JP2826886B2 - Image signal noise reduction device - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to an image signal noise reduction device. Specifically, in inter-frame difference encoding used as a means for high-efficiency encoding of an image signal, while improving faithful reproducibility of the image signal, flicker noise among noise components causing image degradation is reduced. It is an object of the present invention to provide an image signal noise reduction device capable of reducing noise.

[Prior Art] A circuit configuration of a conventional image signal noise reduction device is shown in FIG. 2A and described.

In FIG. 2A, a difference value between a signal 41 indicating the current frame image input from the input terminal 21 and a signal 45B indicating the immediately preceding frame image from the frame memory 25B is subtracted for each pixel. Determined by the container 22. The signal 42 indicating the obtained difference value is input to the non-linear circuit 23B and converted with a predetermined characteristic. The signal 43B indicating the converted difference value is input to the adder 24, and is added to the signal 45B indicating the image of the immediately preceding frame from the frame memory 25B. The signal 44 indicating the obtained added value is output from the output terminal 26 and sent to and stored in the frame memory 25B.

In the above circuit operation, when the absolute value of the amplitude of the signal 42 indicating the difference is smaller than the set value V1, for example, as shown in FIG. 2B, the nonlinear circuit 23B nonlinearly suppresses the amplitude. Has characteristics. That is, the signal
The noise component superimposed on 42, for example, random noise such as flicker noise or camera noise due to illumination by a fluorescent lamp has the property that the amplitude is often smaller than the difference signal between frames in a moving image. Therefore, by passing the signal 42 indicating the difference value between frames through the nonlinear circuit 23B having the characteristic as shown in FIG. 2B, the signal whose absolute value of the amplitude is smaller than the set value V1 is
This is suppressed as a noise component. Therefore, normally, the nonlinear circuit 23B having the set value V1 corresponding to about the maximum value among the peak values of the noise components superimposed on the input signal 41 is used.

Here, the state of noise suppression by such a nonlinear circuit 23B will be specifically described with reference to FIG. 2C. 2nd C
In the figure, a signal 42 shown by a solid line is a subtractor 22 (2nd A) on which a flicker noise component having an amplitude P shown by a broken line
A) represents a signal 42 indicating a difference value from the figure, and A represents a section in which random noise is generated, and B represents a section in which a signal that changes depending on the movement of the imaging target is generated. Therefore, the set value V1 given to the nonlinear circuit 23B
Is set to 1 so as to capture the maximum value of the noise existing in the section A.
If the level is indicated by a dotted line, a signal whose absolute value of the amplitude is smaller than the set value V1 is treated as noise including a valid signal existing in the section B1 of the section B and is suppressed. .

[Problems to be Solved by the Invention] As described above, according to the conventional example shown in FIG. 2A, the subtractor
The amplitude of the signal 42 indicating the difference value from 22 is
When the set value V1 is smaller than the set value V1, the noise is suppressed. Therefore, in order to ensure the faithful reproducibility of an effective moving image signal, the set value V1 is desirably as small as possible. However, in general, inexpensive image encoding devices used for video conferences and videophones used indoors are illuminated by fluorescent lights, and low-cost television cameras are used. In many cases, the flicker noise component and the random noise component caused by the noise are relatively large. Therefore, the set value V1 given to the nonlinear circuit 23B is also set to be relatively large. Therefore, the signal 42 indicating the difference value having an amplitude smaller than the set value V1 is suppressed as noise even if the signal should be effective rather than noise as the signal in the section B1 shown in FIG. 2C. Will be.

As described above, in the conventional example of the configuration shown in FIG. 2A, since the degree of distortion of the effective signal increases, it is difficult to faithfully reproduce the signal of the moving part where the difference value of the moving image signal is minute,
There was a problem to be solved in which images were accompanied by unnaturalness.

[Means for Solving the Problems] In view of the above-mentioned problems, the present invention has been made,
Among the noise components included in the image signal, flicker noise is caused, for example, by illumination of a fluorescent lamp. Generally, the fluorescent lamp is lit by an AC power supply (50 Hz or 60 Hz). Are also low frequency components (100 Hz or 120 Hz). Therefore, focusing on a substantially constant DC component whose intensity does not change in the same line to be scanned, the difference value for each pixel between the current frame and the immediately preceding frame is determined by one or more lines. The average value for the line was determined. This is flicker
Recognition as noise, the difference value is subtracted from the difference value, and the set value of the suppression level in the nonlinear circuit can be made smaller in accordance with the smaller difference value obtained thereby.

[Operation] By using such a means, a small set value corresponding to only the peak value of the random noise itself obtained by subtracting the amplitude of the flicker noise can be given to the nonlinear circuit. The degree of distortion of the signal is also reduced, and the signal of the moving part where the difference value is a small value is less suppressed as noise, and the faithful reproducibility of the image signal can be improved. .

Example A circuit configuration of an example of the present invention is shown in FIG. 1A and described. Here, components corresponding to the components in FIG. 2A are denoted by the same reference numerals.

In FIG. 1A, a first difference value between a signal 41 indicating the image of the current frame input from the input terminal 21 and a signal 45 indicating the image of the immediately preceding frame from the frame memory 25 is a pixel. Is obtained by the subtractor 22 every time. The obtained signal 42 indicating the first difference value is input to the line memory 11 and the average value calculation circuit 12. The average value calculation circuit 12
It is obtained by calculating an average value of one or more lines of the signal 42 indicating the first difference value. Here, the average value calculation circuit 12 obtains an average value only for pixels whose absolute value of the first difference value is equal to or less than a predetermined threshold value. Further, when the number of pixels in which the absolute value of the first difference value is equal to or smaller than the threshold value is smaller than a predetermined number in one or more lines of pixels, the average value is set to “0”. This is because, in the present invention, the average value calculation circuit 12 treats the obtained average value as flicker noise, so the difference signal 42 indicating a large value due to the movement of the imaging target or the like is excluded from the average value calculation target, This is to prevent this from being treated as flicker noise.

In this way, when the average value of the signal 42 indicating the difference value in which the absolute value of the first difference value for one line or a plurality of lines is equal to or smaller than a predetermined threshold is obtained by the average value calculation circuit 12,
The subtractor 13 subtracts this from the signal 31 from the line memory 11, and the signal 33 indicating the obtained second difference value is converted to the nonlinear circuit 23.
And converted by a predetermined characteristic. Converted second
Is input to the adder 24. At the same time, the signal 45 indicating the image of the previous frame from the frame memory 25 is delayed by passing through the line memory 14, and the output signal 34 is input to the adder 24. Here, the delay time of the line memory 14 corresponds to the delay time of the line memory 11 because the delay time in the subtractor 13 and the non-linear circuit 23 can be generally ignored. Is longer than the delay time of the conventional frame memory 25B (Fig. 2A).
The delay time of the memory 11 or the line memory 14 is reduced. Therefore, the adder 24 calculates the signal 43 from the nonlinear circuit 23 and the signal from the line memory 14 for the same pixel.
Add 34.

The signal 44 indicating the obtained added value is output from the output terminal 26 and sent to the frame memory 25 for storage. The above operation is repeated.

Next, how flicker and noise in noise are suppressed by the above circuit operation will be described with reference to FIG. 1B. Here, the signal 33 shown by the solid line in FIG. 1B is the signal 33 in FIG. 2C.
Corresponding to 42, sections A and B correspond to sections A and B shown in FIG. 2C.

In FIG. 1B, the amplitude of the signal 33 indicating the second difference value input to the non-linear circuit 23 (FIG. 1A) is different from the signal 31 indicating the first difference value by the signal 32 indicating the average value of the signal 31 indicating the first difference value. Since it is reduced as flicker noise superimposed in 41,
Compared to the amplitude of signal 42 shown in FIG.
It is smaller by the noise amplitude P (FIG. 2C). Therefore, the set value V2 (two-dot chain line) given to the non-linear circuit 23 corresponding to the maximum value of the peak value of the random noise in the noise
Can be made smaller than the conventional set value V1 indicated by the broken line. Therefore, only the signal contained in the section B2 of the section B in which the effective signal component exists is suppressed without being distinguished from the random noise in the noise, and is suppressed, and the section B1 (2C Compared with FIG. 7), the degree of distortion of the effective signal can be reduced.

[Effect of the Invention] As is clear from the above description, according to the present invention, the average value of the first difference value between the image signal of the current frame and the image signal of the immediately preceding frame is calculated in the input image signal. And operating the non-linear circuit so as to suppress only the random noise component included in the signal indicating the second difference value whose amplitude has been reduced in advance by recognizing it as flicker noise superimposed on the signal. Since a signal that should be effective is suppressed as noise, the degree of distortion is reduced, and faithful reproducibility of a signal of a minute moving portion is improved, so that a natural image can be obtained without noise. Therefore, the effect of the present invention is extremely large.

[Brief description of the drawings]

FIG. 1A is a circuit configuration diagram of one embodiment of the present invention, FIG. 1B is a waveform diagram for explaining the state of noise suppression by the nonlinear circuit shown in FIG. 1A, FIG. 2A is a conventional circuit configuration diagram, FIG. 2B is a characteristic diagram showing an example of input / output characteristics of the nonlinear circuit shown in FIG. 2A, and FIG. 2C is a waveform diagram for explaining a state of noise suppression by the nonlinear circuit shown in FIG. 2A. . 11 ... Line memory, 12 ... Average calculation circuit 13 ... Subtractor, 14 ... Line memory 21 ... Input terminal, 22 ... Subtractor 23,23B ... Non-linear circuit 24 ... Adder 25 , 25B ... frame memory 26 ... output terminals 31 to 34, 41 to 43, 43B, 44, 45, 45B ... signals.

Continuation of the front page (72) Inventor Xiaohong Son 7-6-5 Minami-Aoyama, Minato-ku, Tokyo 6th floor of the column Minami-Aoyama Inside the Research and Development Department, Graphics Communication Technologies, Inc. (56) References JP-A-63-141476 (JP, A) JP-B-57-39705 (JP, B2)

Claims (1)

(57) [Claims] 1. An image signal of an input current frame and 1
First subtraction means (22) for obtaining a difference value for each pixel from the image signal of the immediately preceding frame; and delaying the difference value for each pixel obtained by the first subtraction means by one line. The first line to output
A memory means (11); and an average value calculation means (12) for calculating an average value for one line of a difference value indicating a value within a predetermined value for each pixel obtained by the first subtraction means. ), Second subtraction means (13) for obtaining a difference value for each pixel between the output from the first line memory means and the output from the average value calculation means, and the second subtraction means A non-linear means (23) for suppressing a difference value equal to or less than a predetermined value in the difference value for each pixel obtained by the above, and an image signal of the immediately preceding frame corresponding to an output of the non-linear means, Second line memory means (14) for delaying the output to correspond in time with the output of the non-linear means
An adding means (24) for obtaining an addition value for each pixel of an output from the non-linear means and an output from the second line memory means; and storing the output from the adding means, A frame memory means (25) for outputting the image signal of the immediately preceding frame to the subtraction means and the second line memory means.