Abstract:
A television receiving apparatus for receiving an analog television broadcast and a digital television broadcast is disclosed, that comprises a tuner means for receiving both an analog television broadcast signal and a digital television broadcast signal, selecting a signal having a desired carrier frequency from the received signals, and converting the selected signal having the desired carrier frequency into an intermediate frequency signal, an analog demodulating means for demodulating the analog television broadcast signal to a video signal, a digital demodulating means for demodulating the digital television broadcast signal to a base band signal, a first filter means disposed downstream of the tuner means, a second filter means disposed between the first filter means and the analog demodulating means, and a third filter means disposed between the first filter means and the digital demodulating means.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a television receiving apparatus having a receiving circuit that can receive both a digital television signal and an analog television signal. 
     2. Description of the Related Art 
     In recent years, digital television broadcasts such as digital CS (Communication Satellite) broadcast, BS (Broadcast Satellite) broadcast, and digital ground wave broadcast have been performed. It is predicted that analog television broadcasts will be gradually shifted to digital television broadcasts. 
     However, even most television broadcasts have been shifted to digital broadcasts, existing analog television broadcasts cannot be stopped. Thus, in the transient period of which the analog television broadcasts are shifted to the digital television broadcasts, both analog television digital broadcasts and digital television digital broadcasts will coexist. 
     When both analog television broadcasts and digital television broadcasts are performed, television receivers that can receive both analog television broadcasts and digital television broadcasts are desired. 
     FIG. 1 is a block diagram showing an example of the structure of a receiving circuit of a conventional television receiver. The receiving circuit can receive both digital television broadcasts and analog television broadcasts. 
     Referring to FIG. 1, an RF (Radio Frequency) signal is received from an antenna (not shown). The RF signal is supplied to an antenna input terminal  101 . The signal that is input from the antenna input terminal  101  is supplied to a power splitter  102 . The power splitter  102  splits the received signal into an analog broadcast signal and a digital broadcast signal. 
     The analog broadcast signal is supplied from the power splitter  102  to an analog broadcast tuner  103 . 
     The analog broadcast tuner  103  selects a signal having a desired carrier frequency from the RF signal received through the power splitter  102 . The selected signal is converted into an IF (Intermediate Frequency) signal. A frequency set signal is supplied from a controller  115  to the analog broadcast tuner  103 . Corresponding to the frequency set signal, the oscillation frequency of a PLL (Phase Locked Loop) synthesizer of the analog broadcast tuner  103  is set. Corresponding to the oscillation frequency, the reception frequency is set. An AGC control signal is supplied from an analog demodulating circuit  108  to an AGC (Automatic Gain Control) circuit of the analog broadcast tuner  103 . 
     An output of the analog broadcast tuner  103  is supplied to an SAW (Surface Acoustic Wave) filter  107 . The SAW filter  107  has a pass band for the IF signal received from the analog broadcast tuner  103 . The SAW filter  107  should have an excellent characteristic for the analog broadcast. 
     An output of the SAW filter  107  is supplied to an analog demodulating circuit  108 . The analog demodulating circuit  108  amplitude-demodulates the analog television signal to an NTSC format video signal. The demodulated NTSC format analog video signal is output from an output terminal  109 . 
     The digital broadcast signal is supplied from the power splitter  102  to a digital broadcast tuner  104 . 
     The digital broadcast tuner  104  selects a signal having a desired carrier frequency from the RF signal received through the power splitter  102  and converts the selected signal into a predetermined IF (Intermediate Frequency) signal. A frequency set signal is supplied from the controller  115  to the digital broadcast tuner  104 . Corresponding to the frequency set signal, the oscillation frequency of a PLL synthesizer of the digital broadcast tuner  104  is set. Corresponding to the oscillation frequency, the reception frequency is set. An AGC control signal is supplied from a digital demodulating circuit  112  to an AGC circuit of a digital broadcast tuner  106 . 
     An output of the digital broadcast tuner  104  is supplied to an SAW filter  110 . An output of the SAW filter  110  is supplied to an SAW filter  111 . The SAW filters  110  and  111  have a pass band for the IF signal received from the digital broadcast tuner  104 . A filter composed of the SAW filters  110  and  111  accomplishes an optimum characteristic for the digital broadcast. 
     An output of the SAW filter  111  is supplied to the digital demodulating circuit  112 . The digital demodulating circuit  112  performs a demodulating is process such as QAM (Quadrature Amplitude Modulation), VSB (Vestigial Sideband Amplitude Modulation), or OFDM (Orthogonal Frequency Division Multiplex). An output of the digital demodulating circuit  112  is output from an output terminal  113 . 
     Thus, according to the related art reference, the analog broadcast tuner  103  and the digital broadcast tuner  104  are independently disposed. The analog broadcast signal and the digital broadcast signal are split by the power splitter  102 . 
     This is because the required characteristic for the analog broadcast signal is different from that for the digital broadcast signal. 
     In other words, in the analog broadcast, since a video signal is amplitude-modulated, a high video amplitude characteristic is desired. In addition, a high S/N ratio is required. On the other hand, in the digital broadcast, since QAM, VSB, OFDM, or the like is used, an excellent phase characteristic is required. Moreover, in the digital broadcast, a filter having a sharp characteristic is required. In the analog broadcast, when a filter having a sharp characteristic is used, the resolution deteriorates. 
     To solve such a problem, the power splitter  102  splits the received signal into an analog broadcast signal and a digital broadcast signal. The split analog broadcast signal is processed by the analog broadcast tuner  103 , the SAW filter  107 , and the analog demodulating circuit  108 . The split digital broadcast signal is processed by the digital broadcast tuner  104 , the SAW filters  110  and  111 , and the digital demodulating circuit  112 . 
     The SAW filter  107  has an optimum characteristic for the analog broadcast. 
     The SAW filters  110  and  111  have optimum characteristics for the digital broadcast. In the digital system, the SAW filter  110  and the SAW filter  111  are tandem-connected so that a sharp characteristic for the digital broadcast can be obtained. 
     In the receiving circuit of the conventional television receiver that receives both a conventional analog broadcast and a digital broadcast, since the power splitter  102  splits the RF signal into an analog broadcast signal and a digital broadcast signal, a power loss takes place in the power splitter and thereby the S/N ratio deteriorates. 
     In addition, the receiving circuit of the television receiver that receives both a conventional analog broadcast and a digital broadcast is provided with the analog broadcast tuner  103  and the digital broadcast tuner  104 . In addition, the receiving circuit is provided with the SAW filter  107  for the analog broadcast and the SAW filters  110  and  111  for the digital broadcast. Thus, the circuit scale becomes large. 
     OBJECTS AND SUMMARY OF THE INVENTION 
     Therefore, an object of the present invention is to provide a television receiving apparatus that can receive both an analog television broadcast and a digital television broadcast and that allows the circuit scale to be reduced. 
     Another object of the present invention is to provide a television receiving apparatus that can receive both an analog television broadcast and a digital television broadcast and that allows the S/N ratio to be improved. 
     The present invention is a television receiving apparatus for receiving an analog television broadcast and a digital television broadcast, comprising a tuner means for receiving both an analog television broadcast signal and a digital television broadcast signal, selecting a signal having a desired carrier frequency from the received signals, and converting the selected signal having the desired carrier frequency into an intermediate frequency signal, an analog demodulating means for demodulating the analog television broadcast signal to a video signal, a digital demodulating means for demodulating the digital television broadcast signal to a base band signal, a first filter means disposed downstream of the tuner means, a second filter means disposed between the first filter means and the analog demodulating means, and a third filter means disposed between the first filter means and the digital demodulating means. 
     According to the present invention, an analog/digital common tuner is disposed. The analog/digital common tuner has a flat frequency characteristic in the channel selection band for an analog broadcast. A first SAW filter is disposed downstream of the tuner. A second SAW filter is disposed between the first SAW filter and an analog demodulating circuit. A third SAW filter is disposed between the first SAW filter and a digital demodulating circuit. When an analog broadcast is received, the first SAW filter and the second SAW filter form a filter having a characteristic necessary for the analog broadcast. When a digital broadcast is received, the first SAW filter and the third SAW filter form a filter having a characteristic necessary for the digital broadcast. Since the first SAW filter is shared for both the analog broadcast and the digital broadcast, the circuit scale can be reduced. In addition, since an analog broadcast signal and a digital broadcast signal are not split by a power splitter, the S/N ratio can be improved. 
     The analog demodulating circuit outputs an AGC control signal for an analog broadcast. The digital demodulating circuit outputs an AGC control signal for a digital broadcast. The switch circuit selectively supplies the AGC control signal for the analog broadcast and the AGC control signal for the digital broadcast to the analog/digital common tuner. Thus, when an analog broadcast is received, the reception gain for the analog broadcast can be quickly and optimally set. When a digital broadcast is received, the reception gain for the digital broadcast can be quickly and optimally set. 
     Likewise, the analog demodulating circuit outputs an AFT control signal for an analog broadcast. The digital demodulating circuit outputs an AFT control signal for a digital broadcast. The switch circuit selectively supplies the AFT control signal for the analog broadcast and the AFT control signal for the digital broadcast to the controller. Thus, depending on which of an analog broadcast or a digital broadcast is received, the AFT control can be optimally performed. 
     These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing an example of the structure of a receiving circuit of a television receiver that can receive both a conventional analog broadcast and a digital broadcast; 
     FIG. 2 is a block diagram showing the structure of an embodiment of the present invention; and 
     FIG. 3 is a block diagram for explaining the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Next, with reference to the accompanying drawings, an embodiment of the present invention will be described. FIG. 2 is a block diagram showing an example of the structure of a receiving circuit of a television receiver according to the present invention. Referring to FIG. 2, an RF (Radio Frequency) signal is received from an antenna (not shown). The received RF signal is supplied to an antenna input terminal  1 . The signal that is input from the antenna input terminal  1  is supplied to an analog/digital common tuner  2 . The analog/digital common tuner  2  has a flat frequency characteristic in the channel selection band for the analog broadcast. 
     The analog/digital common tuner  2  selects a signal having a desired carrier frequency from the RF signal received from the antenna input terminal  1 . The selected signal is converted into an IF (Intermediate Frequency) signal. A frequency set signal is supplied from a controller  15  to the analog/digital common tuner  2 . Corresponding to the frequency set signal, the oscillation frequency of a PLL (Phase Locked Loop) synthesizer of the analog/digital common tuner  2  is set. Corresponding to the oscillation frequency, the reception frequency is set. 
     The analog/digital common tuner  2  also has an AGC (Automatic Gain Control) circuit  3 . An AGC control signal is supplied from a switch circuit  12  to the AGC circuit  3 . The characteristic of the AGC circuit  3  is switched by the switch circuit  12  depending on whether an analog broadcast or the a digital broadcast is received. 
     An output of the analog/digital common tuner  2  is supplied to an SAW (Surface Acoustic Wave) filter  5 . The SAW filter  5  has a band passes for the IF signal received from the analog/digital common tuner  2 . The SAW filter  5  should have excellent characteristics for both an analog broadcast and a digital broadcast. 
     In other words, an analog television broadcast signal has been amplitude-modulated. Thus, when an analog broadcast is received, an excellent amplitude characteristic is required. On the other hand, a digital television signal has been modulated by QAM (Quadrature Amplitude Modulation), VSB (Vestigial Sideband Amplitude Modulation), OFDM (Orthogonal Frequency Division Multiplex), or the like. Thus, when a digital broadcast is received, an excellent phase characteristic is required. Consequently, the SAW filter  5  should have an excellent amplitude characteristic and an excellent phase characteristic. 
     An output of the SAW filter  5  is supplied to an SAW filter  6  and an SAW filter  7 . 
     The SAW filter  6  is a filter for an analog broadcast. The SAW filter  6  has a pass band for the IF signal received from the analog/digital common tuner  2 . The SAW filter  6  should have an excellent amplitude characteristic. 
     An output of the SAW filter  6  is supplied to an analog demodulating circuit  8 . The analog demodulating circuit  8  amplitude-demodulates an analog television signal to an NTSC format video signal. The demodulated NTSC format analog video signal is output from an output terminal  10 . 
     In addition, the analog demodulating circuit  8  detects the signal level of the IF signal and generates an AGC control signal AGC 1  for an analog broadcast. The AGC control signal AGC 1  is supplied to a terminal  12 A of the switch circuit  12 . The analog demodulating circuit  8  detects the frequency error of the IF signal. Corresponding to the frequency error, an AFT (Automatic Fine Tuning) signal AFT 1  is generated. The AFT control signal AFT 1  for the analog broadcast is supplied to a terminal  13 A of a switch circuit  13 . 
     The SAW filter  7  for the digital broadcast should have a sharp characteristic and an excellent phase characteristic. An output of the SAW filter  7  is supplied to a digital demodulating circuit  9 . The digital demodulating circuit  9  demodulates the digital broadcast signal to a base band signal corresponding to QAM, VSB, OFDM, or the like. The demodulated base band signal is output from an output terminal  11 . 
     The digital demodulating circuit  9  detects the signal level of the IF signal and generates an AGC control signal AGC 2  for a digital broadcast. The AGC control signal AGC 2  is supplied to a terminal  12 B of the switch circuit  12 . In addition, the digital demodulating circuit  9  detects the frequency error of the IF signal. Corresponding to the frequency error, an AFT signal AFT 2  is generated. The AFT control signal AFT 2  for the digital broadcast is supplied to a terminal  13 B of the switch circuit  13 . 
     An output of the switch circuit  12  is supplied to an AGC circuit of the analog/digital common tuner  2 . A switch control signal is supplied from the controller  15  to the switch circuit  12 . When an analog broadcast is received, the switch control signal causes the switch circuit  12  to be placed in the terminal  12 A position. When a digital broadcast is received, the switch control signal causes the switch circuit  12  to be placed in the terminal  12 B position. 
     Thus, when an analog broadcast is received, the reception gain is controlled corresponding to the AGC control signal AGC 1  generated by the analog demodulating circuit  8 . When a digital broadcast is received, the reception gain is controlled corresponding to the AGC control signal AGC 2  generated by the digital demodulating circuit  9 . 
     In addition, an output of the switch circuit  13  is supplied to the controller  15 . A switch control signal is supplied from the controller  15  to the switch circuit  13 . When an analog broadcast is received, the switch control signal causes the switch circuit  13  to be placed in the terminal  13 A position. When a digital broadcast is received, the switch control signal causes the switch circuit  13  to be placed in the terminal  13 B position. 
     The controller  15  controls the oscillation frequency of the PLL synthesizer of the analog/digital common tuner  2  corresponding to the AFT control signal that is output from the switch circuit  13 . 
     Thus, when an analog broadcast is received, the AFT control is performed corresponding to the AFT control signal AFT 1  generated by the analog demodulating circuit  8 . On other hand, when a digital broadcast is received, the AFT control is performed corresponding to the AFT control signal AFT 2  generated by the digital demodulating circuit  9 . 
     As described above, the receiving circuit of the television receiver according to the present invention has the analog/digital common tuner  2  that has a flat frequency characteristic -in the channel selection band for an analog broadcast. When an analog broadcast is received, the SAW filter  5  and the SAW filter  6  form a filter having a characteristic necessary for the analog broadcast (the formed filter is denoted by F 1  in FIG.  3 ). When a digital broadcast is received, the SAW filter  5  and the SAW filter  7  form a filter having a characteristic necessary for the digital broadcast (the formed filter is denoted by F 2  in FIG.  3 ). Since the SAW filter  5  is shared for both the analog broadcast and the digital broadcast, the circuit scale can be reduced. 
     In the television receiver according to the present invention, the analog demodulating circuit  8  outputs the AGC control signal AGC 1  for an analog broadcast, whereas the digital demodulating circuit  9  outputs the AGC control signal AGC 2  for a digital broadcast. The switch circuit  12  selectively supplies the AGC control signal AGC 1  for an analog broadcast and the AGC control signal AGC 2  for a digital broadcast to the analog/digital common tuner  2 . Thus, when an analog broadcast is received, an optimum reception gain for the analog broadcast is set. When a digital broadcast is received, an optimum reception gain for the digital broadcast is set. 
     In other words, a digital broadcast is transmitted with a lower power than an analog broadcast so as to prevent the digital broadcast from adversely affecting a channel of the adjacent analog broadcast. Thus, the optimum reception gain for the analog broadcast is different from the optimum reception gain for the digital broadcast. The switch circuit  12  selectively supplies the AGC control signal AGC 1  for the analog broadcast and the AGC control signal AGC 2  for the digital broadcast to the analog/digital common tuner  2 . Thus, depending on which of the analog broadcast or digital broadcast is received, the reception gain can be quickly and optimally set. 
     Likewise, the analog demodulating circuit  8  outputs the AFT control signal AFT 1  for the analog broadcast. The digital demodulating circuit  9  outputs the AFT control signal AFT 2  for the digital broadcast. The switch circuit  13  selectively supplies the AFT control signal AFT 1  for the analog broadcast and the AFT control signal AFT 2  for the digital broadcast to the controller  15 . Thus, depending on which of the analog broadcast or digital broadcast is received, the AFT control can be optimally performed. 
     According to the present invention, an analog/digital common tuner is disposed. The analog/digital common tuner has a flat frequency characteristic in the channel selection band for an analog broadcast. A first SAW filter is disposed downstream of the tuner. A second SAW filter is disposed between the first SAW filter and an analog demodulating circuit. A third SAW filter is disposed between the first SAW filter and a digital demodulating circuit. When an analog broadcast is received, the first SAW filter and the second SAW filter form a filter having a characteristic necessary for the analog broadcast. When a digital broadcast is received, the first SAW filter and the third SAW filter form a filter having a characteristic necessary for the digital broadcast. Since the first SAW filter is shared for both the analog broadcast and the digital broadcast, the circuit scale can be reduced. In addition, since an analog broadcast signal and a digital broadcast signal are not split by a power splitter, the S/N ratio can be improved. 
     The analog demodulating circuit outputs an AGC control signal for an analog broadcast. The digital demodulating circuit outputs an AGC control signal for a digital broadcast. The switch circuit selectively supplies the AGC control signal for the analog broadcast and the AGC control signal for the digital broadcast to the analog/digital common tuner. Thus, when an analog broadcast is received, the reception gain for the analog broadcast can be quickly and optimally set. When a digital broadcast is received, the reception gain for the digital broadcast can be quickly and optimally set. 
     Likewise, the analog demodulating circuit outputs an AFT control signal for an analog broadcast. The digital demodulating circuit outputs an AFT control signal for a digital broadcast. The switch circuit selectively supplies the AFT control signal for the analog broadcast and the AFT control signal for the digital broadcast to the controller. Thus, depending on which of an analog broadcast or a digital broadcast is received, the AFT control can be optimally performed. 
     Although the present invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions, and additions in the form and detail thereof may be made therein without departing from the spirit and scope of the present invention.