Abstract:
A video signal processor recursively removes noise from luminance signal data, storing only one frame of the luminance signal data, and detects luminance motion by comparing the current and stored luminance data to decide whether to output the current color difference data, the average of the current color difference data and the color difference data one frame before, or the average of the current color difference data and the color difference data two frames before. The decision may also involve detection of color noise by comparison of the color difference data with the data one frame before, or detection of color motion by comparison of the color difference data with data two or four frames before. Color data stored for color noise detection may also be used for recursive color noise removal. Accurate noise reduction is possible with a comparatively small amount of memory.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a video signal processor that accurately removes cross-color noise, or cross-color noise and random noise, from the demodulated color difference components of a video signal of the National Television System Committee (NTSC) or Phase Alternation by Line (PAL) type in a video device such as a video display device or a video recording and reproducing device, avoiding color smear due to motion.  
         [0003]     2. Description of the Related Art  
         [0004]     Prior art relating to this type of video signal processing is disclosed in, for example, Japanese Patent Application Publication No. 2004-128936.  FIG. 1  of that Publication shows a video signal processor that examines pixels at the same positions in two consecutive frames and detects motion from the frame-to-frame differences of the luminance values of the pixels. If no motion is detected, cross-color noise (crosstalk from the luminance component into the color components) is detected from frame-to-frame differences in the color difference signals, and if cross-color noise is present, it is removed by averaging the color difference signals of the current frame and a previous frame.  
         [0005]     This method relies on the formulas listed in Table 1, which hold when the color difference component signals demodulated from an NTSC or a PAL television video signal do not vary over time, regardless of the presence or absence of cross-color noise.  
                                           TABLE 1                           Cross-color in Color Difference Signals                NTSC   PAL                        Current input   C′(n) = C(n) + ycc(n)   C′(n) = C(n) + ycc(n)       color difference       signal       180° phase-offset   C′(n − 1) = C(n − 1) −   C′(n − 2) = C(n − 2) −       color difference   ycc(n − 1)   ycc(n − 2)       signal                  
 
         [0006]     In this table, n represents the current frame, n−1 represents the immediately preceding frame, n−2 represents the second preceding frame, C represents the true color difference component, ycc represents a luminance component that crosses into the color difference component during luminance/chrominance (Y/C) separation (cross-color noise), and C′ represents the separated color difference signal.  
         [0007]     When the input video data form a still picture, the following formulas hold for NTSC and PAL television signals.  
         [0008]     NTSC 
 
 C ( n )= C ( n −1)  (1) 
 
 ycc ( n )= ycc ( n −1)  (2) 
 
         [0009]     PAL 
 
 C ( n )= C ( n −2)  (3) 
 
 ycc ( n )= ycc ( n −2)  (4) 
 
 Accordingly, the result of averaging the current color difference signal C′(n) and the immediately preceding color difference signal C′(n−1) for NTSC, and averaging the current color difference signal C′(n) and the second preceding color difference signal C′(n−2) for PAL, is the true color difference component C(n), the cross-color component ycc being eliminated. This averaging process is independent of the value of ycc and also works when ycc=0; when there is no temporal variation (no motion), the averaged value is equal to the desired color difference signal C(n) regardless of the presence or absence of cross-color. 
 
         [0010]     A problem with the method disclosed in the above Patent Application Publication is that no motion is assumed to be present whenever there is no detectable luminance motion between two frames, even if motion is present in the color components. This can occur (a) if the only frame-to-frame change in the luminance signal occurs at high frequencies near the chrominance subcarrier frequency (fsc), which are removed from the luminance signal (and become a moving cross-color component) during Y/C separation, or (b) if the luminance signal remains substantially unchanged at all frequencies, but the color difference signals change from one frame to the next. If the averaging process is selected when color motion is present, unwanted color effects such as color smearing may appear.  
         [0011]     A further problem is that in the PAL format, which has a four-frame color difference cycle in which the phase reverses once every two frames, the two frames that are averaged are separated by a two-frame interval, so to detect motion accurately, the luminance signal must be retained for the same two-frame interval; that is, a two-frame luminance signal memory is required instead of a one-frame memory.  
         [0012]     Another problem is that although removal of cross-color noise by the averaging method also has some effect in reducing random noise, the effect is only a halving effect (−6 dB), so half the random noise tends to remain.  
         [0013]     A more effective random noise reduction method that operates recursively is known, although it is not mentioned in the above Patent Application Publication. Recursive noise reduction to remove random noise could be applied separately from the averaging process that removes cross-color noise, but then two frame memories would be required.  
       SUMMARY OF THE INVENTION  
       [0014]     An object of the present invention is to remove cross-color noise accurately from both NTSC and PAL video data without the need to store more than one frame of luminance data.  
         [0015]     Another object of the invention is to remove cross-color noise from both NTSC and PAL video data without the need to store more than two frames of color difference data.  
         [0016]     Another object is to avoid color smear by detecting color motion even when luminance motion cannot be detected.  
         [0017]     Another object is to remove random noise from the color difference data by frame recursive noise reduction without the need for extra color difference data memory.  
         [0018]     The invented video signal processor receives luminance input signal data and color difference input signal data as consecutive frames of video data. In the processor, a frame recursive luminance noise reducer preferably removes noise from the luminance input signal data to obtain luminance output signal data. A first video data memory stores the luminance output signal data for one frame interval, and outputs the stored data as first delayed data. The frame recursive luminance noise reducer, if present, makes use of the first delayed data. A luminance motion detector compares the luminance output signal data with the first delayed data and generates a luminance motion signal: either a detection signal indicating whether luminance motion is present, or a coefficient indicating the degree of luminance motion present. The color difference input signal data are stored and delayed for at least one frame, and an average of the color difference input signal data and the delayed color difference data is taken to obtain averaged color difference data. The color difference input signal data and the averaged color difference data are then combined in a ratio that depends on at least the luminance motion signal to obtain color difference output signal data. If the luminance motion signal is a detection signal, either the color difference input signal data or the averaged color difference data is selected as the color difference output signal data.  
         [0019]     According to a first aspect of the invention, the color difference input signal data are delayed by one frame (NTSC) or two frames (PAL) to generate second delayed data, and the second delayed data are delayed by a like amount to generate third delayed data. The averaged color difference data are the average of the color difference input signal data and the second delayed data. Color motion is detected by comparing the color difference input signal data with the third delayed data, and the result is output as a zero-phase motion signal: either a detection signal indicating the presence or absence of color motion, or a coefficient indicating the degree of color motion present. The color difference input signal data and the averaged color difference data are combined according to both the luminance motion signal and the color motion signal.  
         [0020]     Even when luminance motion is not detected, color motion is detected accurately by comparing two signals with identical luminance-chrominance phase relationships. Color smear due to color averaging when color motion is present can therefore be avoided.  
         [0021]     According to a second aspect of the invention, the color difference input signal data are delayed by one frame to generate second delayed data, either the color difference input signal data or the second delayed data are selected according to the luminance motion signal, and the selected data are delayed by one frame to generate third delayed data, which are averaged with the color difference input signal data to obtain the averaged color difference data.  
         [0022]     When the luminance motion signal is a detection signal, for example, the average of the current color difference input signal data and the color difference input signal data two frames before is output if no luminance motion is detected over either of the last two one-frame intervals. The average of the current color difference input signal data and the color difference input signal data one frame before is output if no luminance motion is detected over the last one-frame interval but luminance motion was present in the one-frame interval preceding that. The current color difference input signal data is output if motion is detected in the last one-frame interval. A generally similar ratio scheme is used when the luminance motion signal is a coefficient signal. These schemes work in particular for PAL video data, enabling PAL video data to be processed with no more memory than required for NTSC video data.  
         [0023]     According to a third aspect of the invention, a frame recursive color difference noise reducer removes noise from the color difference input signal data to obtain noise-reduced color difference signal data. Either the color difference input signal data or the noise-reduced color difference signal data are selected according to a color difference noise reduction decision signal, and the selected data are delayed by one frame to generated second delayed data. The second delayed data are compared with the color difference input signal data to generate the color difference noise reduction decision signal, and are averaged with the color difference input signal data to obtain the averaged color difference data. The color difference input signal data and the averaged color difference data are combined according to both the luminance motion signal and the color difference noise reduction decision signal.  
         [0024]     The third aspect of the invention uses the same frame memory for both frame recursive color difference noise reduction and cross-color noise reduction. When large frame-to-frame changes in the color difference data are present despite a lack of significant luminance motion, indicating that the color difference data include cross-color noise, the cross-color noise is removed by averaging. Otherwise, random noise is removed from the color difference data by the recursive color difference noise reducer.  
         [0025]     In all three aspects of the invention, the frame recursive luminance noise reducer may be disabled when there is little or no noise in the luminance input signal data, in which case the luminance output signal data are identical to the luminance input signal data.  
         [0026]     All three aspects of the invention require storage of the luminance input signal data for only one frame, even when the input video data are PAL video data. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0027]     In the attached drawings:  
         [0028]      FIG. 1  is a block diagram of a video signal processor according to a first embodiment of the invention;  
         [0029]      FIG. 2  is a block diagram of a video signal processor according to a second embodiment of the invention;  
         [0030]      FIG. 3  is a block diagram of a video signal processor according to a third embodiment of the invention;  
         [0031]      FIG. 4  is a block diagram of a video signal processor according to a fourth embodiment of the invention; and  
         [0032]      FIG. 5  is a block diagram of a video signal processor according to a fifth embodiment of the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     Embodiments of the invention will now be described with reference to the attached drawings, in which like elements are indicated by like reference characters.  
       First Embodiment  
       [0034]     Referring to  FIG. 1 , the video signal processor receives luminance input signal data Yin and color difference input signal data Cin as digital input video signals. A luminance signal noise rejection section removes noise from the luminance input signal data Yin and outputs a noise-free luminance output signal data Yout. A cross-color rejection section removes cross-color noise from the color difference input signal data Cin and outputs a color difference signal Cout that is free of rainbow patterns and other unwanted cross-color effects.  
         [0035]     The luminance signal noise rejection section includes a frame recursive luminance noise reducer  1 , a first video data memory (in this embodiment, a frame memory  2 ) connected between the output and input of the frame recursive luminance noise reducer  1 , and a luminance motion detector  3  connected to the output terminal of the frame memory  2 .  
         [0036]     The frame recursive luminance noise reducer  1  removes noise in accordance with the algebraic difference between the luminance input signal data Yin and first delayed data (in this embodiment, one-frame-delayed luminance signal data S 2 ) and generates luminance output signal data S 1  that form the luminance output (Yout) of the video signal processor. The internal structure (not shown) of the frame recursive luminance noise reducer  1  includes subtractors, a filter, an absolute value circuit, and a threshold comparator.  
         [0037]     The frame memory  2  stores the noise-free luminance output signal data Yout for one frame interval and outputs the one-frame-delayed luminance signal data S 2  used by the frame recursive luminance noise reducer  1  to generate the luminance output signal data S 1 . The luminance motion detector  3  outputs a luminance motion signal S 3  based on the difference between the noise-free luminance output signal data S 1  and the one-frame-delayed luminance signal data S 2 .  
         [0038]     The cross-color rejection section includes a second video data memory (in this embodiment, a frame memory  10 ), a third video data memory (in this embodiment, a frame memory  11 ) connected to the output terminal of the second frame memory  10 , and a zero-phase comparison motion detector connected to the output terminal of frame memory  11 . The zero-phase comparison motion detector in this embodiment includes a subtractor  12 , an absolute value circuit (ABS)  13 , and a threshold comparator  14 . The cross-color rejection section further includes a detection signal combiner  15  connected to the output terminal of the threshold comparator  14 , an averager, and an output unit. The averager in this embodiment includes an adder  16  and a divide-by-two circuit  17 . The output unit is a selector (SEL)  18 .  
         [0039]     Frame memory  10  stores the input color difference input signal data Cin and outputs second delayed data (in this embodiment, 180° phase-offset color difference signal data S 10 ). Frame memory  11  stores the 180° phase-offset color difference signal data S 10  and outputs third delayed data (in this embodiment, 0° phase-offset color difference signal data S 11 ). In the zero-phase comparison motion detector, the subtractor  12  takes the difference between the color difference input signal data Cin and the 0° phase-offset color difference signal data S 11 ; the absolute value circuit  13  obtains the absolute value of the difference; the threshold comparator  14  detects color motion from the absolute value and outputs a color difference motion signal S 14 .  
         [0040]     The detection signal combiner  15  combines the luminance motion signal S 3  generated by the luminance motion detector  3  and the color difference motion signal S 14  generated by the threshold comparator  14  and outputs a combined motion detection signal S 15  to the selector  18 . In the averager, the adder  16  adds the color difference input signal data Cin and the 180° phase-offset color difference signal data S 10 ; the divide-by-two circuit  17  halves the sum and outputs averaged color difference signal data S 17 ; the detection signal combiner  15  selects either the color difference input signal data Cin or the averaged color difference signal data S 17  in accordance with the combined motion detection signal S 15 , and outputs the selected signal as a color difference signal Cout that is free of cross-color effects.  
         [0041]     The operation of the first embodiment will be described next.  
         [0042]     The frame recursive luminance noise reducer  1  in the luminance signal noise rejection section takes the algebraic difference between the luminance input signal data Yin and the one-frame-delayed luminance signal data S 2  output from the frame memory  2 . When the filtered absolute value of the algebraic difference is smaller than a threshold, the algebraic difference is treated as noise. When the filtered absolute value is greater than the threshold, the difference is treated as motion. When treated as noise, the algebraic difference is subtracted from the luminance input signal data Yin, and the result is output as the luminance output signal data S 1  forming the luminance output (Yout) of the video signal processor. When the algebraic difference is treated as motion, the luminance input signal data Yin are output without alteration as the luminance output signal data S 1  (Yout).  
         [0043]     The frame memory  2  stores the luminance output signal data S 1  for one frame and outputs the one-frame-delayed luminance signal data S 2 . The luminance motion detector  3  takes the absolute difference between the luminance output signal data S 1  output from the frame recursive luminance noise reducer  1  and the one-frame-delayed luminance signal data S 2  output from the frame memory  2 . If the absolute value is greater than a certain threshold determined in consideration of noise and jitter, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is present. Otherwise, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is absent.  
         [0044]     Frame memory  10  in the cross-color rejection section stores the color difference input signal data Cin for one frame for NTSC or two frames for PAL and outputs the resulting 180° phase-offset color difference signal data S 10 . Frame memory  11  stores the 180° phase-offset color difference signal data S 10  for one frame for NTSC or two frames for PAL and outputs the resulting 0° phase-offset color difference signal data S 11 .  
         [0045]     The 0° phase relationship relies on the formulas listed in Table 2, which hold when the color difference component signals demodulated from an NTSC or a PAL television video signal do not vary over time, regardless of the presence or absence of cross-color noise.  
                                           TABLE 2                           Cross-color in Color Difference Signals                NTSC   PAL                        Current input   C′(n) = C(n) + ycc(n)   C′(n) = C(n) + ycc(n)       color difference       signal       180° phase-offset   C′(n − 1) = C(n − 1) −   C′(n − 2) = C(n − 2) −       color difference   ycc(n − 1)   ycc(n − 2)       signal        0° phase-offset   C′(n − 2) = C(n − 2) +   C′(n − 4) = C(n − 4) +       color difference   ycc(n − 2)   ycc(n − 4)       signal                  
 
         [0046]     In this table, n represents the current frame, n−1 represents the immediately preceding frame, n−2 represents the second preceding frame, n−4 represents the fourth preceding frame, and ycc represents a luminance component that crosses into the color difference component during luminance/chrominance (Y/C) separation (cross-color noise). When the input video data form a still picture, the following formulas hold for NTSC and PAL television signals.  
         [0047]     NTSC 
 
 C′ ( n )≈ C ( n −1)≈ C ( n −2)  (5) 
 
 ycc ( n )≈ ycc ( n −1)≈ ycc ( n −2)  (6) 
 
 C′ ( n )= C′ ( n −2)  (7) 
 
PAL 
 
 C′ ( n )≈ C ( n −2)≈ C ( n −4)  (8) 
 
 ycc ( n )≈ ycc ( n −2)≈ ycc ( n −4)  (9) 
 
 C′ ( n )= C′ ( n −4)  (10) 
 
 Accordingly, averaging the current color difference signal C′(n) and the immediately preceding color difference signal C′(n−1) for NTSC and averaging the current color difference signal C′(n) and the second preceding color difference signal C′(n−2) for PAL eliminates the cross-color component ycc from the color difference input signal data Cin and yields an averaged color difference equal to the true color difference signal C(n). 
 
         [0048]     When there is a scene change, for example, in the input video signal, if the luminance level remains substantially unchanged, the luminance motion detector may indicate that no motion is present, but if the color difference signal changes, so that formulas (7) and (10) do not hold, the change will be detected by the zero-phase comparison motion detector. Specifically, the subtractor  12  takes the difference between the color difference input signal data Cin and the 0° phase-offset color difference signal data S 11  output from frame memory  11  to obtain the change in the color difference signal, the unchanged cross-color component (ycc) canceling out; the absolute value circuit  13  obtains the absolute value of the difference; if the absolute value is greater than the threshold level determined in consideration of noise and jitter, the threshold comparator  14  detects motion and sets the color difference motion signal S 14  to indicate that motion is present. If the color difference signal also remains substantially unchanged and the absolute value is less than the threshold, the threshold comparator  14  sets the color difference motion signal S 14  to indicate that motion is absent.  
         [0049]     The detection signal combiner  15  combines the luminance motion signal S 3  output from the luminance motion detector  3  and the color difference motion signal S 14  output from the threshold comparator  14 . When both the luminance motion signal S 3  and the color difference motion signal S 14  indicate that there is no motion, the combined motion detection signal S 15  is given a value that causes the selector  18  to select the averaged color difference signal data S 17 . If either the luminance motion signal S 3  or the color difference motion signal S 14  indicates that there is motion, the combined motion detection signal S 15  is given a value that causes the selector  18  to select the color difference input signal data Cin. The selected data are output as the color difference output signal data Cout. This makes it possible to avoid the color smear that sometimes occurred in the prior art when color motion was present.  
         [0050]     According to the first embodiment, the cross-color noise rejection section detects color motion by making a zero-phase comparison (a comparison between color difference data with a 0° phase offset), using frame memories  10  and  11  to obtain a delayed signal in which both the color difference component and the cross-color component are in phase with the input signal. If motion is present in a high-frequency portion of the luminance signal that becomes cross-color noise during Y/C separation, it will accordingly be detected as a change in the cross-color (ycc) component in the cross-color noise rejection section, even if no luminance motion is detected in the luminance noise removing section because the low-frequency components of the luminance signal remain substantially unchanged. If color motion is present it will also be detected in the cross-color noise rejection section, even if no luminance motion is present at any frequency and the cross-color component remains unchanged. Consequently, color difference averaging in the presence of motion, which leads to color smear, can be avoided regardless of the signal (luminance or color difference) in which the motion occurs, and regardless of the frequency band in which the motion occurs.  
       Second Embodiment  
       [0051]     The second embodiment of the invention provides a video signal processor that processes PAL signals. Referring to  FIG. 2 , the video signal processor receives PAL luminance input signal data Yin and color difference input signal data Cin as input video data. A luminance signal noise rejection section removes noise from the luminance input signal data Yin and outputs a substantially noise-free luminance signal Yout. A cross-color noise rejection section removes cross-color noise from the color difference input signal data Cin and outputs a color difference signal Cout.  
         [0052]     The luminance signal noise rejection section includes a frame recursive luminance noise reducer  1 , a first video data memory (in this embodiment, a frame memory  2 A), and a luminance motion detector  3 . The frame recursive luminance noise reducer  1  and luminance motion detector  3  are identical to the corresponding elements in the first embodiment. The frame memory  2 A stores the PAL luminance output signal data S 1  generated by the frame recursive luminance noise reducer  1  as the luminance output Yout for one frame and provides one-frame-delayed luminance signal data S 2 A to the frame recursive luminance noise reducer  1  and luminance motion detector  3 .  
         [0053]     The cross-color noise rejection section includes a second video data memory (in this embodiment, a frame memory  10 A), a third video data memory (in this embodiment, a selector  19  and a frame memory  11 A), an averager (in this embodiment, an adder  16  and a divide-by-two circuit  17 ), and an output unit (a selector  18 A). The output terminal of frame memory  10 A is connected via selector  19  to the input terminal of frame memory  11 A.  
         [0054]     Frame memory  10 A stores the input color difference input signal data Cin for one frame and outputs second delayed data (in this embodiment, 90° phase-offset color difference signal data S 10 A). Selector  19  selects either the 90° phase-offset color difference signal data S 10 A from frame memory  10 A or the color difference input signal data Cin in accordance with the luminance motion signal S 3  output by the luminance motion detector  3  and outputs the selected data to frame memory  11 A. Frame memory  11 A stores the data selected by selector  19  for one frame and outputs third delayed data (in this embodiment, 180° phase-offset color difference signal data S 11 A). The adder  16  adds the color difference input signal data Cin and the 180° phase-offset color difference signal data S 10 A; the divide-by-two circuit  17  halves the sum and outputs averaged color difference signal data S 17 A; selector  18 A selects either the color difference input signal data Cin or the averaged color difference signal data S 17  in accordance with the luminance motion signal S 3 , and outputs the selected data.  
         [0055]     The operation of the second embodiment will be described next.  
         [0056]     When the luminance input signal data Yin and the color difference input signal data Cin are input, the frame recursive luminance noise reducer  1  in the luminance signal noise rejection section takes the algebraic difference between the luminance input signal data Yin and the one-frame-delayed luminance signal data S 2 A output from the frame memory  2 A. As in the first embodiment, if the filtered absolute value of the algebraic difference is smaller than a threshold, the difference is treated as noise and is subtracted from the luminance input signal data Yin to obtain the luminance output signal data S 1  (Yout). If the filtered absolute value is greater than the threshold, the algebraic difference is treated as motion and the unaltered luminance input signal data Yin are used as the luminance output signal data S 1  (Yout).  
         [0057]     The frame memory  2 A stores the luminance output signal data S 1  from the frame recursive luminance noise reducer  1  and outputs the one-frame-delayed luminance signal data S 2 A. The luminance motion detector  3  takes the absolute value of the difference between the luminance output signal data S 1  output from the frame recursive luminance noise reducer  1  and the one-frame-delayed luminance signal data S 2 A output from the frame memory  2 A. If the absolute value is greater than a certain threshold determined in consideration of noise and jitter, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is present. Otherwise, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is absent.  
         [0058]     Frame memory  10 A in the cross-color noise rejection section stores the color difference input signal data Cin and outputs 90° phase-offset color difference signal data S 10 A to selector  19 . If the luminance motion detector  3  determines that motion is present, selector  19  selects the color difference input signal data Cin in accordance with the luminance motion signal S 3 . If the luminance motion detector  3  determines that motion is absent, selector  19  selects the 90° phase-offset color difference signal data S 10 A in accordance with the luminance motion signal S 3 . The selected data are stored in frame memory  11 A and output as the 180° phase-offset color difference signal data S 11 A.  
         [0059]     Table 3 shows the relationship among the luminance input signal data Yin, the one-frame-delayed luminance signal data S 2 A stored in the frame memory  2 A, the color difference input signal data Cin, the 90° phase-offset color difference signal data S 10 A stored in frame memory  10 A, and the 180° phase-offset color difference signal data S 11 A stored in frame memory  11 A.  
                                                                   TABLE 3                           Luminance Signal Data and Color Difference Signal       Data Stored in Frame Memories                t − 1   t   t + 1       t + n           (still   (still   (still       (still           picture)   picture)   picture)   . . .   picture)                        Luminance   Y t−1     Y t     Y t+1     . . .   Y t+n         signal Yin       Data in   Y t−2     Y t−1     Y t     . . .   Y t+n−1         frame       memory 2A       Color   C t−1     C t     C t+1     . . .   C t+n         difference       input       signal       data Cin       Data in   C t−2     C t−1     C t     . . .   C t+n−1         frame       memory 10A       Data in   C t−3     C t−2     C t−1     . . .   C t+n−2         frame       memory 11A                  
 
         [0060]     In this table, t represents time, Yt represents the luminance input signal data Yin at time t, and C t  represents the color difference input signal data Cin at time t.  
         [0061]     When the input video data form a still picture, the following formulas hold. 
 
 Y   t−2   ≈Y   t−1   ≈Y   t   ≈Y   t+1   ≈Y   t+n   (11) 
 
 C   t−2   ≈C   t−1   ≈C   t   ≈C   t+1   ≈C   t+n   (12) 
 
         [0062]     From the relationship shown in table 3 and the formulas given above, the color difference signal Cout is obtained as listed in table 4.  
                                                                   TABLE 4                           Stored Luminance Signal and Color Difference Signal and       Output Color Difference Signal when a Still Picture is Formed                t − 1   t   t + 1       t + n           (still   (still   (still       (still           picture)   picture)   picture)   . . .   picture)                        Luminance   Y t−1  ≈   Y t  ≈   Y t+1  ≈   . . .   Y t+n  ≈       signal Yin       Data in   Y t−2     Y t−1     Y t     . . .   Y t+n−1         frame       memory 2A       Color   C t−1  ≈   C t  ≈   C t+1  ≈   . . .   C t+n  ≈       difference       input       signal       data Cin       Data in   C t−2  ≈   C t−1  ≈   C t  ≈   . . .   C t+n−1  ≈       frame       memory 10A       Data in   C t−3     C t−2     C t−1     . . .   C t+n−2         frame       memory 11A       Color   (C t−1  +   (C t  +   (C t+1  +   . . .   (C t+n  +       difference   C t−3 )/2   C t−2 )/2   C t−1 )/2       C t+n−1 )/2       signal       Cout                  
 
         [0063]     Accordingly, the cross-color component can be eliminated through averaging of the color difference input signal data Cin and the 180° phase-offset color difference signal data S 11 A output from frame memory  11 A, performed by the adder  16  and divide-by-two circuit  17 .  
         [0064]     When a moving picture formed by the input video data changes to a still picture at time t+1, the following formulas hold. 
 
 Y   t−2   ≠Y   t−1   ≠Y   t   ≈Y   t+1   ≈Y   t+n   (13) 
 
 C   t−2   ≠C   t−1   ≠C   t   ≈C   t+1   ≈C   t+n   (14) 
 
         [0065]     These formulas are listed in Table 5.  
                                                                   TABLE 5                           Stored Luminance Signal and Color Difference Signal       and Output Color Difference Signal when a Moving Picture       Changes to a Still Picture                t − 1   t   t + 1       t + n           (moving   (moving   (still       (still           picture)   picture)   picture)   . . .   picture)                        Luminance   Y t−1  ≠   Y t  ≠   Y t+1  ≈   . . .   Y t+n  ≈       signal Yin       Data in   Y t−2     Y t−1     Y t     . . .   Y t+n−1         frame       memory 2A       Color   C t−1  ≠   C t  ≠   C t+1  ≈   . . .   C t+n  ≈       difference       input       signal       data Cin       Data in   C t−2  ≠   C t−1  ≠   C t  ≠   . . .   C t+n−1  ≈       frame       memory 10A       Data in   C t−3     C t−2     C t−1     . . .   C t+n−2         frame       memory 11A       Color   C t−1     C t     (C t+1  +   . . .   (C t+n−1  + C t+n−2 )/2       difference           C t−1 )/2       signal       Cout                  
 
         [0066]     At time t+1, when a moving picture changes to a still picture, the luminance motion signal S 3  is set to a value indicating that motion is absent because Y t+1 ≈Y t . The adder  16  adds the color difference input signal data Cin and the 180° phase-offset color difference signal data S 11 A output from frame memory  11 A, and the divide-by-two circuit  17  halves the sum and outputs averaged color difference signal data S 17 . However, because the color difference signal data differ (C t−1 ≠C t+1 ), use of the averaged data (C t+1 +C t−1 )/2 will cause image defects. Whether the color difference signal of the second preceding frame is usable or not cannot be decided from the luminance motion signal S 3  of the current frame alone.  
         [0067]     In order to avoid image defects, accordingly, selector  19  selects the color difference input signal data Cin and frame memory  11 A stores data as listed in Table 6 when the luminance motion signal S 3  is set to a value indicating that motion is present. The arrows in Table 6 indicate that when motion is detected, the data from the preceding frame are stored in both frame memories  10 A and  11 A.  
               TABLE 6                       Stored Luminance Signal and Color Difference Signal       and Output Color Difference Signal when a Moving Picture       Changes to a Still Picture (2)                                                                        
 
         [0068]     The formula at time t+1 (C t+1 ≈C t =C t−1 ) indicates that the color difference signal data C t−1  and C t+1  are substantially equal (C t−1 ≈C t+1 ). Therefore, use of the averaged data (C t+1 +C t )/2 will not cause image defects.  
         [0069]     When the input video data form a moving picture, the following formulas hold. 
 
 Y   t−2   ≠Y   t−1   ≠Y   t   ≠Y   t+1   ≠Y   t+n   (15) 
 
 C   t−2   ≠C   t−1   ≠C   t   ≠C   t+1   ≠C   t+n   (16) 
 
         [0070]     These formulas are listed in Table 7.  
               TABLE 7                       Stored Luminance Signal and Color Difference Signal       and Output Color Difference Signal when a Moving Picture is       Formed                                                                        
 
         [0071]     Because the input video data form a moving picture, frame memory  11 A and frame memory  10 A store the same data. Because of the motion, the absolute value of the difference between the luminance output signal data S 1  and the one-frame-delayed luminance signal data S 2 A output from the frame memory  2 A exceeds the threshold mentioned above, and the luminance motion detector  3  detects the motion. Selector  18 A then selects the color difference input signal data Cin rather than the averaged color difference signal data S 17  obtained from the color difference input signal data Cin and the 180° phase-offset color difference signal data S 11 A output from frame memory  11 A, and outputs the selected color difference input signal data Cin directly as a color difference output signal data Cout. Therefore, no image defects will occur.  
         [0072]     When a still picture formed by the input video data changes to a moving picture, the following formulas hold. 
 
 Y   t−2   ≈Y   t−1   ≈Y   t   ≠Y   t+1   ≠Y   t+n   (17) 
 
 C   t−2   ≈C   t−1   ≈C   t   ≠C   t+1   ≠C   t+n   (18) 
 
         [0073]     These formulas are listed in Table 8.  
               TABLE 8                       Stored Luminance Signal and Color Difference Signal       and Output Color Difference Signal when a Still Picture       Changes to a Moving Picture                                                                        
 
         [0074]     While the input video data form a still picture, the formulas C t−2 ≈C t−1  and C t−1 ≈C t  hold, and selector  18 A selects the averaged color difference signal data S 17  and outputs the data as a color difference signal Cout. While the input video data form a moving picture, the formulas Y t ≠Y t+1  and Y t+1 ≠Y t+n  hold, and selector  18 A selects the color difference input signal data Cin and outputs the signal directly as a color difference signal Cout.  
         [0075]     When PAL video data are input, the luminance frame memory  2 A of a conventional signal processor stores two frames of luminance data. In the second embodiment, the luminance frame memory  2 A stores only one frame, but the cross-color rejection noise section is structured so that so that when a moving picture changes to a still picture, the averager averages the first still frame with the immediately preceding frame (which is substantially identical to the first still frame) instead of with the second preceding frame (which differs from the first still frame). It is therefore avoid image defects without having to compare the current frame with the second preceding frame to detect motion.  
         [0076]     The capacities of the frame memories  10 A and  11 A storing the color difference signals for PAL are determined in accordance with the following ratios:  
         [0077]     NTSC input video data Y:Cb:Cr=4:2:2  
         [0078]     PAL input video data Y:Cb:Cr=4:1:1  
         [0079]     where Y represents the luminance signal, Cb represents the blue color difference signal, and Cr represents the red color difference signal. Cross-color noise can therefore be eliminated from PAL signals with no more memory capacity than required for NTSC signals.  
       Third Embodiment  
       [0080]     The third embodiment of the invention provides a video signal processor that processes NTSC signals. Referring to  FIG. 3 , the video signal processor receives NTSC luminance input signal data Yin and color difference input signal data Cin as input video data. A luminance signal noise rejection section removes noise from the luminance input signal data Yin and outputs a substantially noise-free luminance signal Yout. A cross-color noise rejection section removes cross-color noise and random noise from the color difference input signal data Cin and outputs a substantially noise-free color difference signal Cout.  
         [0081]     The luminance signal noise rejection section includes a frame recursive luminance noise reducer  1 , a first video data memory (in this embodiment, a frame memory  2 B), and a luminance motion detector  3 . The frame recursive luminance noise reducer  1  and luminance motion detector  3  are identical to the corresponding elements in the first embodiment. The frame memory  2 B stores the NTSC luminance output signal data S 1  output by the frame recursive luminance noise reducer  1  as the luminance signal Yout for one frame and provides one-frame-delayed luminance signal data S 2 B to the frame recursive luminance noise reducer  1  and luminance motion detector  3 .  
         [0082]     The cross-color noise rejection section includes a frame recursive color difference noise reduction circuit (in this embodiment, a frame recursive color difference noise reducer  20 ), a selector  21 , a second video data memory (in this embodiment, a frame memory  10 B), a color difference noise reduction decision circuit (in this embodiment, a subtractor  22 , an absolute value circuit  23 , and a threshold comparator  24 ), a detection signal combiner  15 B, an averager (in this embodiment, an adder  16  and a divide-by-two circuit  17 ), and an output unit (a selector  18 B). The output terminal of the frame recursive color difference noise reducer  20  is connected via selector  21  to frame memory  10 B. The output terminal of frame memory  10 B is connected to the frame recursive color difference noise reducer  20 , the subtractor  22  in the color difference noise reduction decision circuit, and the adder  16  in the second selection circuit. The output terminal of the subtractor  22  is connected to the absolute value circuit  23 . The output terminal of the absolute value circuit  23  is connected to the threshold comparator  24 . The output terminal of the threshold comparator  24  is connected to the detection signal combiner  15 B. The adder  16  and divide-by-two circuit  17  are disposed on the input terminal side of selector  18 B. The output terminal of the adder  16  is connected to the divide-by-two circuit  17 . The output terminal of the divide-by-two circuit  17  is connected to selector  18 B.  
         [0083]     The frame recursive color difference noise reducer  20  removes noise on the basis of the difference between the color difference input signal data Cin and second delayed data (in this embodiment, 180° phase-offset color difference signal data S 10 B) and outputs noise-free color difference signal data S 20 . Selector  21  selects either the color difference input signal data Cin or the noise-free color difference signal data S 20  in accordance with the color difference noise reduction decision signal S 24  output from the threshold comparator  24 , and outputs the selected data to frame memory  10 B. Frame memory  10 B stores the data selected by selector  21  for one frame and outputs the 180° phase-offset color difference signal data S 10 B.  
         [0084]     The subtractor  22  in the color difference noise reduction decision circuit takes the difference between the color difference input signal data Cin and the 180° phase-offset color difference signal data S 10 B. The absolute value circuit  23  obtains the absolute value of the difference. The threshold comparator  24  detects color difference noise from the absolute value obtained by the absolute value circuit  23  and outputs the result as color difference noise reduction decision signal S 24 . The detection signal combiner  15 B combines the luminance motion signal S 3  output from the luminance motion detector  3  and the color difference noise reduction decision signal S 24  output from the threshold comparator  24 , and outputs a combined motion detection signal S 15 B to selector  18 B of the second selection circuit.  
         [0085]     The adder  16  adds the color difference input signal data Cin and the 180° phase-offset color difference signal data S 10 B. The divide-by-two circuit  17  halves the sum and outputs averaged color difference signal data S 17 . Selector  18 B selects either the averaged color difference signal data S 17  or the noise-free color difference signal data S 20  from the frame recursive color difference noise reducer  20  in accordance with the combined motion detection signal S 15 B from the detection signal combiner  15 B, and outputs the selected data as a color difference signal Cout.  
         [0086]     The operation of the third embodiment will be described next.  
         [0087]     When the luminance input signal data Yin and the color difference input signal data Cin are input, the frame recursive luminance noise reducer  1  in the luminance signal noise rejection section takes the algebraic difference between the luminance input signal data Yin and the one-frame-delayed luminance signal data S 2 B output from the frame memory  2 B. As in the first embodiment, if the filtered absolute value of the algebraic difference is smaller than a threshold, the algebraic difference is treated as noise and is subtracted from the luminance input signal data Yin to obtain the luminance output signal data S 1  (Yout). Otherwise, the algebraic difference is treated as motion and the unaltered luminance input signal data Yin are used as the luminance output signal data S 1  (Yout).  
         [0088]     The frame memory  2 B stores the luminance output signal data S 1  for one frame and outputs the one-frame-delayed luminance signal data S 2 . The luminance motion detector  3  takes the absolute value of the difference between the luminance output signal data S 1  output from the frame recursive luminance noise reducer  1  and the one-frame-delayed luminance signal data S 2 B output from the frame memory  2 B. If the absolute value is greater than a threshold, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is present. Otherwise, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is absent.  
         [0089]     The frame recursive color difference noise reducer  20  in the cross-color noise rejection section takes the algebraic difference between the color difference input signal data Cin and the 180° phase-offset color difference signal data SLOB from frame memory  10 B. If the absolute value of the algebraic difference is smaller than a threshold, the algebraic difference is treated as color difference noise. If the absolute value of the algebraic difference is greater than a threshold, the algebraic difference is treated as cross-color noise. When treated as color difference noise, the algebraic difference is subtracted from the color difference input signal data Cin, and the result is output as the noise-free color difference signal data S 20 . When the algebraic difference is treated as cross-color noise, the color difference input signal data Cin is output without alteration as the noise-free color difference signal data S 20 .  
         [0090]     When cross-color noise is present, large-amplitude flicker noise appears, making other color difference noise inconspicuous. If cross-color noise is absent, flicker noise is also absent, making other noise conspicuous. The subtractor  22  and absolute value circuit  23  obtain the absolute difference between the 180° phase-offset color difference signal data SLOB and the color difference input signal data Cin. If the absolute difference is greater than a threshold, the threshold comparator  24  sets the color difference noise reduction decision signal S 24  to a value indicating that cross-color noise is present. If the absolute difference is smaller than the threshold, the threshold comparator  24  sets the color difference noise reduction decision signal S 24  to a value indicating that color difference noise is absent.  
         [0091]     When the color difference noise reduction decision signal S 24  indicates that cross-color noise is present, selector  21  selects the color difference input signal data Cin. When the color difference noise reduction decision signal S 24  indicates that cross-color noise is absent, selector  21  selects the noise-free color difference signal data S 20  from the frame recursive color difference noise reducer  20 . Frame memory  10 B stores the selected data for one frame and outputs the 180° phase-offset color difference signal data S 10 B.  
         [0092]     The detection signal combiner  15 B combines the luminance motion signal S 3  output from the luminance motion detector  3  and the color difference noise reduction decision signal S 24  output from the threshold comparator  24 . When the luminance motion signal S 3  indicates that motion is absent and when the color difference noise reduction decision signal S 24  indicates that cross-color noise is present, the combined motion detection signal S 15 B is given a value that causes selector  18 B to select the averaged color difference signal data S 17 . When the luminance motion signal S 3  indicates that motion is present or the color difference noise reduction decision signal S 24  indicates that cross-color noise is absent, the combined motion detection signal S 15 B is given a value that causes selector  18 B to select the noise-free color difference signal data S 20  from the frame recursive color difference noise reducer  20 . The data selected by selector  18 B become the color difference output (Cout) of the video signal processor.  
         [0093]     Conventional video signal processors require a frame memory for cross-color noise reduction to be used even in frames in which cross-color noise reduction is not performed. If frame recursive noise color difference noise reduction is performed in these frames, another memory is required in addition to the memory for cross-color noise reduction. In the third embodiment, the single  10 B serves for both cross-color noise reduction and frame recursive color difference noise reduction, one type of noise reduction or the other being selected in each frame.  
         [0094]     Conventional cross-color noise reduction uses averaging. Random noise is always halved (−6 dB), regardless of the type of random noise, and much random noise inevitably remains. According to the third embodiment, cross-color noise reduction and frame recursive color difference noise reduction are switched on a frame basis. In frames in which recursive color difference noise reduction is selected, random color difference noise can be eliminated more effectively, further improving picture quality.  
       Fourth Embodiment  
       [0095]     The video signal processor in the fourth embodiment processes both PAL and NTSC signals, as selected by a video system control signal, more specifically a PAL-NTSC selection signal PN. Referring to  FIG. 4 , the video signal processor receives luminance input signal data Yin and color difference input signal data Cin as input video data. A luminance signal noise rejection section removes noise from the luminance input signal data Yin and outputs a noise-free luminance signal Yout. A cross-color rejection section removes cross-color noise from the color difference input signal data Cin and outputs a color difference signal Cout.  
         [0096]     The luminance signal noise rejection section includes a frame recursive luminance noise reducer  1 , a frame memory  2 , and a luminance motion detector  3  as described in the first embodiment.  
         [0097]     The cross-color rejection section of the fourth embodiment includes a frame recursive color difference noise reducer  20  and other elements included in the first to third embodiments, as well as some additional elements. The output terminal of the frame recursive color difference noise reducer  20  is connected to a selector  21 . The output terminal of selector  21  is connected to a frame memory  10 C- 1  and a pair of selectors  19 C- 1 ,  19 C- 2 . The output terminal of frame memory  10 C- 1  is connected to the frame recursive color difference noise reducer  20 , a subtractor  22 , and another pair of selectors  25 - 1  and  25 - 2 . Subtractor  22  is followed by an absolute value circuit  23  and a threshold comparator  24  as in the third embodiment. The output terminal of selector  25 - 1  is connected to another frame memory  10 C- 2 . The output terminal of frame memory  10 C- 2  is connected to selector  25 - 2 . The output terminal of selector  25 - 2  is connected to selector  19 C- 1 . The output terminal of selector  19 C- 1  is connected to yet another frame memory  11 C- 1 . The output terminal of frame memory  11 C- 1  is connected to selector  19 C- 2 . The output terminal of selector  19 C- 2  is connected to still another frame memory  11 C- 2 . The output terminal of frame memory  11 C- 2  is connected to another selector  19 C- 3 . The output terminal of selector  19 C- 3  is connected to a subtractor  12 , which is followed by an absolute value circuit  13  and a threshold comparator  14  as in the first embodiment. The output terminals of the two threshold comparators  14 ,  24  are connected to a detection signal combiner  15 C. The output terminal of the detection signal combiner  15 C is connected to a selector  18 C.  
         [0098]     As in the third embodiment, the frame recursive color difference noise reducer  20  removes noise on the basis of the difference between the color difference input signal data Cin and delayed color difference signal data S 10 C- 1  and outputs a noise-free color difference signal data S 20 . Selector  21  selects either the color difference input signal data Cin or the noise-free color difference signal data S 20  in accordance with a color difference noise reduction decision signal S 24 . Frame memory  10 C- 1  stores the data selected by selector  21  for one frame interval and outputs a delayed color difference signal data S 10 C- 1 . Subtractor  22  takes the difference between the color difference input signal data Cin and the delayed color difference signal data S 10 C- 1  from frame memory  10 C- 1 . Absolute value circuit  23  obtains the absolute value of the difference. Threshold comparator  24  detects cross-color noise from the absolute value obtained by absolute value circuit  23  and outputs the color difference noise reduction decision signal S 24 .  
         [0099]     Selector  25 - 1  selects either the delayed color difference signal data S 10 C- 1  from frame memory  10 C- 1  or the data selected by selector  21  in accordance with the luminance motion signal S 3  output from the luminance motion detector  3 . Frame memory  10 C- 2  stores the data selected by selector  25 - 1  for one frame interval and outputs delayed color difference signal data S 10 C- 2 . Selector  25 - 2  selects either the delayed color difference signal data S 10 C- 1  from frame memory  10 C- 1  or the delayed color difference signal data S 10 C- 2  from frame memory  10 C- 2  in accordance with the PAL-NTSC selection signal PN, and outputs the selected data as 180° phase-offset color difference signal data S 25 - 2 .  
         [0100]     Selector  19 C- 1  selects either the 180° phase-offset color difference signal data S 25 - 2  selected by selector  25 - 2  or the data selected by selector  21  in accordance with the luminance motion signal S 3 . Frame memory  11 C- 1  stores the data selected by selector  19 C- 1  for one frame interval and outputs delayed color difference signal data S 11 C- 1 . Selector  19 C- 2  selects either the delayed color difference signal data S 11 C- 1  or the data from selector  21  in accordance with the luminance motion signal S 3 . Frame memory  11 C- 2  stores the data selected by selector  19 C- 2  for one frame interval and outputs delayed color difference signal data S 11 C- 2 .  
         [0101]     Selector  19 C- 3  selects either the delayed color difference signal data S 11 C- 2  from frame memory  11 C- 2  or the delayed color difference signal data S 11 C- 1  from frame memory  11 C- 1  in accordance with the PAL-NTSC selection signal PN, and outputs 0° phase-offset color difference signal data S 19 C- 3 . The subtractor  12  takes the difference between the color difference input signal data Cin and the 0° phase-offset color difference signal data S 19 C- 3  from selector  19 C- 3 . The absolute value circuit  13  obtains the absolute value of the difference. The threshold comparator  14  determines whether motion is present in accordance with the absolute value obtained by the absolute value circuit  13 , and outputs the result as a color difference motion signal S 14 .  
         [0102]     The detection signal combiner  15 C combines the color difference motion signal S 14 , the luminance motion signal S 3  output from the luminance motion detector  3 , and the color difference noise reduction decision signal S 24  output from the threshold comparator  24 , and outputs a combined motion detection signal S 15 C to selector  18 C. Selector  18 C selects either the averaged color difference signal data S 17 , which are obtained from the color difference input signal data Cin and the 180° phase-offset color difference signal data S 25 - 2  by the adder  16  and the divide-by-two circuit  17 , or the noise-free color difference signal data S 20  from the frame recursive color difference noise reducer  20 , in accordance with the combined motion detection signal S 15 C, and outputs the selected data as a color difference signal Cout.  
         [0103]     The operation of the fourth embodiment will be described next.  
         [0104]     When the luminance input signal data Yin and the color difference input signal data Cin are input, the frame recursive luminance noise reducer  1  of the luminance signal noise rejection section takes the algebraic difference between the luminance input signal data Yin and the one-frame-delayed luminance signal data S 2  output from the frame memory  2 , and operates as described in the first embodiment to generate the luminance output signal data S 1  used as luminance output Yout of the signal processing unit.  
         [0105]     Frame memory  2  stores the luminance output signal data S 1  for one frame interval and outputs the one-frame-delayed luminance signal data S 2 . The luminance motion detector  3  takes the absolute value of the difference between the luminance output signal data S 1  output from the frame recursive luminance noise reducer  1  and the one-frame-delayed luminance signal data S 2  output from the frame memory  2 . If the absolute value is greater than a threshold, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is present. Otherwise, the luminance motion detector  3  sets the luminance motion signal S 3  to a value indicating that motion is absent.  
         [0106]     The frame recursive color difference noise reducer  20  in the cross-color noise rejection section takes the algebraic difference between the color difference input signal data Cin and the delayed color difference signal data S 10 C- 1  output from frame memory  10 C- 1 . As in the third embodiment, if the absolute value of the algebraic difference is smaller than a threshold, the algebraic difference is treated as color difference noise and is subtracted from the color difference input signal data Cin to obtain the noise-free color difference signal data S 20 . If the absolute value of the algebraic difference is greater than the threshold, the algebraic difference is treated as cross-color noise and the color difference input signal data Cin are output without alteration as the noise-free color difference signal data S 20 .  
         [0107]     Also as in the third embodiment, subtractor  22  and absolute value circuit  23  obtain the absolute difference between the delayed color difference signal data S 10 C- 1  from frame memory  10 C- 1  and the color difference input signal data Cin. If the absolute difference is greater than a threshold, the threshold comparator  24  sets the color difference noise reduction decision signal S 24  to a value indicating that cross-color noise is present and selector  21  selects the color difference input signal data Cin. If the absolute difference is smaller than the threshold, the threshold comparator  24  sets the color difference noise reduction decision signal S 24  to a value indicating that cross-color noise is absent, and selector  21  selects the noise-free color difference signal data S 20  output from the frame recursive color difference noise reducer  20 . Frame memory  10 C- 1  stores the data selected by selector  21  for one frame interval and outputs delayed color difference signal data S 10 C- 1 .  
         [0108]     When the luminance motion detector  3  determines that motion is present, the luminance motion signal S 3  causes selector  25 - 1  to select the color difference input signal data Cin or the noise-free color difference signal data S 20 , whichever was selected by selector  21 . When the luminance motion detector  3  determines that motion is absent, the luminance motion signal S 3  causes selector  25 - 1  to select the delayed color difference signal data S 10 C- 1  from frame memory  10 C- 1 . Frame memory  10 C- 2  stores the selected data for one frame interval and outputs delayed color difference signal data  10 C- 2 .  
         [0109]     When the PAL-NTSC selection signal PN selects NTSC, selector  25 - 2  selects the delayed color difference signal data S 10 C- 1  from frame memory  10 C- 1 . When the PAL-NTSC selection signal PN selects PAL, selector  25 - 2  selects the delayed color difference signal data S 10 C- 2  from frame memory  10 C- 2 . The selected data are output as 180° phase-offset color difference signal data S 25 - 2 .  
         [0110]     When the luminance motion detector  3  determines that motion is present, the luminance motion signal S 3  causes selector  19 C- 1  to select the data output from selector  21 : either the color difference input signal data Cin or the noise-free color difference signal data S 20 . If the luminance motion detector  3  determines that motion is absent, the luminance motion signal S 3  causes selector  19 C- 1  to select the 180° phase-offset color difference signal data S 25 - 2  from selector  25 - 2 . Frame memory  11 C- 1  stores the selected data for one frame interval and outputs delayed color difference signal data S 11 C- 1 .  
         [0111]     When the luminance motion detector  3  determines that motion is present, the luminance motion signal S 3  causes selector  19 C- 2  to select the output of selector  21 , which is the color difference input signal data Cin or the noise-free color difference signal data S 20 . When the luminance motion detector  3  determines that motion is absent, the luminance motion signal S 3  causes selector  19 C- 2  to select the delayed color difference signal data S 11 C- 1  from frame memory  11 C- 1 . Frame memory  11 C- 2  stores the selected data for one frame interval and outputs delayed color difference signal data S 11 C- 2 .  
         [0112]     When the PAL-NTSC selection signal PN selects NTSC, selector  19 C- 3  selects the delayed color difference signal data S 11 C- 1  from frame memory  11 C- 1 . When the PAL-NTSC selection signal PN selects PAL, selector  19 C- 3  selects the delayed color difference signal data S 11 C- 2  from frame memory  11 C- 2 . The selected data are output as 0° phase-offset color difference signal data S 19 C- 3 .  
         [0113]     Subtractor  12  takes the difference between the color difference input signal data Cin and the 0° phase-offset color difference signal data S 19 C- 3 . Absolute value circuit  13  obtains the absolute value of the difference. If the absolute value is greater than a certain threshold determined in consideration of noise and jitter, the threshold comparator  14  sets the color difference motion signal S 14  to a value indicating that motion is present. Otherwise, the threshold comparator  14  sets the color difference motion signal S 14  to a value indicating that motion is absent.  
         [0114]     The detection signal combiner  15 C combines the luminance motion signal S 3 , the color difference motion signal S 14 , and the color difference noise reduction decision signal S 24 , and outputs a combined motion detection signal S 15  to selector  18 C. When cross-color noise is detected by threshold comparator  24  and motion is not detected by either the luminance motion detector  3  or threshold comparator  14 , selector  18 C selects the averaged color difference signal data S 17 , which are obtained by adding the color difference input signal data Cin and the 180° phase-offset color difference signal data S 25 - 2  from selector  25 - 2  in the adder  16  and halving the sum in the divide-by-two circuit  17 . When the luminance motion detector  3  determines that luminance motion is present, threshold comparator  14  determines that color difference motion is present, or threshold comparator  24  fails to detect cross-color noise, selector  18 C selects the noise-free color difference signal data S 20  output from the frame recursive color difference noise reducer  20 . Selector  18 C outputs the selected data as a color difference output signal data Cout.  
         [0115]     The fourth embodiment produces the following effects:  
         [0116]     (i) The threshold comparator  14  detects motion by making a 0° phase (in-phase) comparison, using the frame memories  10 C- 1  and  11 -C to obtain a two-frame delay for NTSC or the frame memories  10 C- 1 ,  10 C- 2 ,  11 C- 1 , and  11 C- 2  to obtain a four-frame delay for PAL. The color-difference in-phase comparison can detect motion when the only motion in the luminance signal is present in a high frequency component such as a component near the fsc frequency, which is removed from the luminance signal during Y/C separation and becomes cross-color noise, for example, or even if motion is present only in the color difference component signals, so that the color difference input signal data Cin changes while the luminance signal remains substantially unchanged at all frequencies. Consequently, color difference averaging in the presence of motion, which leads to color smear, can be avoided regardless of the signal (luminance or color difference) in which the motion occurs, and regardless of the frequency band in which the motion occurs.  
         [0117]     (ii) When PAL video data are input, the frame memory  2  does not have to store two frames of luminance signal data as in conventional video signal processors. Color difference motion across two frames can be detected even though the only one luminance frame is stored. Accordingly, the cross-color noise rejection section can avoid color motion image defects without having to store more than one frame of luminance signal data.  
         [0118]     (iii) When cross-color noise is absent, frame recursive color difference noise reduction is used so that a greater random noise reduction effect can be achieved than the −6-db effect achieved by conventional averaging. Cross-color noise reduction and frame recursive color difference noise reduction are switchable on a frame basis, further improving picture quality.  
         [0119]     (iv) Conventional video signal processors require a frame memory for cross-color noise reduction to be used even in frames in which cross-color noise reduction is not performed. If frame recursive color difference noise reduction is performed in these frames, another memory is required in addition to the memory for cross-color noise reduction. In the fourth embodiment, the frame memories for color difference noise reduction  10 C- 1 ,  10 C- 2 ,  11 C- 1 , and  11 C- 2  serve for both cross-color noise reduction and frame recursive color difference noise reduction, one type of noise reduction or the other being selected in each frame.  
       Fifth Embodiment  
       [0120]     In a fifth, sixth, and seventh embodiments, the color difference input signal data and the averaged color difference data are combined in a ratio that depends on the degree of motion detected. More specifically, degrees of motion are detected by making comparisons with a plurality of thresholds to obtain a motion coefficient K. The color difference input signal data are multiplied by K, the averaged color difference data are multiplied by 1−K, and the two products are added together to obtain the output color difference signal Cout.  
         [0121]     The fifth embodiment is based on the first embodiment and includes all of the elements shown in  FIG. 1  except the luminance motion detector  3 , threshold comparator  14 , detection signal combiner  15 , and selector  18 , which are replaced by the luminance motion detector  3 D, threshold comparator  14 D, motion coefficient generator  35 , and output unit  40  shown in  FIG. 5 . In addition, a filter  36  is inserted between the subtractor  12  and absolute value circuit  13 , as also shown in  FIG. 5 . The signals S 2 , Sit, S 17 , and Cin in  FIG. 5  are the same as in  FIG. 1 .  
         [0122]     The luminance motion detector  3 D comprises a subtractor  31 , a filter  32 , an absolute value circuit  33 , and a motion detector  34 , which are interconnected in this order. The subtractor  31  takes the difference between the luminance output signal data S 1  output from the frame recursive luminance noise reducer  1  and the one-frame-delayed luminance signal data S 2  read from the frame memory  2 . The filter  32  filters the difference to reduce the effects of noise and jitter. The absolute value circuit  33  takes the absolute value of the filtered difference. The motion detector  34  compares the resulting absolute frame-to-frame difference with a series of threshold values as explained below to obtain a luminance motion coefficient Ky, which is supplied to the motion coefficient generator  35 .  
         [0123]     In the cross-color rejection section, the filter  36  provided between the subtractor  12  and the absolute value circuit  13  filters the difference output from the subtractor  12  to reduce the effects of noise and jitter. The absolute value circuit  13  takes the absolute value of the filtered difference and outputs it as an absolute 0° phase-offset color difference value. The threshold comparator  14 D compares the absolute 0° phase-offset color difference value with a series of threshold values to obtain a color difference motion coefficient Kc, which is also supplied to the motion coefficient generator  35 .  
         [0124]     The motion coefficient generator  35  functions as a detection signal combiner by selecting the greater of the two coefficients Ky and Kc received from the motion detector  34  and the threshold comparator  14 D and outputs the selected value and a complementary value to the output unit  40 . The output unit  40  includes a first multiplier  41 , a second multiplier  42 , and an adder  43  connected to the output terminals of the multiplier  41  and multiplier  42 . Multiplier  41  multiplies the color difference input signal data Cin by the selected motion coefficient. Multiplier  42  multiplies the averaged color difference signal data S 17  from the divide-by-two circuit  17  by the complementary coefficient. The adder  43  adds the two products.  
         [0125]     The function of the filters  32  and  36  is to eliminate data other than motion, such as noise and jitter. The filters  32  and  36  may be low-pass filters (LPFs), median filters, or combinations of a plurality of filters, including a ‘filter’ that simply passes the difference value through without change. If a combination of filters is used, the maximum value of the plurality of resulting signals may be taken (to stress motion detection), the minimum value may be taken (to stress noise reduction), or the mean or median value may be taken. The filters  32  and  36  may also be omitted.  
         [0126]     The operation of the fifth embodiment will now be described in further detail.  
         [0127]     When the luminance input signal data Yin and the color difference input signal data Cin are input, the luminance output signal data S 1  output from the frame recursive luminance noise reducer  1  and the one-frame-delayed luminance signal data S 2  output from the frame memory  2  are input to the luminance motion detector  3 D. At the same time, the color difference input signal data Cin and the 0° phase-offset color difference signal data S 11  output from frame memory  11  are input to the subtractor  12 .  
         [0128]     In the luminance motion detector  3 D, the subtractor  31  takes the difference between the luminance output signal data S 1  and the one-frame-delayed luminance signal data S 2 ; the filter  32  filters the resulting frame-to-frame luminance difference signal to reduce the effect of noise and jitter; the absolute value circuit  33  takes the absolute value of the filtered difference; the motion detector  34  compares the absolute frame-to-frame luminance difference signal with a luminance motion threshold TH 1  and a series of successively higher thresholds to determine the value of the luminance motion coefficient Ky output to the motion coefficient generator  35 . Specifically, the value of the luminance motion coefficient Ky is derived from the series of comparisons in Table 9, which are performed in the listed order, from top to bottom in the table, until one of the comparisons succeeds.  
                             TABLE 9                           Determination of Ky from Absolute Frame-to-Frame       Luminance Difference                Comparison   Value of Ky                       Absolute difference &lt; TH1   0           Absolute difference &lt; TH1 + 1   ¼           Absolute difference &lt; TH1 + 2   ½           Absolute difference &lt; TH1 + 3   ¾           Absolute difference ≧ TH1 + 3   1                      
 
         [0129]     In the cross-color rejection section, the subtractor  12  takes the difference between the color difference input signal data Cin and the 0° phase-offset color difference signal data S 11 , which are delayed by two frames for NTSC or by four frames for PAL by the frame memories  10  and  11 ; the filter  36  filters the difference to remove the effect of noise and jitter; the absolute value circuit  13  takes the absolute value of the filtered difference; and the threshold comparator  14 D compares the resulting absolute 0° phase color difference value with a 0° phase color difference motion threshold TH 2  and a series of successively higher thresholds to determine the color difference motion coefficient Kc output to the motion coefficient generator  35 . Specifically, the value of the color difference motion coefficient Kc is calculated by the series of comparisons shown in Table 10, which are performed in the listed order until one of the comparisons succeeds.  
                             TABLE 10                           Determination of Kc from Absolute 0° Phase Color       Difference                Comparison   Value of Kc                       Absolute difference &lt; TH2   0           Absolute difference &lt; TH2 + 1   ¼           Absolute difference &lt; TH2 + 2   ½           Absolute difference &lt; TH2 + 3   ¾           Absolute difference ≧ TH2 + 3   1                      
 
         [0130]     The motion coefficient generator  35  selects either the luminance motion coefficient Ky or the color difference motion coefficient Kc, whichever is greater, as the coefficient K, calculates the complement K with respect to unity, and outputs the coefficients (K and 1−K) thus obtained to the multipliers  41  and  42 . Multiplier  41  multiplies the color difference input signal data Cin by the selected motion coefficient K; multiplier  42  multiplies the averaged color difference signal data S 17  by the complementary motion coefficient 1−K; the adder  43  adds the two products and outputs the sum as the color difference signal Cout. The value of the color difference signal Cout depends on the value of K as shown in Table 11.  
                         TABLE 11                           Determination of Cout from Selected Motion       Coefficient K            K   Value of Cout               0   (S17 × 1)       ¼   (Cin × ¼) + (S17 × ¾)       ½   (Cin × ½) + (S17 × ½)       ¾   (Cin × ¾) + (S17 × ¼)       1   (Cin × 1)                  
 
         [0131]     The fifth embodiment accordingly detects different degrees of luminance motion and color difference motion, combines them into a single motion coefficient K, and then combines the color difference input signal data with the averaged color difference signal data in a proportion determined by the coefficient K to obtain the color difference output signal data Cout. As in the preceding embodiments, motion can be reliably detected from the color difference signal even when motion is undetectable in the luminance signal. In addition, cross-color noise reduction is switched on and off in a semi-continuous series of steps, corresponding to different proportions in which the input color difference input signal data Cin and the averaged color difference signal are combined, making the operation of the cross-color rejection section smoother than in the first embodiment.  
         [0132]     The first embodiment can be considered as a special case of the fifth embodiment, in which the selected motion coefficient K is always either 1 or 0.  
       Sixth Embodiment  
       [0133]     The sixth embodiment is similar to the fifth embodiment, but is based on the second embodiment instead of the first embodiment, and is used with a PAL component video signal.  
         [0134]     The video signal processor in the sixth embodiment has a frame recursive luminance noise reducer  1 , frame memory  2 A, frame memories  10 A,  11 A, an adder  16 , a divide-by-two circuit  17 , and a selector  19  as shown in  FIG. 2 , and a luminance motion detector  3 D and output unit  40  substantially as shown in  FIG. 5 .  
         [0135]     The frame recursive luminance noise reducer  1  removes noise in accordance with a difference between the luminance input signal data Yin and first delayed data (in this embodiment, the one-frame-delayed luminance signal data S 2 A) and outputs luminance output signal data S 1 . The frame memory  2 A stores the luminance output signal data S 1  for one frame interval and outputs one-frame-delayed luminance signal data S 2 A. The luminance motion detector  3 D takes an absolute difference between the luminance output signal data S 1  and the one-frame-delayed luminance signal data S 2 A, compares the absolute difference with a luminance motion threshold TH 1  and a series of higher thresholds as explained in the fifth embodiment, and outputs a luminance motion coefficient K.  
         [0136]     Frame memory  10 A stores the color difference input signal data Cin for one frame interval and outputs second delayed data (in this embodiment, the 90° phase-offset color difference signal data S 10 A). Selector  19  selects either the color difference input signal data Cin or the 90° phase-offset color difference signal data S 10 A in accordance with the luminance motion coefficient K. Frame memory  11 A stores the selected data for one frame interval and outputs third delayed data (in this embodiment, 180° phase-offset color difference signal data S 11 A). The adder  16  adds the color difference input signal data Cin and the 180° phase-offset color difference signal data S 11 A; the divide-by-two circuit  17  halves the sum and outputs averaged color difference signal data S 17 .  
         [0137]     In the output unit  40 , the multiplier  41  multiplies the color difference input signal data Cin by the luminance motion coefficient K to obtain a first product; the multiplier  42  multiplies the averaged color difference signal data S 17  by a complementary coefficient (1−K), obtained by subtracting the luminance motion coefficient K from unity, to obtain a second product; the adder  43  adds the first and second products and outputs the sum as color difference output signal data Cout.  
         [0138]     The sixth embodiment has the same effects as the second embodiment, with the additional effect that cross-color noise reduction is switched on and off in a series of steps, as in the fifth embodiment, so the transition is smoother than in the second embodiment.  
       Seventh Embodiment  
       [0139]     The seventh embodiment is also similar to the fifth embodiment, but is based on the third embodiment instead of the first embodiment, and is used with an NTSC component video signal.  
         [0140]     The video signal processor in the seventh embodiment has a frame recursive luminance noise reducer  1 , a frame memory  2 B, a frame memory  10 B, an adder  16 , a divide-by-two circuit  17 , a frame recursive color difference noise reducer  20 , a selector  21 , a subtractor  22 , an absolute value circuit  23 , and a threshold comparator  24  as shown in  FIG. 3 , and a luminance motion detector  3 D, a motion coefficient generator  35 , and a output unit  40  substantially as shown in  FIG. 5 .  
         [0141]     The frame recursive luminance noise reducer  1  removes noise in accordance with a difference between the luminance input signal data Yin and first delayed data (in this embodiment, one-frame-delayed luminance signal data S 2 ) and outputs luminance output signal data S 1 . The frame memory  2 B stores the luminance output signal data S 1  for one frame interval and outputs one-frame-delayed luminance signal data S 2 B. The luminance motion detector  3 D takes an absolute difference between the luminance output signal data S 1  and the one-frame-delayed luminance signal data S 2 B, compares the absolute difference with a luminance motion threshold TH 1  and a series of higher thresholds as explained in the fifth embodiment, and outputs a luminance motion coefficient Ky.  
         [0142]     The frame recursive color difference noise reducer  20  removes noise in accordance with a difference between the color difference input signal data Cin and second one-frame-delayed data (in this embodiment, 180° phase-offset color difference signal data S 10 ) and outputs noise-free color difference signal data S 20 . Selector  21  selects either the color difference input signal data Cin or the noise-free color difference signal data S 20  in accordance with the color difference noise reduction decision signal S 24 . Frame memory  10 B stores the data selected by selector  21  for one frame interval and outputs 180° phase-offset color difference signal data S 10 B.  
         [0143]     The subtractor  22  takes the difference between the color difference input signal data Cin and the 180° phase-offset color difference signal data SLOB, and the absolute value circuit  23  obtains the absolute value of the difference. If the absolute difference is greater than a threshold, the threshold comparator  24  sets the color difference noise reduction decision signal S 24  to a value indicating that cross-color noise is present. If the absolute difference is smaller than the threshold, the threshold comparator  24  sets the color difference noise reduction decision signal S 24  to a value indicating that cross-color noise is absent.  
         [0144]     The motion coefficient generator  35  combines the luminance motion coefficient Ky with the color difference noise reduction decision signal S 24  to generate a motion coefficient K and complementary coefficient 1−K. In one exemplary scheme, the motion coefficient generator  35  sets the motion coefficient K equal to the luminance motion coefficient (K=Ky) when the color difference noise reduction decision signal S 24  indicates that cross-color noise is present, and sets the motion coefficient K to unity (K=1) when the color difference noise reduction decision signal S 24  indicates that cross-color noise is absent.  
         [0145]     The adder  16  adds the color difference input signal data Cin and the 180° phase-offset color difference signal data SLOB, and the divide-by-two circuit  17  halves the sum and outputs averaged color difference signal data S 17  to the output unit  40 .  
         [0146]     In the output unit  40 , multiplier  41  multiplies the noise-free color difference signal data S 20  by the motion coefficient K to obtain a first product; multiplier  42  multiplies the averaged color difference signal data S 17  by the complementary coefficient 1−K to obtain a second product; the adder  43  adds the first and second products and outputs the sum as a color difference signal Cout.  
         [0147]     The seventh embodiment has the same effects as the third embodiment, with the additional effect that when cross-color noise is present, cross-color noise reduction is switched on and off in a series of steps according to the amount of luminance motion detected, so the transition is smoother than in the third embodiment.  
       Variations  
       [0148]     Possible modifications of the preceding embodiments include, but are not limited to, the following (1) to (6).  
         [0149]     (1) In the first embodiment, frame memories  10  and  11  may each be furnished with an input formatter and an output reformatter. The formatters and reformatters control the writing and reading of data according to a PAL-NTSC selection signal PN so that the output is delayed by one frame for NTSC or two frames for PAL. The same circuit can then be used for processing both NTSC and PAL signals. Similar formatters and reformatters can be added to the third and fourth embodiments. A formatter and reformatter for frame memory  10 B in  FIG. 3  enable the third embodiment to be used with a PAL signal. In  FIG. 4 , frame memories  10 C- 1  and  10 C- 2  and selectors  25 - 1  and  25 - 2  can be combined into a single frame memory with a formatter and reformatter. Frame memories  11 C- 1  and  11 C- 2  and selectors  19 C- 1  and  19 C- 2  can be combined into another single frame memory with another formatter and reformatter. A similar modification of the second embodiment is possible.  
         [0150]     (2) In any of the first to fourth embodiments, frame memories having the same capacity can be used for both NTSC and PAL signals if the signals are sampled so that the data ratios are Y:Cb:Cr=4:2:2 for NTSC and Y:Cb:Cr=4:1:1 for PAL. This sampling scheme permits total frame memory capacity to be reduced.  
         [0151]     (3) The luminance motion detector  3  in any of the first to fourth embodiments can be equipped with input filters such as LPFs for its noise-free luminance input (S 1 ) and one-frame-delayed luminance input (S 2 , S 2 A, or S 2 B). These filters are spatial filters operating on the data in one frame by combining vertically or horizontally adjacent data, or data adjacent in both directions, to reduce the effects of noise and jitter.  
         [0152]     (4) When the luminance input signal Yin includes little or no noise, such as in the vertical blanking interval, the frame recursive luminance noise reducer  1  of the first or second embodiment may stop operating and pass the luminance input signal Yin through without alteration. If the luminance input signal Yin always includes little or no noise, as in a digital versatile disc (DVD) player, for example, the frame recursive luminance noise reducer  1  may be eliminated to reduce the size of the circuit.  
         [0153]     (5) The color difference input signal Cin and the 0° phase-offset color difference signal data S 11  may be spatially filtered by LPFs or another type of filter before input to the subtractor  12  in the first embodiment to reduce the effects of noise and jitter by combining vertically or horizontally adjacent data, or data adjacent in both directions, in the same frame.  
         [0154]     (6) The fourth embodiment illustrated in  FIG. 4  shows one possible combination of the first, second, and third embodiments. The combined features include improved motion detection by in-phase color difference comparison, PAL signal processing with storage of only one luminance signal frame, and improved noise reduction by switching between cross-color noise reduction and frame recursive color difference noise reduction. Other combinations of the first to third embodiments are possible without altering these independent effects.  
         [0155]     Possible uses of the invented video signal processor include, but are not limited to, (A) to (C) below.  
         [0156]     (A) In video display apparatus such as a television set, the input signals may be obtained by two-dimensional Y/C separation of a composite video signal followed by conversion of the separated signals to demodulated component video signals, or by reproduction of demodulated component video signals that have been obtained in this way and then recorded on recording media. In either case, the invention provides an easy way to display a video image with reduced noise, including reduced cross-color noise.  
         [0157]     (B) Similarly, in video recording apparatus such as a video tape recorder (VTR), video cassette recorder (VCR), DVD recorder, or hard disk drive (HDD) recorder, the input signals may be obtained by two-dimensional Y/C separation of a composite video signal followed by conversion of the separated signals to demodulated component video signals, or by reproduction of demodulated component video signals that have been obtained in this way and then recorded on recording media. In either case, the invention provides an easy way to record a video image with reduced noise, including reduced cross-color noise.  
         [0158]     (C) In a video reproducing apparatus, noise such as cross-color noise included in a video signal recorded as a component signal can be easily reduced when a video image is output.  
         [0159]     Those skilled in the art will recognize that further variations are possible within the scope of the invention, which is defined in the appended claims.