Abstract:
There is provided a color killer circuit comprising a burst amplitude detection unit that detects an amplitude of a color burst; a delay unit that stores, delays, by a predetermined period of time, and outputs the detected amplitude; a subtractor that calculates a difference between an output from the burst amplitude detection unit and an output from the delay unit; a first comparator that compares the output from the burst amplitude detection unit with a first threshold value to determine whether or not the output from the burst amplitude detection unit is less than the first threshold value; and a second comparator that compares an output from the subtractor with a second threshold value to determine whether or not the output from the subtractor is greater than the second threshold value, wherein the color killer circuit masks the color component of the composite video signal when the first comparator determines that the output from the burst amplitude detection unit is less than the first threshold value, or when the second comparator determines that the output from the subtractor is greater than the second threshold value.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The disclosure of Japanese Patent Application No. 2007-265256 including specification, claims, drawings and abstract is incorporated herein by reference in its entirety. 
       BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    The present invention relates to a color killer circuit that masks a color component when a composite video signal including a color component signal and a luminance component signal is processed. 
         [0004]    2. Related Art 
         [0005]    In video signals for the NTSC system or other systems (composite video signals), as shown in  FIGS. 9 and 10 , one field of a video signal SIN includes a vertical synchronization period T V , a horizontal blanking period T H , and a video signal period T S , and the video signal S IN  for one horizontal line includes a horizontal blanking period T H , a color burst period T C , and an effective video signal period T I . It should be noted that, for the sake of clarity of description,  FIGS. 9 and 10  show an example in which the ratios of the respective periods are changed from those of an actual video signal S IN . 
         [0006]    In the processing of such a video signal to display a video image, when the strength of an input video signal is weak, or when noise is large with respect to the strength of the video signal, routinely processing the video signal may result in occurrence of uneven color or other irregularities, causing a video image to distort. With this being the situation, a color component output may be blocked so that a monochrome video image containing luminance information only is displayed. 
         [0007]    In order to perform processing while thus masking a color component, a color killer circuit has been used. The color killer circuit includes, as shown in  FIG. 11 , an analog-to-digital converter  10 , a burst amplitude detection unit  12 , a noise detection unit  14 , and an OR element  16 . 
         [0008]    The analog-to-digital converter  10  converts an input video signal to a digital signal, which is input to the burst amplitude detection unit  12  and the noise detection unit  14 . The analog-to-digital converter  10  samples signals at a sampling rate four times a frequency of the color burst included in the video signal. In this process, as shown in  FIG. 12 , sampling timing is adjusted so that a sampling point matches with a peak of one cycle of the color burst. 
         [0009]    The burst amplitude detection unit  12  receives the digitalized video signal, and extracts an amplitude of the color burst for each video signal of one horizontal line. The burst amplitude detection unit  12  determines whether or not the extracted amplitude is less than a preset first threshold value. An output to the OR element  16  is set to HIGH level when the extracted amplitude is less than the preset first threshold value, and otherwise is set to LOW level. 
         [0010]    The noise detection unit  14  receives the digitalized video signal, and determines a strength of noise superimposed on the video signal. The strength of noise can be estimated based on whether or not an average strength of input video signals is less than a preset second threshold value. The noise detection unit  14  sets an output to the OR element  16  to HIGH level when an average strength of input video signals is less than the preset second threshold value, and otherwise sets it to LOW level. 
         [0011]    The OR element  16  receives outputs from the burst amplitude detection unit  12  and the noise detection unit  14 , sets a killer determination signal to HIGH level when at least one of those outputs has HIGH level, and sets a killer determination signal to LOW level when both of those outputs have LOW level. Thus, when the strength of an input video signal is weak, or when noise is large with respect to the strength of the video signal, it is possible to determine, based on the killer determination signal, whether or not the color signal should be masked to perform processing. 
         [0012]    However, the above-described related-art color killer circuit requires, in addition to the burst amplitude detection unit  12  for extracting a maximum peak value of the color burst, the noise detection unit  14  for determining whether or not the electric field strength of the video signal is under weak electric field conditions under which it is less than a predetermined threshold value. 
         [0013]    Therefore, there have been problems in that the color killer circuit has a complicated circuit structure, and is large in circuit scale. As a result, there is also a problem in that the cost of manufacturing an apparatus including the color killer circuit is increased. 
       SUMMARY 
       [0014]    According to one aspect of the present invention, there is provided a color killer circuit comprising a burst amplitude detection unit that detects an amplitude of a color burst included in a composite video signal including signals for a color component and a luminance component; a delay unit that stores, delays, by a predetermined period of time, and outputs the amplitude of the color burst detected by the burst amplitude detection unit; a subtractor that calculates a difference between an output from the burst amplitude detection unit and an output from the delay unit; a first comparator that compares the output from the burst amplitude detection unit with a first threshold value to determine whether or not the output from the burst amplitude detection unit is less than the first threshold value; and a second comparator that compares an output from the subtractor with a second threshold value to determine whether or not the output from the subtractor is greater than the second threshold value, wherein the color killer circuit masks the color component of the composite video signal when the first comparator determines that the output from the burst amplitude detection unit is less than the first threshold value, or when the second comparator determines that the output from the subtractor is greater than the second threshold value. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
           [0016]      FIG. 1  is a block diagram showing a structure of a color killer circuit according to an embodiment of the present invention; 
           [0017]      FIG. 2  is a block diagram showing a structure of a color killer detection unit according to the embodiment of the present invention; 
           [0018]      FIG. 3  illustrates a sampling method according to the embodiment of the present invention; 
           [0019]      FIG. 4  illustrates hysteresis characteristics of a comparator according to the embodiment of the present invention; 
           [0020]      FIG. 5  illustrates a difference calculation method according to the embodiment of the present invention; 
           [0021]      FIG. 6  illustrates hysteresis characteristics of a comparator according to the embodiment of the present invention; 
           [0022]      FIG. 7  is a block diagram showing a structure of a color killer detection unit according to a modification of the embodiment; 
           [0023]      FIG. 8  shows a relationship between a register value ACC_FRAME and the number of frames over which averaging is performed, according to the modification of the embodiment; 
           [0024]      FIG. 9  shows a structure of a video signal; 
           [0025]      FIG. 10  shows a structure of a video signal; 
           [0026]      FIG. 11  is a block diagram showing a structure of a color killer circuit according to the related art; and 
           [0027]      FIG. 12  illustrates a sampling method. 
       
    
    
     DETAILED DESCRIPTION 
       [0028]    According to an embodiment of the present invention, a color killer circuit  100  includes a burst gate pulse generation unit  102 , a color killer detection unit  104 , and a color killer processing unit  106 , as shown in  FIG. 1 . The color killer circuit  100  processes a video signal, such as an NTSC system video signal including a color burst, as shown in  FIGS. 9 and 10 . 
         [0029]    The burst gate pulse generation unit  102  receives, from a preceding-stage synchronization signal detection unit (not shown), a horizontal synchronization timing pulse Hsync indicating timing of a horizontal synchronization signal, and starts timing detection using a counter. Then, after a predetermined period of time has passed from a rise time of the horizontal synchronization timing pulse Hsync, or in other words, during a color burst period of the video signal, the burst gate pulse generation unit  102  generates and outputs, to the color killer detection unit  104 , a burst gate pulse BGP having a pulse width corresponding to one cycle of the color burst. 
         [0030]    The color killer detection unit  104  includes an analog-to-digital converter  20 , a burst amplitude detection unit  22 , a delay unit  24 , a subtractor  26 , comparators  28  and  30 , and an OR element  32 , as shown in  FIG. 2 . The color killer detection unit  104  has the function of determining whether or not the color killer processing unit  106  should perform processing to mask a color signal of the video signal. 
         [0031]    The analog-to-digital converter  20  receives a video signal, converts the video signal into a digital signal through sampling, and outputs the digital signal. In this process, the sampling is performed at a frequency at least four times the frequency of the color burst. In this process, a peak position of an amplitude of the color burst is detected through peak detection, and the sampling is performed such that each peak position of the color burst corresponds to a sampling point, as shown in  FIG. 3 . 
         [0032]    The burst amplitude detection unit  22  receives the video signal sampled by the analog-to-digital converter  20  and the burst gate pulse BGP, and extracts and outputs a peak value of the color burst. The burst gate pulse BGP is a signal having a pulse width corresponding to one cycle of the color burst in the color burst period. Therefore, a maximum sample value of sampling points included in a period during which the burst gate pulse BGP is at HIGH level is detected and output as the peak value of the color burst. 
         [0033]    For example, in the example of sampling of the color burst shown in  FIG. 3 , a sample value D 6  of a sampling point S 6  having a maximum value of sampling points S 4 , S 5 , S 6 , and S 7  included in a period during which the burst gate pulse BGP rises is extracted and output as a peak value P 1  of the color burst. 
         [0034]    The comparator  28  receives the peak value P 1  of the color burst output from the burst amplitude detection unit  22 , and determines whether or not the peak value P 1  is less than a predetermined amplitude determination threshold value (first threshold value) TH 1 . The amplitude determination threshold value TH 1  may be prestored in a register (not shown) included in the color killer circuit  100 . The comparator  28  sets an output C 1  to HIGH level when the peak value P 1  is less than the amplitude determination threshold value TH 1 , and otherwise sets the output C 1  to LOW level. 
         [0035]    It should be noted that the amplitude determination threshold value TH 1  of the comparator  28  may have hysteresis characteristics. More specifically, as shown in  FIG. 4 , the amplitude determination threshold value TH 1   a  at which the output C 1  is switched from LOW level to HIGH level is set to be less than an amplitude determination threshold value TH 1   b (=TH 1   a +ΔTH 1 ) at which the output C 1  is switched from HIGH level to LOW level. Thus, after the peak value P 1  becomes less than the amplitude determination threshold value TH 1   a,  and the output C 1  is set to HIGH level, the amplitude determination threshold value TH 1   b  is used as a determination reference value so that the output C 1  will not be switched back to LOW level until the peak value P 1  becomes greater than or equal to the amplitude determination threshold value TH 1   b.  Thus, even when the peak value P 1  varies with time near the amplitude determination threshold value TH 1   a,  the output C 1  of the comparator  28  can be prevented from unnecessarily switching between LOW level and HIGH level. 
         [0036]    The delay unit  24  receives the peak value of the color burst output from the burst amplitude detection unit  22 , counts reference clocks such as externally input system clocks, and delays, by one horizontal period of the video signal, and outputs the peak value P 1  of the color burst. It should be noted that the delay time is one horizontal period in the present embodiment, but may be any constant multiple of the horizontal period. 
         [0037]    The subtractor  26  receives the peak value P 1  of the color burst output from the burst amplitude detection unit  22  and a peak value P 0  delayed by one horizontal period, which is output from the delay unit  24 , and calculates and outputs a difference between those values. 
         [0038]    Through this process performed by the subtractor  26 , as shown in  FIG. 5 , a difference ΔP(=P 1 -P 0 ) between the peak value P 0  of the color burst included in the previous horizontal line and the peak value P 1  of the color burst included in the current horizontal line is obtained. Because noise superimposed on a video signal constitutes random noise varying with respect to time, the greater the noise, the more likely a difference will occur between a noise component included in the peak value P 0  of the color burst included in the previous horizontal line and a noise component included in the peak value P 1  of the color burst included in the current horizontal line, and the greater the variation in peak values tends to be. Therefore, the greater the noise, the higher the possibility that the difference value ΔP between these peak values will become larger. 
         [0039]    The comparator  30  receives the difference value ΔP of the peak value output from the subtractor  26 , and determines whether or not the difference value ΔP is greater than a predetermined noise determination threshold value (second threshold value) TH 2 . The noise determination threshold value TH 2  may be prestored in a register (not shown) included in the color killer circuit  100 . The comparator  30  sets an output C 2  to HIGH level when the difference value ΔP is greater than the noise determination threshold value TH 2 , and otherwise sets the output C 2  to LOW level. 
         [0040]    It should be noted that, as in the comparator  28 , the amplitude determination threshold value TH 2  may have hysteresis characteristics. More specifically, as shown in  FIG. 6 , the amplitude determination threshold value TH 2   a  at which the output C 2  is switched from LOW level to HIGH level is set to be greater than a amplitude determination threshold value TH 2   b (=TH 2   a -ΔTH 2 ) at which the output C 2  is switched from HIGH level to LOW level. Thus, after the difference value ΔP becomes greater than the amplitude determination threshold value TH 2   a,  and the output C 2  is set to HIGH level, the amplitude determination threshold value TH 2   b  is used as a determination reference value so that the output C 2  will not be switched back to LOW level until the difference value ΔP becomes less than or equal to the amplitude determination threshold value TH 2   b.  Thus, even when the difference value ΔP varies with time near the amplitude determination threshold value TH 2   a,  the output C 2  of the comparator  30  can be prevented from unnecessarily switching between LOW level and HIGH level. 
         [0041]    The OR element  32  receives the output C 1  of the comparator  28  and the output C 2  of the comparator  30 , calculates a logical OR of the two, and outputs the result as a killer determination signal C 3 . More specifically, the killer determination signal C 3  has HIGH level when either one of the output C 1  and the output C 2  is at HIGH level, and otherwise the killer determination signal C 3  has LOW level. The killer determination signal C 3  is input to the color killer processing unit  106 . 
         [0042]    The color killer processing unit  106  receives a color signal separated from the video signal and the killer determination signal C 3 , and performs processing to mask the color signal in accordance with the killer determination signal C 3 . The color signal input to the color killer processing unit  106  may be a signal which is separated, together with a luminance signal, from the video signal in a processing circuit preceding the color killer circuit  100 , and is digitally converted. However, it is optional whether or not to perform digitalization. The color killer processing unit  106  does not output the color signal when the killer determination signal C 3  has HIGH level, and outputs the color signal to a following-stage processing circuit (not shown) when the killer determination signal C 3  has LOW level. 
         [0043]    The following-stage processing circuit performs processing to reproduce a color video image including the color signal when the color signal is not masked, and to reproduce a monochrome video image only from the luminance signal separated from the video signal when the color signal is masked. However, the processing performed by the following-stage processing circuit is not limited to the above-described processing. 
         [0044]    As described above, by employing the color killer circuit  100 , when the strength of the video signal is low, or when noise is large, it is possible to perform processing while masking a color signal. More specifically, without performing a weak electric field detection process in which an amplitude of a video signal is compared with a predetermined threshold value, by comparing color bursts included in the video signal with each other, it is possible to determine whether or not noise is included. Thus, color killer determination can be performed in a simple structure with an adequate detection sensitivity. 
       Modification  1   
       [0045]    A color killer circuit according to a modification of the embodiment of the present invention has an overall structure similar to that of the color killer circuit  100 , but differs in that the color killer detection unit  104  includes averaging units  34  and  36 , as shown in  FIG. 7 . 
         [0046]    The averaging unit  34  averages peak values P 1  of color bursts output from the burst amplitude detection unit  22  over a plurality of horizontal lines. For example, the averaging is performed over effective horizontal lines included in one frame of a video signal. The term “effective horizontal lines” refers to horizontal lines which include effective signals for representing a video image, excluding horizontal lines such as those included in vertical blanking periods included in video signals. Alternatively, the averaging may be performed over a plurality of frames for effective horizontal lines included in a video signal. 
         [0047]    Specifically, when one frame of a video signal has  128  effective horizontal lines ranging from the 10th horizontal line to the 137th horizontal line, and the averaging is performed over a specified number of frames as shown in FIG.  8  using a value ACC_FRAME (2 bits) set in a register (not shown) attached to the averaging unit  34 , an average value P 1   AVE  can be obtained by the following equation (1): 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                      
                     
                         
                     
                      
                     
                       1 
                       AVE 
                     
                   
                   = 
                   
                     
                       1 
                       
                         128 
                         × 
                         
                           2 
                           ACC_FRAME 
                         
                       
                     
                      
                     
                       
                         ∑ 
                         
                           F 
                           = 
                           0 
                         
                         
                           2 
                            
                           ACC_FRAME 
                         
                       
                        
                       
                         
                           ∑ 
                           
                             H 
                             = 
                             9 
                           
                           136 
                         
                          
                         
                           P 
                            
                           
                               
                           
                            
                           
                             1 
                             FH 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0048]    One frame of an NTSC signal has a total of 525 horizontal lines, among which color burst carrier lines are from the 10th horizontal line to the 263rd horizontal line, and from the 273rd horizontal line to the 525th horizontal line. In this example, for the sake of circuit simplicity, the 10th horizontal line to the 137th horizontal line of one frame are used for detection. 
         [0049]    The output P 1   a  from the averaging unit  34  is input to the comparator  28 . The processing performed by the comparator  28  is similar to that of the above-described embodiment. Here, because the peak values P 1  of the color bursts are averaged, the instability in the processing caused by the variation in peak value P 1  for each horizontal line can be reduced. More specifically, by averaging amplitudes of color bursts, which are under determination, over a plurality of horizontal lines, it is possible to more reliably determine whether or not the strength of a video signal is weak. 
         [0050]    The averaging unit  36  averages differences ΔP between peak values of color bursts output from the subtractor  26  over a plurality of horizontal lines. For example, the averaging is performed over effective horizontal lines included in one frame of a video signal. Alternatively, the averaging may be performed over a plurality of frames for effective horizontal lines included in a video signal. 
         [0051]    Similarly as with the above-described example, when one frame of a video signal has 128 effective horizontal lines ranging from the 10th horizontal line to the 137th horizontal line, and the averaging is performed over a specified number of frames as shown in  FIG. 8  using a value ACC_FRAME (2 bits) set in a register (not shown) attached to the averaging unit  36 , an average value P 2   AVE  can be obtained by the following equation (2): 
         [0000]    
       
         
           
             
               
                 
                   
                     P 
                      
                     
                         
                     
                      
                     
                       2 
                       AVE 
                     
                   
                   = 
                   
                     
                       1 
                       
                         128 
                         × 
                         
                           2 
                           ACC_FRAME 
                         
                       
                     
                      
                     
                       
                         ∑ 
                         
                           F 
                           = 
                           0 
                         
                         
                           2 
                            
                           ACC_FRAME 
                         
                       
                        
                       
                         
                           ∑ 
                           
                             H 
                             = 
                             9 
                           
                           136 
                         
                          
                         
                           P 
                            
                           
                               
                           
                            
                           
                             2 
                             FH 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0052]    The output ΔPa from the averaging unit  36  is input to the comparator  30 . The processing performed by the comparator  30  is similar to that of the above-described embodiment. Here, because the difference values ΔP are averaged, the instability in the processing caused by the variation in difference value ΔP for each horizontal line can be reduced. Especially, when noise is large, because the variance value of a difference value ΔP for each horizontal line is also large, variation tends to occur in determining the strength of noise in the comparator  30 , but, by averaging difference values ΔP over a plurality of horizontal lines, it is also possible to reduce the variation in determination. 
         [0053]    It should be noted that both or either one of the averaging units  34  and  36  may be provided. Even when either one of the averaging units  34  and  36  is provided, an advantage can be obtained in that the stability of a color killer circuit can be increased.