Abstract:
There is provided a television tuner for improving a frequency characteristic of a receiver signal after a FM trap and enhancing a picture quality.  
     There is provided a television tuner comprising a detour unit  15  that allows a receiver signal directly to pass; a first trap unit  14  that attenuates a predetermined signal band included in the receiver signal; a first switch unit  13  that switches a signal pathway in which the receiver signal passes to any one of the first trap unit and the detour unit; an image detecting unit that detects a signal having passed the first trap unit or the detour unit; a video equalizer unit  40  that compensates a frequency characteristic within the predetermined signal band; a second switch unit that switches an operation of the video equalizer unit; and a selection control unit  21  that controls switching operations of the first switch unit  13  and the second switch unit.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a television tuner, and more specifically, to a television tuner capable of improving a frequency characteristic of a receiver signal after a FM trap.  
         [0003]     2. Description of the Related Art  
         [0004]     In the US TV broadcast, a frequency band allocated for channel 6 (hereinafter, referred to as ‘US 6ch’) is from 82 to 88 MHz, and a sound carrier frequency thereof is fixed at 87.75 MHz. Alternatively, in the US FM radio broadcast, the frequency band is extended from 88 to 108 MHz, and the broadcast of a frequency approximated to the frequency band of channel 6 in TV broadcast is existed. That is, 88 MHz is a frequency (hereinafter, referred to as ‘boundary frequency’) partitioning an upper end of the channel 6 (US 6ch) of the US TV broadcast and a lower end of the US FM radio broadcast.  
         [0005]     Therefore, when a receiver electric field intensity is comparatively large, beat noise is represented in an image as noise is generated in a regenerated sound at the time of receiving the TV broadcast of US 6ch.  
         [0006]     In a typical television tuner for receiving the United States TV broadcast, FM trap circuit of approximately 91 to 93 MHz band is connected to a receiver signal input terminal, and a receiver signal adjacent to the aforementioned boundary frequency which is the lower end of the United States FM radio broadcast is made to be attenuated by the trap circuit thereof.  
         [0007]     In such television tuners, it is ideal to attenuate only the band of FM radio broadcast when trying to select to receive the TV broadcast of US 6ch by the FM trap circuit functioning.  
         [0008]     However, it is difficult to attenuate only the FM radio broadcast without giving any effect on the US 6ch band, and problem arises in a conventional television tuner that a video signal (particularly, chroma signal component) of high-pass end near to the aforementioned boundary frequency (88 MHz) among the aforementioned US 6ch occupies a band frequencies extending from 82 to 88 MHz is particularly affected by the aforementioned FM trap circuit and is attenuated, and thus a picture quality of television is degraded.  
         [0009]     In other words, it is ideal for the aforementioned FM trap circuit to attenuate only the signal within a predetermined frequency band (FM radio broadcast band) in which the boundary frequency (88 MHz of which is the United States FM radio broadcast) is the lower limit, however in reality, band of the US 6ch which is below the boundary frequency is taken over and is attenuated together, and problem arises in that a part loss of frequency characteristic (characteristic f) of the video signal is thus generated.  
       SUMMARY OF THE INVENTION  
       [0010]     The present invention is to solve the aforementioned conventional problems and the object is to provide an improved frequency characteristic of the receiver signal (video signal) after an FM trap, and a television tuner to give a highly modified picture quality.  
         [0011]     The present invention comprises a detour unit that allows a receiver signal to pass directly, a first trap unit that attenuates a predetermined signal band included in the receiver signal, a first switch unit that switches a signal pathway in which the receiver signal passes to any of the first trap unit or the detour unit, an image detecting unit that detects a signal having passed the first trap unit or the detour unit so as to generate an image base band signal comes from a predetermined signal band, a video equalizer unit that compensates a frequency characteristic within the predetermined signal band, a second switch unit that switches an operation of the video equalizer unit, and a selection control unit that controls switching operations of the first switch unit and the second switch unit.  
         [0012]     In the present invention, when receiving predetermined television channels (e.g., US 6ch), the video equalizer unit can compensate a portion attenuated by the first trap unit, and when receiving channels other than the predetermined television channels, generation of attenuation can be prevented by being passed through the detour unit. Therefore, since all signals included within a band (band width) of the predetermined television channels are assuredly received, degradation of the picture quality is particularly prevented.  
         [0013]     Accordingly, the video equalizer unit comprises a circuit consisting of an inductance element and a capacitor element, a first resistor, and a second resistor having a smaller resistance value than the first resistor, and it is preferable to connect any one of the first resistor and the second resistor in parallel to the inductance element by the switching operation of the second switch unit.  
         [0014]     In the above manner, when the first resistor is selected as an element connected to the inductance element in parallel, frequency characteristic of the video equalizer unit can be a raised characteristic of high-pass, and when the second resistor is selected, frequency characteristic of the video equalizer unit can be a frequency characteristic of high-pass less in raise compare to a case selected the first resistor.  
         [0015]     Accordingly, when predetermined television channels are selected, the selection control unit controls a first switch unit to select a pathway of which passes through the first trap unit, and a second switch unit to output a selection signal for selecting the first resistor.  
         [0016]     Alternatively, when channels other than predetermined television channels are selected, the selection control unit controls a first switch unit to select the detour unit and a second switch unit to output a selection signal for selecting the second resistor.  
         [0017]     In the above manner, circuits appropriate in situations of when US 6ch are selected and any other channels are selected can be formed respectively.  
         [0018]     For example, the first trap unit is preferable to attenuate a signal within a predetermined frequency band in which a lower limit is 88 MHz, and further a signal band of the predetermined television channel is preferable to be within the range of 82 MHz to 88 MHz.  
         [0019]     In the above manner, the 6 channel of TV broadcast in the United States can be received assuredly. The selection control unit generates a selection signal in response to a channel switching signal.  
         [0020]     In the above manner, operations of the first switch unit and the second switch unit are coincided. Therefore, circuits appropriate in situations when US 6ch is selected and any other channels are selected can be correctly selected.  
         [0021]     Additionally, the present invention comprises a FM trap unit that attenuates a frequency signal of FM broadcast band, an image detecting unit that is connected to a rear end of the FM trap unit and outputs an image base band signal, and a video equalizer unit that is connected to the image detecting unit, wherein the FM trap unit is configured to turn ON when predetermined television channels are received and turn OFF when predetermined television channels are not received, and the video equalizer unit consists of a circuit by an inductance element and a capacitor element, and two resistors connected to the inductance element, and when the predetermined television channel is received, it selects the one resistor and when the predetermined television channel is not received, it selects the other resistor that comes from a smaller resistance value than the first resistor.  
         [0022]     Accordingly, it is preferable that the input terminal of a switching circuit having a first output terminal, a second output terminal and an input terminal alternatively conducted between the first output terminal or between the second output terminal is connected to one end at the side of the image detecting circuit of the inductance element, one end of the first resistor is connected to the first output terminal, one end of the second resistor is connected to the second output terminal, the other ends of the first resistor and the second resistor are connected to each other, the connection points thereof are connected to the other end of the inductance element, a resistance value of the first resistor is set to be larger than that of the second resistor, and that when the FM trap circuit is turned ON, at the same time the switching circuit at the side of the first resistor is switched. In the above way, switching operation of the video equalizer unit can be performed assuredly.  
         [0023]     Television tuner of the present invention can flatten a frequency characteristic for a predetermined television channel such as channel 6 fixed within a band of lower end of the FM radio broadcast in the United States. Thus, the signal within the all ranges of low-pass to high-pass band width which forms such predetermined television channels can be received assuredly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0024]      FIG. 1  is a block diagram showing the overall configuration of a television tuner according to the present invention.  
         [0025]      FIG. 2  is a block diagram showing the configuration of an IF unit.  
         [0026]      FIG. 3  is a circuit diagram showing a video equalizer as a major unit according to the present invention.  
         [0027]      FIG. 4  is a schematic diagram showing a base band frequency characteristic of the IF unit in the case of assuming that the video equalizer unit is not connected.  
         [0028]      FIG. 5  is a schematic diagram showing the base band frequency characteristic of the video equalizer unit.  
         [0029]      FIG. 6  is a schematic diagram showing an overall frequency characteristic of the television tuner according to the present invention. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0030]      FIG. 1  is a block diagram showing the overall configuration of a television tuner according to the present invention,  FIG. 2  is a block diagram showing the configuration of an IF unit,  FIG. 3  is a circuit diagram showing a video equalizer unit as a major unit according to the present invention.  
         [0031]     According to the present invention, when two frequency bands are adjacent to each other such as the predetermined television channels (e.g., channel 6 of the TV broadcast in the United States (US 6ch: 82 MHz to 88 MHz)) and the broadcast band fixed to the upper end thereof for example FM radio broadcast (88 MHz to 108 MHz), and when desired receiver signals are picked up from other signal bands (predetermined television channels) in an attenuated state by a trap (FM trap) at a lower end of one broadcast band and adjacent to a boundary frequency (88 MHz), effect of the trap is beyond the receiver signal detected from the aforementioned signal bands of others, and particularly a frequency characteristic of upper end (high-pass) near to the boundary frequency of the aforementioned signal bands of others is flattened.  
         [0032]     According to the  FIG. 1 , the television tuner of the present invention consists of a signal input unit  11 , a band pass filter unit (BPF)  12 , a first switch unit  13 , a FM trap unit  14  (first trap), a detour unit  15 , a single tuning unit  16 , a radio-frequency amplifying unit (RFAMP)  17 , a double tuning unit  18 , a mixing unit (MIX)  19 , a local oscillating unit  20 , a channel selection control unit  21 , an IF unit  30 , a video equalizer unit  40 , a buffer unit  25 , and a signal output unit  26 .  
         [0033]     The IF unit  30  consists of a SAW filter  31 , an intermediate frequency amplifying unit  32 , an image detecting unit  33 , a second trap unit  34  and an image amplifying unit  35  as shown in  FIG. 2 .  
         [0034]     The video equalizer unit  40  is constructed as shown in  FIG. 3  by consisting of a second switch unit  41 , a first resistor  42  formed with resistance R 1 , a second resistor  43  formed with resistance R 2 , an inductance element  44  formed with an inductance L, and a capacitor element  45  formed with a capacity C. Basic configuration of the video equalizer unit  40  is circuits formed by the inductance element  44  and the capacitor  45 , and both ends of the inductance element  44  alternatively selects any one of the first resistor  42  and the second resistor  43 .  
         [0035]     The channel selection control unit  21  shown in  FIG. 1  receives a channel switching signal given by a remote control apparatus or the like, and channel voltage e responding therefor is given to the local oscillating unit  20 . The local oscillating unit  20  that received the channel voltage e, functions to fit the local oscillation signal into a predetermined frequency (local oscillation frequency) which corresponds to a tuned channel by a use of PLL circuit (not shown) employed in an inner part. The, the local oscillation frequency signal f 1  which is fitted in the manner thereof is given to the mixing unit  19 .  
         [0036]     In addition, the selection control unit  21  functions for example to generate a selection signal S 1  of 0 v when US 6ch is selected, and a selection signal S 1  of 5 v when other than US 6ch is selected, so as to be outputted to the first switch unit  13  and to the second switch unit  41  respectively.  
         [0037]     As shown in  FIG. 3 , the first resistor  42  and the second resistor  43  constructing the video equalizer unit  40  are equipped in a state connectable to a signal pathway of which is the main. The second switch unit  41  is equipped in between an output terminal of the IF unit  30  and each input terminal of the first resistor  42  and the second resistor  43 , and this second switch unit  41  selects any of the first resistor  42  or the second resistor  43  in response to the selection signal S 1  which comes from the selection control unit  21 , so as to be freely switched.  
         [0038]     The inductance element  44  is connected in parallel to the first resistor  42  and the second resistor  43 . The terminals at the output side of the first resistor  42 , the second resistor  43  and the inductance element  44 , are input to the buffer unit  25  and connected to the ground via a capacitor element  45 .  
         [0039]     The video equalizer unit  40  employs peaking circuits of high-pass, particularly near to a color subcarrier frequency (3.58 MHz) by the inductance element  44 , the capacitor element  45 , and the resistor  42  or the resistor  43 , and a peaking amount is adjusted by resistance of the first resistor  42  or the second resistor  43 . That is, when the resistance is large, the peaking amount becomes large and at the same time becomes to a raised frequency characteristic of high-pass, particularly nearby a color subcarrier frequency (3.58 MHz), and alternatively when the resistance is small, the peaking amount becomes small, and becomes to a frequency characteristic of high-pass less in raise.  
         [0040]     Relationship between the first resistor  42  and the second resistor  43  is set for a resistance R 1  of the first resistor  42  to be a larger resistance than a resistance R 2  of the second resistor  43  (R 1 &gt;R 2 ). Configuration of the video equalizer  40  to be described later is not limited to the configuration shown in diagram  3  above.  
         [0041]     Hereinafter, operation of the television tuner of the present invention will be described.  
         [0042]     As shown in  FIG. 1 , a receiver signal rf which comes from a TV broadcast or FM radio broadcast or the like taken from an external antenna is given to the signal input unit  11 . The receiver signal rf given to the signal input unit  11  is inputted to the band pass filter  12 . This band pass filter  12  is functioned to remove unwanted signals existing outside broadcasts band, the TV broadcast or FM broadcast, in approximate range, and to pass the wanted receiver signal (TV broadcast or FM radio broadcast) rf existing in the broadcasts band.  
         [0043]     The receiver signal rf which has passed the band pass filter  12  is selected in the first switch unit  13 , and one part is inputted to the single tuning unit  16  via the FM trap unit  14 , and other part is directly inputted to the single tuning unit via the detour unit  15 .  
         [0044]     The first switch unit  13  can perform switching in response to the selection signals S 1  of the 0 v or 5 v given by the channel selection control unit  21 . For example, when the selection signal (at the time of receiving the US 6ch) S 1  of 0 v is given, part between a terminal SW 0  and a terminal SW 2  of the first switch unit  13  is opened to connect terminal the SW 0  and a terminal SW 1  (first conduction state shown as a full line in  FIG. 1 ), and when the selection signal (at the time of receiving other than the US 6ch) S of 5 v is given, part between the terminal SW 0  and the terminal SW 1  is opened to connect the terminal SW 0  and the terminal SW 2  (second conduction state shown as a dotted line in  FIG. 1 ). Herein, as the first switch unit  13 , examples of switching diode disclosed in Japanese Patent Application No. 1999-205699 can be included.  
         [0045]     Since the detour unit  15  is cut off from a signal pathway when the first conduction state is selected (at the time of receiving the US 6ch), the receiver signal rf is inputted to the single tuning unit  16  via the FM trap unit  14 . Alternatively, when the second conduction state is selected (at the time of receiving other than the US 6ch), the receiver signal rf does not pass the FM trap unit  14 , and instead passes the detour unit  15  to be directly inputted to the single tuning unit  16 .  
         [0046]     The FM trap unit  14  is constructed as an attenuator to attenuate only the signals within a predetermined frequency band consisting of FM radio broadcast frequency adjacent to the US 6ch. Therefore, when the first conduction state is selected (at the time of receiving the US 6ch), a receiver signal rf of which the FM radio broadcast frequency component is attenuated is inputted to the single tuning unit  16 , and when the second conduction state is selected (at the time of receiving other than the US 6ch), a receiver signal rf containing the FM radio broadcast frequency component is inputted to the single tuning unit  16 .  
         [0047]      FIG. 4  is a schematic diagram showing a base band frequency characteristic of the IF unit in the case of assuming that the video equalizer unit is not connected.  
         [0048]     Since the frequency characteristic of the FM trap unit  14  is made to be attenuated by a predetermined inclination (for example, −20 dB/dec) from just before the boundary frequency (88 MHz) which is the boundary of US ch6 and FM radio broadcast, according to the effect of the FM trap unit  14 , as shown in  FIG. 4 , the base band frequency characteristic seen from an output terminal of the IF unit is in a state of circuit of high-pass, particularly near by a color subcarrier frequency (3.58 MHz), being attenuated (signal of a hatching part).  
         [0049]     A receiver signal of desired channel is selected in the single tuning unit  16 . Next, this receiver signal is amplified in the radio-frequency amplifying unit  17  and the desired receiver signal is further selected in the double tuning unit subsequently connected thereof, the selected receiver signal is mixed with the local oscillation frequency signal f 1  outputted from the local oscillating unit  20  in the mixing unit  19  to be changed into an intermediate frequency signal if, and the intermediate frequency signal if is outputted to the IF unit  30 . In the intermediate frequency signal if, an image intermediate frequency signal, a color subcarrier frequency signal, a sound intermediate frequency signal and the like are included.  
         [0050]     As shown in  FIG. 2 , the intermediate frequency signal if inputted to the IF unit  30  is inputted to the intermediate frequency amplifying unit  32  via the SAW filter  31 . The SAW filter  31  has a transmission characteristic to level an image intermediate frequency signal, a color subcarrier frequency signal, and a sound intermediate frequency signal if which are within an intermediate frequency band, into a predetermined level. The intermediate frequency signal if amplified to the predetermined level in the intermediate frequency amplifying unit  32  is detected by the image detecting unit  33  and is changed into a video signal (also known as an ‘image base band’).  
         [0051]     The video signal v is inputted to the image amplifying unit  35  via the second trap unit  34 , and then the video signal v amplified by the amplifying unit  35  is inputted to the video equalizer unit  40 . Herein, the second trap unit  34  functions to stop a sound signal mixed to the video signal v or a signal level of other channel adjacent to the desired channel so as to suppress a bad impact such as an interference fringe causing to the desired channel by the signals thereof.  
         [0052]      FIG. 5  is a schematic diagram showing the base band frequency characteristic of the video equalizer unit when US 6ch is selected.  
         [0053]     When US 6ch is selected, part between the terminals of the second switch unit  41  SW 0  and SW 1  is connected by the selection signal S 1  outputted from the channel selection control unit, thus the first resistor  42  from a large resistance R 1  is connected in parallel to both terminals of the inductance element  44 .  
         [0054]     Hereat, the base band frequency characteristic of the video equalizer unit  40  is as shown in  FIG. 5 . Since a frequency characteristic at output terminal of the IF unit when assumed that the video equalizer unit is not connected, is that the high-pass unit of the base band signal, that is nearby a subcarrier frequency (3.58 MHz), is attenuated (referred to  FIG. 4 ) by an impact of the FM trap unit  14 , the frequency characteristic of the video equalizer unit has a characteristic of rise in the high-pass unit such as nearby the subcarrier frequency by raising a level corresponding to the part so as to compensate the attenuation.  
         [0055]     The frequency characteristic of the video equalizer unit thus is a frequency characteristic of a high-pass raised by peaking circuits from the inductance element  44 , the capacitor element  45  and the first resistor  42 .  
         [0056]     A peaking frequency of the video equalizer unit  40  can be set into a desirable frequency as shown in  FIG. 5  by an applicable use of an inductance L of the inductance element  44  and a capacity C of the capacitor element  45 .  
         [0057]      FIG. 6  is a schematic diagram showing an overall frequency characteristic of the television tuner of the present invention. As shown in  FIG. 6 , according to the television tuner of the present invention, the video equalizer unit  40  can compensate a part of receiver signal attenuated by the FM trap unit  14 . In this manner, all of frequency characteristics of the video signal v including the FM trap unit  14  and the video equalizer unit  40  basically can be turned into flat frequency characteristics as shown in  FIG. 6 .  
         [0058]     Therefore, an impact on the signal band of the US 6ch set in a lower end of the boundary frequency can be further minimized than in the past, and at the same time the FM radio broadcast allocated in an upper end of the boundary frequency can be more effectively attenuated. Thus, it becomes possible to improve a picture quality when receiving the US 6ch.  
         [0059]     Meanwhile, when channels other than the US 6ch are selected, part between the terminal SW 0  and the terminal SW 2  of the second switch unit  41  is connected as shown as a dotted line in  FIG. 3 , by a selection signal S 1  of the channel selection control unit  21 . In this manner, the second resistor  43  from a resistance R 2  which is smaller than the resistance R 1  of first resistor is connected in parallel to both terminals of the inductance element  44 .  
         [0060]     The resistance R 2  of the second resistor  43  is relatively small compared to the resistance R 1  of the first resistor  42  (R 1 &gt;R 2 ). At the same time, amount of peaking of peaking circuits from the inductance element  44 , the capacitor element  45  and the second resistor  43  in the video equalizer unit therefor becomes relatively small compared to a situation when the first resistor  42  is connected.  
         [0061]     That is, if the second resistor  43  with a small resistance is connected to both ends of the inductance element  44 , the video signal v outputted from the IF unit  30  is raised in level by some extent in a high-pass unit that is nearby a subcarrier frequency (3.58 MHz) in the frequency characteristic thereof, in accordance with the peaking circuits of the video equalizer unit, and as a result, almost flattened characteristic as shown in  FIG. 6  is outputted in the output terminal of the video equalizer unit.  
         [0062]     In the aforementioned manner, it becomes possible to improve a picture quality of channels of the lower end band (US 6ch) by compensating a declining of low end band of the boundary frequency (US 6ch), particularly an attenuated high-pass, by making the second switch  41  to be switched by coupling a selection operation of the US 6ch.  
         [0063]     In the aforementioned embodiment, configuration capable of switching the first resistor or the second resistor by the second switch unit  41  is shown, however the present invention is not limited this. For example, it can be a configuration equipping an adjustable resistor instead of the first resistor or the second resistor, and changing a resistance for the time of receiving a predetermined channel or for the time of receiving other channels accordingly.  
         [0064]     Also in the aforementioned embodiment, configuration of the first switch unit  13  equipped in latter to the band pass filter  12  and former to the FM trap unit  14  and the detour unit  15  is shown, however the present invention is not limited to this, and it can also be in a configuration of equipped in latter to the FM trap unit  14  and the detour unit  15  and former to the single tuning unit  16 .  
         [0065]     Further, in the aforementioned embodiment, case of channel 6 in the United States and FM radio broadcast are explained as examples of a predetermined TV channel and a broadcast band, however the present invention is not limited to this and it can be subjected to other broadcast bands.