Abstract:
Disclosed herein is an analog television broadcast signal receiving apparatus including: a tuner section configured to convert an analog television broadcast signal into a predetermined intermediate frequency band signal; a demodulation circuit section configured to obtain a picture output signal and a sound intermediate frequency signal from the predetermined intermediate frequency band signal coming from the tuner section; a picture processing circuit section configured to convert the picture output signal into a display-ready picture signal; a sound demodulation processing circuit section configured to demodulate the sound intermediate frequency signal; and a control section.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to an analog TV (television) broadcast signal receiving apparatus and an analog TV broadcast signal demodulating apparatus for use therewith, the analog TV broadcast signal receiving apparatus being designed illustratively to deal with diverse analog TV broadcasts used all over the world. 
         [0003]    2. Description of the Related Art 
         [0004]    A number of analog TV broadcast systems exist throughout the world. They differ from one another in terms of broadcast bandwidths and in frequency assignments of the picture and sound carriers involved. More specifically, there are three principal broadcast bandwidths: 6 (4.2) MHz, 7 (5) MHz, and 8 (6) MHz (the numbers in parentheses denote picture signal bandwidths). 
         [0005]    The 6 MHz band is the broadcast band for the analog TV system used mainly in the United States. The 7/8 MHz band is the broadcast band for the analog TV system employed primarily in the European Union (Europe). 
         [0006]    On ordinary broadcast bands (RF bands), the sound carrier is assigned a higher frequency than the picture carrier. In Germany, for example, Channel 2 has its picture and sound carrier frequencies appropriated at 48.25 MHz and 53.75 MHz, respectively. 
         [0007]    In some regions of France, however, the picture carrier is assigned a higher frequency than the sound carrier. For example, the station called Channel A has its picture and sound carrier frequencies appropriated at 47.75 MHz and 41.25 MHz, respectively. 
         [0008]    Except for the different frequency assignments of its picture and sound carriers, however, the above-mentioned French analog TV broadcast system has the same picture signal bandwidth and the same frequency interval between the picture and the sound carriers as the analog TV broadcast systems in the other regions. The channel of which the picture signal is assigned a higher frequency than the sound carrier is commonly referred to as Channel L′. 
         [0009]    There have been attempts to devise analog TV broadcast signal receiving apparatuses capable of demodulating the TV signals of all the analog TV broadcast systems outlined above. Generally, these analog TV broadcast signal receiving apparatuses have their filters arranged to limit the picture and sound signal bands on the IF (intermediate frequency) band for each of the broadcast bandwidths involved, and are each furnished with mechanisms to switch between such band limitation filters. 
         [0010]    More specifically, the vestigial sideband (VSB) filter for limiting the picture signal band and the band limitation filter for limiting the sound signal band are switched in keeping with the picture and sound carriers on each broadcast band. Generally, surface acoustic wave (SAW) filters are used to serve as the VSB filter for limiting the picture signal band and as the filter for limiting the sound signal. 
         [0011]      FIG. 9  schematically shows a typical structure of an ordinary analog TV broadcast signal receiving apparatus designed for use in the United States and in the European Union. In  FIG. 9 , an analog TV broadcast signal received by a receiving antenna  101  is shown forwarded to an RF tuner  102 . In the RF tuner  102 , the received analog TV broadcast signal (i.e., RF signal) is converted in frequency to an IF signal TVif by a mixer  104  using a frequency signal coming from a variable frequency oscillator  103 . The IF signal TVif is output by the RF tuner  102  by way of an amplifier  105 . 
         [0012]      FIGS. 10A ,  10 B and  10 C schematically show how the IF signal TVif behaves on the 6, 7, and 8 MHz bands, respectively.  FIG. 10A  indicates the IF signal on the 6 MHz band used mainly in the United Sates.  FIG. 10B  represents the IF signal on the 7 MHz band employed primarily in Europe.  FIG. 10C  denotes the IF signal on the 8 MHz band utilized also in Europe. As shown in these figures, the IF signal differs from one band to another. That means each IF signal needs to have the appropriate SAW filter selected for use therewith. 
         [0013]    The SAW filter selection is accomplished by first feeding the IF signal TVif from the RF tuner  102  to VSB filters (SAW filters)  106  and  107  for picture signal use and to sound selection SAW filters  108  and  109  for sound signal use. The VSB filter  106  is a SAW filter for use on the 6 MHz band (4.2 MHz for the picture signal band); the VSB filter  107  is a SAW filter used on the 7/8 MHz band (5/6 MHz for the picture signal band); the sound selection SAW filter  108  is utilized on the 6 MHz band; and the sound selection SAW filter  109  is employed on the 7/8 MHz band. 
         [0014]    The outputs of the VSB filters  106  and  107  are supplied to a selector  110 . The outputs of the sound selection SAW filters  108  and  109  are fed to a selector  111 . The selectors  110  and  111  are controlled selectively by a host control section  130  providing overall control on the receiving apparatus as a whole. 
         [0015]    The host control section  130  is composed of a microcomputer typically called a TV microcomputer. For each of the different markets (e.g., U.S., Europe) for which the receiving apparatus is destined, destination-oriented settings are registered in the host control section  130 . 
         [0016]    The host control section  130  receives user operation input information from a remote control transmitter  137 . The user operation input information includes broadcast channel selection operation information. Upon receipt of the broadcast channel selection operation information from the remote control transmitter  137 , the host control section  130  generates a selection control signal reflecting the destination-oriented setting information that has been registered inside and forwards the generated selection control signal to the variable frequency oscillator  103 . In turn, the TV broadcast signal of the selected broadcast channel is converted to the corresponding IF signal TVif. 
         [0017]    The host control section  130  also supplies the selectors  110  and  111  with selection signals in keeping with the destination-oriented setting information that has been registered in this receiving apparatus. Illustratively, if the receiving apparatus is set to receive the analog TV broadcast signal in the United States, then the microcomputer  130  causes the selector  110  to select the VSB filter  106  for the 6 MHz band. Similarly, if the receiving apparatus is arranged to receive the analog TV broadcast signal in the European Union, then the microcomputer  130  causes the selector  110  to select the VSB filter  107  for the 7/8 MHz band. The selector  110  outputs a picture IF signal Vif having undergone VSB demodulation. 
         [0018]    Meanwhile, each sound signal requires that the applicable SAW filter be used to remove the picture signal from the IF signal TVif. For example, if the receiving apparatus is set to receive the analog TV broadcast signal on the 6 MHz band, the microcomputer  130  causes the selector  110  to select the sound selection SAW filter  108  for the 6 MHz band so as to remove the picture signal. Likewise, if the receiving apparatus is designed to receive the analog TV broadcast signal on the 7/8 MHz band, then the microcomputer  130  causes the selector  110  to select the sound selection SAW filter  109  for the 7/8 MHz band in order to eliminate the picture signal. 
         [0019]    The picture IF signal Vif from the selector  110  and a sound band signal Sa from the selector  111  are supplied to an analog TV demodulation circuit  120 . This analog TV demodulation circuit  120  is constituted by an LSI (large scale integrated circuit). 
         [0020]    In the analog TV demodulation circuit  120 , the picture IF signal Vif is fed to a multiplier  122  via an amplifier  121 . The multiplier  122  multiplies the supplied picture IF signal by a frequency signal having the picture carrier frequency coming from a variable frequency oscillator  123 . Although not shown, the variable frequency oscillator  123  is controlled by control signals from the host control section  130  in such a manner that the oscillation frequency of the oscillator  123  stays equal to the picture carrier frequency of the analog TV broadcast desired to be received. 
         [0021]    A multiplication output signal from the multiplier  122  and the frequency signal from the variable frequency oscillator  123  are sent to a phase comparator  124 . In turn, the phase comparator  124  generates a control signal reflecting the detected phase error between the two inputs and feeds the generated control signal to the variable frequency oscillator  123 . These arrangements constitute a phase-locked loop (PLL). 
         [0022]    In the PLL, the variable frequency oscillator  123  is controlled to output its frequency signal in synchronism with the picture carrier frequency. As a result, the multiplier  122  provides a baseband signal obtained by converting the picture carrier frequency of the picture IF signal Vif resulting from VSB demodulation into the baseband. 
         [0023]    The baseband signal of the picture signal passes through a low-pass filter  125  that removes unnecessary signal components and is output by the analog TV demodulation circuit  120  as its output picture signal. This output picture signal is supplied to a video processor  131 . 
         [0024]    The video processor  131  performs color signal processing and other processes. The picture signal processed by the video processor  131  is fed to an LCD (liquid crystal display) panel  132  through an LCD panel driver, not shown. The picture signal is then output as a picture on the display screen of the LCD panel  132 . 
         [0025]    Meanwhile, the sound band signal Sa from the selector  111  is sent to a multiplier  127  via an amplifier  126  in the analog TV demodulation circuit  120 . The frequency signal of the sound carrier frequency from a variable frequency oscillator  128  is also fed to the multiplier  127  and multiplied therein by the sound band signal Sa. Although not shown, the variable frequency oscillator  128  is controlled by control signals from the host control section  130  in such a manner that the oscillation frequency of the oscillator  128  stays equal to the sound carrier frequency of the analog TV broadcast desired to be received. The variable frequency oscillator  128 , controlled in frequency by the control signal coming from the phase comparator  124 , outputs the frequency signal in synchronism with the sound carrier frequency. 
         [0026]    In the manner outlined above, the multiplier  127  converts the sound band signal Sa into the sound signal on the 4 to 6 MHz band. The 4 to 6 MHz band sound signal from the multiplier  127  is submitted to a band-pass filter  129  for removal of unnecessary signal components. Thereafter the sound signal from the band-pass filter  129  is output to a sound demodulation circuit  133 . The sound demodulation circuit  133 , also constituted by an LSI, demodulates the input sound signal into sounds that are supplied to and output acoustically by speakers  134 . 
         [0027]    As mentioned above, the signal L′ used in parts of France is configured such that the way its picture and sound carriers are assigned in inverse relation with each other in frequency as opposed to the carriers of the signals employed elsewhere. The reversed carrier assignments require having the SAW filters switched accordingly. A typical structure of a traditional receiving apparatus that takes these specificities into account is shown schematically in  FIG. 11 . The difference of the structure of  FIG. 11  from that of  FIG. 9  is that the VSB filter  106  for 6 MHz and the sound selection SAW filter  109  for 6 MHz are removed and replaced simply by a sound SAW filter  135  for the signal L′. For the related art regarding the present information, reference should be made to Japanese Patent Laid-Open No. 2006-191388. 
       SUMMARY OF THE INVENTION 
       [0028]    As explained above, there are a plurality of signal bands along with different IF signal bands for the analog TV signals used all over the world. There also exist the signals such as L′ of which the picture and sound carriers are assigned in inverse relation with each other in frequency as opposed to the carriers of the other signals as mentioned earlier. 
         [0029]    Where the signals of these multiple broadcast systems are to be demodulated, it is necessary for traditional analog TV broadcast signal receiving apparatuses to switch their SAW filters in accordance with each of the broadcast systems involved. This has been a costly solution to the problem of the coexisting multiple signal systems. It has been difficult to devise an apparatus capable of receiving and demodulating all analog TV broadcasts the world over. 
         [0030]    The present invention has been made in view of the above circumstances and provides an analog TV broadcast signal receiving apparatus capable of receiving and demodulating the analog TV broadcast signals anywhere in the world. 
         [0031]    In carrying out the present invention and according to one embodiment thereof, there is provided an analog TV broadcast signal receiving apparatus including: a tuner section configured to convert an analog TV broadcast signal into a predetermined intermediate frequency band signal; a demodulation circuit section configured to obtain a picture output signal and a sound intermediate frequency signal from the predetermined intermediate frequency band signal coming from the tuner section; a picture processing circuit section configured to convert the picture output signal into a display-ready picture signal; a sound demodulation processing circuit section configured to demodulate the sound intermediate frequency signal; and a control section; wherein the demodulation circuit section includes: an A/D conversion section configured to convert the predetermined intermediate frequency band signal coming from the tuner section into an intermediate frequency band digital signal; a complex frequency conversion section configured to convert the intermediate frequency band digital signal into a baseband frequency band digital signal by multiplying the intermediate frequency band digital signal coming from the A/D conversion section by a frequency signal coming from a complex variable frequency oscillator; a variable band limitation filter configured to band-limit the digital signal output by the complex frequency conversion section; a picture/sound separation section configured to have a picture/sound separation filter section, the picture/sound separation section being further configured to separate the digital signal coming from the variable band limitation filter into a picture signal band digital signal and a sound signal band digital signal; a VSB (vestigial sideband) filter configured to demodulate the picture signal band digital signal coming from picture/sound separation section into a picture signal component; a first digital-to-analog conversion section configured to convert a digital signal from the VSB filter into an analog picture signal before feeding the analog picture signal to the picture processing circuit section; and a second digital-to-analog conversion section configured to convert the sound signal band digital signal coming from the picture/sound separation filter section into an analog sound signal before supplying the analog sound signal to the sound demodulation processing circuit section; the control section includes a storage section configured to store a plurality of sets of a control signal for controlling the complex variable frequency oscillator in oscillation frequency, a filter coefficient for the variable band limitation filter, a filter coefficient for the picture/sound separation filter section, and a filter coefficient for the VSB filter with regard to each of a plurality of TV broadcasts; and when settings are input to receive selectively a TV broadcast, the control section retrieves from the storage section the set of the control signal and of the filter coefficients corresponding to the selected TV broadcast and feeds the retrieved set to the sections involved. 
         [0032]    In the above-outlined structure of the analog TV broadcast signal receiving apparatus according to the invention, the demodulation circuit section converts the picture and sound signals on the intermediate frequency signal band altogether into a digital signal. The digital signal is turned into a baseband signal through complex frequency conversion. Following the frequency conversion, the signal is submitted to the variable band limitation filter for removal of unnecessary band components. The signal past the variable band limitation filter is separated by the picture/sound separation filter section into a picture signal band digital signal and a sound signal band digital signal. 
         [0033]    The picture signal band digital signal is fed to the VSB filter for demodulation of the picture signal component into an analog picture signal. The analog picture signal is then sent to the picture processing circuit section for conversion into a display-ready picture signal. The sound signal band digital signal is supplied to the sound demodulation processing circuit section for demodulation into sounds. 
         [0034]    According to the analog TV broadcast signal receiving apparatus of the present invention, it is possible to switch the frequency of the complex frequency conversion section for frequency conversion as well as the filter coefficients for the variable band limitation filter and for the picture/sound separation filter section in accordance with the TV broadcast selected to be received. 
         [0035]    That is, the inventive analog TV broadcast signal receiving apparatus is different structurally from traditional setups in which a plurality of SAW filters are provided to address diverse TV broadcasts and are switched in accordance with the selected TV broadcast to be received. The analog TV broadcast signal receiving apparatus of the invention need only switch the signal frequency for frequency conversion as well as the filter coefficients in keeping with the analog TV broadcast to be received. 
         [0036]    The inventive analog TV broadcast signal receiving apparatus is capable of demodulating picture and sound signals without recourse to a plurality of SAW filters. Because the same demodulation circuit can be used in different geographical regions adopting different TV broadcast systems, significant cost savings are accomplished in manufacturing the receiving apparatus. According to an embodiment of the present invention, as outlined above, the analog TV broadcast signal receiving apparatus can demodulate a plurality of analog TV broadcast signals used all over the world using an inexpensive structure free of SAW filters. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]    Further advantages of the present invention will become apparent upon a reading of the following description and appended drawings in which: 
           [0038]      FIG. 1  is a block diagram showing a typical structure of an analog TV broadcast signal receiving apparatus practiced as one embodiment of the present invention; 
           [0039]      FIGS. 2A ,  2 B,  2 C and  2 D are schematic views explanatory of how the embodiment of the invention typically works during its demodulating process; 
           [0040]      FIG. 3  is a block diagram showing a typical structure of a variable band picture/sound separation filter used by the embodiment of this invention; 
           [0041]      FIGS. 4A ,  4 B,  4 C,  4 D and  4 E are schematic views explanatory of how the variable band picture/sound separation filter of  FIG. 3  typically works; 
           [0042]      FIG. 5  is a graphic representation showing typical frequency characteristics of variable band VSB filters used by the embodiment of this invention; 
           [0043]      FIG. 6  is a tabular view showing typical setting data about various analog TV broadcast signals used by the embodiment of this invention; 
           [0044]      FIGS. 7A and 7B  are schematic views explanatory of the IF signal in effect when the signal L′ is received; 
           [0045]      FIGS. 8A and 8B  are schematic views showing how the embodiment of this invention converts the IF signal upon receipt of the signal L′ into a complex baseband signal; 
           [0046]      FIG. 9  is a block diagram showing a typical structure of an ordinary analog TV broadcast signal receiving apparatus designed for use mainly in the United States and in the European Union; 
           [0047]      FIGS. 10A ,  10 B and  10 C are schematic views explanatory of the frequency bands of various analog TV broadcast signals used primarily in the United States and in the European Union; and 
           [0048]      FIG. 11  is a block diagram showing a typical structure of an analog TV signal receiving apparatus capable of demodulating the signal L′ used in Europe. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]    The preferred embodiments of the analog TV broadcast signal receiving apparatus according to the present invention will now be described in reference to the accompanying drawings. 
       A Typical Hardware Structure of one Analog TV Broadcast Signal Receiving Apparatus Embodying the Invention  
       [0050]      FIG. 1  is a block diagram showing a typical structure of an analog TV broadcast signal receiving apparatus practiced as one embodiment of the present invention. In  FIG. 1  as well as in  FIGS. 9 and 11  mentioned earlier, like reference numerals denote like or corresponding parts. For example, an RF tuner section  102 , a video processor  131 , an LCD panel  132 , a sound demodulation circuit  133 , and speakers  134  are structurally the same as their counterparts shown in  FIGS. 9 and 11 . 
         [0051]    The embodiment of  FIG. 1  is free of SAW filters  106  through  109 , selectors  110  and  111 , and an analog TV demodulation circuit  120  indicated in  FIG. 9 . Instead, the embodiment of  FIG. 1  includes a digital demodulation circuit section  200  that may be constituted by a single-chip IC (integrated circuit) arrangement. 
         [0052]    The digital demodulation circuit section  200  serves as an analog TV broadcast signal demodulating apparatus of the present invention. Illustratively, the digital demodulation circuit section  200  includes a demodulation circuit control section  200  having a microprocessor capability and a setting data memory  221  for holding the setting data about various analog TV broadcast systems. Preset in the setting data memory  221  are varieties of setting data such as the control signal for a variable frequency oscillator  203  (to be discussed later) and the control signals (filter coefficients) for the filter section in conjunction with each of the diverse analog TV broadcast systems. 
         [0053]    The host control section  130  is capable of controlling the entire receiving apparatus embodying the invention, in such a manner that each of the analog TV broadcast systems with their settings registered illustratively by an operator is suitably dealt with. The host control section  130  notifies a demodulation circuit control section  220  of a control command identifying one of the analog TV broadcast systems of which the registered settings have been selected. 
         [0054]    Given such a control command from the host control section  130 , the demodulation circuit control section  220  retrieves the setting data of the selected analog TV broadcast system from the setting data memory  221 . The retrieved setting data is forwarded to the relevant sections of the apparatus, as will be discussed later. 
         [0055]    Alternatively, the demodulation circuit control section  220  need not be installed. Instead, the host control section  130  may be furnished with the setting data memory  221 . Using what is held in the setting data memory  221 , the host control section  130  may establish the settings of a given analog TV broadcast system with the digital demodulation circuit section  200 . 
         [0056]    In this embodiment of the invention, the IF signal TVif from the RF tuner  102  is forwarded to the digital demodulation circuit section  200 . The IF signal TVif is first converted by an A/D conversion section  201  into a digital signal which is then sent to a complex multiplier  202 . The complex variable frequency oscillator  203  supplies the complex multiplier  202  with two frequency signals (not shown) such that their phases are positioned perpendicular to each other. The two frequency signals coming from the complex variable frequency oscillator, with their phases intersecting at right angles, will not be discussed further for purpose of simplification. 
         [0057]    The complex multiplier  202  provides a digital signal (complex baseband signal) C 1  as the result of having the IF signal TVif frequency-converted to the baseband frequency band. It should be noted that in  FIG. 1 , two parallel lines represent a complex signal. However, the oscillation frequency signal coming out of the complex variable frequency oscillator  203  is indicated by a single line for purpose of simplification. These conventions will hold for the ensuing description. 
         [0058]    The demodulation circuit control section  220  supplies the complex variable frequency oscillator  203  with a control signal such that the complex oscillation frequency signal of the oscillator  203  has a frequency for frequency-converting the IF signal TVif of the selected analog TV broadcast system into the baseband frequency band. 
         [0059]    The complex baseband signal C 1  from the complex multiplier  202  is fed to a variable band low-pass filter  204  for band limitation whereby unnecessary frequency components are removed. At this point, the demodulation circuit control section  220  reads a suitable control signal from the setting data memory  221  and supplies the variable band low-pass filter  204  therewith, the control signal being such as to control the filter  204  to limit the band corresponding to the selected analog TV broadcast system. 
         [0060]    The variable band low-pass filter  204  is constituted by a finite impulse response (FIR) digital filter or an infinite impulse response (IIR) digital filter. For this reason, the control signal representing the setting data is made up of a filter coefficient. The filter structure and the control signals representative of the setting data regarding the filter  204  in the digital demodulation circuit section  200  will also hold for description that follows. 
         [0061]    A complex baseband signal C 2  having undergone band limitation by the variable band low-pass filter  204  is sent to a picture/sound separation section  205 . In this example, the picture/sound separation section  205  is constituted by a frequency conversion section for frequency-converting the complex baseband signal C 2  and by a variable band picture/sound separation filter  209 . 
         [0062]    The frequency conversion section for frequency-converting the complex baseband signal C 2  provides preprocessing prior to the separation of the signal into a picture signal component and a sound signal component by the variable band picture/sound separation filter  209 . The picture signal component or the sound signal component is frequency-converted into a predetermined frequency band. The frequency conversion section is made up of circuits for frequency-converting the picture signal component into the predetermined frequency band in such a manner that the picture carrier frequency is shifted to zero (DC). 
         [0063]    The frequency conversion section of this example for shifting the picture carrier frequency to zero (DC) is formed by a complex multiplier  206 , a complex variable frequency oscillator  207  that supplies the complex multiplier  206  with a complex frequency signal, and a phase detection section  208 . 
         [0064]    The phase detection section  208  compares in phase the picture carrier of a complex baseband signal C 3  from the complex multiplier  206  with the target frequency f=0. Based on the result of the comparison, the phase detection section  208  gives an output that controls the oscillation frequency of the complex variable frequency oscillator  207 . That is, the complex multiplier  206 , complex variable frequency oscillator  207 , and phase detection section  208  make up a PLL (phase-locked loop). In the PLL, the oscillation frequency of the complex variable frequency oscillator  207  is controlled to be equal to, and in phase with, the picture carrier frequency. It follows that the complex baseband signal C 3  from the complex multiplier  206  is a signal in which the picture carrier frequency is shifted to zero. 
         [0065]    The demodulation circuit control section  220  reads from the setting data memory  221  a control signal for using a free-running center frequency of the complex variable frequency oscillator  207  as the picture carrier frequency corresponding to the selected analog TV broadcast system. The demodulation circuit control section  220  supplies the retrieved control signal to the complex variable frequency oscillator  207 . 
         [0066]    The complex baseband signal C 3  from the complex multiplier  206  is fed to the variable band picture/sound separation filter  209 . In turn, the variable band picture/sound separation filter  209  separates the complex baseband signal C 3  into a picture signal component and a sound signal component, as will be discussed later. 
         [0067]    Before a detailed explanation is made of the variable band picture/sound separation filter  209 , the workings upstream of the filter  209  are described below using specific examples. Suppose now that the IF signal on the 6 MHz band shown in  FIG. 10A , used mainly in the United States, is input to the digital demodulation circuit section  200 . 
         [0068]    The digital demodulation circuit section  200  of this embodiment converts the IF signal TVif into a digital signal for complex signal treatment. The complex multiplier  202  converts the IF signal TVif into a digital signal using a complex oscillation frequency signal coming from the complex variable frequency oscillator  203 . At this point, the frequency of the complex oscillation frequency signal from the complex variable frequency oscillator  203  is set to 44 MHz under control by the control signal from the demodulation circuit control section  220 . The digital signal of the IF signal TVif is multiplied by the frequency of the complex variable frequency oscillator  203  for conversion into the complex baseband signal C 1 . 
         [0069]    What takes place at this point is illustrated in  FIGS. 2A and 2B . The IF signal TVif having a negative frequency is multiplied by the complex oscillation frequency signal at 44 MHz for frequency conversion. The multiplication provides the complex baseband signal C 1  shown in  FIG. 2B . 
         [0070]    The complex baseband signal C 1  shown in  FIG. 2B  is sent to the variable band low-pass filter  204  for band limitation whereby the complex baseband signal C 2  indicated in  FIG. 2C  is obtained. In this example, as shown in  FIG. 2C , the variable band low-pass filter  204  is controlled by the control signal from the demodulation circuit control section  220  in a manner band-limiting the input signal to a bandwidth of ±3 MHz. 
         [0071]    The complex band signal C 2  shown in  FIG. 2C  is fed to the complex multiplier  206  of the frequency conversion section for frequency conversion wherein the picture carrier frequency is reproduced in the PLL. The frequency conversion converts the complex baseband signal C 2  into the complex baseband signal C 3  in which the picture carrier frequency is shifted to zero (DC) as shown in FIG.  2 D. 
         [0072]    The variable band picture/sound separation filter  209  will now be explained. In this example, the variable band picture/sound separation filter  209  extracts the sound signal component (i.e., sound carrier component) from the complex baseband signal C 3 , and subtracts the extracted sound signal component from the complex baseband signal C 3  to acquire the picture signal component only. 
         [0073]      FIG. 3  shows a detailed typical structure of the variable band picture/sound separation filter  209 . The complex baseband signal C 3  from the complex multiplier  206  is fed to a complex multiplier  401 . In the complex multiplier  104 , the complex baseband signal C 3  is multiplied by a complex oscillation frequency signal of the sound carrier frequency from a complex variable frequency oscillator  402 . Following the multiplication, the complex multiplier  401  provides a complex baseband signal C 4  in which the sound carrier frequency is shifted to zero (DC). 
         [0074]    At this point, the demodulation circuit control section  220  reads from the setting data memory  221  a control signal for using a free-running center frequency of the complex variable frequency oscillator  402  as the sound carrier frequency corresponding to the selected analog TV broadcast system. The demodulation circuit control section  220  supplies the retrieved control signal to the complex variable frequency oscillator  402 . 
         [0075]    The complex baseband signal C 4  from the complex multiplier  401  is fed to a variable band low-pass filter  403  for band limitation whereby unnecessary frequency components are removed from the signal C 4 . As a result, the variable band low-pass filter  403  produces a complex baseband signal C 5  made up of solely the sound signal component, with the sound carrier frequency shifted to zero. At this point, the demodulation circuit control section  220  reads from the setting data memory  221  a control signal for limiting the bandwidth corresponding to the selected analog TV broadcast system. The demodulation circuit control section  220  supplies the retrieved control signal to the variable band low-pass filter  403 . 
         [0076]    The complex baseband signal C 5  obtained by the variable band low-pass filter  403  through band limitation is supplied to a complex multiplier  404  serving as a frequency conversion section for returning the sound carrier frequency to the original frequency. The complex oscillation frequency signal from the complex variable frequency oscillator  402  is multiplied by the value “−1” in a multiplier  405 . Following the multiplication by the multiplier  405 , the resulting signal is forwarded to the complex multiplier  404  as the signal for use in frequency conversion. 
         [0077]    In turn, the complex multiplier  404  produces a complex baseband signal formed solely by the sound signal component in which the sound carrier frequency is returned to the original frequency. An output sound signal component So that is the real signal component of the complex baseband signal comes out as the output of the variable band picture/sound separation filter  209 . 
         [0078]    Furthermore, the complex baseband signal formed solely by the sound signal component from the complex multiplier  404  is sent to a complex subtractor  407 . The complex subtractor  407  is also supplied with the complex baseband signal C 3  from the complex multiplier  206  through a delay circuit  406 . The delay circuit  406  delays the complex baseband signal C 3  by the time it takes to extract the sound signal component mentioned above. 
         [0079]    The complex subtractor  407  subtracts from the complex baseband signal C 3  the complex baseband signal formed only by the sound signal component from the complex multiplier  403 , thereby producing a picture signal component Vc. 
         [0080]    Described below in reference to  FIGS. 4A through 4E  is how the variable band picture/sound separation filter  209  works when the above-mentioned IF signal on the 6 MHz band used mainly in the United States is input to the digital demodulation circuit section  200 . 
         [0081]    As already indicated in  FIG. 2D , the complex baseband signal C 3  input to the variable band picture/sound separation filter  209  behaves as shown in  FIG. 4A . The complex baseband signal C 3  is frequency-converted by the complex multiplier  401  into the complex baseband signal C 4  in which the sound carrier frequency is shifted to zero as shown in  FIG. 4B . At this point, the oscillation frequency of the complex variable frequency oscillator  402  is controlled to be 4.5 MHz by the demodulation circuit control section  220  of this example. 
         [0082]    The complex baseband signal C 4  from the complex multiplier  401  is sent to the variable band low-pass filter  403  for carrying out band limitation in a manner shown in  FIG. 4C . The band limiting process produces the complex baseband signal C 5  made up of solely the sound signal component. In this example, as shown in  FIG. 4C , the bandwidth of the variable band low-pass filter  403  is controlled to be ±250 kHz by the demodulation circuit control section  220 . 
         [0083]    The complex baseband signal C 5  is then frequency-converted by the complex multiplier  404  as shown in  FIG. 4D , with the sound carrier frequency returned to 4.5 MHz in this example. The output sound signal component So is thus obtained from the complex multiplier  404 . 
         [0084]    Finally, the complex subtractor  407  subtracts from the complex baseband signal C 3  the complex baseband signal made up of solely the sound signal component from the complex multiplier  404 . The subtracting process provides the picture signal component Vc formed only by the picture signal component, as shown in  FIG. 4E . 
         [0085]    In the manner described above, the complex baseband component Vc formed only by the picture signal component from the variable band picture/sound separation filter  209  is sent to a variable band VSB filter  210  whereby the picture signal component is demodulated. At this point, the demodulation circuit control section  220  reads from the setting data memory  221  the control signal (i.e., filter coefficient) corresponding to the selected analog TV broadcast system, and sends the retrieved control signal to the variable band VSB filter  210 . 
         [0086]      FIG. 5  is a graphic representation showing typical frequency characteristics of VSB filters for the picture bands of 4 MHz, 5 MHz and 6 MHz with regard to the analog TV signal bandwidths of 6 MHz, 7 MHz, and 8 MHz respectively. The variable band VSB filter  210  is controlled by the demodulation circuit control section  220  using the filter coefficient representing one of these filter characteristics. 
         [0087]    The digital picture signal from the variable band VSB filter  210  is fed to a D/A (digital to analog) conversion section  211  for conversion into an analog picture signal. The resulting analog picture signal is then output by the digital demodulation circuit section  200 . As with the traditional setup discussed above, the picture signal from the digital demodulation circuit section  200  is fed to the video processor  131  and output as a picture on the display screen of the LCD panel  132 . 
         [0088]    The sound signal component So from the variable band picture/sound separation filter  209  is converted by a D/A conversion section  212  into an analog sound signal. The resulting analog sound signal is then output by the digital demodulation circuit section  200 . As with the traditional setup discussed above, the sound signal from the digital demodulation circuit section  200  is supplied to the sound demodulation circuit  133  and reproduced acoustically by the speakers  134 . 
       The Workings of the Digital Demodulation Circuit Section  200  of the Embodiment  
       [0089]    Below is a description of how the digital demodulation circuit section  200  of this embodiment works when the analog TV signal on the 6 MHz band is received. Given a control command from the host control section  130 , the demodulation circuit control section  220  sends the setting data necessary for modulating the 6 MHz band signal to the relevant sections of the digital demodulation circuit section  200  discussed above. Each of the sections involved is set as follows: 
         [0090]    Setting P 1 : The complex variable frequency oscillator  203  is set to oscillate at a complex oscillation frequency of +44 MHz so as to convert the IF signal into the complex baseband signal C 1 . In this case, the oscillator  203  is set so that the picture carrier frequency following the frequency conversion becomes −1.75 MHz. 
         [0091]    Setting P 2 : The variable band low-pass filter  204  is set with the filter coefficient for attaining the low-pass filter characteristic of ±3 MHz. 
         [0092]    Setting P 3 : The complex variable frequency oscillator  402  in the variable band picture/sound separation filter  209  is set to oscillate at a complex oscillation frequency of −4.5 MHz so as to convert the sound carrier into the baseband. 
         [0093]    Setting P 4 : The variable band low-pass filter  403  in the variable band picture/sound separation filter  209  is set with the filter coefficient for attaining the low-pass filter characteristic of about ±250 kHz. 
         [0094]    Setting P 5 : The variable band VSB filter  210  is set with the filter coefficient for attaining the VSB filter characteristic for the picture band at 4 MHz. 
         [0095]    When the above settings are completed, the digital demodulation circuit section  200  performs its demodulation operations in eight steps (1) through (8) described below. 
         [0096]    (1) The IF signal shown in  FIG. 2A  is converted by the A/D conversion section  201  into digital data. 
         [0097]    (2) The IF signal in digital form is fed to the complex multiplier  202  for complex multiplication wherein the IF signal is multiplexed by the complex oscillation frequency signal with the complex frequency of +44 MHz coming from the complex frequency oscillator  203 . The complex multiplier  202  thus converts the IF into the complex baseband signal C 1  such as one shown in  FIG. 2B . In this example, the negative IF signal is multiplied by the complex oscillation frequency signal of +44 MHz and thereby shifted into the complex baseband range. 
         [0098]    (3) The variable band low-pass filter  204  for ±3 MHz is then set to turn the complex baseband signal C 1  into the complex baseband signal C 2  devoid of unnecessary signals such as those of adjacent channels, as shown in  FIG. 2C . 
         [0099]    (4) In the PLL made up of the complex multiplier  206 , complex variable frequency oscillator  207 , and phase detection section  208 , the complex variable frequency oscillator  207  works to output a complex oscillation frequency signal in synchronism with the picture carrier (−1.75 MHz). This converts the complex baseband signal C 2  into the complex baseband signal C 3  in which the picture carrier is accurately shifted to zero (DC) in frequency, as shown in  FIG. 2D . 
         [0100]    (5) The variable band picture/sound separation filter  209  proceeds to separate the picture and sound signals as discussed above. Specifically, the complex variable frequency oscillator  402  oscillates at a complex frequency of −4.5 MHz so as to extract only the sound signal from the complex baseband signal C 3  in which the picture and sound signals are mixed. The complex multiplier  401  multiplies the complex baseband signal C 3  by the complex oscillation frequency signal of −4.5 MHz, thus converting the sound signal into the baseband. This sound signal is free of the picture signal that has been removed by the variable band low-pass filter  403  from the high frequency range. 
         [0101]    Devoid of the picture signal, the sound signal is fed to the complex multiplier  404  and multiplied therein by the complex frequency of +4.5 MHz. The sound signal is thus returned to the original frequency of +4.5 MHz. A real part signal So of this sound signal is output to the outside for sound processing. 
         [0102]    Meanwhile, the complex baseband signal C 3  in which the picture and sound signals are mixed is input to the delay circuit  406  for a delay allowing for the time it takes to extract the sound signal. The complex baseband signal C 3  thus delayed is forwarded to the complex subtractor  407 . The complex subtractor  407  subtracts the sound signal component returned to the frequency of +4.5 MHz from the delayed complex baseband signal C 3 . The subtraction produces solely the picture signal component Vc free of the sound signal component. The picture signal component Vc is sent to the variable band VSB filter  210 . 
         [0103]    (6) The variable band VSB filter  210  selects the VSB filter for the picture band of 4 MHz of which the characteristic is indicated by solid lines in  FIG. 5 . The picture signal component Vc, having undergone VSB demodulation, is output to the D/A conversion section  211 . 
         [0104]    (7) The D/A conversion section  211  converts the demodulated picture signal in digital form into an analog signal which is then output to the video processor  131  located downstream. The subsequent processes are the same as those of the traditional analog TV demodulation described earlier. 
         [0105]    (8) The D/A conversion section  212  converts into an analog sound signal the digital sound signal So coming from the variable band picture/sound separation filter, and forwards the analog sound signal to the sound demodulation circuit  133  located downstream. The subsequent processes are the same as those of the traditional analog TV demodulation. 
         [0106]    The foregoing paragraphs have described the processes in which to receive the analog TV signal on the 6 MHz band. For each of the other analog TV signals on the 7 MHz and 8 MHz bands, the settings P 1  through P 5  described above may be modified accordingly for suitable demodulation. 
         [0107]      FIG. 6  shows typical setting data corresponding to the above-described settings P 1  through P 5  for receiving the analog TV signals on the 6 MHz, 7 MHz, and 8 MHz bands. The values listed in  FIG. 6  are for explanation purposes and may vary depending on the frequency assignments of the picture and sound carriers in the IF signal, on the circuits to be implemented, and on the clock signals to be used. 
       Processing Involved with the Signal L′ 
       [0108]    What follows is a description of how the so-called signal L′ is demodulated by the digital demodulation circuit section  200  of this embodiment, the signal L′ being a signal in which the sound carrier is assigned a higher frequency than the picture carrier.  FIGS. 7A and 7B  show how the IF signal behaves. More specifically,  FIG. 7A  indicates the IF signal for the ordinary analog TV broadcast signal used in Europe.  FIG. 7B  depicts the IF signal for use with the signal L′. 
         [0109]    In the case of the signal L′, the IF signal TVif coming from the RF tuner  102  has the carrier signal assignments shown in  FIG. 7B . When the signal L′ is to be demodulated, a setting command from the host control section  130  causes the demodulation circuit control section  220  to set suitably the relevant sections in the digital demodulation circuit section  200  using the setting data kept in the setting data memory  221 . The settings necessary for demodulating the signal L′ are listed in  FIG. 6 . 
         [0110]    The settings needed to demodulate the signal L′ are the same as those for demodulating the ordinary signal on the 8 MHz band except for the complex frequency set on the complex variable frequency oscillator  203  for converting the IF signal TVif into the complex baseband signal C 1 . That is, the complex oscillation frequency of the complex variable frequency oscillator  203  is set to +36.15 MHz for demodulating the ordinary 8 MHz band signal and to −35.45 MHz for demodulating the signal L′. The remaining settings are the same for the two signals. 
         [0111]    What is carried out by the digital demodulation circuit section  200  in demodulating the signal L′ is described below.
   (1) The IF signal shown in  FIG. 8A  is converted into digital data by the A/D conversion section  201 .   (2) The IF signal in digital form is sent to the complex multiplier  202  for complex multiplication by the complex oscillation frequency signal having the complex frequency of −35.45 MHz coming from the complex variable frequency oscillator  203 . The complex multiplier  202  thus converts the digital IF signal into the complex baseband signal C 1  shown in  FIG. 8B .   
 
         [0114]    Where the ordinary 8 MHz band signal is to be demodulated, the negative IF signal is multiplied by the complex oscillation frequency signal at +36.15 MHz and thereby shifted into the complex baseband range. In the case of the signal L′, the positive IF signal is multiplied by the complex oscillation frequency signal at −35.45 MHz and thereby shifted into the complex baseband range. The subsequent processes are the same as those of the demodulation of the 8 MHz band signal. 
         [0115]    That is, the digital demodulation circuit section  200  of this embodiment is capable of handling both positive and negative complex frequencies because the section  200  deals with signals through complex frequency processing. For this reason, if the frequency assignments of the picture and sound carriers are found to be changed, the complex variable frequency oscillator  203  need only be switched in oscillation frequency accordingly to address the change. Upon conversion into the complex baseband, it is possible for the embodiment to handle different picture and sound frequency assignments in a unified manner. Unlike traditional circuits, there is no need to switch between the SAW filters dedicated for the signal L′. 
       Other Embodiments and Variations  
       [0116]    In the foregoing description of the embodiment of the invention, the picture/sound separation section  205  was shown extracting the sound signal component from the complex baseband signal C 2  and then subtracting the extracted sound signal component from the complex baseband signal so as to produce the picture signal component. Conversely, the picture/sound separation section  205  may be arranged to extract the picture signal component from the complex baseband signal C 2  and to subtract the extracted picture signal component from the complex baseband signal in order to produce the sound signal component. 
         [0117]    In the foregoing description of the embodiment, the digital demodulation circuit section  200  was shown furnished with the demodulation circuit control section  220  as well as the setting data memory  221 . The digital demodulation circuit section  200  was shown to make the settings regarding various analog TV signals based on the control command from the host control section  130  and by resorting to the setting data memory  221 . Alternatively, the setting data memory may be connected to the host control section  130  to let the section  130  carry out the function of the demodulation circuit control section  220 . In this alternative example, there is no need to install the demodulation circuit control section  220  or the setting data memory in the digital demodulation circuit section  200 . 
         [0118]    The present application contains subject matter related to that disclosed in Japanese Priority Patent Applications JP 2008-089574 and JP 2008-195921 filed in the Japan Patent Office on Mar. 31, 2008 and Jul. 30, 2008, respectively, the entire content of which is hereby incorporated by reference. 
         [0119]    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 factor in so far as they are within the scope of the appended claims or the equivalents thereof.