Abstract:
A signal separator for separating luminance and color signals from a composite video signal includes: a first line delay element for delaying the composite video signal by one line period; a second line delay element for delaying the composite video signal by two line periods; an adder element for adding the first and second delay signals; a subtraction element for subtracting the second delay signal from the first delay signal; a control signal generator for passing a part of the addition signal within a predetermined frequency band therethrough; and a gain control element for reducing a level of the subtraction signal based on the control signal.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is based on Japanese Patent Application No. 2006-277789 filed on Oct. 11, 2006, the disclosure of which is incorporated herein by reference. This application is also related to U.S. application Ser. No. ______, entitled “SIGNAL SEPARATOR”, filed on ______. 
     
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to a signal separator. 
       BACKGROUND OF THE INVENTION 
       [0003]    A television receiver requires separating a color signal and a luminance signal from a composite video signal. A simple method for separating the signals is, for example, a method with using a band pass filter (i.e., BPF) or a method with using a comb filter. 
         [0004]    The method with using a BPF is described as follows. A composite video signal is input into the BPF. The color signal desired is an output signal of the BPF. The luminance signal desired is the subtraction result of the predetermined color signal from the composite video signal. Here the BPF has the ability that the BPF passes only a signal component within a color-signal-band. The method with using a comb filter is described as follows. The luminance signal desired is the addition result between a one-line-delayed composite video signal and a non-delayed composite video signal. The color signal desired is the subtraction result of the one-line delayed composite-video-signal from the non-delayed composite video signal. Here, the separation is executed with using the information on a phase correlation between the lines. 
         [0005]    The two methods mentioned above are easy to perform but result in quality losses of videos such as that due to cross color and dot interference. 
         [0006]    An addition of a field memory and a complicated circuit on a display is a method for improving the video quality loss. An example is shown in JP-A-H09-163398, in which the following technique is presented. Output signals of the field delay circuit and the line delay circuit are input into filters. Then output signals of these filters are to be desired signals by selecting the output signals. The selection of the output signals is made based on the decorrelation-detection-result obtained by limiting a band in relation to time, vertical, and horizontal components. 
         [0007]    An application of a complicated circuit exampled in JP-A-H09-163398 to a small display such as a vehicle display and a mobile display, however, leads to display devices having a much bigger sized circuit than its screen size. Moreover, some information losses of a video signal may not be relevant to an actual video quality for audiences in use in taking into account the screen area of the display. An application of the large circuit exampled in JP-A-H09-163398 to the small display provides an overmuch resolution in use. The small display having a complicated circuit exampled in JP-A-H09-163398 poses problems of a non-suitable sized circuit for the display and the overmuch resolution in use. Thus, what is required is a signal separator without having a large circuit. Further, it is required for the signal separator to provide an appropriate video quality with reducing quality loss such as that due to cross color and dot interference. 
       SUMMARY OF THE INVENTION 
       [0008]    In view of the above-described problem, it is an object of the present disclosure to provide a signal separator. 
         [0009]    According to an aspect of the present disclosure, a signal separator for separating a luminance signal and a color signal from a composite video signal includes: a first line delay element for delaying the composite video signal by one line period, and outputting a delayed signal as a first delay signal; a second line delay element for delaying the composite video signal by two line periods, and outputting a delayed signal as a second delay signal; an adder element for adding the first and second delay signals, and outputting an added signal as a addition signal; a subtraction element for subtracting the second delay signal from the first delay signal, and outputting a subtracted signal as a subtraction signal; a control signal generator for passing a part of the addition signal within a predetermined frequency band therethrough, and outputting a filtered signal as a control signal; and a gain control element for reducing a level of the subtraction signal based on the control signal, and outputting a reduced signal as a gain-controlled signal. 
         [0010]    According to the above separator, the level of the subtraction signal is reduced when a part of the addition signal includes a signal component within the predetermined frequency band. The separator provides an appropriate video quality without having a large circuit. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    The above and other objects, features and advantages of the present invention will become more apparent from the following detailed description made with reference to the accompanying drawings. In the drawings: 
           [0012]      FIG. 1  is a schematic block diagram of a signal separator  1  according to a first embodiment; 
           [0013]      FIGS. 2A ,  2 B,  2 C and  2 D are graphs explaining characteristics of a frequency detection circuit; 
           [0014]      FIG. 3  is a schematic block diagram of a signal separator  2  according to a second embodiment; and 
           [0015]      FIGS. 4A and 4B  are graphs explaining characteristics of a first and a second BPFs. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment 
       [0016]      FIG. 1  shows a schematic block diagram of a signal separator  1 . A signal-input-terminal  11  is inputting a NTSC-standard composite-video-signal IS (i.e., input signal) including a color-signal and a luminance-signal into the separator  1 . The composite-video-signal input through the signal-input-terminal  11  is output to a first line-delay-circuit  13 , a second line-decay-circuit  15  and a correlation-determination-circuit  17 . 
         [0017]    The first line-delay-circuit  13  delays the composite-video-signal by one line period. The one-line-delayed composite-video-signal is output to the correlation-determination-circuit  17 , a BPF  18 , an adder-circuit  25 , a first subtracting-circuit  23  and a second subtracting-circuit  27 . 
         [0018]    A second line-delay-circuit  15  delays the composite-video-signal by two line periods. The two-line-delayed composite-video-signal is output to the correlation-determination-circuit  17 , the adder-circuit  25  and the second subtracting-circuit  27 . 
         [0019]    The correlation-determination-circuit  17  executes a correlation determination among the composite-video-signal, the one-line-delayed composite-video-signal and the two-line-delayed composite-video-signal. Then the correlation-determination-circuit  17  outputs a switching-signal to a first selector  33  and a second selector  35 . Here, a switching signal “ON” is output when the correlation among the above signals exceeds a threshold value, and a switching signal “OFF” is output when the correlation does not exceed the threshold value. The threshold value is adjusted based on the videos that are produced by color-signals and luminance-signals, where the color signal CS and luminance signal LS are output from a color-signal-output-terminal  39  and a luminance-signal-output-terminal  37 , respectively. 
         [0020]    The one-line-delayed composite-video-signal is input into the BPF  18 . The BPF passes only a signal component within a predetermined color-signal-band. The signal passed is output to a first subtracting-circuit  23  and the second selector  35 . Here the frequency band of the color signal is in a range between 3 MHz and 4 MHz. 
         [0021]    The first subtracting-circuit  23  executes a subtraction of the output signal of the BPF  18  from the one-line-delayed composite-video-signal. Here the output signal of the BPF has only a signal component within the color-signal-band. The signal subtracted is output to the first selector  33 . 
         [0022]    The adder-circuit  25  executes an addition between the one-line-delayed composite-video-signal and the two-line-delayed composite-video-signal. The signal added is output to the first selector  33 . 
         [0023]    The second subtracting-circuit  27  executes a subtraction of the two-line-delayed composite-video-signal from the one-line-delayed composite-video-signal. The signal subtracted is output to the second selector  35 . 
         [0024]    The signal from the adder-circuit  25  is input into the frequency-detection-circuit  29 . The frequency-detection-circuit  29  passes a signal component within a predetermined frequency band. Then the signal filtered is outputs to the gain-control-circuit  31 .  FIG. 2A  illustrates levels of a composite-video-signal as a function of frequencies. As shown in  FIG. 2A , the color signal is disposed in the specific frequency band and is superposed on the luminance signal (see a solid curve P in the  FIG. 2A ). Although a filtering of signals along with the curve P could separates the color signal and the luminance signal, it is difficult to execute a complete separation in a practical meaning because the color signal and luminance signal are mixed together on the region around the curve P. Moreover, as separation accuracy increases, circuits become more complicated. Here, the band where the two signals are mixed together shall be described as “the frequency bands of difficult separation” (that corresponds to IIA in  FIG. 2A ). 
         [0025]    The frequency detection circuit  29  is described as the combination of a band pass filter (i.e., BPF) and a trap filter. Frequency characteristics of the BPF, the trap filter and the frequency detection circuit  29  are illustrated in  FIG. 2B ,  FIG. 2C  and  FIG. 2D , respectively. 
         [0026]    A frequency characteristic of the BPF illustrated in  FIG. 2B  is similar to that shown as the curve P in  FIG. 2A . The trap filter having the characteristic shown in  FIG. 2C  dumps a specific frequency band. The band to be dumped partly overlaps the curve P in  FIG. 2A . 
         [0027]      FIG. 2D  shows a frequency characteristic of the combination circuit, which is described as the combination of the BPF shown in  FIG. 2B  and the trap filter shown in  FIG. 2C . The combination circuit is able to pass only a signal component around both ends of the curve P. 
         [0028]    Specifically, the function of the frequency-detection-circuit  29  is such that it outputs a voltage to a gain-control-circuit  31  when the signal being input into the frequency-detection-circuit  29  includes the signal component around both ends of the curve P, which is disposed within “the frequency bands of difficult separation”. The frequency detection circuit  29  is a sort of a control signal generator. 
         [0029]    The gain-control-circuit  31  controls the gain of the signal from the second subtracting-circuit  27  based on a voltage input from the frequency-detection-circuit  29 . Then, the gain-control-circuit  31  output a signal to the second selector  35 . Specifically, the gain-control-circuit  31  reduces the gain of the signal from the second subtracting-circuit  27  when a voltage is input from the frequency-detection-circuit  29 . The gain-control circuit is composed of, for example, an adder circuit or a dividing circuit. 
         [0030]    The first selector  33  chooses a signal to be output between the signal from the first subtracting-circuit  23  and the signal from the adder circuit  25  based on information on a signal from the correlation-determination-circuit  17 . Specifically, the signal from the first subtracting-circuit  23  is output to the luminance-signal-output-terminal  37  when the “OFF” signal comes from the correlation-determination-circuit  17 . When the “ON” signal comes from the correlation-determination-circuit  17 , the signal from the adder circuit  25  is output to the luminance-signal-output-terminal  37 . 
         [0031]    The second selector  35  also chooses a signal to be output between the signal from the BPF  18  and the signal from the gain-control-circuit  31  based on information of a signal from the correlation-determination-circuit  17 . Specifically, the signal from the BPF  18  is output to the color-signal-output-terminal  39  when “OFF” signal comes from the correlation determination circuit  17 . When the “ON” signal comes from the correlation determination circuit  1 , the signal from the gain-control-circuit  31  is output to the color-signal-output-terminal  39 . 
         [0032]    The luminance-signal-output-terminal  37  is able to output the signal from the first selector  33 . The color-signal-output-terminal  39  is able to output the signal from the second selector  35 . 
         [0033]    The signal separator  1  operates as follows. 
         [0034]    The separation of the color-signal from the composite-video-signal by the BPF  18  gives the color signal. The subtraction of the color signal from the composite-video-signal by the subtracting-circuit  23  gives the luminance signal. 
         [0035]    An addition of composite video signals between lines cancels a color signal component and produces a signal only including a luminance-signal component in the case of little differences of the composite video signal between the lines (i.e., when a correlation value given in the correlation-determination-circuit  17  exceeds a threshold value). This is because color signal components within composite-video-signals are opposite phases between the lines. Thus a luminance signal is output from the adder-circuit  25 . 
         [0036]    A subtraction of composite-video-signals between lines removes a luminance signal component and produces a color signal of twice amplitude in the case of little differences of the composite video signal between the lines (i.e., when a correlation given by the correlation-determination-circuit  17  exceeds a threshold value). Therefore a color signal is output from the second subtracting-circuit  27 . 
         [0037]    The frequency-detection-circuit  29  detects a frequency-band of the luminance-signal from the adder-circuit  25 . Based on information of a signal from the frequency-detection-band  29 , a level of the color signal from the second subtracting-circuit  27  is controlled. 
         [0038]    The signal-separator  1  includes the following two circuits. The one is a BPF circuit having the BPF  18  and the first subtracting-circuit  23 . The BPF circuit corresponds to “BPF CIR.” in  FIG. 1 . The other is a reformed comb filter circuit having the adder-circuit  25 , the second subtracting-circuit, the frequency-detection-circuit  29  and the gain-control-circuit  31 . The reformed comb filter circuit corresponds to “FILTER CIR.” in  FIG. 1 . The output signal of the correlation-determination-circuit  17  chooses the circuit for outputting desired signals between the BPF circuit and the reformed comb filter circuit. Thus, the characteristic advantages of each circuit are available. 
         [0039]    The reformed comb filter circuit reduces a level of the color signal when a part of the luminance signal is within “the frequency bands of difficult separation”. Conventionally, a color signal is superimposed on a luminance signal when a part of the luminance signal is within “the frequency bands of difficult separation” so that the color signal may not be separated sufficiently. However, the reduction of the color-signal-level is able to let audiences hardly recognize the quality loss of video due to cross color. 
       Second Embodiment 
       [0040]      FIG. 3  shows a schematic block diagram of a signal separator  2 . The main differences between the signal separator  1  of the first embodiment and the signal separator  2  of the second embodiment are as follows. The BPF  18  is replaced by a first BPF  19 . A second BPF  21  is newly placed on the latter part of the first BPF  19 . A signal from the second BPF  21  is output to the second selector  35 . 
         [0041]      FIG. 4A  shows a frequency characteristic of the first BPF  19 . The frequency characteristic of the first BPF  19  is slightly wider than that shown as the curve P in  FIG. 2A . Therefore, the first BPF  19  passes a signal component within “the frequency bands of the difficult separation”. If a frequency characteristic of the first BPF  19  is too much wider than that shown as the curve P, too much luminance signal components are removed. This means that the luminance signal components are excessively eliminated, and therefore, necessary components may be eliminated. Thus, it is necessary to select a filter having an optimal characteristic in taking into account the size of a screen etc. 
         [0042]      FIG. 4B  shows a frequency characteristic of the second BPF  21 . The frequency characteristic of the second BPF  21  is slightly narrower than that shown as the curve P in  FIG. 2A . Therefore, the second BPF  21  passes a signal component outside “the frequency bands of difficult separation”. If a frequency characteristic of the second BPF  21  is too much narrower than that shown as the curve P, too much color signal components are removed. This means that the color signal components are excessively eliminated, and therefore, necessary components may be eliminated. Thus, it is necessary to select a filter having an optimal characteristic in taking into account the size of screen etc. 
         [0043]    The signal separator  2  operates as follows. 
         [0044]    The first BPF  19  outputs a signal including a signal component within “the frequency bands of difficult separation” (see  FIG. 4B ). The second BPF  21  removes the signal component within “the frequency bands of difficult separation” from the color signal including the signal component within “the frequency bands of difficult separation”. Thus, the two BPFs produce a color signal that does not include the signal component within “the frequency bands of difficult separation”. 
         [0045]    The first subtracting circuit  23  outputs a signal that does not include the signal component within “the frequency bands of difficult separation” because the first subtracting circuit  23  subtracts the output signal of the first BPF (which includes the signal component within “the frequency bands of difficult separation”) from the composite video signal. 
         [0046]    A circuit having the first BPF  19 , the second BPF  21  and first subtracting circuit  23  (i.e., a reformed comb filter circuit) outputs the color and the luminance signals that do not include the signal component within “the frequency bands of difficult separation”. This filter circuit suppresses cross color and dot interference more than a conventional circuit. 
         [0047]    The frequency detection circuit  29  detects a frequency band of the luminance signal from the adder circuit  25 . An output signal of the frequency detection circuit  29  determines the reduction level of the color signals, which is the output signal of the second subtracting circuit  27 . 
         [0048]    Conventionally, at the moment that a part of the luminance signal is within “the frequency bands of difficult separation”, a wrong signal is given in many cases. This is because a signal component of the color signal overlaps a luminance signal band. However, the reduction of the color signal level is able to let audiences hardly recognize the quality loss of video due to cross color. 
         [0049]    While the invention has been described with reference to preferred embodiments thereof, it is to be understood that the invention is not limited to the preferred embodiments and constructions. The invention is intended to cover various modification and equivalent arrangements. In addition, while the various combinations and configurations, which are preferred, other combinations and configurations, including more, less or only a single element, are also within the spirit and scope of the invention.