JPH04222178A - Exposure control circuit for video camera - Google Patents

Abstract

PURPOSE:To optimumly control the brightness characteristic of the video camera by switching the detection characteristic of an exposure control loop corresponding to the state of a picture. CONSTITUTION:A peak detection circuit 10 is provided to detect the peak value of an image pickup signal, and an average detection circuit 9 is provided to detect the average value of the image pickup signal. The detected peak value and the detected average value are weighted and added by multiplier circuits 11 and 12 and an adder circuit 14 and according to the result, the gain of an AGC amplifier 4 and the ON/OFF of an iris 2 are controlled. The luminance distributing state of the image pickup picture is detected by a division circuit 13. According to this luminance distributing state, weight coefficients KA and Kp to be applied to the multiplier circuits 11 and 12 are set. Thus, since the weight coefficients KA and Kp are set for the detected average value and the detected peak value of the image pickup signal and exposure is corrected corresponding to the luminance distributing state, the brightness characteristic of the video camera can be optimumly controlled.

Description

[Detailed description of the invention]

0001

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure control circuit for a video camera.

0002

2. Description of the Related Art A video camera is provided with an AGC circuit in order to keep the level of an imaging signal constant. FIG. 4 is an example of a conventional AGC circuit. In FIG. 4, a subject image is formed through a lens 101 through an iris 102 on a light-receiving surface of a CCD image sensor 103. The output of the CCD image sensor 103 is supplied to a signal processing circuit 105 via an AGC amplifier 104 and also to a detection circuit 106 .

[0003] In the detection circuit 106, the CCD image sensor 103
The imaging signal level of is detected. The output of the detection circuit 106 is supplied to a comparator 107. Comparator 10
7, a predetermined reference level is set. A comparator 107 compares the level of the imaging signal detected by the detection circuit 106 with a predetermined reference level.

[0004] The output of comparator 107 is supplied to loop filter 108 . The output of the loop filter 108 is supplied to the AGC amplifier 104. Loop filter 10
The gain of the AGC amplifier 104 is controlled according to the output of the AGC amplifier 104.

[0005] When the imaging signal level is lower than a predetermined reference level, the gain of the AGC amplifier 104 is increased. When the imaging signal level is higher than a predetermined reference level, the gain of AGC amplifier 104 is lowered. Thereby, the level of the imaging signal supplied to the signal processing circuit 105 is controlled to a predetermined level.

[0006]

[Problem to be Solved by the Invention] In such an AGC circuit, the brightness characteristics of the screen differ depending on whether the detection characteristics of the detection circuit 106 are based on peak detection or average detection. .

That is, as shown in FIG. 6, the imaging signal level is adjusted so that a standard screen 110 such as a color bar is displayed at a predetermined position, and the imaging signal level at this time becomes a predetermined value. Ru. As shown in Figure 6, the detection level VA when the color bar screen is detected by the average value is the peak value (
The value is 35% of the white value). Therefore, when performing AGC control using the average detected value of the image pickup signal, the reference level is set to a value of 35% of the peak value.

[0008] Here, it is assumed that the peak detection value and the average detection value are mixed at a weighting ratio of (0.3:0.7). As shown in Figure 5, if the peak value is 100, the average value is 35, so if the peak detection value and the average detection value are mixed with a weight of (0.3:0.7), Detection level is 100 x 0.3 + 35 x 0.7 = 54.5
becomes. Therefore, the mixing ratio (
When weighted and mixed by 0.3:0.7), the detection level becomes 54.5% of the peak value. Therefore, the mixing ratio of the peak detection value and the average detection value is (0.3:0
．． When performing AGC control using the detected value mixed with the weight of 7), the reference level is 54.5, which is the peak value.
Set to a percentage value.

[0009] As described above, when the detection characteristic is based on peak detection, the A
The reference level when performing GC control increases. Therefore, when displaying a screen with little change in brightness level, the screen becomes brighter. On the other hand, if the detection characteristics are based on peak detection, if there is a bright part within the screen, the entire screen tends to be affected by the bright part and become dark. On the other hand, if the characteristics are based on average detection, when projecting a screen with little change in brightness level, the entire screen will be dark, but even if there are bright parts within the screen,
Bright areas of the screen do not affect the overall brightness of the screen.

Therefore, an object of the present invention is to perform exposure control by switching between characteristics based on peak detection and characteristics based on average detection depending on the brightness distribution state of the screen, so that the brightness of the screen can always be set to the optimum level. Our goal is to provide a video camera that is designed to meet your needs.

[0011]

[Means for Solving the Problems] The present invention provides an exposure control circuit for a video camera that detects an imaging signal level and controls the opening degree of an iris and/or the gain of an AGC according to the imaging signal level. a peak detection circuit that detects the peak value of the imaging signal level; an average detection circuit that detects the average value of the imaging signal; and weighted addition means that weights and adds the peak detection value and the average detection value of the imaging signal, respectively; and a brightness distribution state detection means for detecting the brightness distribution state of the imaging screen, and sets a weight for the peak detection value and a weight for the average detection value, respectively, according to the brightness distribution state detected by the brightness distribution state detection means. This is an exposure control circuit for a video camera.

0012

[Operation] Weighting coefficients corresponding to the brightness distribution state are stored in the ROMs 15 and 45, and weights are set for each of the peak detection value and the average detection value in accordance with the brightness distribution state. Therefore, when the brightness of the screen is uniform, the characteristics are better than peak detection, and the entire screen can be made brighter. If there is a bright part within the screen, the characteristics will be based on average detection, and it is possible to prevent the entire screen from becoming dark due to the influence of the bright part within the screen.

[0013]

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described below with reference to the drawings. FIG. 1 shows an embodiment of the invention. In FIG. 1, a subject image is formed through a lens 1 through an iris 2 on a light-receiving surface of a CCD image sensor 3. As shown in FIG. The opening and closing of the iris 2 is performed by the iris drive circuit 20.
controlled by The output of the CCD image sensor 3 is supplied to an AGC amplifier 4. The gain of AGC amplifier 4 is D/
It is set according to the output of the A converter 21. The output of AGC amplifier 4 is supplied to A/D converter 6. A/
A D converter 6 digitizes the image signal. This digital imaging signal is supplied to the signal processing circuit 7 and also to the area setting circuit 8.

The area setting circuit 8 is a gate circuit for setting a predetermined exposure detection area. The output of the area setting detection circuit 8 is supplied to an average detection circuit 9 and also to a peak detection circuit 10. The output of the average detection circuit 9 is supplied to a multiplication circuit 11 and also to a division circuit 13 . The output of the peak detection circuit 10 is supplied to a multiplication circuit 12 and also to a division circuit 13.

A division circuit 13 calculates the ratio between the peak detection value and the average detection value. The output of this division circuit 13 is RO
Supplied to M15. As shown in FIG. 2, the ROM 15 stores a mixing ratio of peak detection values corresponding to the ratio of the peak detection value to the average detection value. When the ratio between the peak detection value and the average detection value is given from the division circuit 13, the ROM 15 outputs a weighting coefficient KP for the peak detection value corresponding to the ratio between the peak detection value and the average detection value.

Weighting coefficient KP output from ROM 15
is applied to the multiplication circuit 12. Further, the weighting coefficient KP output from the ROM 15 is supplied to the subtraction circuit 16. The subtraction circuit 16 performs the calculation (1-KP) to obtain a weighting coefficient KA for the average detected value. This weighting coefficient KA is supplied to the multiplication circuit 11.

The outputs of the multiplier circuits 11 and 12 are added to the adder circuit 1.
4. Multiplication circuits 11 and 12, addition circuit 14
As a result, the average detection value output from the average detection circuit 9 and the peak detection value output from the peak detection circuit 10 are weighted and added. The output of the adder circuit 14 is the comparator 1
7. A predetermined reference level is set in the comparator 17. The output of the coparator 17 is supplied to a control amount calculation circuit 19 via a loop filter 18.

The control amount calculation circuit 19 determines the opening amount of the iris 2 and the gain of the AGC amplifier 4 in accordance with the image pickup signal level. The output of the control amount calculation circuit 19 is supplied to the iris drive circuit 20 and also to the D/A converter 21 . By the iris drive circuit 20, the iris 2
The opening and closing of is controlled. The output of the D/A converter 21 controls the gain of the AGC amplifier 4.

In one embodiment of the present invention, as shown in FIG. 2, weighting coefficients corresponding to the ratio between the peak detection value and the average detection value are stored in the ROM 15. Then, the ratio between the peak detection value and the average detection value is determined by the division circuit 13, and weights are set for each of the peak detection value and the average detection value according to the ratio between the peak detection value and the average detection value. .

[0020] When the brightness of the screen is uniform, the ratio of the peak detection value to the average detection value is approximately equal. As shown in FIG. 2, when the ratio between the peak detection value and the average detection value is close to 1, the mixing ratio of the peak detection values is set to be large. Therefore, the characteristics are better than peak detection, and the entire screen can be brightened.

[0021] If there is a bright portion within the screen, the peak detection value will be larger than the average detection value. As shown in FIG. 2, when the ratio between the peak detection value and the average detection value becomes large, the mixing ratio of the peak detection values is set to be small. Therefore, the characteristics are based on average detection, and it is possible to prevent the entire screen from becoming dark due to the influence of bright areas within the screen.

FIG. 3 shows another embodiment of the invention. In the above embodiment, the ratio between the peak detection value and the average detection value is determined, and the weights for the peak detection value and the average detection value are set according to the ratio between the peak detection value and the average detection value. ing. In contrast, in this embodiment, weights are set for each of the peak detection value and the average detection value, depending on the proportion of the portion of the screen that has a luminance level equal to or higher than a predetermined level.

That is, in FIG. 3, a subject image is formed through a lens 31 through an iris 32 on a light-receiving surface of a CCD image sensor 33. In FIG. The output of the CCD image sensor 33 is supplied to an A/D converter 36 via an AGC amplifier 34. The output of the A/D converter 36 is supplied to a signal processing circuit 37 and also to an area setting circuit 38. The output of the area setting circuit 38 is supplied to an average detection circuit 39 and also to a peak detection circuit 40. Further, the output of the A/D converter 36 is supplied to the histogram circuit 43 .

The histogram circuit 43 divides the screen into a plurality of areas, counts the number of samples having a luminance level equal to or higher than a predetermined level in each area, and detects the luminance distribution state. The output of this histogram circuit 43 is supplied to the ROM 45. The ROM 45 stores a mixture ratio of peak detection values according to the brightness distribution state. When the brightness distribution state is given from the histogram 43, a weighting coefficient KP for the peak detection value corresponding to this brightness distribution state is output from the ROM 45.

Weighting coefficient KP output from ROM 45
is applied to the multiplication circuit 42. Further, the weighting coefficient KP outputted from the ROM 45 is supplied to the subtraction circuit 46. The subtraction circuit 46 performs the calculation (1-KP) to obtain a weighting coefficient KA for the average detected value. This weighting coefficient KA is supplied to a multiplication circuit 41.

The outputs of the multiplier circuits 41 and 42 are added to the adder circuit 4.
4. Multiplication circuits 41 and 42, addition circuit 44
As a result, the average detection value output from the average detection circuit 39 and the peak detection value output from the peak detection circuit 40 are weighted and added. The output of the adder circuit 44 is supplied to a comparator 47. A predetermined reference level is set in the comparator 47. The output of the coparator 47 is supplied to a control amount calculation circuit 49 via a loop filter 48.

A control amount calculation circuit 49 determines the opening amount of the iris 32 and the gain of the AGC amplifier 34 in accordance with the image pickup signal level. The output of the control amount calculation circuit 49 is supplied to the iris drive circuit 50, and the D/A converter 5
1. An iris drive circuit 50 controls opening and closing of the iris 32. The output of the D/A converter 51 controls the gain of the AGC amplifier 34.

In this embodiment, the ROM 45 stores weighting coefficients corresponding to the brightness distribution state. Then, the number of samples with a luminance level equal to or higher than a predetermined level is determined by the histogram circuit 43, and weights are set for each of the peak detection value and the average detection value according to the number of samples with a luminance level equal to or higher than the predetermined level.

When the brightness of the screen is uniform, the number of samples of brightness levels equal to or higher than a predetermined level in each area detected by the histogram circuit 43 will all be almost equal. At this time,
The mixing ratio of peak detection values is set to a large value. For this reason,
The characteristics are better than peak detection, and the entire screen can be brightened.

[0030] If there is a bright portion within the screen, the number of samples with a luminance level equal to or higher than a predetermined level in each area detected by the histogram circuit 43 will differ. At this time, when the ratio between the peak detection value and the average detection value becomes large, the mixing ratio of the peak detection value is set to be small. Therefore, the characteristics are based on average detection, and it is possible to prevent the entire screen from becoming dark due to the influence of bright areas within the screen.

In the above-described embodiment, exposure control is performed by one exposure detection, but the present invention can be similarly applied to a case where a plurality of exposure detection areas are provided.

[0032]

[Effects of the Invention] According to the present invention, weighting coefficients corresponding to the brightness distribution state are stored in the ROMs 15 and 45, and weights are set for each of the peak detection value and the average detection value according to the brightness distribution state. . That is, when the brightness of the screen is uniform, the mixing ratio of the peak detection value is set to a large value, and when there is a portion of high brightness in the screen, the characteristic is set to be better than the average detection value. Therefore, if the brightness of the screen is uniform, the entire screen can be made brighter, and if there is a bright area on the screen, the entire screen will not become dark due to the influence of the bright area. can.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the present invention.

FIG. 2 is a graph used to explain one embodiment of the present invention.

FIG. 3 is a block diagram showing another embodiment of the invention.

FIG. 4 is a block diagram showing an example of a conventional AGC circuit.

FIG. 5 is a schematic diagram used to explain a conventional AGC circuit.

FIG. 6 is a waveform diagram used to explain a conventional AGC circuit.

[Explanation of symbols]

1,31 Lens 2,32 Iris 4,34 AGC amplifier 9,39 Average detection circuit 10,40 Peak detection circuit 11, 41 Multiplication circuit 12, 42 Multiplication circuit 13 Division circuit 15,45 ROM 43 Hist circuit

Claims (3)

[Claims] 1. An exposure control circuit for a video camera, which detects an imaging signal level and controls an iris opening and/or an AGC gain according to the imaging signal level, comprising: a peak value of the imaging signal level; an average detection circuit that detects the average value of the imaging signal; a weighted addition means that weights and adds the peak detection value and the average detection value of the imaging signal, respectively; and a brightness distribution of the imaging screen. and a brightness distribution state detection means for detecting a state, and a weight for the peak detection value and a weight for the average detection value are respectively set according to the brightness distribution state detected by the brightness distribution state detection means. Video camera exposure control circuit. 2. The brightness distribution state detection means calculates a ratio between a peak detection value of the imaging signal and an average detection value of the imaging signal, and calculates a ratio between the peak detection value of the imaging signal and the average detection value of the imaging signal. 2. The exposure control circuit for a video camera according to claim 1, wherein the brightness distribution state is detected from the ratio of . 3. The brightness distribution state detecting means determines a percentage of an area in the imaging screen that is at a predetermined level or higher, and detects a brightness distribution state from the ratio of the area in the imaging screen that is at a predetermined level or higher. 2. The exposure control circuit for a video camera according to claim 1.