JPH05130452A - Variable filter device - Google Patents

Abstract

(57) [Abstract] [Purpose] In a coefficient variable filter device that adaptively controls coefficients in accordance with screen position and time, generation of fixed pattern noise due to gain variation is prevented. [Structure] A coefficient generator 4 that changes a coefficient according to a position on a screen, a circuit 3 that detects a change in a filter gain by comparing a total value of coefficients with a predetermined value, and a new gain correction unit that corrects the gain change. It is composed of a circuit 2 for generating a coefficient and a variable coefficient filter 1 controlled by this coefficient. [Effect] Since the DC gain can be kept constant for any filter coefficient, it is possible to suppress the occurrence of fixed pattern noise caused by gain variation, and prevent deterioration of image quality due to this noise.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable filter device, and more particularly to a filter device for processing signals such as image signals and audio signals, and more particularly to a transversal type filter device capable of controlling filter characteristics. Regarding configuration.

[0002]

2. Description of the Related Art Conventionally, in the field of image signal processing, a two-dimensional filter (spatial filter) has been used for improving the sharpness of an image or removing noise. An example of this will be described by taking a contour compensation circuit of a television camera as an example. In television cameras, the resolution of the image taken may be insufficient due to the characteristics of the lens and image sensor used, and in order to improve this, a contour compensation circuit has been developed to compensate for the high frequency part of the spatial frequency of the image signal. ing. FIG. 2 shows an example of a conventionally known contour compensation circuit, which is composed of a transversal filter including delay elements 7 and 8, a multiplication circuit 10, and addition circuits 9 and 11. The compensation characteristic of the contour compensation circuit can be controlled by the gain (filter coefficient) of the multiplication circuit 10. However, in the conventional contour compensation circuit, the characteristic is generally uniform in the screen of the processed image. ..

An ordinary camera lens has a characteristic that the resolution in the vicinity of the optical axis of the lens is high, but the resolution in the peripheral portion of the screen away from the optical axis is low. It's becoming a problem. Especially in high-definition type cameras whose main purpose is to provide high-resolution video services, non-uniform resolution is an obstacle to image quality. In order to solve the above problems, the applicants of the present invention have applied for a patent for an adaptive contour compensation circuit capable of controlling the compensation characteristics according to the screen position. (Japanese Patent Application No. 63-1566)
40, JP-A-2-7678).

The adaptive contour compensation circuit of the above application is shown in FIG.
A brief description will be given with reference to FIGS. Figure 3 shows a TV screen 1
4 is a diagram schematically showing 4 and 15 and 16 respectively indicate the center part and the peripheral part of the screen. In the lens of the television camera, as shown in FIGS.
5 and the peripheral portion 16 respectively, the resolution characteristics 19
And 20 characteristics. The resolution 20 in the peripheral portion of the screen has a low response in a high spatial frequency region. On the other hand, the resolution 19 in the central portion of the screen has a higher response in the high spatial frequency region than in the peripheral portion. Therefore, when a high-definition image is taken, the peripheral part 16 lacks resolution, and is taken differently from the high-resolution central part 15. In order to compensate for this resolution lowering characteristic and always obtain a uniform resolution on the screen, the compensation characteristics of the adaptive contour compensating circuit are shown as 21 and 22. 5A and 5B are coefficients of the digital filter of the contour compensation circuit corresponding to the compensation characteristics 21 and 22 of FIG. 4, respectively. The horizontal axis of FIG. 5 is the tap position of the filter, which corresponds to the delay time of the delay circuit. Therefore, in order to realize a uniform resolution within the screen, the filter coefficient is adapted to the position on the screen and sequentially updated to control the multiplication circuit of FIG.

[0005]

In order to adaptively enhance the contour according to the screen position, the invention of the above application uses a digital spatial filter whose coefficient can be changed for each screen position. When the image is processed by such a variable coefficient filter, if the DC gain of the filter changes,
Fixed pattern noise is generated on the screen due to the mismatch of gains, and the image quality is degraded. For example, when position-corresponding filtering is performed on an image signal, such as a wall of a building or the sky, which has little DC fluctuation, the DC gain changes at the switching point of the filter characteristics, resulting in a level difference. Vision is extremely sensitive to such DC fluctuations, and even fluctuations of 1% or less of the signal are detected. This DC level fluctuation becomes fixed pattern noise, which causes image quality disturbance. Therefore, it is an object of the present invention to provide a variable filter device which does not have a DC level fluctuation.

[0006]

In order to achieve the above object, a variable variable filter device according to the present invention comprises means for controlling the coefficient of a variable coefficient filter, and means for keeping the DC gain of the variable coefficient filter constant. And a filter device was constructed. As a preferable mode for keeping the DC gain of the variable coefficient filter constant, a filter device is constructed using the following means. That is, (1)
Means for obtaining the sum of the coefficients of the digital filter, (2) means for obtaining the difference value between the predetermined value and the sum of the filter coefficients, and (3) means for variably correcting the coefficient value of the filter according to the difference value. Configured using.

[0007]

The means for obtaining the sum of the coefficients of the digital filter calculates the sum of the coefficients to be filtered. As a result, the DC gain of this filter is obtained. The means for obtaining the difference value between the predetermined value and the total value of the filter coefficients is
A gain difference between the predetermined DC gain and the calculated DC gain of the filter is obtained. Further, the circuit that variably corrects the coefficient value of the filter according to the difference value has a function of correcting the gain fluctuation of the spatial filter so that the same DC gain is obtained for all the spatial frequency characteristics. With the above operation, the DC gain fluctuation does not occur even when the filter coefficient is changed and the frequency characteristic thereof is controlled, and it is possible to prevent the occurrence of image disturbance due to fixed pattern noise.

[0008]

Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 is a block diagram showing the configuration of an embodiment of a variable filter device according to the present invention. The present embodiment comprises a filter 1, a coefficient generation circuit 2, a gain detection circuit 3, and a coefficient correction circuit 4. The coefficient generating circuit 2 obtains the information of the position of the image and the lens and generates the coefficient corresponding to the screen position. The signal representing the coefficient is branched into two systems, one is input to the gain detection circuit 3 and the other is input to the coefficient correction circuit 4. The gain detection circuit 3 calculates the total sum of the input coefficient values, and outputs a difference signal between the total sum value and a predetermined value to the coefficient correction circuit 4. The coefficient correction circuit 4 corrects the filter coefficient using the input difference signal and outputs the result to the coefficient variable filter 1. The filter 1 performs a filtering process on the image signal input from the terminal 5 using the coefficient output from the coefficient correction circuit 4, and outputs the result to the terminal 6 as image data output.

FIG. 6 is a circuit diagram showing the configuration of an embodiment of the variable coefficient filter 1 of FIG. Here, in order to facilitate understanding, the operation of the filter will be described by taking a three-tap one-dimensional filter as an example. It goes without saying that the variable coefficient filter 1 can be easily expanded into two dimensions. The variable coefficient filter shown in FIG. 6 includes delay circuits (abbreviated as D) 25, 26,
It is a transversal filter including multiplication circuits 27, 28, 29 and an addition circuit 30. The image data input signal input from the terminal 31 is guided to the delay circuits 25 and 26 connected in series, and delays the delay time D respectively. An input signal from the terminal 31 and a delay circuit 25,
The output signals from 26 are multiplication circuits 27, 28,
It is input to 29, multiplied by filter coefficients (C-1, C0, C + 1) by the multiplication circuits 27, 28 and 29, and then input to the addition circuit 30. The adder circuit 30 adds all the pixel data signals multiplied by the coefficient and outputs the result as image data output to the output end 32. Through the above operation, the filtering process is performed on the input image data input. To the multiplication circuits 27, 28, and 29, the filter coefficient for obtaining a desired frequency characteristic is input from the coefficient correction circuit 4 of FIG. This coefficient value is updated and generated each time the compensation characteristic is changed, and is changed according to, for example, the screen position or the focus condition of the lens.

FIG. 7 is a configuration block diagram of an embodiment of the gain detection circuit 3 shown in FIG. The gain detection circuit 3 includes a multi-input addition circuit 33, a subtraction circuit 34, and a predetermined value memory 35.
Composed of. First, all filter coefficients (C-1, C0, C + 1) are input to the multi-input addition circuit 33 and added. The output of the multi-input addition circuit 33 is the subtraction circuit 34.
Entered in. The output of the predetermined value memory 35 is added to the other input of the subtraction circuit 34, and the difference between the total value of the coefficients and the predetermined value is obtained. This difference value is input to the coefficient correction circuit 4 of FIG. 1 as a coefficient correction signal.

FIG. 8 is a configuration block diagram of an embodiment of the coefficient correction circuit 4 shown in FIG. The coefficient correction circuit 4 in this embodiment is configured to add to the coefficient of the center tap of the filter 1. The input coefficients C-1 and C + 1 are output as they are. The center tap coefficient C0 and the difference signal from the gain detection circuit are added to the adder circuit 37, and the result is sent to the filter 1 of FIG. 1 as a new coefficient C'0. The gain of the new coefficient (C-1, C'0, C + 1) corrected in this way becomes equal to the value set in the predetermined value memory. By the above-described gain detection and correction, it is possible to always obtain a constant filter gain even when the coefficient is changed, and it is possible to prevent generation of fixed pattern noise due to gain variation. In addition,
In the above embodiment, in order to correct the gain fluctuation, the coefficient at the center tap position of the transversal filter is corrected. The correction with the center tap is a preferable method because the correction can be performed without changing the phase characteristic of the filter. Further, the correction may be performed at other tap positions, but in the case of correction other than the center tap, it is preferable to divide the correction value and add it to each coefficient on average. If the correction is concentrated on a specific tap, the fluctuation of the phase characteristic of the filter becomes large. By dispersing, the phase fluctuation can be reduced.

When the coefficient correction is performed as described above, the filter coefficient differs from the normal value, and the frequency characteristic of the filter also slightly changes. However, visually, the sensitivity to a change in resolution is lower than the sensitivity to a change in gain, and a slight change in resolution is not detected. Therefore, in the corrected image, only the effect of suppressing the gain variation can be obtained. In the above embodiments, the case where there is a spatial gain variation has been mainly described, but even if the gain varies for each field and frame, it is detected as a flicker, which also causes image quality obstruction. According to the variable filter method for performing gain correction of the present invention, it is possible to correct even the gain fluctuation due to the time transition, and it is also possible to prevent image quality obstruction due to flicker or the like.

In the embodiment shown in FIG. 1, the case where the filter coefficient is corrected by calculation has been described. However, in the filter processing for changing the coefficient, the DC gain is fixed to a constant value even when the filter coefficient is changed. There is a point. A table in which the filter coefficient is calculated in advance according to the position of the screen or the like may be created to provide a table lookup type variable filter device. In the calculation process at the time of creating the table, as described above, the sum of the coefficients is obtained, and the coefficient is corrected by the difference signal between this and a predetermined value to create the table.

FIG. 9 is a flow chart of the calculation procedure when creating the lookup table. In this processing procedure, the coefficient of the center tap or other taps is corrected so that the gain becomes constant after calculating the filter coefficient,
Normalize to obtain a uniform DC gain. For this reason,
A process 38 for obtaining the sum of coefficients, a difference value detection 39 with respect to a predetermined gain, and a process 40 for correcting the coefficient are provided. In addition,
In the above embodiment, the method of making the filter gain constant has been described by taking the contour compensation circuit of the television camera as an example. In addition to the contour compensation circuit, the present invention depends on time and space, such as a geometric transformation filter (affine transformation filter) that performs distortion correction according to the screen position and an adaptive automatic equalizer used in magnetic recording and the like. It can be applied to shift-variant filters whose characteristics are varied.

[0015]

As described above, according to the present invention, the filter gain can be kept constant even when the coefficient variable spatial filter processing is performed, and the fixed pattern noise caused by the gain variation can be prevented. ..

[Brief description of drawings]

FIG. 1 is a block diagram of an embodiment of a variable filter device according to the present invention.

FIG. 2 is a block diagram of a conventional contour compensation circuit.

FIG. 3 is a schematic diagram of a television screen.

FIG. 4 is a diagram showing resolution characteristics and compensation characteristics.

FIG. 5 is a diagram schematically showing coefficients of a digital filter.

6 is a circuit diagram showing a configuration of an embodiment of the coefficient variable filter 1 of FIG.

7 is a configuration block diagram of an embodiment of the gain detection circuit 3 of FIG. 1. FIG.

8 is a configuration block diagram of an embodiment of the coefficient correction circuit 4 of FIG.

FIG. 9 is a flow chart diagram for creating a lookup table used in another embodiment of the variable filter device according to the present invention.

[Explanation of symbols]

1 ... Filter, 30 ... Adder circuit,
2 ... Coefficient generation circuit, 33 ... Multi-input addition circuit, 3 ... Gain detection circuit, 34 ... Addition circuit, 4 ...
Coefficient correction circuit, 35 ... Predetermined value memory, 7, 8 ... Delay element, 37 ... Addition circuit, 10 ... Multiplication circuit, 38 ... Procedure for obtaining total value of coefficients, 9, 11 ... Addition circuit,
39 ... Means for obtaining difference from predetermined gain, 25,
26 ... Delay circuit, 40 ... Means for correcting coefficient. 27, 28, 29 ... Multiplier circuits.

Claims (5)

[Claims] 1. An apparatus having a variable coefficient filter for processing a signal, comprising: means for controlling a coefficient of the variable coefficient filter; and means for holding a DC gain of the variable coefficient filter constant. Characteristic variable filter device. 2. The variable filter device according to claim 1, wherein the variable coefficient filter is a transversal type filter including a digital signal processing circuit. 3. The variable filter device according to claim 2, wherein the means for holding the direct current gain of the variable coefficient filter constant comprises a circuit which acts on a center tap of the transversal filter to make the direct current gain constant. A variable filter device characterized in that 4. A variable filter device comprising a look-up table in which a plurality of sets of filter characteristics are recorded and means for controlling the coefficients of a variable coefficient digital filter by switching the plurality of sets of filter characteristics. The table is characterized in that the temporal or spatial position information of the signal processed by the variable coefficient digital filter and the coefficient for the position information are recorded so as to keep the DC gain of the variable coefficient digital filter constant. Variable filter device. 5. A variable filter device having a transversal filter and coefficient control means for controlling tap coefficients of the transversal filter, wherein the coefficient control means corresponds to a spatial or temporal position of a signal input to the transversal filter. A coefficient generation circuit for generating a tap coefficient, a gain detection circuit for obtaining a difference value between the sum of coefficients and a constant value from the output of the coefficient generation circuit, and a tap coefficient generated by the coefficient generation circuit by the output of the gain detection circuit. A variable filter device, comprising: a coefficient correction circuit for correcting the above-mentioned condition; and a multiplication circuit for controlling the tap coefficient of the transversal filter by the output of the coefficient correction circuit.