Abstract:
In detection of noise by comparing a digital input image signal and an output image signal in one previous frame with each other, when the input image signal, the output image signal in one previous frame, and a predetermined reference value are represented by a, b, and x, respectively, and when |a−b|≦x and a−b≠1 are satisfied, the presence of noise is determined.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and device for detecting a noise component included in a digital video signal, and a noise reduction method and device. 
   2. Description of the Related Art 
   A conventional noise reduction device for reducing a noise component included in a video signal is shown in  FIG. 4 . In  FIG. 4 , reference numeral  1  denotes a video signal input unit to which a digital image signal is input;  2 , a noise reducing process unit for processing pixel data d and e between adjacent sequential frames to perform a noise reducing process; and  3 ′, a noise detecting process unit for comparing the pixel data a and b between adjacent sequential frames to perform noise detection. Reference numeral  4  denotes a selecting unit which selects and output pixel data i processed by the noise reducing process unit  2  when noise detection is performed by the noise detecting process unit  3 ′ and which selects and directly output pixel data h input to the video signal input unit  1  when noise detection is not performed. Reference numeral  5  denotes a 1-frame delay unit constituted by a 2-port memory or the like which delays an output image signal from the selecting unit  4  by one frame, and reference numeral  6  denotes a video signal output unit. 
   The noise reducing process unit  2 , as shown in  FIG. 5 , is constituted by an adder  21  for adding pixel data d of a current image signal input to the video signal input unit  1  and pixel data e, output from the 1-frame delay unit  5 , of the same pixel of in one previous frame, and a divider  22  which divides output data from the adder  21  by ½. In the divider  22 , when a fractional figure (less than “1”) is obtained, the fractional figure is rounded down because the value is less than the LSB. 
   The noise detecting process unit  3 ′, as shown in  FIG. 6 , a subtractor  31  which calculates a difference between pixel data a of the current video signal input to the video signal input unit  1  and pixel data b, output from the 1-frame delay unit  5 , of the same pixel in one previous frame, an absolute value calculator  32  which calculates an absolute value of the difference obtained by the subtractor  31 , and a comparator  33  which compares a pixel data p from the absolute value calculator  32  with a preset reference value x to output a determination signal of “noise is present” when p≦x is satisfied. 
   An operation of the noise reduction device will be described below with reference to  FIG. 7 . An upper half in  FIG. 7  shows pixel data (4×4 pixels for descriptive convenience) of an input image input to the video signal input unit  1 , and the lower half shows similar pixel data of an output image output from the video signal output unit  6 . A case in which the reference value x in the noise detecting process unit  3 ′ is given by x=4 on the assumption that luminance levels of the pixels are set to be levels of “0” to “255” (8 bits) will be described below. 
   In the noise reducing process unit  2  and the noise detecting process unit  3 ′, pixel data of a current input image at the same pixel and pixel data of an output image in one previous frame are to be processed. For example, an input image IN 2  and an output image OUT 1  in one previous frame are compared with each other. The pixel data of all the pixels of the input image IN 2  are “ 20 ”, and pixel data of all the pixels are “ 19 ”. For this reason, the noise detecting process unit  3 ′ determines the value as “noise is present”. The noise reducing process unit  2  calculates “( 19 + 20 )/2→ 19 ” (number less than 1 is rounded down). Therefore, at this time, the output from the noise reducing process unit  2  is selected by the selecting unit  4 , and pixel data “ 19 ” is output as the output image OUT 2 . 
   However, in this processing method, a problem is posed when a thing traverses a certain background, for example. A case in which an image having pixel data “ 80 ” moves from the right to the left on a background having pixel data “ 20 ” like a change of input image IN 4 →input image IN 5  will be considered. 
   In an output image OUT 4  processed by the input image IN 4  and the output image OUT 3 , in a pixel where pixel data “ 20 ” of the input image IN 4  is compared with the pixel data “ 19 ” of the output image OUT 3 , the pixel data is changed into pixel data “ 19 ”. In a pixel where pixel data “ 80 ” of the input image IN 4  is compared with the pixel data “ 19 ” of the output image OUT 3 , the difference is larger than “4”, noise detection is not performed, and the pixel data is kept at “ 80 ”. 
   In an output image OUT 5  processed by the input image IN 5  and the output image OUT 4 , pixel data is changed into the pixel data “ 19 ” in a pixel where the pixel data “ 20 ” of the input image IN 5  is compared with the pixel data “ 19 ” of the output image OUT 4 . However, since a difference between the pixel data “ 80 ” and the pixel data “ 19 ” is larger than “4” in the pixel where the pixel data “ 80 ” of the input image IN 5  and the pixel data “ 19 ” of the output image OUT 4  are compared with each other, noise detection is not performed, and the pixel data is kept at “ 80 ”. Since a difference between the pixel data “ 20 ” and the pixel data “ 80 ” in the pixel where the pixel data “ 20 ” of the input image IN 5  is compared with the pixel data “ 80 ” of the output image OUT 4 , noise detection is not performed, and the pixel data is kept at “ 20 ”. 
   In an output image OUT 6  processed by an input image IN 6  and the output image OUT 5 , pixel data is changed into the pixel data “ 19 ” in a pixel where the pixel data “ 20 ” of the input image IN 6  is compared with the pixel data “ 19 ” of the output image OUT 5 . However, since a difference between the pixel data “ 20 ” and the pixel data “ 80 ” is larger than “4” in a pixel where the pixel data “ 20 ” of the input image IN 6  is compared with the pixel data “ 80 ” of the output image OUT 5 , the pixel data is kept at “ 20 ”. Furthermore, the pixel data is kept at “ 20 ” in the pixel where the pixel data “ 20 ” of the input image IN 6  is compared with the pixel data “ 20 ” of the output image OUT 5 . 
   In this manner, in the output images OUT 5  and OUT 6 , a new boundary between the pixel data “ 19 ” and the pixel data “ 20 ” is generated and disadvantageously appears as a large noise component. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to provide a noise reduction method and device which performs appropriate noise reduction without the inconvenience described above. 
   In a first aspect of the present invention, in a noise detection method for comparing an input image signal with an output image signal in one previous frame to detect noise, when the input image signal, the output image signal in one previous frame, and a predetermined reference value are represented by a, b, and x, respectively, and when |a−b|≦x and a−b≠1 are satisfied, the presence of noise is determined. 
   In a second aspect of the present invention, in a noise detection method for comparing an input image signal with an output image signal in one previous frame to detect noise, when the input image signal, the output image signal in one previous frame, and a predetermined reference value are represented by a, b, and x, respectively, and when |a−b|≦x and 1&lt;|a−b| are satisfied, the presence of noise is determined. 
   In a third aspect of the present invention, when the presence of noise is determined by applying the noise detection method according to the first or second aspect, the input image signal and the output image signal in one previous frame are input to output a signal obtained by averaging both the image signals as a noise reducing image signal, and when the presence of noise is not detected, the input image signal is directly used as an output image signal. 
   In a fourth aspect of the present invention, in the noise reduction method according to the third aspect to which the noise detection method according to the first aspect is applied, when the input image signal and the output image signal in one previous frame are input and averaged, a fractional figure less than “1” of an obtained value is rounded down. 
   In a fifth aspect of the present invention, in the noise reduction method according to the third aspect to which the noise detection method according to the second aspect is applied, when the input image signal and the output image signal in one previous frame are input and averaged, a fractional figure less than “1” of an obtained value is rounded down or rounded up. 
   In a sixth aspect of the present invention, in a noise detection device for comparing an input image signal with an output image signal in one previous frame to detect noise, when the input image signal, the output image signal in one previous frame, and a predetermined reference value are represented by a, b, and x, respectively, and when |a−b|≦x and a−b≠1 are satisfied, the presence of noise is determined. 
   In a seventh aspect of the present invention, in a noise detection device for comparing an input image signal with an output image signal in one previous frame to detect noise, when the input image signal, the output image signal in one previous frame, and a predetermined reference value are represented by a, b, and x, respectively, and when |a−b|≦x and 1≦|a−b| are satisfied, the presence of noise is determined. 
   In an eighth aspect of the present invention, a noise reduction device includes the noise detection device according to the sixth or seventh aspect, noise reducing process means for inputting the input image signal and the output image signal in one previous frame to obtain an average value between both the image signals, and selecting means for outputting an image signal obtained by the noise reducing process means as the output image signal when the presence of noise is detected by the noise detection device and for directly outputting the input image signal as the image output signal when the presence of noise is not detected. 
   In a ninth aspect of the present invention, in the noise reduction device according to the eighth aspect to which the noise detection device according to the sixth aspect is applied, the noise reducing process means receives the input image signal and the output image signal in one previous frame to round down a fractional figure less than “1” of a value obtained by averaging both the image signals. 
   In a tenth aspect of the present invention, in the noise reduction device according to the eighth aspect to which the noise detection device according to the seventh aspect is applied, the noise reducing process means receives the input image signal and the output image signal in one previous frame to round down or round up a fractional figure less than “1” of a value obtained by averaging both the image signals. 
   In the noise detection according to the first, second, sixth, or seventh aspect, when a difference between an input image signal and an output image signal in one previous frame is not 1 or less than 1, even though the difference is detected as noise in a conventional technique, noise is not detected. For this reason, when the noise detection is applied to the third to fifth aspects and the eighth to tenth aspects, appropriate noise reduction can be performed. The noise detection according to the first or sixth aspect is effective under the condition that a figure less than “1” is rounded down in the noise reducing process. The noise detection according to the second or seventh aspect is effected under both the conditions that a figure less than “1” is rounded up and rounded down in the noise reducing process. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a noise detecting process unit according to a first embodiment of the present invention; 
       FIG. 2  is a block diagram of a noise detecting process unit according to a second embodiment of the present invention; 
       FIG. 3  is a diagram for explaining a noise reducing process performed by a noise reduction device using the noise detecting process unit in  FIG. 1 ; 
       FIG. 4  is a block diagram showing the configuration of a conventional noise reduction device; 
       FIG. 5  is a detailed block diagram of a noise reducing process unit of the noise reduction device in  FIG. 4 ; 
       FIG. 6  is a detailed block diagram of a noise detecting process unit of the noise reduction device in  FIG. 4 ; and 
       FIG. 7  is a diagram for explaining a noise reducing process performed by a noise reduction device using the noise detecting process unit in  FIG. 4 . 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention will be described below. The embodiments of noise detecting process units will be described below. However, when the noise detecting process unit is incorporated in a noise reduction device, the noise detecting process unit is replaced with a noise detecting process unit  3 ′ in a configuration in  FIG. 4 . 
   First Embodiment 
     FIG. 1  is a diagram showing a block of a noise detecting process unit  3  according to the first embodiment. In a conventional technique, in a noise reducing process unit  2 , a fractional figure (less than “1”) of a value obtained by a divider  22  is rounded down. For this reason, when an absolute value |a−b| of a difference between pixel data a and b is “1”, the output pixel data must be smaller pixel data, and the problem described above is posed. 
   Therefore, in the first embodiment, when the value |a−b| is “1”, “absence of noise” is determined. More specifically, the noise detecting process unit  3  performs a process of outputting a signal representing “presence of noise” only when the pixel data a input from a video signal input unit  1  and the pixel data b input from a 1-frame delay unit  5  satisfy:
 
| a−b|≦x  and  a−b≠ 1.
 
Although the condition “|a−b|≦x” is the same as that in the noise detecting process unit  3 ′, the condition “a−b≠1” is additionally set in the embodiment. Therefore, for example, x=“4” is set, “presence of noise” is determined only when the difference between the data a and b is “−4”, “−3”, “−2”, “−1”, “0”, “2”, “3”, or “4”. When the difference is “0”, the noise reducing process unit  2  outputs pixel data equal to the pixel data a. For this reason, output pixel data of a selecting unit  4  is the same as input pixel data to the image input unit  1 .
 
     FIG. 3  is a diagram for explaining a noise reducing process when the noise reduction device in  FIG. 4  is constituted by using the noise detecting process unit  3 . In this case, as in a change of input image IN 4 →input image IN 5 , a case in which an image having pixel data “ 80 ” moves from the right to the left on a background having pixel data “ 20 ” will be considered. 
   In an output image OUT 2  processed by an input image IN 2  and an output image OUT 1 , a pixel (in this case, all pixels) where the pixel data “ 20 ” of the input image IN 2  and the pixel data “ 19 ” of the output image OUT 1  are compared with each other has a difference of “1”, noise detection is not performed, and the pixel data is kept at “ 20 ”. 
   In an output image OUT 3  processed by an input image IN 3  and the output image OUT 2 , all pixel data of all the input image IN 3  and the output image OUT 2  are “ 20 ”, the difference is “0”, and “presence of noise” is detected. However, since output pixel data of the noise reducing process unit  2  is “ 20 ”, the pixel data is kept at “ 20 ”. 
   In an output image OUT 4  processed by an input image IN 4  and the output image OUT 3 , a pixel where pixel data “ 20 ” of the input image IN 4  and the pixel “20” of the output image OUT 3  are compared with each has a difference of “0”, and “presence of noise” is detected. However, since output pixel data from the noise reducing process unit  2  is “ 20 ”, the pixel data is kept at “ 20 ”. A pixel where pixel data “ 80 ” of the input image IN 4  and the pixel data “ 20 ” of the output image OUT 3  are compared with each other has a difference of “60”, “absence of noise” is detected, and the pixel data is kept at “ 80 ”. 
   In an output image OUT 5  processed by an input image IN 5  and the output image OUT 4 , a pixel where pixel data “ 20 ” of the input image IN 5  and the pixel data “ 20 ” of the output image OUT 4  are compared with each other has a difference of “0”, and “presence of noise” is detected. However, since output pixel data from the noise reducing process unit  2  is “ 20 ”, the pixel data is kept at “ 20 ”. A pixel where pixel data “ 80 ” of the input image IN 5  and the pixel data “ 20 ” of the output image OUT 4  are compared with each other has a difference of “60”, “absence of noise” is detected, and the pixel data is kept at “ 80 ”. 
   In an output image OUT 6  processed by an input image IN 6  and the output image OUT 5 , a pixel where pixel data “ 20 ” of the input image IN 6  and the pixel data “ 20 ” of the output image OUT 5  are compared with each other has a difference of “0”, and “presence of noise” is detected. However, since output pixel data from the noise reducing process unit  2  is “ 20 ”, the pixel data is kept at “ 20 ”. A pixel where pixel data “ 20 ” of the input image IN 6  and the pixel data “ 80 ” of the output image OUT 5  are compared with each other has a difference of “60”, “absence of noise” is detected, and the pixel data is kept at “ 20 ”. 
   In this manner, in the output images OUT 5  and OUT 6 , noise except for a boundary between the pixel data “ 20 ” and “ 80 ” is not generated, and the problem explained in  FIG. 7  can be solved. As described above, in the noise reducing process unit  2 , the problem posed in  FIG. 7  when the divider  22  rounds down a fractional figure can be solved. 
   Second Embodiment 
     FIG. 2  is a diagram showing a block of a noise detecting process unit  3 A according to the second embodiment. In the second embodiment, “absence of noise” is determined when a difference between input pixels a and b, i.e., a−b is “−1”, “0”, or “1”, “absence of noise” is determined. For this reason, even if the divider  22  rounds down or up a fractional figure of a value, the noise detecting process unit  3 A can be prevented from being adversely affected by the round up or down operation. More specifically, in the noise detecting process unit  3 A, when the pixel data a input from a video signal input unit  1  and the pixel data b input from a 1-frame delay unit  5  satisfy the following conditions:
 | a−b|≦x  and 1&lt;| a−b|,    
a process of outputting a signal representing “presence of noise” is performed. Although the condition “|a−b|≦x” is the same as that in the noise detecting process unit  3 ′ shown in  FIG. 6 , the condition “a−b≠1” is additionally set in the embodiment. Therefore, for example, x=“4” is set, “presence of noise” is determined only when the difference between the data a and b is “−4”, “−3”, “−2”, “2”, “3”, or “4”.
 
   Third Embodiment 
   In the first and second embodiments, when an image signal to be input is a signal of an interlace system, a 2-field signal is digitally processed into a 1-frame signal in advance, and the resultant pixel data is input to the image signal input unit  1  of the noise reduction device according to the embodiment. When the image signal to be input is an RGB signal, noise reducing processes are performed in red, green, and blue, respectively.