Abstract:
Disclosed are an apparatus and a method for YC separation and three-line correlation detection with high accuracy, which allow the YC separator to generate reliable Y signals. The correlation detecting apparatus includes a sub-correlation detector. The detector checks input signals for the presence or absence of vertical correlation, and provides the signals with a judgment “high-correlation exists” or “otherwise”. According to the judgment, the correlation detecting apparatus changes the process; i) when accepted the judgment “high-correlation exists”, the apparatus determines the judgment to be reliable and adopts it as the output, ii) when accepted “otherwise”, the apparatus increases its detecting accuracy and provides the signals with multi-leveled outputs according to the correlation levels. In this way, the main apparatus switches the detecting mode according to the result from sub-correlation detector, with the detecting accuracy greatly improved. This also allows the YC separator to flexibly cope with input signals.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for YC separation and three-line correlation detection providing luminance (often represented as Y) signals with high accuracy in luminance and chrominance signals separation, i.e., YC separation using between-lines correlation found in composite video signals. 
     BACKGROUND OF THE INVENTION 
     In recent years, three-line correlation detection has received much attention for its luminance and chrominance signals separation (YC separation) with high accuracy, which is effectively used in a cost-valued television-set having no three-dimensional YC separator with frame memory. 
     Now will be described an example of the prior-art three-line correlation detecting apparatus, referencing to the accompanying drawings. 
     FIG. 8 shows a block diagram of an YC separator employing the correlation detecting apparatus disclosed in Japanese Patent Laid-Open No. 8-65706. In the figure, receiving composite video signals as an input, three-line signal separator (three-line comb filter)  1  outputs a separated chrominance signal C′. Band-pass filter (BPF)  3  isolates high-band components from the composite video signals to output a chrominance signal C″. 
     According to the output from correlation detector  2 , i.e., the output from OR circuit  9 , selector  11  chooses either the signal C′ fed from three-line comb filter  1 , or the signal C″ fed from BPF  3  filtering chrominance signals. Selector  11  then passes the selected signal CC to one end of subtractor  15  as the chrominance signal. 
     The composite video signal are also fed into delay circuit  13 , which controls output timing by providing the signal with a delay, and then passed to the other end of subtractor  15 . Receiving the delayed composite video signal from delay circuit  13 , subtractor  15  subtracts signal CC from the delayed signal to generate luminance (Y) signal. 
     Here will be described the object of correlation detector  2  and the structure of the three-line correlation detecting apparatus both of which are introduced in the prior-art. 
     Suppose that processing the composite video signal having a high correlation in a direction perpendicular to the horizontal lines—the signal with a high vertical correlation with respect to the screen—for example, an image showing vertical stripes. In this case, allowing selector  11  to output signal C′ fed from three-line C separator (three-line comb filter)  1  as signal CC to subtractor  15  can generate a Y signal with a good quality. 
     Now suppose that processing the composite video signal with a low vertical correlation with respect to the screen—for example, an image showing one horizontal red scanning lines against a white background. If a Y signal is generated from output signal C′ determined as signal CC, dot interference caused by chrominance signals occurs at the horizontal red lines on the screen—a structural weak point of three-line comb filter  1 . That is, because the chrominance level of output signal C′ at the horizontal red lines is decreased to half its normal value, subtractor  15  cannot completely cancel out the chrominance signal. As a result, the residual chrominance signals in the Y signal cause dot interference. In such a screen with a low vertical correlation, allowing selector  11  to output signal C″ fed from BPF  3  as signal CC can generate a Y signal, with dot interference from the chrominance signal suppressed. In this case, however, the high band characteristics of the Y signal are deteriorated. 
     As described above, the YC separator using the correlation detecting apparatus can properly switch between output signal C′ and output signal C″ according to the level of the detected vertical correlation with respect to the screen, which can generate a good Y signal. 
     FIG. 9 is a block diagram of the YC separation circuit that is embodied in Japanese Patent Laid-Open No. 8-65706. In the figure, frame  66  surrounded by the dotted lines represents the three-line correlation detecting apparatus, the rest in the figure shows the YC separator. 
     FIG. 10 shows a block diagram indicating the vertical impulse detector of three-line correlation detecting apparatus  66 . 
     In FIG. 9, the composite video signals are separated into the  0 H signal, the  1 H signal (delayed by delayed element  21  for one horizontal scanning period), and the  2 H signal (delayed by delayed elements  21  and  23  for two horizontal scanning periods), each of which is filtered by low-pass filters (LPFs)  41 ,  43 , and  45 , respectively. The filtered signals f, g, and h—the low-band components (luminance signals) of the composite video signal passed through LPFs  41 ,  43 , and  45 , respectively—are fed into low-band vertical impulse detector  47 . On the other hand, high-band components (chrominance signals) of the composite video signal, which have passed through band-pass filters (BPFs)  49 ,  51 , and  53 , have opposite phases by  1 H. Inverters  55  and  57  process the signals having different phases into in-phase chrominance signals i, j, and k, all of which are fed into high-band vertical impulse detector  59 . 
     FIG. 10 shows the structure of the vertical impulse detector, which is employed for detector  47  for low-band and detector  59  for high-band. In the figure, accepting signals f, g, and h, subtractors  71  and  73  calculate differential signals by subtracting signal f from signal g, and by subtracting signal h from signal g, respectively. Absolute-value calculators (ABSs)  75  and  77  obtain each absolute value of respective differential signals. Receiving the two values, comparators  79  and  81  compare each value with respective predetermined reference values REFs, which are predetermined by comparators  79  and  81 . The two outputs from comparators  79  and  81  are applied to AND circuit  83 . 
     To provide the detection through the process above with accuracy, exclusive NOR circuit  85  is placed between the subtractor and ABS. If circuit  85  detects that the two differential signals have same signs, the output from circuit  85  and the output from AND circuit  83  are further applied to AND circuit  87 , with the final output in FIG. 10 obtained. 
     High-band vertical impulse detector  59  shown in FIG. 9 can be the same as the structure illustrated in FIG.  10 . 
     The output from detector  47  and the output from detector  59  are applied to OR circuit  61 , and the result is determined as the output of three-line correlation detector  66 . If vertical impulse is detected either detector  47  or  59 , detector  66  determines that the correlation is low. The output from detector  66  takes the form of “1” or “0”: “1” indicating low correlation, “0” indicating the presence of the correlation. 
     As described above, the prior-art three-line correlation detecting apparatus detects correlation between the lines carrying the chrominance signal and the luminance signal of the composite video signal, and then outputs “0” or “1” depending on the presence or absence of the correlation. 
     According to the output from the correlation detecting apparatus, YC separator switches the filter used in separation; when accepted the output that represents the presence of the correlation, the separator uses three-line comb filter (5 tap median filter), otherwise uses BPF. In the case that a screen shows one horizontal red scanning lines against a white background described earlier, the correlation detector determines that the correlation is low, thereby uses BPF to generate the Y signal. This therefore suppresses dot interference in the Y signal. It still has, however, room for improvement in performance—a series of noises vertically generated on the screen. 
     The vertically generated in-series noises may occur between adjacent video processing devices. Compared to a noise occurred randomly, the noise spoils the view due to its occurrence in series on a regularly basis. 
     The frequency spectrum of such a noise is distributed over the range from the lower-middle band to high band of the luminance signal, especially the component of the high-band is to be an “eyesore” on the screen. In the YC separator, the Y signal generated through a comb filter has better high-band characteristics than that generated through a BPF. The fact makes noises to be conspicuous. 
     Because the vertically generated in-series noises have a vertical correlation, the detecting apparatus mistakenly determined that there is a correlation, accordingly the YC separator performs the separation through the comb filter. As a result, the misjudgment makes the noise conspicuous. 
     SUMMARY OF THE INVENTION 
     The present invention addresses the problem described above. It is therefore the object to provide an apparatus and method for YC separating and detecting correlation in order to accurately detect correlation between video signals, which is able to determine that the correlation is not so high as for the vertically generated in-series noises at the same time. 
     Now will be described the workings of the three-line correlation detecting apparatus of the present invention. 
     The apparatus includes a sub-correlation detector, by which input signals are sub-checked for the vertical correlation. From the sub-check, the characteristics of a signal is sub-determined to be “having a high-correlation” or “otherwise”. Because the vertically in-series noises include jitter components and variations in amplitude, the sub-correlation detector is controlled to have a level of the threshold to get the determination of “otherwise”. If accepted the result of “high correlation” from sub-checking, the three-line correlation detecting apparatus determines that the result can be reliable and outputs “high correlation” signal preferentially. On the other hand, as for the signals sub-determined as “otherwise”, the apparatus increases the level of detecting accuracy and places the output in “m” levels according to the degree of the correlation. In this way, the vertically in-series noises are properly positioned in the m-leveled judgment. 
     The YC separator generates the Y signal according to the result fed from the three-line correlation detecting apparatus; i) generating the Y signal through the three-line comb filter for the signal “having a high correlation”, ii) generating the Y signal through the BPF for the signal “having the least correlation”, and iii) generating the Y signal by mixing the output from the comb filter with the output from the BPF, or by using a specific filter for the middle-ranged signal. 
     Since the three-line correlation detecting apparatus detects the vertical correlation in the chrominance signal, it is possible to detect the vertical correlation from the chrominance differential signal by isolating the chrominance signal from the composite video signal. Generally, however, the presence or absence of the vertical correlation in the chrominance signal and the luminance signal are closely related to each other. Therefore, providing the correlation detecting apparatus capable of detecting the vertical correlation of the two signals simultaneously contributes to higher accuracy in the detection. 
     Furthermore, incorporating the sub-correlation detector described earlier into the structure above can provide the correlation detecting apparatus with much higher accuracy and reliability. 
     With such structured correlation detecting apparatus, as described above, the YC separator allows to generate the Y signal with the interference minimized including dot interference, the vertically in-series noises and the noise interference of the signal having a middle-ranged vertical correlation. Thus, the practical method can provide the video images with high quality. 
     Basically structured the same as the three-line correlation detecting apparatus, the sub-correlation detector is primarily designed with the aim of detecting the signal having a high-correlation. From the purpose, detecting the impulse state in the differential signal between lines is not required to the sub-correlation detector; the required thing to the detector is to provide the output only if the differential signal has a small value. In addition, the sub-correlation detector has a lot common in the circuit design with the three-line correlation detecting apparatus. This allows them to share some circuits, realizing a low parts count. 
     The structure and workings of each component will be explained in detail in the description of the preferred embodiments of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the three-line correlation detecting apparatus in accordance with a first preferred embodiment of the present invention. 
     FIG. 2 is a block diagram of the three-line correlation detecting apparatus in accordance with a second preferred embodiment of the present invention. 
     FIG. 3 is a block diagram of the three-line correlation detecting apparatus in accordance with a third preferred embodiment of the present invention. 
     FIG. 4 is a block diagram of the three-line correlation detecting apparatus in accordance with a fourth preferred embodiment of the present invention. 
     FIG. 5 is a specific block diagram of the YC separator and the three-line correlation detecting apparatus in accordance with the first preferred embodiment of the present invention. 
     FIG. 6 is a specific block diagram of the YC separator and the three-line correlation detecting apparatus in accordance with the second preferred embodiment of the present invention. 
     FIG. 7 is a specific block diagram of the YC separator and the three-line correlation detecting apparatus in accordance with the third preferred embodiment of the present invention. 
     FIG. 8 is a block diagram of the prior-art YC separator and three-line correlation detecting apparatus. 
     FIG. 9 is a specific block diagram of the prior-art YC separator and three-line correlation detecting apparatus. 
     FIG. 10 is a block diagram of the prior-art three-line correlation detecting apparatus. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The preferred embodiments of the present invention are described hereinafter with reference to the accompanying drawings. 
     First Preferred Embodiment 
     FIG. 1 shows a block diagram of the three-line correlation detecting apparatus with the first preferred embodiment of the present invention. 
     FIG. 5 shows a block diagram of the YC separator employing the three-line correlation detecting apparatus shown in FIG.  1 . Now will be described how such structured three-line correlation detecting apparatus works, referring to FIGS. 1 and 5. 
     FIG. 5 shows a specific example in which the detecting apparatus has three-leveled outputs, i.e., 2 bits (the number of comparaters n=2, where n takes on numeral numbers). 
     Suppose that the current line signal is indicated by  0 H-delayed signal, the signal delayed for one horizontal scanning period is indicated by  1 H-delayed signal, and the signal delayed for 2 horizontal scanning periods is indicated by  2 H-delayed signal.  0 H-delayed signal  10 A,  1 H-delayed signal  10 B, and  2 H-delayed signal  10 C are fed into input connectors  101 ,  102 , and  103 , respectively. Band-pass filters (BPFs)  120 ,  121 , and  122  filter the entered signals and obtain chrominance signals  1 AA,  1 AB, and  1 AC. As the chrominance signals have opposite phases by one horizontal scanning period, the differential signal between these chrominance signals is obtained by adding each other. 
     That is, adder  123  calculates differential signal  1 AD from signals  1 AA and  1 AB by adding each other. Similarly, adder  124  calculates the differential signal  1 AE from signals  1 AB and  1 AC. 
     Absolute-value calculators (ABSs)  125  and  126  receive the differential signals to obtain the absolute value of the correlation difference signal. If the differential signal has a large value, there is no correlation between the input signals. Comparing the values of each differential signal  10 K,  10 L, MAX  127  determines the larger signal as MAX signal  1 AJ. Similarly, MIN  128  determines the smaller signal as MIN signal  1 AK by comparing the values of each signal  10 K  10 L. 
     On the other hand, differential signals  1 AD and  1 AE are fed into exclusive NOR circuit  134 . Output signal  1 AL from circuit  134  represents whether the vertical correlation of input signal is in the impulse state. In the impulse state, signals  1 AD and  1 AE have the same sign, allowing signal  1 AL to take on logic “1”. 
       0 H-,  1 H-, and  2 H-delayed signals are also fed into sub-correlation detector  130 . When detecting a high correlation between the entered signals, detector  130  outputs logic “0” as output signal  1 AO so that selector  129  outputs the MIN signal protecting the detection of the main detector from misjudgment. 
     On the other hand, when detecting a low correlation between the signals, detector  130  outputs logic “1”, by which the vertically in-series noises are escaped from being judged as “correlation exists”, and selector  129  selects the output from MAX detector  127 . 
     Furthermore, when the output from detector  130  will be applied to OR circuit  136 , with the output signal from exclusive NOR circuit  134 , detector  130  outputs logic “1”. This invalidates the judgment of exclusive NOR circuit  134 , thereby signal  1 AN takes on logic “1” regardless of whether differential signals  1 AD and  1 AE are being in the impulse state or not. 
     AND circuit  131  outputs signal  1 AP. When the output from OR circuit  136  is logic “1”, AND circuit  131  determines the signal fed from selector  129  as the output signal, while the output from circuit  136  is logic “0”, AND circuit  131  outputs a reference value. 
     Comparators  132  and  133  compare output signal  1 AP with each reference value REFC  1  and REFC  2  (where REFC  1  is not equal to REFC  2 ), and outputs signals  1 AQ and  1 AR, respectively. 
     Chrominance signal  10 T is obtained from three-line comb filter  137 . On the other hand, chrominance signal  10 U is obtained from BPF  138 . 
     Median signal  10 V, which is ranged between signal  10 T and signal  10 U, is obtained from adder  139  through ½ amplifier  140 . The three signals  10 T,  10 U, and  10 V are fed into selector  141 . Comparators  132  and  133 , where the value of REFC  1  is set to be larger than that of RFFC  2 , determine the output according to the three levels below. 
     a) in the case of signal  1 AP&gt;REFC  1   
     Comparators  132  and  133  output selecting signals  1 AQ and  1 AR both of which take on logic “1”. Receiving the two signals, selector  141  selects signal  10 U. As a result, the Y signal, with dot interference and the vertically in-series noises suppressed, is obtained. 
     b) in the case of REFC  2 &lt;signal  1 AP&lt;REFC  1   
     Signal  1 AQ takes on logic “0”, while signal  1 AR takes on logic “1”. From the two signals, selector  141  selects median signal  10 V. As a result, the Y signal, with dot interference and vertically in-series noises reduced to half its occurrence. 
     c) in the case of signal  1 AP&lt;REFC  2   
     This result represents that a high correlation exists. It means that the chrominance signal should taken from the output signal of the three-line comb filter  10 T to generate the Y signal and signal  1 AQ,  1 AR take on logic “0” as a result, selector  141  selects signal  10 T. 
     According to the embodiment, as described above, the detected result of the differential signal of the chrominance signal is controlled by the result obtained from the sub-correlation detector, then accuracy of correlation detection is improved. Furthermore, by employing two comparators having different reference values, the output from the three-line correlation detecting apparatus is subdivided into three levels. 
     Thus, using the output with three levels can suppress dot interference and the vertically in-series noises in the YC separator. 
     Second Preferred Embodiment 
     FIG. 2 is a block diagram of the three-line correlation detecting apparatus in accordance with the second preferred embodiment of the present invention. 
     FIG. 6 shows an YC separator employing the detecting apparatus shown in FIG.  2 . Now will be described how such structured three-line correlation detecting apparatus works, referring to FIGS. 2 and 6. 
     FIG. 6 shows an example in which the detecting apparatus has three-leveled outputs (n=2), and employs two OR circuits  143 ,  144  as logic operating section  135 . 
     As the chrominance signal vertical correlation detector  150 —lower part framed by dotted lines in FIG.  6 —has the structure the same as the apparatus described in the first embodiment, the explanation will be omitted. 
     Input signals, i.e.,  0 H-,  1 H-, and  2 H-delayed signals captured into each terminal, enter low-pass filters (LPFs)  104 ,  105 , and  106 , respectively. LPFs  104 ,  105 , and  106  filter off the chrominance-signal band in each input signal, and output luminance signal  10 D,  10 E, and  10 F. Receiving these luminance signals, subtractors  107  and  108  calculate luminance differential signal  10 G—the difference component between  0 H- and  1 H-delayed signals—and signal  10 H—the difference component between  1 H- and  2 H-delayed signals. ABSs  109  and  110  accept signals  10 G and  10 H to obtain each absolute value. If the obtained signal (the correlation differential signal) has a large value, there is no correlation between the input signals. Comparing the values of each correlation differential signal, MAX  111  determines the larger signal as MAX signal  10 M. Similarly, MIN  112  determines the smaller signal as MIN signal  10 N by comparing the values of each signal. 
     On the other hand, luminance differential signals  10 G and  10 H are fed into exclusive NOR circuit  116 . Output signal  10 Z from circuit  116  represents whether signals  10 G and  10 H are in the impulse state. 
       0 H-,  1 H-, and  2 H-delayed signals are also fed into sub-correlation detector  130 . When detecting a high correlation between the entered signals, detector  130  outputs logic “0” as output signal  1 AO so that selector  113  outputs the MIN signal to ensure the correct functioning without misjudgment in the three-line correlation detecting apparatus. 
     On the other hand, when detecting a low correlation between the signals, detector  130  outputs logic “1”, by which the vertically in-series noises are considerably suppressed, and selector  113  selects the output from MAX detector  111 . Furthermore, when the output from detector  130  will be applied to OR circuit  117 , with the output signal from exclusive NOR circuit  116 , detector  130  outputs logic “1”. This invalidates the judgment of exclusive NOR circuit  116 , thereby signal  1 AO takes on logic “1” regardless of whether differential signals  10 G and  10 H are being in the impulse state or not. 
     AND circuit  118  outputs signal  10 Q. Comparators  114  and  115  compare output signal  10 Q with each reference value REFC  1  and REFC  2  (where REF  1  is not equal to REF  2 ), and outputs signals  10 R and  10 S, respectively. 
     AND circuit  118  accepts the output from selector  113  as its input. When the output from OR circuit  117  is logic “1”, AND circuit  118  determines the signal fed from selector  113  as the output signal, while the output from circuit  117  is logic “0”, AND circuit  118  outputs a reference value. 
     Comparators  114  and  115  have reference values REF  1  and REF  2 , respectively, where REF  1  is larger than RFF  2 . Similarly, comparators  132  and  133  have reference values REFC  1  and REFC  2 , respectively, where REFC  1  is larger than REFC  2 . When the correlation is low between Y signals or between C signals, signal  10 Q or signal  1 AP has a large value. Comparing signals  10 Q,  1 AP with each reference value, the Y signal suitable for the following three levels is generated. 
     a) in the case of signal  10 Q&gt;REF  1 , and  1 AP&gt;REFC  1   
     Both selecting signals  1 AS and  1 AT take on logic “1”. Receiving the two signals, selector  141  selects signal  10 U. As a result, the Y signal, with dot interference and the vertically in-series noises suppressed, is obtained. 
     b) in the case of REF  2 &lt;signal  10 Q&lt;REF  1 , and REFC  2 &lt;signal  1 AP&lt;REFC  1   
     Signal  1 AS takes on logic “0”, while signal  1 AT takes on logic “1”. From the two signals, selector  141  selects median signal  10 V. As a result, the Y signal, with dot interference and vertically in-series noises reduced to half its occurrence. 
     c) in the case of signal  10 Q&lt;REF  2 , and signal  1 AP&lt;REFC  2   
     This represents that there is a high correlation exists. In this case, the chrominance signal detected by the three-line comb filter can be reliable. Therefore, selector  141  selects signal  10 T and obtains the Y signal generated by the comb filter. 
     According to the embodiment, as described above, the low-band differential signal of the luminance signal is detected, at the same time, the result detected from the differential signal found in the chrominance signal is controlled with the result from the sub-correlation detector. Furthermore, the output signals detected correlation between the luminance signals and between the chrominance signals are processed in logical operation. Through the process, the output from the three-line correlation detecting apparatus is subdivided into three levels. 
     Thus, using the output with three levels can suppress dot interference and the vertically in-series noises in the YC separator. 
     Third Preferred Embodiment 
     FIG. 3 is a block diagram of the three-line correlation detecting apparatus in accordance with the third preferred embodiment of the present invention. 
     Referring to FIG. 3, hereinafter will be described how the sub-correlation detector works. 
       0 H-delayed signal  20 A,  1 H-delayed signal  20 B, and  2 H-delayed signal  20 C are fed into input terminals  201 ,  202 , and  203 , and then filtered by BPFs  204 ,  205 , and  206 , respectively. Adders  207  and  208  calculate  20 G and  20 H—between-lines chrominance differential signals—from signals  20 D and  20 E, from signals  20 E and  20 F, respectively. ABSs  209  and  210  calculate each absolute value of signals  20 G,  20 H and obtain signal  20 I—the correlation level signal between  0 H- and  1 H-delayed signals—and signal  20 J—the correlation level signal between  1 H- and  2 H-delayed signals. 
     If a signal having high frequency a lot, such as, a thin vertical line or diagonal line, is captured as the composite video signal, each BPF cannot filter it properly. Accepting such a signal, adders  207  and  208 , which calculate the differential signal between the chrominance signal for correlation detecting, add each luminance signal component as a chrominance component. As a result, an extremely large value that indicates a low-correlation is obtained. 
     In order to cope with such “confusing” signals, the sub-correlation detector should be reinforced for a reliable detection. To address the problem, according to the embodiment, sub-correlation detector selects output signal  20 K fed from MIN  211 . Comparator  212  compares received signal  20 K with its own predetermined value REFCL. If signal  20 K is larger than REFCL, comparator  212  outputs control signal  20 L taking on logic “1”. It will be understood that signal  20 L in FIG. 3 is identical with signal  1 AO in FIG.  6 . Allowing the sub-correlation detector to control the correlation detecting apparatus in the second preferred embodiment enhances accurate correlation detecting even for the signal indicating a vertical-line image. Furthermore, the vertically in-series noises are also properly detected, contributing to decreased noises. 
     Fourth Preferred Embodiment 
     FIG. 4 is a block diagram of the three-line correlation detecting apparatus in accordance with the fourth preferred embodiment of the present invention. 
     FIG. 7 shows the YC separator employing the three-line correlation detecting apparatus illustrated in FIG.  4 . Here will be described how such structured detecting apparatus works, referring to FIGS. 2,  3  and  7 . 
     Adapting some components in FIG. 7 to corresponding ones in FIG. 3 can form the sub-correlation detector illustrated in FIG. 3, that is: BPFs  120 ,  121 ,  122  to BPFs  204 ,  205 ,  206 ; adders  123 ,  124  to adders  207 ,  208 ; ABSs  125 ,  126  to ABSs  209 ,  210 ; MIN  128  to MIN  211 ; comparator  212  (FIG. 7) to comparator  212  (FIG.  3 ). That is, the working of the detecting apparatus shown in FIG. 7, which incorporates the apparatus shown in FIG. 3 into the sub-correlation detector shown in FIG. 2, is exactly the same as that of the apparatus described in the second preferred embodiment. In other words, the structure of the embodiment already satisfy the desired function for far less component count than incorporating the whole structure shown in FIG. 3 into the sub-correlation detector shown in FIG.  2 . 
     These four embodiments of the present invention relate to an NTSC system but may also be applied to a PAL system, provided that  1 H- and  2 H-delayed signals as the input signals are replaced with  2 H- and 4H-delayed signals, respectively. 
     As described above, the three-line correlation detecting apparatus of the present invention offers the sub-correlation detecting, enhancing the accuracy of correlation detecting between three lines. The apparatus can provide the multi-leveled output, by which vertical correlation detecting for the video signal and the vertically in-series noises are properly detected. 
     Furthermore, the three-line correlation detecting apparatus of the present invention can be applied to the YC separator separating the composite video signal. Such YC separator properly isolates the luminance signal, with noises or dot-interference significantly suppressed, from various composite video signals. As a result, a reliable video image of high quality is obtained.