JP4412042B2 - Frame cyclic noise reduction method - Google Patents

Description

  The present invention divides one field of an input image signal into a plurality of subfields, and controls the light emission of the display means by the combination of the light emission for each subfield to represent a frame cycle used in a subfield image display device for expressing gradation. The present invention relates to a mold noise reduction method and a noise device.

  When displaying a multi-gradation image using a display device such as a plasma display panel (PDP) that is based on binary display, one field of the image is divided into a plurality of subfields, and each subfield has a predetermined value. There is known a method of displaying an image by controlling the presence or absence of light emission for each subfield with a luminance weight of. For example, to display 256 gradations, as shown in FIG. 1, one field of the input signal is divided into eight subfields (SF), and the luminance weight of each subfield is set to “1”, “2”. , “4”, “8”, “16”, “32”, “64”, “128”. If the input signal is an 8-bit digital signal, it is assigned to eight subfield images in order from the least significant bit. The arrangement of the subfields is not particularly limited. For example, as shown in FIG. 1A, the subfields are arranged in descending order of luminance weight (hereinafter referred to as ascending coding) or FIG. As shown in b), there are those arranged in order from the one with the largest luminance weight to the smallest one (hereinafter referred to as descending coding). A halftone is expressed by integrating the luminance of each subfield in the time direction. In this case, gradation display of 8 bits and 256 gradations can be performed by combining these eight subfields (for example, Non-Patent Document 1). ).

In addition, many noise reduction control methods and noise reduction control devices that reduce noise on the input video signal and improve the S / N ratio have been proposed. In a subfield image display device such as a plasma display. Is also used (see, for example, Patent Document 1). Among them, a frame cyclic noise reduction method is known as a relatively effective noise reduction method (for example, Non-Patent Document 2). In general, an image signal has strong autocorrelation of image information between frames, while a noise component included in the image signal has no autocorrelation. The frame recursive noise reduction device pays attention to this, and can reduce noise components by averaging the image signals for each frame period by a method of performing noise reduction. However, if the time average of the frame period is taken for the moving image portion, moving images over a plurality of frames are also averaged, resulting in afterimages such as blurring and tailing, resulting in a reduction in resolution. Therefore, as a practical noise reduction device, several frame cyclic noise reduction devices that detect the movement of an image signal and control the degree of time average (cyclic amount) according to the amount of motion have been proposed (for example, patents). Reference 2). FIG. 7 is a block diagram showing a schematic configuration of a frame cyclic noise reduction device described in Patent Document 2. In FIG. In this way, the amount of motion is obtained based on the difference signal between frames, and in the moving image area, the cyclic amount k (k is set between 0 and 1) is set small according to the amount of motion to suppress afterimages and still images. In the region, the traveling amount k is set large to obtain a noise suppression effect.
JP 2001-36770 A JP-A-6-225178 Co-authored by Hiraki Uchiike and Shigeo Miko “All about Plasma Displays” 165-177 Takahiko Fukiki "Multi-dimensional signal processing of TV images" Nikkan Kogyo Shimbun, p. 190

  However, in the frame cyclic noise reduction device, various methods have been proposed such as controlling the cyclic coefficient so as not to cause image quality degradation such as moving image blur in the moving image portion. Since the reduction effect and the degree of blur in the moving image portion are in a trade-off relationship, it is difficult to completely suppress the blur in the moving image portion while obtaining a good noise reduction effect. In most cases, the blur of the moving image portion is suppressed while sacrificing the noise reduction effect, or a certain amount of blur in the moving image portion is compromised and adjusted to obtain a noise reduction effect while suppressing the blurring to a less noticeable level.

  Also, in a subfield image display apparatus that divides one field of an input image signal into a plurality of subfields and controls the light emission of the display means by the combination of light emission for each subfield to express gradation, Since we are trying to express halftones by integrating the luminance in the time direction, when the line of sight moves in a moving image, etc., integrate the weight of each bit of the image at a position different from the original pixel position over time. When halftone display is greatly distorted and image quality deterioration such as distorted contours called moving image pseudo contours or blurred edges of images (hereinafter referred to as subfield blur) is observed. There is. The subfield blur generation mechanism is described below.

  FIG. 2 shows the subfield emission state and the visual intensity distribution on the retina over time when the input video signal level 63 is displayed using the ascending coding of FIG. When the level 63 is expressed, luminance weights “1”, “2”, “4”, “8”, “16”, and “32” are lit in the pixels A to E as shown in the drawing. In the still image, the light emission “1”, “2”, “4”, “8”, “16”, “32” of each subfield is emitted on the retina along the line of sight indicated by the broken line in the figure. Since integration is performed sequentially at the same position, the visual strength is uniform. On the other hand, as shown in FIG. 3, when the image moves to the left, the line of sight moves along the line of sight as indicated by the dashed arrow in FIG. For this reason, the visual strength becomes non-uniform, and edge blurring occurs at the edge portion as shown in the figure. As described above, in the subfield image display, the subfield blurring in which the resolution of the edge portion particularly decreases occurs in the moving image portion. FIG. 4 shows the occurrence of sub-field blur when the input video signal level is 255. From the figure, when the input signal level is 255 compared with 63, the number of lighting subfields is large, that is, the light emission time in one field is long, so the degree of subfield blurring becomes large.

  Here, the ascending order coding shown in FIG. 1A has been described as an example, but in principle, the subfield blur is not eliminated by changing the arrangement of the subfields. In addition, since the movement of the line of sight on the display image of the observer when a moving image is displayed on a normal image display apparatus has a strong correlation with the movement of the image on the display image apparatus, in the following, the movement of the line of sight and the image It shall be written without distinction from the movement of

  Subfield blur has been described with reference to FIG. 2, FIG. 3, and FIG. 4 using an input video signal with a sharp edge as an example. However, in fact, the edge portion in a video shot by a TV camera is often blurred. . In such a case, in the ascending coding subfield display as shown in FIG. 1A, as shown in FIG. 5, in the moving image portion in the input video, the visual recognition is performed at the edge portion where the signal level decreases along the moving direction. The degree of blurring is greater than the input video due to subfield blurring. Further, in descending order coding as shown in FIG. 1B, as shown in FIG. 6, the degree of blurring visually recognized at the edge portion where the signal level increases in the moving direction in the moving image portion in the input video is increased.

  The degree of sub-field blur becomes more conspicuous as the degree of blur at the edge portion in the input video increases. In addition, the higher the input video signal level is, the higher the moving speed of the moving image portion is, the greater the degree of subfield blur that is visually recognized.

  In addition, here, the case where the arrangement of the subfields is regularly arranged in ascending order of luminance weight as shown in FIGS. 1A and 1B and the case where the subfields are arranged in order from the largest is described as an example. Even when arranged irregularly, subfield blur occurs.

  The frame recursive noise reduction device detects the moving amount of the moving image part from the frame difference value, and controls the recurring amount according to the moving amount. FIG. 8 shows the relationship between the pixel position and the signal level in the upper part, and a state in which the video signal having the sharp edge on the right side and the edge part having the left slope move leftward with the same movement width A. Is shown. In the lower part, the frame difference value between the current frame video signal and the previous frame video signal at each edge is obtained. From the comparison of the frame difference values shown in the lower part of FIG. 8, it can be seen that the size of the obtained frame difference value differs depending on the state of the signal level change of the edge even if the movement width is the same. This means that when the motion amount is obtained from the frame difference value, the same motion amount cannot always be obtained even if the display image has the same motion width. In addition, although the frame difference value can be sufficiently large at the steep edge portion, the amount of motion is small at the edge portion having an inclination because the obtained frame difference value is small. Therefore, the amount of patrol is determined rather than stationary, and as a result, tailing and afterimages are likely to occur. In addition, sub-field blur occurs more significantly when the signal level is high, so that the sub-field blur is further deteriorated when the signal level near the inclined edge is high.

  Therefore, in the subfield image display device, when noise removal is performed using the frame cyclic noise reduction device introduced above, depending on the spatial signal level change of the moving image portion, the obtained frame difference value is small, Since the amount is also reduced, the amount of circulation from stillness is determined, tailing and afterimages are likely to occur, and as a result, subfield blur is worsened.

  The present invention has been made to solve such a problem, and by correcting the amount of motion according to the signal level of the input image signal, motion blur is generated even in a portion where the difference value is small. The present invention provides a frame recursive noise reduction method and a noise reduction apparatus that can suppress noise, prevent deterioration of subfield blur, and maintain noise reduction effect while reducing noise.

  The frame cyclic noise reduction method of the present invention is characterized in that the amount of motion is corrected according to the signal level of the input image signal. The noise reduction device of the present invention is a frame cyclic noise reduction device using the frame cyclic noise reduction method.

  According to the frame cyclic noise reduction method and the noise reduction apparatus of the present invention, it is possible to suppress the occurrence of subfield blur while maintaining the noise reduction effect by being used for the subfield image display apparatus.

  Embodiments of the present invention will be described below with reference to the drawings.

(Embodiment 1)
FIG. 9 is a block diagram showing a schematic configuration of a frame cyclic noise reduction apparatus for explaining the frame cyclic noise reduction method according to Embodiment 1 of the present invention. The noise reduction apparatus includes a frame memory 10, a difference value calculation unit 20, a motion amount detection unit 30, a cyclic amount determination unit 40, a multiplier 50, a multiplier 60, and an adder 70.

  The difference value calculation unit 20 inputs the image signal of the previous frame and the image signal of the current frame, calculates the difference between them, and outputs it as a difference value signal. The difference value signal is input to the motion amount detection unit 30. The motion amount detection unit 30 detects the motion amount of the moving image part in the image based on the difference signal between the frames output from the difference value calculation unit 20 and the current frame video signal, and the detected motion amount is detected as the current frame. A signal subjected to correction processing according to the signal level of the video signal is output as a motion amount signal. The motion amount signal is input to the tour amount determination unit 40.

  Further, the travel amount determination unit 40 receives the motion amount signal, determines the travel amount for each motion amount signal level, and outputs it as the travel amount k. The cyclic amount k is input to the multiplier 50 and the multiplier 60.

  The cyclic amount k represents the degree of time averaging of the image signal for each frame period. When k = 0, time averaging is not performed, and the degree of time averaging increases as k increases (however, 0 ≦ k ≦ 1). In other words, this means that the noise reduction effect increases as k increases. On the other hand, it also means that motion blur tends to occur in the moving image portion.

  The multiplier 50 multiplies the input image signal, that is, the image signal of the current frame by (1-k). The multiplier 60 multiplies the output from the frame memory 10, that is, the image signal of the previous frame by the cyclic amount k. The adder 70 adds the outputs from the multiplier 50 and the multiplier 60 and outputs the result as an output image signal. The output from the adder 70 is accumulated in the frame memory 10 and used for processing in the next frame.

  Next, the configuration and operation of the motion amount detection unit 30 will be described. FIG. 10 is a block diagram showing a schematic configuration of the motion amount detection unit 30 in the present embodiment. The absolute value unit 31 calculates the absolute value of the difference signal input from the difference value calculation unit 20 and outputs the absolute value signal to the correction unit 32. The correction unit 32 receives the absolute value signal input from the absolute value unit 31 and the current frame video signal. The correction unit 32 calculates a correction value signal for correcting the absolute value signal output from the absolute value unit 31 according to the signal level of the current frame video signal. The correction value signal is output to the adder 33. The adder 33 adds the absolute value signal that is output from the absolute value unit 31 and the correction value signal that is output from the correction unit 32, and outputs the result to the limiter unit 34 as a correction signal. The limiter unit 34 limits the correction signal that is output from the adder 33 so as not to exceed a certain value. For example, if the signal handled as the motion amount is 8 bits and the maximum value is 255, a signal exceeding 255 is limited and 255 is output. The limited limiter signal is input to the low-pass filter unit 35. The low-pass filter unit 35 is provided to suppress the influence of noise in the frame difference signal as much as possible, and performs a smoothing process in order not to treat a difference value due to sudden noise as a motion. As a smoothing processing method, for example, a median filter is generally used, but other filters may be used as long as the filter can suppress detection by sudden noise by smoothing. Then, the signal smoothed by the low-pass filter unit 35 is output as a motion amount signal to the circulation amount determination unit 40.

  The travel amount determination unit 40 is a ROM that stores a conversion table for converting the value of the motion amount signal that is output from the motion amount detection unit 30 into the value of the tour amount k, and is a motion amount signal that is an input signal. The amount of patrol k corresponding to is output. For example, the conversion table is stored in the ROM so that the relationship between the motion amount and the circulation amount is as shown in FIG. In the first embodiment of the present invention, the relationship between the amount of movement and the amount of circulation is an example of the relationship shown in FIG. 12, but the relationship is not limited to this, and the same effect can be obtained in other relationships. .

  A specific motion amount correction method and correction signal calculation method will be described.

  The motion amount detection unit 30 calculates a motion amount for determining the cyclic amount from the difference value between the current frame image signal and the previous frame image signal. At this time, the correction value calculated by the correction unit 32 according to the signal level of the current frame image signal is added to the motion amount, and the motion amount is as much as possible from the motion in a portion where the signal level of the current frame image signal is spatially high. Correction is performed to take the value of. The correction unit 32 calculates a correction value by multiplying the absolute value signal, which is the absolute value of the difference between the current frame image signal and the previous frame image signal, obtained by the absolute value unit 31 by a coefficient. The coefficient is obtained by dividing the absolute value signal by the maximum value of the current frame image signal. For example, in the case of 8-bit 256 gradation, the maximum value is 255, and when expressed by a calculation formula, coefficient = absolute value signal / 255. The absolute value signal is added to the product of this coefficient multiplied by the absolute value signal. In addition, the value resulting from the addition may exceed 255 in some cases. For example, when the motion amount signal is handled as an 8-bit signal, the maximum value is 255, and thus the limiter unit 34 performs limiter processing on the exceeding signal. Next, the limiter signal subjected to the limiter process is subjected to a smoothing process in the low-pass filter unit 35 in order to suppress the influence of the frame difference value generated by sudden noise on the motion amount. The result of the smoothing process is output as a motion amount signal to the tour amount determination unit 40, and the tour amount determination unit 40 outputs the value of the tour amount k corresponding to the motion amount signal from the conversion table.

  FIG. 11 is a graph showing the relationship between the frame difference absolute value and the motion amount. The relationship between the frame difference absolute value indicated by the broken line and the motion amount before correction, and the relationship between the corrected frame difference absolute value and the motion amount indicated by the solid line. A comparison is made. For example, according to the correction method described above, when the current frame video signal level is 50, the relationship between the corrected frame difference absolute value and the amount of motion is represented by a line indicated by the input signal level 50 in the figure. Can do. Further, the relationship between the absolute value of the frame difference and the amount of motion when the current frame video signal level is 100 and 255 by the same processing can be expressed by the relationship indicated by the input signal levels 100 and 255 in the figure. Thus, there are 255 types of corrected lines corresponding to the current frame image signal levels 1 to 255, respectively. In the present embodiment, the motion amount detection unit 30 outputs the corrected motion amount by real-time processing.

  FIG. 13 shows a sloped edge portion shown on the left side of FIG. 8 that moves to the left with a maximum signal level value of 255 and a width A = 5 pixels / F (F: 16.7 ms which is a 1 TV field period). .) Shows the relationship between the amount of motion at each pixel position. The right side of FIG. 13 shows the amount of movement after the correction process performed in the first embodiment, and the left side shows the amount of movement before the correction process. Comparing before and after the correction of the motion amount in the lower part of FIG. 11, it can be seen that the motion amount after the correction processing is corrected in a direction in which the motion amount becomes larger than before the correction.

  As shown in FIG. 8, even in the same movement width, in the part where the difference in the magnitude of the frame difference value differs greatly, the part where the signal level is large is more moved according to the input image signal level. By correcting so that the amount of motion is larger than the amount of motion, that is, the amount of motion is larger, the amount of circulation is determined from the motion, so that tailing and afterimages in the motion portion are suppressed and subfield blurring is prevented from deteriorating. Can do.

  Thus, unlike the conventional example shown in FIG. 7, the noise reduction apparatus according to the present embodiment corrects the amount of motion according to the current frame image signal level, thereby maintaining the noise reduction effect while moving the moving image portion. The blur of the video signal can be suppressed, and when used in the subfield image display device, the deterioration of the subfield blur can be suppressed.

(Embodiment 2)
A frame cyclic noise reduction method according to Embodiment 2 of the present invention will be described below. The schematic configuration of the noise reduction apparatus for realizing the frame cyclic noise reduction method according to the present embodiment is the same as the block diagram shown in FIG. 9, but the motion amount detection unit described in the first embodiment is 30 configurations are different. That is, the motion amount detection unit 30 is configured to perform correction processing by real-time processing by a circuit as shown in FIG. 10 in the first embodiment, but in the second embodiment, FIG. As shown in the block diagram of the schematic configuration, the calculation method of the correction process is exactly the same as in the first embodiment, but it does not perform real-time processing, but for each current frame image signal level obtained by calculation in advance. The motion amount after correction in each frame difference absolute value is stored in a conversion table, and correction is performed using the conversion table.

  The configuration and operation of the motion amount detection unit 30 shown in FIG. 14 will be described below. An absolute value signal of a frame difference value between the current frame image signal and the previous frame image signal output from the absolute value unit 31 is input to the correction conversion table 36, and a correction signal after correction according to the current frame video signal level is generated. As in the first embodiment, the low-pass filter unit 35 performs smoothing processing using, for example, a median filter in order to suppress sudden detection due to noise by smoothing processing. The signal after the low-pass filter processing is output to the travel amount determination unit 40 as a motion amount signal. The subsequent processing is the same as in the first embodiment.

  In the second embodiment of the present invention, by performing the motion amount correction using the conversion table, the circuit scale can be reduced and realized with a simple configuration. The effect is the same as in the first embodiment. By correcting the amount of motion according to the current frame image signal level, it is possible to suppress blurring of the video signal in the moving image portion while maintaining the noise reduction effect. When used in an image display device, it is possible to suppress deterioration of subfield blur.

  As described above, according to the frame cyclic noise reduction method of the present invention, one field of the input image signal is divided into a plurality of subfields, and the light emission of the display means is controlled by the combination of the light emission for each subfield. A frame cyclic noise reduction method used for a sub-field image display device that expresses gradation, which causes degradation of moving image quality such as resolution degradation of a moving image portion in a display device such as a DLP using a PDP or DMD element. Therefore, a good noise reduction effect can be obtained for the video signal.

The figure for explaining the example of the arrangement of the subfield Diagram for explaining the principle of halftone expression in subfield image display The figure for explaining the mechanism of the resolution degradation of moving picture part in the subfield picture display The figure for demonstrating the difference of the grade by the video signal level of the moving image part resolution degradation in a subfield image display The figure for demonstrating the example of the animation part resolution degradation in ascending order coding The figure for demonstrating the example of the animation part resolution degradation in descending order coding A block diagram showing a schematic configuration of a conventional frame cyclic noise reduction device The figure for demonstrating the difference in the magnitude | size of the frame difference value by the difference in the spatial change of a video signal level The block diagram which shows schematic structure of the frame cyclic noise reduction apparatus for demonstrating the frame cyclic noise reduction method by Embodiment 1 of this invention. The block diagram which shows schematic structure of the motion amount detection part shown in FIG. The figure which shows the relationship between the frame difference value before and after the motion amount correction | amendment by the present frame image signal level in Embodiment 1 of this invention, and a motion amount. The figure which shows the relationship between the amount of movement and the amount of patrols in Embodiment 1 of this invention. The figure which shows the effect of the motion amount correction process in Embodiment 1 of this invention. The block diagram which shows schematic structure of the motion amount detection part 30 in Embodiment 2 of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Frame memory 20 Difference value calculation part 30 Motion amount detection part 31 Absolute value part 32 Correction part 33 Adder 34 Limiter part 35 Low pass filter part 36 Correction conversion table 40 Cyclic amount determination part 50, 60 Multiplier 70 Adder

Claims (1)

One frame of the input image signal is divided into a plurality of subfields and used for a subfield image display device that expresses gradation by controlling the light emission of the display means according to the combination of light emission for each subfield, and the amount of motion is A frame cyclic noise reduction method for detecting and determining a cyclic amount according to the amount of motion,
The amount of motion is obtained by multiplying the absolute value by a coefficient obtained by dividing the absolute value of the difference between the current frame image signal and the previous frame image signal by the maximum value of the current frame image signal. A frame cyclic noise reduction method, wherein the amount of motion is corrected so as to take a value greater than motion as the signal level is higher .

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