Patent Publication Number: US-8111332-B2

Title: Noise suppression method, noise suppression method program, recording medium recording noise suppression method program, and noise suppression apparatus

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present invention contains subject matter related to Japanese Patent Application JP 2006-156803 filed in the Japanese Patent Office on Jun. 6, 2006, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a noise suppression method, a noise suppression method program, a recording medium recording a noise suppression method program, and a noise suppression apparatus. More particularly, the present invention can be applied to a time-cyclic noise filter eliminating noise of a video signal. The present invention makes it possible to suppress noise more sufficiently and at a higher speed than in the past by counting the number of fields or the number of frames after a sudden change in the signal level of an input video signal, and dynamically controlling the feedback ratio to increase gradually in accordance with the count value. 
     2. Description of the Related Art 
     To date, as shown in  FIG. 13 , a time-cyclic noise filter has been used to suppress noise of a video signal using a field difference of a frame difference. That is to say, in this noise filter  1 , a delay-signal generation section  2  delays an output video signal S 2  for a period of one field or one frame, and outputs a reference video signal S 3  for extracting noise components. In this regard, the reference video signal S 3  may be generated by performing motion compensation on the output video signal S 2  here. A subtraction circuit  3  subtracts the reference video signal S 3  from the input video signal S 1  to generate a difference signal S 4 . A compensation-signal generation section  4  multiples the difference signal S 4  by a feedback ratio k to generate a noise compensation signal S 5 . In this regard, at this time, the noise compensation signal S 5  is sometimes generated such that, for example, the smaller the amplitude of a change is, the more likely that the change is noise, and the feedback ratio is set to a high value, on the contrary, the larger the amplitude of a change is, the more likely that the change is not noise, and the feedback ratio is set to a low value. Also, the difference signal S 4  is sometimes bank-divided and processed. A subtraction circuit  5  subtracts the noise compensation signal S 5  from the input video signal S 1  to generate the output video signal S 2 . 
     On such a time-cyclic noise filter, a scheme for measuring a noise level and automatically setting the signal level of the noise compensation signal S 5  on the basis of the noise-level measuring result has been proposed in Japanese Unexamined Patent Application Publication No. 2001-136416, etc. 
     The noise reduction processing in such a time-cyclic noise filter can be expressed by the following recurrence relation. In this regard, here, I t  is the signal level of the input video signal S 1 , and O t  is the signal level of the output video signal S 2 . Also, the subscript t of each symbol is time. When the processing unit of the input video signal S 1  is a frame, or a field, the subscript t is the number of the frame, or the field from the start, respectively. Accordingly, in the configuration in which the output video signal S 2  is simply delayed and fed back, O t-1  is the signal level of the reference video signal S 3 , and (I t −O t-1 ) is the signal level of the difference signal S 4 . In this regard, here, it is assumed that the average signal level of the input video signal S 1  is 0 level, and this input video signal S 1  includes only a noise component having variance σ 0   2 . 
     
       
         
           
             
               
                 
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     The signal level of noise average 0 level. There is no correlation between successive fields or frames. Thus, the variance σ 0   2  of the reference video signal S 3  at time t can be expressed by the following expression on the basis of Expression (1). 
     [Expression 2]
 
σ 0   2 =(1− k ) 2 ·σ 0   2   +kσ   t-1   2   (2)
 
     By solving the recurrence relation of Expression (2), the variance of the output video signal S 2  at time t can be expressed by the following general expression. 
     
       
         
           
             
               
                 
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     Here, since the feedback ratio k is less than 1(k&lt;1), when time t is infinity, the variance 2  of the output video signal S 2  can be expressed by the following expression. 
     
       
         
           
             
               
                 
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     Here, σ t   2 /σ 0   2  represents the noise reduction rate at time t. Accordingly, the noise reduction rate at infinite time by the feedback ratio k is shown by  FIG. 14 . Thus, from Expression (4) in  FIG. 14 , it is understood that in a known time-cyclic noise filter, the noise reduction effect becomes higher as the feedback ratio k is set higher. 
     From the above relational expression, the relationship between the variance σ t   2  of the output video signal S 2  and time t is shown in  FIG. 15 . Accordingly, it is understood that, in the known time-cyclic noise filter, if the feedback ratio k is set high, it takes time to converge. 
     Thus, in the known time-cyclic noise filter, there has been a problem in that if the setting is determined so as to increase the noise reduction effect, it is difficult to ensure the noise reduction effect of a portion in fast motion. That is to say, for example as shown in  FIG. 16 , if moving objects  7 A and  7 B are moving in front of a still background at a high speed, when viewing a part of the background, indicated by the arrow A, over which the moving objects  7 A and  7 B are crossing, the background appears and the moving objects  7 A and  7 B appears in this part at points t 1 , t 2 , and t 3  in time as shown in  FIGS. 17 and 18 . In this case, if the feedback ratio k is set to a low value, noise can be reduced sufficiently at a high speed in response to the moving speed of the moving objects  7 A and  7 B as shown in  FIG. 17 , whereas the noise reduction effect becomes little. Also, in this case, in the part of the background over which the moving objects  7 A and  7 B are not crossing, it becomes difficult to sufficiently ensure the noise reduction effect because the feedback ratio k is low. 
     On the contrary, if the feedback ratio k is set to a high value, in the part of the background over which the moving objects  7 A and  7 B are not crossing, it is possible to ensure the noise reduction effect sufficiently. However, in the part of the background over which the moving objects  7 A and  7 B are crossing, it becomes difficult to suppress noise sufficiently at a high speed in response to the moving speed of the moving objects  7 A and  7 B as shown in  FIG. 18 . 
     SUMMARY OF THE INVENTION 
     The present invention has been made in view of the above points. It is desirable to propose a noise suppression method, a noise suppression method program, a recording medium recording a noise suppression method program, and a noise suppression apparatus which are capable of suppress noise more sufficiently and at a higher speed than in the past. 
     According to an embodiment of the present invention, there is provided a method of suppressing noise of an input video signal and outputting an output video signal, the method of suppressing noise, including the steps of: generating a difference signal being a field difference or a frame difference between the input video signal and the output video signal; generating a noise compensation signal by multiplying the difference signal by a feedback ratio; subtracting the noise compensation signal from the input video signal; and controlling a feedback ratio, wherein the step of controlling the feedback ratio includes the steps of determining a signal level of the difference signal by a criterion value for each area set for the difference signal, and detecting an abrupt change in a signal level of the input video signal, counting the number of fields or the number of frames of the input video signal from the time when the signal-level determination section detects the abrupt change in the signal level to the time when the signal-level determination section subsequently detects the abrupt change in the signal level, and setting a feedback ratio for each of the areas in accordance with a count value by the step of counting, wherein the step of setting a feedback ratio sets the feedback ratio such that the feedback ratio gradually increases from 0 as the count value increases. 
     According to another embodiment of the present invention, there is provided a program of a method of suppressing noise of an input video signal and outputting an output video signal, the program including the steps of: generating a difference signal being a field difference or a frame difference between the input video signal and the output video signal; generating a noise compensation signal by multiplying the difference signal by a feedback ratio; subtracting the noise compensation signal from the input video signal; and controlling a feedback ratio, wherein the step of controlling the feedback ratio includes the steps of determining a signal level of the difference signal by a criterion value for each area set for the difference signal, and detecting an abrupt change in a signal level of the input video signal, counting the number of fields or the number of frames of the input video signal from the time when the signal-level determination section detects the abrupt change in the signal level to the time when the signal-level determination section subsequently detects the abrupt change in the signal level, and setting a feedback ratio for each of the areas in accordance with a count value by the step of counting, wherein the step of setting a feedback ratio sets the feedback ratio such that the feedback ratio gradually increases from 0 as the count value increases. 
     According to another embodiment of the present invention, there is provided A recording medium for recording a program of a method of suppressing noise of an input video signal and outputting an output video signal, the program including the steps of: generating a difference signal being a field difference or a frame difference between the input video signal and the output video signal; generating a noise compensation signal by multiplying the difference signal by a feedback ratio; subtracting the noise compensation signal from the input video signal; and controlling a feedback ratio, wherein the step of controlling the feedback ratio includes the steps of determining a signal level of the difference signal by a criterion value for each area set for the difference signal, and detecting an abrupt change in a signal level of the input video signal, counting the number of fields or the number of frames of the input video signal from the time when the signal-level determination section detects the abrupt change in the signal level to the time when the signal-level determination section subsequently detects the abrupt change in the signal level, and setting a feedback ratio for each of the areas in accordance with a count value by the step of counting, wherein the step of setting a feedback ratio sets the feedback ratio such that the feedback ratio gradually increases from 0 as the count value increases. 
     According to another embodiment of the present invention, there is provided a noise suppressing apparatus for suppressing noise of an input video signal and outputting an output video signal, the noise suppressing apparatus including; a difference-signal generation section generating a difference signal being a field difference or a frame difference between the input video signal and the output video signal; a noise compensation-signal and the section generating a noise compensation signal by multiplying the difference signal by a feedback ratio; a noise-compensation signal subtraction section subtracting the noise compensation signal from the input video signal; and a feedback-ratio control section controlling the feedback ratio, wherein the feedback ratio control section includes a signal-level determination section determining a signal level of the difference signal by a criterion value for each area set for the difference signal, and detecting an abrupt change in a signal level of the input video signal, a count section counting the number of fields or the number of frames of the input video signal from the time when the signal-level determination section detects the abrupt change in the signal level to the time when the signal-level determination section subsequently detects the abrupt change in the signal level, and a feedback-ratio setting section setting the feedback ratio for each of the areas in accordance with a count value of the count section, wherein the feedback-ratio setting section sets the feedback ratio such that the feedback ratio gradually increases from 0 as the count value increases. 
     By the configuration of the above embodiments, the feedback ratio is dynamically changed in the field or the frame after the detection of an abrupt change in the signal level of the input video signal, and the feedback ratio is gradually increased. Thus, it is possible to have a great noise reduction effect when the feedback ratio is high, and at the same time, it is possible to shorten the necessary time for the convergence when the feedback ratio is low. Accordingly, it is possible to suppress noise more sufficiently and at a high speed than in the past. 
     By the present invention, it is possible to suppress noise more sufficiently and at a higher speed than in the past. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating a noise filter according to a first embodiment of the present invention; 
         FIG. 2  is a block diagram illustrating the configuration of a reference-video-signal generation section in the noise filter of  FIG. 1 ; 
         FIG. 3  is a block diagram illustrating the configuration of another example of the reference-video-signal generation section in  FIG. 1 ; 
         FIG. 4  is a block diagram illustrating the configuration of a cyclic-history information storage section in the noise filter of  FIG. 1 ; 
         FIG. 5  is a plan view illustrating the cyclic-history information storage section of  FIG. 4 ; 
         FIG. 6  is a plan view illustrating a feedback-ratio setting section in the noise filter of  FIG. 1 ; 
         FIG. 7  is plan view illustrating the setting of the feedback ratio of the feedback-ratio setting section in the noise filter of  FIG. 1 ; 
         FIG. 8  is a characteristic curve illustrating the characteristic of the noise filter of  FIG. 1 ; 
         FIG. 9  is a block diagram illustrating the noise filter according to a second embodiment of the present invention; 
         FIG. 10  is a plan view illustrating a cycle determination section in the noise filter of  FIG. 9 ; 
         FIG. 11  is a block diagram illustrating the configuration of the cyclic-history information storage section in the noise filter of  FIG. 9 ; 
         FIG. 12  is a plan view illustrating the cyclic-history information storage section of  FIG. 10 ; 
         FIG. 13  is a block diagram illustrating a known time-cyclic noise filter; 
         FIG. 14  is a characteristic curve illustrating a feedback ratio; 
         FIG. 15  is a characteristic curve illustrating a change in a characteristic by a feedback ratio; 
         FIG. 16  is a plan view illustrating an example of a video with a fast motion; 
         FIG. 17  is a characteristic curve illustrating a characteristic when a feedback ratio is low; and 
         FIG. 18  is a characteristic curve illustrating a characteristic when a feedback ratio is high. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In the following, a detailed description will be given of embodiments of the present invention with reference to the drawings. 
     First Embodiment 
     1. Configuration of Embodiment 
       FIG. 1  is a block diagram illustrating a noise filter according to a first embodiment of the present invention. This noise filter  10  is a time-cyclic noise filter, and suppresses noise of an input video signal S 1  using the difference between fields or the difference between frames to output an output video signal S 2 . 
     In this noise filter  10 , a reference-video-signal generation section  11  delays the output video signal S 2  for a period of one field or one frame, and outputs a reference video signal S 11 . More specifically, the reference-video-signal generation section  11  performs motion compensation on the output video signal S 2  to generate the reference video signal S 11 . 
     That is to say, as shown in  FIG. 2 , in the reference-video-signal generation section  11 , a delay-signal generation section  12  delays an output video signal S 2  for a period of one field or one frame to generate a delay video signal S 12 . A motion-vector detection section  13  detects a motion vector MV from the input video signal S 1  on the basis of the delay video signal S 12 . In this regard, various vector detection methods, such as a block matching method, a gradient method, etc., can be applied to the detection of the motion vector here. Also, the accuracy of the motion vector may be either integer-pixel accuracy or factional-pixel accuracy. 
     A motion-compensation video signal generation section  14  performs motion compensation on the delay video signal S 12  using the motion vector MV, and outputs a reference video signal S 3 . In this regard, as shown in  FIG. 3 , if a sufficient characteristic can be ensured practically, the reference video signal S 3  here may be generated by simply delaying the output video signal S 2  for one field or one frame by the delay-signal generation section  12 . 
     A difference-signal generation section  16  is a subtraction circuit  17 , and subtracts the reference video signal S 3  from the input video signal S 1 , and outputs a difference signal S 4 , which is a field difference or a frame difference between the input video signal S 1  and the output video signal S 2 . 
     In a compensation-signal generation section  18 , an amplifier circuit  19  multiplies the difference signal S 4  by the feedback ratio k to generate a noise compensation signal SN. 
     In a noise-subtraction processing section  20 , a subtraction circuit  21  subtracts the noise compensation signal SN from the input video signal S 1  in order to suppress noise of the input video signal S 1 , and outputs the output video signal S 2 . 
     In this noise filter  10 , a feedback-ratio control section  23  controls the feedback ratio k, and suppresses noise of the input video signal S 1  more sufficiently and at a higher speed than in the past by controlling the feedback ratio k. 
     Here, in the feedback-ratio control section  23 , a noise-level measuring section  24  measures the noise level of the input video signal S 1  for each pixel of the input video signal S 1 , and outputs a noise-level measuring result NL. In this regard, the measurement of the noise level here can be carried out, for example, by extracting high frequency components in a certain range with a pixel to be measured as center in the horizontal direction and in the vertical direction using a two-dimensional high-pass filter, and detecting the signal level, etc. Also, the noise level of the input video signal S 1  may be detected using the difference signal S 4  in place of the input video signal S 1 . 
     A scene-change detection section  25  detects a scene change of the input video signal S 1 , and outputs a scene-change detection flag FS. In this regard, various scene-change detection methods, such as a determination of the sum of the absolute value of a frame difference by a predetermined threshold value, for example, can be widely applied to the scene change detection. Also, a scene change of the input video signal S 1  may be detected using the difference signal S 4  in place of the input video signal S 1 . 
     A cycle determination section  27  detects an abrupt change in the signal level of the input video signal S 1  on the basis of the noise level NL detected by the noise-level measuring section  24  for each pixel. That is to say, the cycle determination section  27  multiplies the noise level NL detected by the noise-level measuring section  24  by a predetermined constant α for each pixel, and generates a criterion value Cth (NL·α) of the input video signal S 1 . Furthermore, the cycle determination section  27  determines the amplitude value |Diff (x, y)| of the difference signal S 4  by the criterion value Cth, and detects a portion where the signal level of the input video signal S 1  changes abruptly on the basis of the noise level NL. Here, it is unlikely that the portion where the signal level changes abruptly is noise, and that portion is likely to be a change in the original input video signal S 1 . More specifically, it is just like the case in which a background is hidden by the moving objects described in  FIG. 16  and the hidden background appears. 
     Thus, if the amplitude value |Diff (x, y)| is less than the criterion value Cth, the cycle determination section  27  sets a cycle identification flag F 1  indicating the suppression of this signal component. In this regard, the criterion value Cth may be adjustable by the user by allowing the user to set the constant α. Also, the criterion value Cth may be adjustable by the user irrelevantly to the noise level NL. Further, if sufficient characteristic can be ensured practically, the criterion value Cth may be a fixed value. 
     A cyclic-history information storage section  28  counts the number of setting times n of the continuous cycle identification flag F 1  for each area set for the input video signal S 1 . Thereby, the cyclic-history information storage section  28  counts and outputs the number of fields or the number of frames during the time while the signal level of the input video signal S 1  abruptly changed and then the signal level abruptly changed succeedingly. 
     That is to say, as shown in  FIG. 4 , in the cyclic-history information storage section  28 , a subarea cycle determination section  29  determines the setting of the cycle identification flag F 1  for each area set for the input video signal S 1 . Here, as shown in  FIG. 5 , in this embodiment, one screen of the input video signal S 1  is divided into a predetermined number of pixels in the horizontal direction and in the vertical direction to set a plurality of areas. The subarea cycle determination section  29  sums up the number of setting times of the cycle identification flag F 1  for each area. Also, the subarea cycle determination section  29  determines the sum result by each predetermined threshold value. The subarea cycle determination section  29  sets an area cycle identification flag F 1 P of an area whose number of setting times of the cycle identification flag F 1  is greater than this threshold value. In this regard, in the example in  FIG. 5 , one are is set to have 6 pixels and 4 pixels in the horizontal direction and in the vertical direction, respectively. However, the setting of the area is not limited to this, and various settings are possible. Also, the area may vary in size at each portion of one screen. The subarea cycle determination section  29  detects, for example in  FIG. 16 , the background area hidden by the moving objects  7 A and  7 B, and further, the background area that appears by the movement of the moving objects  7 A and  7 B by the determination of the number of setting times of the cycle identification flag F 1  by the threshold value. 
     A cycle number memory  30  records and holds the number of setting times of consecutive area cycle identification flags F 1 P for each area of the input video signal S 1 . A cycle-number counter section  31  updates the number of setting times of the corresponding area held in the cycle number memory  30  in accordance with the area cycle identification flag F 1 P output from the subarea cycle determination section  29 . That is to say, the cycle-number counter section  31  initializes, to 0, the number of setting times of the corresponding area held in the cycle number memory  30  for the area in which the area cycle identification flag F 1 P is not set by the subarea cycle determination section  29 . Also, the cycle-number counter section  31  increments the number of setting times of the corresponding area held in the cycle number memory  30  by one when the area cycle identification flag F 1 P is set by the subarea cycle determination section  29 . At this time, if the number of setting times of the corresponding area held in the cycle number memory  30  has increased to a predetermined value, the increment processing is stopped. 
     When scene-change detection section  25  detects a scene change and sets the scene-change detection flag FS, the cycle number memory  30  initializes the number of setting times for all the held area to 0. Also, the cycle number memory  30  outputs the number of setting times n for each held area at timing corresponding to the processing in the amplifier circuit  19  of the corresponding area of the succeeding field or frame. Accordingly, if the input video signal S 1  is an interlaced video signal, one-field period is delayed and output. Also, if the input video signal S 1  is a non-interlaced video signal, one-frame period is delayed and output. 
     A cycle-number compensation section  32  compensates the number of setting times n of each area output from the cycle number memory  30  in accordance with the format of the input video signal S 1 , and outputs the number. That is to say, when the input video signal S 1  is an interlaced video signal and the difference signal S 4  is a field difference, or when the input video signal S 1  is a non-interlaced video signal and the difference signal S 4  is a frame difference, the cycle-number compensation section  32  outputs the number of setting times n for each area output from the cycle number memory  30  without any compensation. On the other hand, when the input video signal S 1  is an interlaced video signal and the difference signal S 4  is a frame difference, the cycle-number compensation section  32  compensates the number of setting times n for each area output from the cycle number memory  30  into one-half, and outputs the number. In this regard, when the input video signal S 1  is an interlaced video signal and the difference signal S 4  is a frame difference, the difference signals S 4  can be obtained for even fields and for odd fields, respectively. Accordingly, the cyclic-history information storage section  28  may be provided with an even-field processing system and an odd-field processing system in accordance with this, and the number of setting times n detected by each system may be output interchangeably. In this regard, in this case, the cycle-number compensation section  32  may be omitted. Also, the configuration of the cycle-number compensation section  32  may be omitted, and the processing to compensate the number of setting times n may be executed at the same time in the setting of the feedback ratio in a feedback-ratio setting section  35  described below. 
     The feedback-ratio setting section  35  sets the feedback ratio kt for each pixel in accordance with the number of setting times n output from the cyclic-history information storage section  28 . The feedback-ratio setting section  35  calculates a feedback ratio kh, which changes its value in accordance with the number of setting times n, and a feedback ratio kc in accordance with the amplitude of the difference signal S 4  for the noise level NL, individually, and then multiplies kh and kc to obtain the final feedback ratio kt. 
     Here, Expression (2) indicates the variance of the output video signal s 2  at time t. In order to obtain the feedback ratio k that minimizes this variance, Expression (2) is first differentiated to obtain the following relational expression. 
     [Expression 5]
 
(σ t   2 )′=−2(1 −k )·σ 0   2 +2 kσ   t-1   2   (5)
 
     Here, when solving Expression (5) for k, the feedback ratio kh minimizing variance σ t   2  at time t can be expressed by the following expression. 
     
       
         
           
             
               
                 
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     Accordingly, as shown in  FIG. 6 , the feedback ratio kh minimizing variance σ t   2  at each time t can be expressed as n/(n+1) using the number of continuous setting times n. In this regard, n=0 represents the frame or the field immediately after a scene change as described above, or a background area hidden by the moving objects  7 A and  7 B as shown in  FIG. 16 , and a background area which appears by the movement of the moving objects  7 A and  7 B. Thus, in this case, for the noise compensation signal SN by the difference signal S 4  of the preceding field or the preceding frame, the feedback ratio kt is set to 0, and is set not to subtract from the input video signal S 1 . Also, in the subsequent frames and fields, the feedback ratio kh is set such that the value increases in accordance with the value n corresponding to the continuous frame or field in sequence. In this regard, the feedback-ratio setting section  35  obtains the feedback ratio kt by referring to a look-up table by the number of setting times n output from the cyclic-history information storage section  28 . In this regard, the feedback ratio kh may be obtained by calculation. Also, the feedback ratio kt may be limited to a value having a finite bit length. 
     Also, the feedback-ratio setting section  35  multiplies the noise level NL by a predetermined constant β to generate a first criterion value Lth (NL·β) of the difference signal S 4 . Furthermore, the feedback-ratio setting section  35  multiplies the criterion Lth by a constant γ less than 1 to generate a second criterion value Coth (Lth·γ) of the difference signal S 4 . As shown in  FIG. 7 , the feedback-ratio setting section  35  determines the amplitude value |Diff (x, y)| of the difference signal S 4  by the first and the second criteria Lth (NL·β) and Coth (Lth·γ). If the amplitude value |Diff (x, y)| is greater than the first criterion value Lth (NL·β), the feedback ratio kc is set to 0. Also, if the amplitude value |Diff (x, y)| is less than the second criterion value Coth (Lth·γ), the feedback ratio kc is set to a fixed value less than 1. Also, if the amplitude value |Diff (x, y)| is between the first criterion value Lth (NL·β) and the second criterion value Coth (Lth·γ), the feedback ratio kc is set to a value by linear interpolation corresponding to the amplitude value |Diff (x, y)|. By this means, the feedback-ratio setting section  35  sets the feedback ratio kc such that the value of the feedback ratio kc decreases as the amplitude value of the difference signal for the noise-level measuring result increases. In this regard, the feedback-ratio setting section  35  sets the feedback ratio kc for each pixel. 
     2. Operation of Embodiment 
     In the above configuration, the input video signal S 1  ( FIG. 1 ) input sequentially is input into the noise filter  10  in sequence. Then, the noise compensation signal SN is subtracted from the signal by the subtraction circuit  21  to be subjected to the noise suppression, and is output as an output video signal S 2 . The output video signal S 2  is processed by the reference-video-signal generation section  11  to generate the reference video signal S 3 , and the reference video signal S 3  is subtracted from the input video signal S 1  by the subtraction circuit  17  to generate the difference signal S 4 . Also, this difference signal S 4  is processed by the compensation-signal generation section  18  to generate the noise compensation signal SN. Thus, the input video signal S 1  is subjected to noise suppression by the characteristic in accordance with the setting of the feedback ratio kt in the compensation-signal generation section  18  in the time-cyclic noise filter  10 . 
     However, as shown in  FIG. 8 , if the feedback ratio kt is set to a high constant value, the final noise reduction effect becomes great, but it takes long time for the convergence, and it becomes difficult to correspond to a fast motion. On the other hand, if the feedback ratio kt is set to a low constant value, the convergence time becomes short, but the final noise reduction effect becomes little. 
     Thus, in the noise filter  10 , the noise level NL of the input video signal S 1  is measured by the noise-level measuring section  24 , and the criterion value Cth of the difference signal S 4  is set on the basis of the noise level NL. Also, the amplitude value |Diff (x, y)| of the difference signal S 4  is determined by the criterion value Cth. If the amplitude value |Diff (x, y)| of the difference signal S 4  is less than the criterion value Cth, a cycle identification flag F 1  indicating the noise suppression is set as the pixel in which the signal level of the difference signal S 4  has been changed by noise. 
     For the input video signal S 1 , the cycle identification flag F 1  is summed up for each area ( FIG. 5 ) set for one screen of the input video signal S 1  by the subarea cycle determination section  29  ( FIG. 4 ), and an area cycle identification flag F 1 P is set. Also, the consecutive setting of the cycle identification flag F 1  is counted for each area by the cycle-number counter section  31 , and the feedback ratio kh ( FIG. 6 ) corresponding to the number of the setting times n, the count result, is set by the feedback-ratio setting section  35 . 
     Here, the feedback-ratio setting section  35  sets the feedback ratio to a low value at first, and then sets the value to gradually increase as the number of setting times n increases ( FIG. 6 ). Thus, for the input video signal S 1 , the variance value decreases by a low feedback ratio at first in the consecutive fields or the frames in order to converge at a high speed ( FIG. 8 ). After that, the setting is determined such that the feedback ratio gradually increases, and the final noise suppression effect becomes high. Accordingly, in this noise filter  10 , the dynamic setting of the feedback ratio makes it possible to have a great noise reduction effect when the feedback ratio is high, and at the same time, it is possible to shorten the necessary time for the convergence when the feedback ratio is low. Thus, it is possible to suppress noise more sufficiently and at a higher speed than in the past. 
     At this time, only if the amplitude value |Diff (x, y)| of the difference signal S 4  is less than the criterion value Cth, the cycle identification flag F 1  of the input video signal S 1  is set, and the feedback ratio value is varied in sequence. If the amplitude value |Diff (x, y)| of the difference signal S 4  is greater than the criterion value Cth, the cycle identification flag F 1  is not set, and the feedback ratio kt is set to 0. Thus, the setting is determined for the input video signal S 1  not to be subjected to the noise suppression in the moving area effectively using the configuration to set the feedback ratio dynamically. Thus, it is possible to prevent the occurrence of a blur, which is so-called a smear of a moving object. 
     Also, at this time, for the input video signal S 1 , the criterion value Cth of the amplitude value |Diff (x, y)| of the difference signal S 4  is set in accordance with the noise level NL. Thus, it is possible to effectively avoid the situation in which a still-image area is mistakenly determined to be a moving area by the increase of the noise level. As a result, even if the noise level of the input video signal S 1  is high, it is possible to sufficiently suppress noise. 
     Furthermore, if the scene-change detection section  25  detects a scene change, that is to say, a scene change occurs in the input video signal S 1 , in the same manner as a moving area, the number of setting times n is set in the initial value, and the feedback ratio kh is set to an initial value 0. Accordingly, when a scene change occurs in the input video signal S 1 , the setting is determined not to perform noise suppression by effectively using the configuration to set the feedback ratio dynamically. Thus, it is possible to prevent the occurrence of a blur in the portion of a scene change. 
     Also, for the input video signal S 1 , the feedback-ratio setting section  35  determines the amplitude value |Diff (x, y)| of the difference signal S 4  by the first criterion value Lth (NL·β) produced by the multiplication of the noise level NL and a predetermined constant β, and the second criterion value Coth (Lth·γ) produced by the multiplication of the criterion value Lth and a constant γ less than 1. The feedback ratio kc is generated so as to have a higher value as the amplitude value |Diff (x, y)| is lower ( FIG. 7 ). Also, the feedback ratio kh in accordance with the number of setting times n, which is the number of repetition, is multiplied by the feedback ratio kc in order to set the final feedback ratio kt. 
     By this means, the high amplitude components of the input video signal S 1  is suppressed to generate the noise compensation signal SN by effectively using the configuration of dynamically setting the feedback ratio. Thus, the deterioration of the original high frequency components of the input video signal S 1 , such as an edge, etc., is prevented. Also, at this time, the first and the second criteria Lth (NL·β) and Coth (Lth·γ) are set in accordance with the noise level NL. Thus, the deterioration of the original high frequency components of the input video signal S 1  is prevented appropriately in accordance with the amount of noise. 
     3. Effect of Embodiment 
     With the above configuration, it is possible to suppress noise more sufficiently and at a higher speed than in the past by counting the number of fields or the number of frames after a sudden change in the signal level of an input video signal, and dynamically controlling the feedback ratio to the increase gradually in accordance with the count value. 
     Also, at this time, a criterion is set to detect an area in which the signal level has changed abruptly on the basis of the measuring result of the noise level. This makes it possible to effectively avoid the erroneous detection of a still-image portion and a moving image portion by the noise level, and to reliably obtain the noise reduction effect. 
     Also, the count values of all areas are initialized to 0 by the detection of a scene change. Thus, it is possible to suppress noise more sufficiently and at a higher speed than in the past after a scene change. 
     Also, a feedback ratio kc is generated so as to have a lower value as the amplitude of the difference signal for the noise-level measuring result becomes high, and the feedback ratio is compensated by multiplying the feedback ratio kc by the feedback ratio kh. Thus, it is possible to suppress noise level by suppressing a high amplitude component using the configuration of dynamically varying and controlling the feedback ratio, and to appropriately suppress the noise level. 
     Second Embodiment 
       FIG. 9  is a block diagram illustrating the noise filter according to a second embodiment of the present invention. In this noise filter  40 , the same components as those in the noise filter  10  of the first embodiment are marked with the corresponding reference numerals, and the duplicated descriptions will be omitted. 
     In the noise filter  40 , the band division section  41  transforms the difference signal S 4 , which is a signal in a pixel area, into sub-difference signals S 4 B having a plurality of bands of the frequency domain, and outputs the signal. Here, various methods, such as an orthogonal transformation processing, for example, Hadamard transformation, Haar transformation, a discrete cosine transformation, etc., a wavelet transformation using a filter bank, sub-band division, etc., can be applied to the transformation processing into the frequency domain. 
     In a compensation-signal generation section  42 , a amplifier circuit  44  multiplies the sub-difference signals S 4 B having a plurality of bands divided into a plurality of bands by the band division section  41 , and the feedback ratios ktB of individual bands, respectively to generate the noise compensation signals SNB for a plurality of bands. 
     A band synthesis section  45  converts the noise compensation signals SNB for a plurality of bands output from the compensation-signal generation section  42  into a signal of a pixel area, and generates the compensation signal SN. Thus, the noise filter  40  divides the difference signal S 4  into bands, sets the feedback ratio ktB for each band, and suppresses the noise level of the input video signal S 1 . 
     In accordance with this configuration, the feedback-ratio control section  47  sets the feedback ratio ktB for each band. Thus, in the feedback-ratio control section  47 , a noise-level measuring section  48  and a scene-change detection section  49  process sub-difference signals S 4 B divided into bands individually, and set a noise level NLB and a scene-change detection flag FSB for each band. 
     A cycle determination section  51  is the same as the cycle determination section  27  described above in the first embodiment, and determines the amplitude value of the sub-difference signal S 4 B for each band, and outputs a cycle identification flag F 1 B. Accordingly, as shown in  FIG. 10 , if a difference signal S 4 B is divided into 8 bands and 4 bands in the horizontal direction and in the vertical direction, respectively, sub-difference signal S 4 B is divided into 32-line signals. Thus, a cycle determination section  51  multiplies the noise level NLB corresponding to each pixel by a constant α for each band to generate the criterion value Cth B (NL·α) of each band, determines the amplitude value |Diff (x, y) B| of the corresponding sub-difference signal S 4 B by the criterion value Cth B, and sets the cycle identification flag F 1 B. In this regard, here, the following expression is a relational expression related to the setting of this threshold value and the setting of the flag. Here, bx and by are variables for identifying each band in the horizontal direction and the vertical direction, respectively. NL bx, by  is the noise level of the band identified by variables bx and by. Cth bx, by  is the threshold value identified by the variables bx and by. |Diff bx, by (x, y)| is the amplitude value of the band identified by variables bx and by in the pixel having the coordinate (x, y) in the horizontal direction and the vertical direction. When the relational expression Expression (8) holds, the cycle determination section  51  sets the cycle identification flag F 1 B of the corresponding band. In this regard, the threshold value can be set in various methods in the same manner as described above in the first embodiment. 
     [Expression 7]
 
 Cth   bx,by   =NL   bx,by ·α  (7)
 
     [Expression 8]
 
|Diff bx,by ( x,y )|&lt; Cth   bx,by   (8)
 
     In the same manner as the cyclic-history information storage section  28  described above in the first embodiment, a cyclic-history information storage section  52  obtains the number of repetitive setting times nB of the feedback ratio kcB for each band, and outputs the number. Here,  FIG. 11  is a block diagram illustrating the cyclic-history information storage section  52 . In the same manner as the subarea cycle determination section  29  described above in the first embodiment, in the cyclic-history information storage section  52 , a subarea cycle determination section  53  determines the number of the cycle identification flag F 1 B for each area, and sets a area cycle identification flag F 1 PB. At this time, the subarea cycle determination section  53  sets the area cycle identification flag F 1 PB for each band. Also, as shown in  FIG. 12 , the subarea cycle determination section  53  further sets the area cycle identification flag F 1 PB together in a plurality of bands having adjacent frequencies in the horizontal direction and in the vertical direction in order to simplify the subsequent processing. In this regard, in the example shown in  FIG. 12 , 2×2 bands adjacent each other in the horizontal direction and in the vertical direction are grouped into one, and the area cycle identification flag F 1 PB is set for each grouped bands. In this example, 8 area cycle identification flags F 1 PB are set for one area. In this regard, the area cycle identification flag F 1 PB may be set for each original band without grouping a plurality of bands into one in this manner. 
     A cycle-number counter section  55 , a cycle number memory  56 , and a cycle-number compensation section  57  are the same as the cycle-number counter section  31 , the cycle number memory  30 , and the cycle-number compensation section  32  described in the first embodiment, respectively, and calculate the number of setting times nB for each band, and output the number. 
     A feedback-ratio setting section  59  is the same as the feedback-ratio setting section  35  described in the first embodiment, and calculates the feedback ratio kt for each band, and outputs the feedback ratio kt. 
     With this embodiment, by setting the feedback ratio for each band, it is possible to reduce noise more reliably than the first embodiment. 
     Third Embodiment 
     In this regard, in the above described embodiments, a description has been given of the case of obtaining the final feedback ratio kt by multiplying the feedback ratio kh set for each area and feedback ratio kc set for each pixel. However, the present invention is not limited to this. When a sufficient characteristic can be practically ensured, the noise compensation signal may be generated only by the feedback ratio kh set for each area. 
     Also, in the above-described embodiments, a description has been given of the case of constituting the noise filter by hardware. However, the present invention is not limited to this, and may be constituted by the execution of a program by the calculation processing means. In this regard, in this case, this program may be provided by pre-installation, or may be provided by being recorded on a recording medium, such as an optical disc, a magnetic disk, a memory card, etc. Furthermore, the program may be provided by being loaded down through a network, such as the Internet, etc. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.