  is set to +16 dB. The AMP  433  increases an input signal level of the intermediate distance level  4331  (an attenuation amount of a signal level is about −35 dBm to −55 dBm) by 16 dB. The gain of the AMP  434  is set to +36 dB. The AMP  434  increases an input signal level of the long distance level  4341  (an attenuation amount of a signal level is −55 dBm to −75 dBm) by 36 dB. 
     The SW  435 , the SW  436 , the SW  437 , and SW  438  perform, according to an SW control signal input via the hold circuit  441 , switching control to output a converted output of a selected AMP to the AGC  450 . For example, when the input signal is within the shortest distance level  4311  in which the attenuation amount is the smallest, the AMP  431  (G=−12 dB) and the SW  435  are turned on and the other amplifiers and switches are turned off. When the input signal is within the short distance level  4321 , the AMP  432  (G=−4 dB) and the SW  436  are turned on and the other amplifiers and switches are turned off. 
     When the input signal is within the intermediate distance level  4331 , the AMP  432  (G=16 dB) and the SW  437  are turned on and the other amplifiers and switches are turned off. When the input signal is within the long distance level  4341 , the AMP  433  (G=40 dB) and the SW  437  are turned on and the other amplifiers and switches are turned off. In this way, any one of the AMPs connected in parallel and the SW connected in series to the AMP are selected according to the input signal level. 
     The RSSI detection circuit  439  outputs an RSSI voltage signal corresponding to a signal level (−75 dBm to −5 dBm) of the input signal. 
     The SW control unit  440  selects, according to the RSSI voltage signal of the RSSI detection circuit  439 , an AMP optimum for converting the level of the input signal into a fixed level range. Specifically, the SW control unit  440  generates an SW control signal for controlling switching of the AMPs and outputs the SW control signal to the hold circuit  441 . 
     The hold circuit  441  selects, according to a hold control signal input from the outside, whether the SW control signal input from the SW control unit  440  should be transmitted to the AMPs and the SWs. When an instruction for fixing an AMP currently selected by instruction button operation or the like is input from the outside such as a user, the hold control signal is set to “hold”. When the hold control signal is “hold”, the hold circuit  441  does not transfer the SW control signal input from the SW control unit  440 . 
     Consequently, a switching state of the AMP  431 , the AMP  432 , the AMP  433 , and the AMP  434  and the SW  435 , the SW  436 , the SW  437 , and the SW  438  is not changed and an immediately preceding switching state is continued. When the hold control signal is set to “not hold”, the hold circuit  441  transfers the SW control signal input from the SW control unit  440  to the AMP  431 , the AMP  432 , the AMP  433 , and the AMP  434  and the SW  435 , the SW  436 , the SW  437 , and the SW  438 . Consequently, an AMP to be used is selected anew on the basis of an SW control signal input from the SW control unit  440 . 
     The fixing of the switching state by the hold circuit  441  is performed for the purpose of, for example, preventing signal interruption caused by switching of the AMPs. The switching of the AMPs is performed according to occurrence of switch changeover due to attenuation amount fluctuation in a signal level caused by a temperature change or the like of the cable and fluctuation in a detection signal of the RSSI detection circuit  439  caused by an interference signal such as interference or jump-in. 
     If such switching is performed during an actual broadcast, an image is interrupted a moment. 
     Therefore, the switching state is fixed by the hold circuit  441  when the AMPs are switched at the start of the actual broadcast. This makes it possible to prevent occurrence of a problem such as momentary interruption of the image during the actual broadcast. 
       FIG. 5  is a diagram of level changes in an input signal and an output signal by the step AGC circuit in this example. A fluctuation range of the input signal on the ordinate indicates the level range of the input signal (the abscissa) shown in  FIG. 4 . An output signal range  4302  indicates a level range of the output signal after being converted by the step AGC circuit  430 . 
     As shown in the figure, the fluctuation range of the input signal includes the shortest distance level  4311  in which the signal level is the largest, the short distance level  4321 , the intermediate distance level  4331 , and the long distance level  4341 . Overlaps are secured in the respective level areas. Since the level  4321  at the time of the cable short distance is set as a reference, the output signal range  4302  is within the range of the level  4321  at the time of the cable short distance. 
     When the input signal level is within the level  4321  at the time of the cable short distance, the input signal is directly converted into the short distance range  4322 . When the input signal level is within the shortest distance level  4311 , the input signal is attenuated at a gain of −12 dB and converted into the shortest distance range  4312  in the output signal range  4302 . 
     When the input signal level is within the intermediate distance level  4331 , the input signal is amplified at a gain of 16 dB and converted into the intermediate distance range  4332  in the output signal range  4302 . When the input signal level is within the long distance level  4341 , the input signal is amplified at a gain of +36 dB and converted into the long distance range  4342  in the output signal range  4302 . 
     In this way, the signal levels of the signal level areas obtained by dividing the fluctuation range of the input signal into four are converted and compressed to the output signal range  4302 . 
     Any one of the AMP  431  to the AMP  434  is selectively applied in this way. This makes it possible to compress a signal level of a reception signal from the input signal range  4301  (−75 dBm to −5 dBm) to the output signal range  4302  (−39 dBm to −17 dBm). 
     The configuration shown in  FIG. 5  is only an example. The number of AMPs to be set and gains are set as appropriate. The hold circuit  441  controls, according to a hold control signal, propriety of a change of a switching state by the SW  435 , the SW  436 , the SW  437 , and the SW  438 . 
     Configuration Example of the AGC at the Post-Stage of the Step AGC Circuit 
     A configuration example of the AGC  450  at the post-stage of the step AGC circuit  430  is explained below with reference to  FIGS. 6 to 10 . The AGC  450  in this example supplies a signal, which is input from the step AGC circuit  430 , to the frequency converting unit  460  after fixing the level of the signal to a predetermined level and removing an image frequency included in the reception signal. 
     First, processing for fixing a signal level in the AGC  450  is explained.  FIG. 6  is a schematic diagram of the signal level fixing processing in the AGC  450 . A signal of −30 dBm to 0 dBm, a band of which is compressed by the STEP AGC circuit  430 , is input to the AGC  450 . The AGC  450  fixes (converts) the level of the input signal to −15 dBm and outputs the signal to the frequency converting unit  460 . 
     −15 dBm is a level at which a maximum value of output S/N of the mixer (not shown in the figure) in the frequency converting unit  460  is obtained. The value depends on an output S/N characteristic of the mixer.  FIG. 7  is a graph of the output S/N characteristic of the mixer. The abscissa indicates a level (dBm) of an input signal and the ordinate indicates output S/N (dBc). It is seen that, in  FIG. 7 , a maximum dynamic range is obtained near the input signal level of −15 dBm of the mixer. 
     The AGC  450  in this example typically supplies a signal of −15 dBm to the mixer having such a characteristic. 
     An internal configuration example of the frequency converting unit  460  is explained below with reference to  FIG. 8 . The frequency converting unit  460  includes a PLL (Phase-Locked Loop) unit  461 , a local oscillator  462 , a mixer  463 , a variable-gain amplifier  464 , a SAW (Surface Acoustic Wave) filter  465 , and a variable-gain amplifier  466 . 
     The local oscillator  462  generates, on the basis of the control by the PLL unit  461 , a local oscillation signal necessary for generating an intermediate frequency signal in the mixer  463  and supplies the local oscillation signal to the mixer  463 . The mixer  463  mixes the signal of −15 dBm input from the AGC  450  (see  FIG. 6 ) and the local oscillation signal input from the local oscillator  462  and converts the mixed signals into an intermediate frequency signal. 
     An output S/N characteristic of the mixer  463  is as shown in  FIG. 7 . Therefore, when the signal of −15 dBm is input, deterioration in an output signal from the mixer  463  is minimized. 
     The variable-gain amplifier  464  amplifies the intermediate frequency signal generated by the mixer  463  and outputs the intermediate frequency signal to the SAW filter  465 . The SAW filter  465  allows only a frequency band for one channel of an OFDM signal to pass and supplies the signal to the variable-gain amplifier  466 . The variable-gain amplifier  466  amplifies the gain of the signal having passed through the SAW filter  465  and outputs the signal to the OFDM demodulation unit  125  (see  FIG. 6 ). 
     A configuration example of an image frequency removal processing section in the AGC  450  in this example is explained below with reference to a block diagram of  FIG. 9 . In order to secure a wide cover range, the AGC  450  shown in  FIG. 9  includes two stages of a variable-gain amplifier  451  and a variable-gain amplifier  452 . 
     The amplifiers are coupled by a capacitor coupling system and a two-stage high-pass filter (HPF)  453  is configured. In the two-stage HPF  453 , A capacitor C 1  used for the coupling and a resistor R 1  as an input resistor in the inside of the variable-gain amplifier  452  configure an HPF in the first stage and a capacitor C 2  and the resistor R 1  configure an HPF in the second stage. 
     In order to secure the amplitude of an output signal, an amplifying unit including a resistor R 2 , a resistor R 3 , and an amplifier  454  is provided at a post-stage of the variable-gain amplifier  452 . A series circuit of a resistor R 4  and a capacitor C 3  connected in parallel to the resistor R 3  and a resistor R 5  and a capacitor C 4  provided at a post-stage of the amplifier  454  configure a low-pass filter (LPF)  455 . 
     Specifically, the two-stage high-pass filter and the LPF  455  configure a band-pass filter (BPF). A pass frequency characteristic in the AGC  450  including such a band-pass filter is shown in  FIG. 10 .  FIG. 10  indicates that the AGC  450  allows signals in a range of 10 MHz to 100 MHz as a desired frequency band to pass and cuts frequencies lower than the range and frequencies higher than the range. 
     In other words, a function of the band-pass filter is imparted to the AGC  450 . This makes it possible to delete an image frequency higher than a reception frequency and extract only a signal component necessary for demodulation. 
     EFFECTS BY THIS EMBODIMENT 
     According to the embodiment explained above, when OFDM signals are transmitted between the camera  110  and the control apparatus  120 , the plural AMPs corresponding to a signal level are switched to perform level conversion. This makes it possible to compress a fluctuation amount of the signal level, which increases according to an increase in a cable distance, to a signal level range suitable for the OFDM demodulation unit  125 . 
     According to the embodiment, the level of the signal input to the mixer  463  is fixed to a value (in the example explained above, −15 dBm) at which the output S/N of the mixer is the highest. Therefore, compared with the configuration in the past in which signals of frequency bands in a wide range are input to the mixer  463 , it is possible to improve the S/N of a demodulated signal (hereinafter also referred to as reception signal) in the OFDM demodulation unit  115  or the OFDM demodulation unit  125  (see  FIG. 1 ). 
     In  FIG. 11 , an output S/N characteristic of the AGC  450  and an output S/N characteristic of the mixer  463  are shown on one graph. The abscissa of the graph indicates a level (dBm) of an input signal and the ordinate indicates output S/N (dBc). A broken line indicates a characteristic of the mixer  463  and a solid line indicates a characteristic of the AGC  450 . 
     When the AGC  450  in this example is not used, a signal in a range of −39 dBm to −17 dBm output from the step AGC circuit  430  is directly input to the mixer  463 . Therefore, when a signal level of a signal input to the mixer  463  is −39 dBm, the output S/N is about 27 dBc. 
     On the other hand, output S/N of the AGC  450  obtained when the input signal level is −39 dBm increases to about 43 dBc. In other words, when the AGC  450  in this example is used, the output S/N of the mixer  463  is improved in a range indicated by dots in the figure. 
       FIG. 12  is a graph of an S/N characteristic of a signal (a reception signal) after a signal gain-adjusted by the AMP  432  (see  FIG. 3 ) in the step AGC circuit  430 , i.e., a signal classified into the short distance level  4321  is demodulated by the OFDM demodulation unit  125 . The ordinate indicates S/N (dB) of the reception signal and the abscissa indicates a cable distance (cable length). 
     The cable length can be replaced with the level of the signal. The signal level is smaller as the cable length is larger. The signal level is larger as the cable length is smaller. A broken line in the figure indicates the S/N of a reception signal in the past and a solid line indicates the S/N of a reception signal in this embodiment. 
     In the configuration in the past, the S/N of the signal is suddenly deteriorated around the cable length exceeding 250 m. The S/N falls to about 21 dB near 700 m. On the other hand, in the configuration according to this embodiment, S/N of about 28 dB can be maintained even near 700 m. 
     Therefore, with the camera  100  and the control apparatus  120  according in this example including the step AGC circuit  430  and the AGC  450 , since deterioration of a signal caused in a process of transmitting the signal through the communication cable is suppressed, the communication cable length can be extended. Specifically, compared with the configuration in which only the step AGC circuit  430  is provided, the communication cable length can be extended by about several hundreds m. 
     Since the S/N of the reception signal is improved in this way, the number of errors included in the reception signal is also reduced. It is confirmed that, when the S/N of the reception signal is improved by 1 dB, errors are reduced to about 1/10. Consequently, since sections to be corrected by error correction such as Reed-Solomon or Viterbi are also reduced, deficiencies in a video signal that occur when the error correction does not work can be minimized. 
     Specifically, it is possible to improve a quality of signal transmission and improve reliability of the video signal transmission system. In particular, when a video signal for a television broadcast is handled, the receiving apparatus according to the embodiment is effective because occurrence of interruption during a broadcast of a video by the video signal is not permitted. 
     According to this embodiment, a range in which the S/N of the reception signal with respect to a cable length change is high is wider than in the past, even if there is fluctuation in switching accuracy of the step AGC circuit  430 , it is possible to prevent deterioration in the S/N. 
       FIG. 13  is a graph of S/N characteristics of a reception signal of the long distance level  4341  gain-adjusted by the AMP  433  (see  FIG. 3 ) and a reception signal of the intermediate distance level  4331  gain-adjusted by the AMP  434 . 
     A scale of the figure is the same as that shown in  FIG. 12 . A broken line indicates the S/N of a reception signal by the receiving apparatus  200  in the past. A solid line indicates the S/N of a reception signal by the camera  110  or the control apparatus  120  according to the embodiment. The reception signal gain-adjusted by the AMP  433  is indicated by a line plotted by circles. The reception signal gain-adjusted by the AMP  434  is indicated by a line plotted by squares. 
     In the receiving apparatus in the past, the S/N of the reception signal gain-adjusted by the AMP  434  suddenly falls to about 27 dB at a point of the cable length of 400 m as indicated by the broken line on which square identifiers are arranged. Therefore, to secure the S/N of the reception signal, it is necessary to switch the amplifier from the AMP  434  to the AMP  433  at this point of time. The S/N of the reception signal can be kept at about 28.5 dB by switching the amplifier to the AMP  433 . 
     Conversely, when the switching of the amplifier is not performed at this timing, the S/N of the reception signal is deteriorated. 
     On the other hand, with the configuration in this example, the S/N of the reception signal gain-adjusted by the AMP  434  is maintained at a high value of about 28.4 dB even at the point of 400 m as indicated by the solid line plotted by squares. This makes it possible to keep the S/N of the reception signal without strictly performing the switching of the amplifier at this point of time. 
     There is fluctuation in manufacturing of the RSSI detection circuit  439  necessary for the switching of the amplifier. When there is a shift in a value of an RSSI voltage signal depending on a product, a shift also occurs in timing of the switching of the amplifier. However, with the configuration in this example, even if the switching timing for the amplifier is slightly earlier or later, it is possible to keep the S/N of the reception signal at a high level. Therefore, mass-productivity is improved because a request for manufacturing accuracy of the RSSI detection circuit  439  is low. 
     In this embodiment, since the band-pass filter is incorporated in the AGC  450 , the image frequency component is not supplied to the mixer  463 . This makes it possible to improve the output S/N of the mixer  463  compared with that obtained when the BPF is not provided. According to an example of an experiment performed by the inventor, improvement of about 3 dB was obtained at the maximum. The S/N of a video signal demodulated by the OFDM demodulation unit  125  was also improved. 
     In this case, since the BPF is incorporated in the AGC  450 , the number of circuit components can also be reduced. 
     MODIFICATIONS OF THE EMBODIMENT 
     In the example explained in the embodiment, the signal area of the reception signal is divided into four by the reception-signal processing unit  124  (see  FIG. 3 ). However, the present invention is not limited to this. For example, the signal area may be divided into three, five, or the like. 
     In the embodiment, the gain of the AMP  431  of the step AGC circuit  430  is set to −12 dB, the gain of the AMP  432  is set to −4 dB, the gain of the AMP  433  is set to +16 dB, and the gain of the AMP  434  is set to +36 dB. However, the present invention is not limited to this. Different gains may be set for the respective AMPs. 
     In the embodiment, the level of the output signal from the AGC  450  is set to −15 dBm. However, this value is only an example. Other values may be set as the level of the output signal according to the performance of the mixer  463 . 
     In the example explained in the embodiment, the AMP  431 , the AMP  432 , the AMP  433 , and the AMP  434  in the step AGC circuit  430  are connected in parallel. However, the present invention is not limited to this configuration. The AMPS may be connected in cascade or the parallel connection and the cascade connection may be combined. 
     In the embodiment, the reception-signal processing unit  114  of the camera  110  has the same configuration as the reception-signal processing unit  124  of the control apparatus  120 . However, the hold circuit  441  may be removed in the reception-signal processing unit  114  of the camera  110 . Since the reception signal of the camera  110  is the camera HD return signal, a serious problem does not occur even when interruption occurs. Therefore, the hold circuit  441  provided to prevent interruption of a signal does not have to be included in the reception-signal processing unit  114 . 
     In the example explained in the embodiment, the gain of the mixer  463  is 0 dB. However, the present invention is not limited to this. For example, when the gain of the mixer  463  is +5 dB or the like, an AGC for adjusting the level of an input signal to the OFDM demodulation unit  125  may be provided at a pre-stage of the OFDM demodulation unit  125 . 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2009-028821 filed in the Japan Patent Office on Feb. 10, 2009, the entire contents of which is hereby incorporated by reference. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.