JP2936824B2 - Exposure control device for video camera - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exposure control apparatus for a video camera, and more particularly to an exposure control apparatus for a video camera adapted to a video camera having two kinds of sensitivity adjustment functions of an iris and an automatic gain control (AGC). About.

[0002]

2. Description of the Related Art A conventional exposure control device for a video camera mainly controls an iris opening based on an output signal from an AGC when an object is bright, and outputs an output signal from the AGC when an object is dark. Mainly based on AG
Some control the gain of the C amplifier. In addition, when the subject is dark, there is one that controls the gain of the AGC amplifier together with the iris opening based on the output signal from the AGC (Japanese Patent Publication No. 60-22863).

[0003] This type of exposure control device for a video camera includes:
To perform two types of sensitivity adjustment based on one evaluation value,
Usually, one of the controls is prioritized according to the brightness of the subject. On the other hand, an electric signal photoelectrically converted by an image sensor such as a CCD and amplified by a preamplifier is input as an iris detection signal, and the iris detection signal is processed to obtain a photometric value. And a control system for controlling the AGC to become a reference value for iris, and an electric signal output from an AGC amplifier subsequent to the preamplifier as an AGC detection signal, and processing the AGC detection signal by signal processing. An integrated value corresponding to the brightness of the image is obtained, and the AG value is set so that the integrated value becomes the AGC reference value.
2. Description of the Related Art An exposure control apparatus for a video camera having two independent control systems, that is, a control system for controlling the gain of a C amplifier, is known.

This exposure control device for a video camera uses two independent control systems to control iris opening and AGC.
In order to control the gain of the power amplifier, they can be operated simultaneously. That is, the AGC operates independently on the video signal resulting from the iris adjustment. Normally, when the subject is bright, the video signal is adjusted to a predetermined level by the iris. Therefore, the gain of the latter AGC amplifier is constant, but the circuit operates. Therefore, when the brightness suddenly changes and adjustment by the iris cannot be made in time, adjustment is performed by AGC.

[0005]

In the latter case of the video camera exposure control device, two independent control systems for controlling the iris opening and the gain control of the AGC amplifier operate at the same time. As the adjustment speed increases, the overshoot of the brightness increases, which may result in a visually unpleasant image. This is the AG
This is due to the fact that C is adjusted to the same extent at the same time as the previous stage iris, resulting in excessive adjustment.

The present invention has been made in view of such circumstances, and an object of the present invention is to provide an exposure control apparatus for a video camera capable of reducing an overshoot of a change in brightness caused by a dual operation of an iris and an AGC. Aim.

[0007]

SUMMARY OF THE INVENTION In order to achieve the above object, the present invention converts subject light incident on an image pickup device via a photographic lens and an iris into an electric signal by the image pickup device, and converts the electric signal to a previous signal. A video camera which outputs to a video signal processing circuit via a preamplifier and an AGC amplifier and performs appropriate signal processing by the video signal processing circuit to obtain a required video signal is output from the preamplifier. An iris control means for inputting an electric signal as an iris detection signal, controlling the iris based on the iris detection signal, and an electric signal output from the AGC amplifier as an AGC detection signal; AGC means for controlling a gain of the AGC amplifier based on a detection signal for detection, detection means for detecting brightness of a subject, and If the detected brightness of the subject is equal to or higher than a predetermined brightness at which exposure control can be performed only by the iris, the gain control in the AGC means is performed so that the gain of the AGC amplifier is fixed to a constant value. And a stopping means.

[0008]

According to the present invention, an iris control means for inputting an electric signal output from an image sensor via a preamplifier as an iris detection signal, and controlling the iris based on the iris detection signal; An electric signal output from the AGC amplifier at the subsequent stage of the preamplifier is input as an AGC detection signal, and two control systems of AGC means for controlling the gain of the AGC amplifier based on the AGC detection signal are provided. When the brightness of the subject exceeds a predetermined brightness, AG
The gain control in the C means is stopped.
That is, 2 only when the brightness of the subject is less than the predetermined brightness.
The two control systems are operated simultaneously, and when the brightness of the subject is equal to or higher than a predetermined brightness, only the iris control means is operated.

As a result, a natural image can be obtained in which there is almost no overshoot in brightness change caused by the dual operation of the iris and the AGC.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of an exposure control apparatus for a video camera according to the present invention will be described below in detail with reference to the accompanying drawings. FIG.
1 is a block diagram of a video camera including an embodiment of a video camera exposure control device according to the present invention.

Referring to FIG.
Then, an image is formed on a light receiving surface of an image sensor (CCD) 14 via the iris 12. The CCD 14 accumulates the incident light,
The stored charge is read out in synchronization with a timing pulse applied from the timing generator 16. The electric signal read from the CCD 14 in this way is amplified by the preamplifier 18 and then applied to the AGC amplifier 20, the detection circuit 22, and the LPF (low-pass filter) 24.

An AGC gain control signal, which will be described later, is added to the AGC amplifier 20. The AGC amplifier 20 amplifies the electric signal with a gain controlled by the AGC gain control signal. And output to the detection circuit 28. The video signal processing circuit 26 includes a white balance circuit, a gamma correction circuit, a matrix circuit, an encoder circuit, and the like. After performing predetermined signal processing by these circuits, the video signal processing circuit 26 outputs an NTSC video signal to the recording device 29, for example. The recording device 29 includes a recording circuit,
Here, after converting the video signal into a recording signal suitable for magnetic recording, the recording signal is magnetically recorded on a recording medium such as a video tape via a magnetic head.

On the other hand, a detection circuit 22 for inputting an electric signal from the preamplifier 18 detects an input signal and outputs
0, the integration circuit 30 integrates the input signal for one field, and outputs the integrated value to the iris drive circuit 32 as an average photometric value. An iris reference value is set in the iris drive circuit 32 in advance.
Compares this iris reference value with the average photometric value,
If the average photometric value is larger than the iris reference value, it is determined that the image is too bright, and the iris 12 is driven in the closing direction. On the other hand, if the average photometric value is smaller than the iris reference value, it is determined that it is too dark. Then, the iris 12 is driven in the opening direction.

An exposure correction signal and a drive speed control signal are added to the iris drive circuit 32, respectively, and the iris reference value is corrected by these signals.
Also, the operation speed of the iris drive circuit 32 is controlled,
The details will be described later. Similarly, a detection circuit 28 for inputting an electric signal from the AGC amplifier 20 detects the input signal and outputs the detected signal to an integration circuit 34. The integration circuit 34 integrates the input signal for one field and controls the integrated value by AGC control. Output to the circuit 36.

An AGC reference value is set in the AGC control circuit 36 in advance.
The reference value is compared with the integrated value. If the integrated value is larger than the AGC reference value, it is determined that the image is too bright.
An AGC gain control signal is output to the AGC amplifier 20 so as to reduce the gain of the C amplifier 20. On the other hand, if the integrated value is smaller than the AGC reference value, it is determined that the darkness is too dark. AGC to increase gain
The gain control signal is output to the AGC amplifier 20.

An exposure correction signal and a drive speed control signal are added to the iris drive circuit 32 and the AGC control circuit 36, respectively. The iris reference value and the AGC reference value are corrected by these signals. The operation speeds of the iris drive circuit 32 and the AGC control circuit 36 are controlled. Also, a control stop signal for stopping the gain control is added to the AGC control circuit 36, and details thereof will be described later.

The LPF 24 which receives an electric signal from the preamplifier 18 detects the input signal into a signal indicating a luminance signal. The A / D converter 38 converts the luminance signal that has passed through the LPF 24 into a digital signal, and outputs the converted luminance data to the center-of-screen integration circuit 40 and the entire-screen integration circuit 42. In the screen center integration circuit 40, the window signal generation circuit 44 supplies the screen center in one field (FIG. 2).
A window signal indicating the timing at which the luminance data (in the range indicated by the broken line in FIG. 3) is output is added, and the screen central part integrating circuit 40 uses this window signal to determine the luminance of the central part of the screen in one field of the input luminance data. Only the data is integrated, and the central average luminance data I ₁ obtained by dividing the integrated data by the area in the range of the broken line in FIG. 2 is output to the MPU (microprocessor unit) 46.

Similarly, a window signal indicating the timing at which luminance data of substantially the entire screen (in the range of the dashed line in FIG. 2) is output from the window signal generation circuit 44 is added to the entire screen integrating circuit 42. , the entire screen integration circuit 42 by the window signal, substantially integrated one field of luminance data to be input, the integrated data entire section, divided by the area in the range of one-dot chain line in FIG. 2 the average luminance data I ₂ a
Is output to the MPU 46.

The other input of the MPU 46 includes a Hall element 4
8 through the Hall element amplifier 50 and the A / D converter 52, a signal corresponding to the opening of the iris 12 is output.
And the AGC gain control signal output from the AGC control circuit 36 is added via the A / D converter 54 as AGC gain data.

The MPU 46 receives the input luminance data I ₁ , I
_2. The opening degree data of the iris 12 and the AGC gain data are processed based on a program stored in a ROM (read only memory) 56 and a look-up table described later, and are respectively processed via D / A converters 58 and 60. An exposure correction signal and a driving speed control signal are output. That is,
The MPU 32 first finds the logarithm of the ratio of the central portion average luminance data I ₁ and the whole portion average luminance data I ₂ , D = log (I ₁ / I ₂ ). Also, the opening degree data of the iris 12 and the AGC
A value is calculated by taking the logarithm of the brightness of the subject from the gain data.

[0021] Then, the absolute value | D | if exceeds the threshold Th _D is such that the center-weighted metering (exposure control mode that emphasizes the central section of the screen), the exposure correction signal D / A converter 58 The exposure correction signal is output to the iris drive circuit 32 and the AGC control circuit 36 via the iris drive circuit 32, and the exposure correction signal is limited according to the calculated brightness of the subject so that the exposure correction amount falls within the range of FIG. That is, as shown in FIG. 3, when the brightness of the subject increases, the correction amount of the spot light with respect to the average photometry value decreases, and when the brightness decreases, the correction amount of the backlight against the average photometry value decreases.

Thus, when the luminance distribution in the subject screen is in a backlight or spotlight illumination state, the exposure correction is performed so that the center of the screen is appropriate, but the correction amount is corrected for extremely bright abnormal light. Is reduced to prevent the surroundings from becoming too dark, and it is also possible to prevent the surroundings from becoming too bright due to excessive backlight correction when an extremely dark subject is photographed.

When the correction amount is reduced as described above, the correction amount is changed so as to gradually decrease in accordance with the brightness of the subject, whereby a natural image without a rapid change in exposure can be obtained. Like that. Moreover, | D | average becomes metering in the case of a ≦ Th _D, exposure correction signal becomes a standard value. In the above-described embodiment, the exposure correction amount is reduced when the brightness of the subject increases and decreases. However, the present invention is not limited to this, and the exposure correction amount is reduced only when the brightness of the subject increases. May be reduced, or the exposure correction amount may be reduced only when the brightness of the subject is reduced.

The MPU 46 determines whether or not the iris 12 is within a range where the amount of incident light can be sufficiently adjusted based on the input opening degree data (output of the Hall element) of the iris 12. If the control stop signal is within the adjustable range, the control stop signal is set to A.
The signal is output to the GC control circuit 36, and the gain control in the AGC amplifier 20 is stopped (the gain is fixed to a constant value). Also,
If it is out of the adjustable range, the output of the control stop signal is stopped, and the gain control in the AGC amplifier 20 is performed.

In the case where the iris 12 has a multi-blade configuration, the change in exposure to the driving amount of the iris 12 (corresponding to the output of the Hall element 48) occurs in a portion where the iris 12 is almost open. The volume becomes quite rapid, as shown in the lower part of FIG. Therefore, by turning on / off the output of the control stop signal near the start of the broadening (point A in the figure), the brightness of the iris 12 is adjusted as if it were as indicated by the dotted line in the figure. Is performed more naturally. still,
The iris characteristics are not limited to those shown in FIG. 4 and vary depending on the number of aperture blades and the like. Therefore, the iris opening position (ON / OFF of the control stop signal according to the iris actually used) (brightness of the subject) Is preferably determined.

The MPU 46 controls ON / OFF of the output of the above-mentioned exposure correction and control stop signal, and furthermore, the central part average luminance data I ₁ and the whole part average luminance data I _2.
Are compared before and after a predetermined time has elapsed. Then, it is estimated how the shooting scenes before and after the change have changed. That is, as shown in FIG. 5, with respect to the central part average luminance data I ₁ and the whole part average luminance data I ₂ , respective values at the time point T ₀ are represented by I ₁₀ , I ₂₀ , and at the time point T ₁ after ΔT. Expressing absolute value symbol ABS as the values respectively I _11, I _21, the value of _{K 1 = ABS (I 11 -I} 10) K 2 = ABS (I 21 -I 20) is obtained, these values Each threshold value Th ₁ , Th
Compared to ₂ . The comparison results are classified as shown in Table 1 below. Note that when | D ₀ |> Th _D , this corresponds to the fact that the exposure control was performed by center-weighted photometry at the time of T ₀ , and | D ₀ |
_{The case of 0} | ≦ Th _D corresponds to the fact that exposure control by average photometry has been performed. Table 1 is stored in the ROM 56 as a look-up table.

When these conditions are actually associated with the photographing scene before and after ΔT, the conditions: only the background changes; the main subject moves; the switching of the photographing conditions; Condition: Only the background changes. Condition: The main subject moves to the center of the screen. Condition: Switching of the shooting scene. Condition: It can be estimated that there is almost no change. The sub-classification of the conditions is as follows: "| D ₁ |> | D ₀ |", condition A, "| D ₁ |
> | D ₀ | ”, the condition B is satisfied. As the case where the condition A is satisfied, for example, a scene in which the main subject has moved to the center of the screen is assumed.

After performing the above arithmetic processing, the MPU 46 estimates the state of the photographic scene around the time ΔT and changes the operation speed of the exposure / gain control. That is, the operation speed of the iris 12 and the AGC amplifier 20 is reduced when the shooting scene is determined to be the condition, B,..., And the iris 12 and the AGC are determined when the shooting scene is determined to be the condition A. Amplifier 20
A drive speed control signal is output to the iris drive circuit 32 and the AGC control circuit 36 via the D / A converter 60 so that the operation speed of the iris drive becomes faster. Note that the drive speed control signal is a control signal such as how many EVs can be moved in one second. For example, the operation speed is high when the speed is 10 EV / second, and when the speed is 0.5 EV / second. The operating speed is slow.

The operation of the exposure control device for a video camera constructed as described above will be described with reference to the flowchart shown in FIG. Exposure / gain control is started by the start of recording, and at the same time, the central part average luminance data I ₁ and the whole part average luminance data I ₂ are subjected to divided photometry. The first exposure and gain control by averaging metering is performed based on the entire portion average luminance data I _2. In this case, the exposure correction signal is not output from the MPU 46, so that the adjustment is made so that the total amount of light received by the CCD 14 is constant.
In this case, a high-speed signal is output from the MPU 46 as a drive speed control signal. As described above, when the recording is started, the exposure and the gain are instantaneously adjusted in the average metering mode, so that the exposure amount of the main subject can be prevented from being extremely inappropriate.

Subsequently, the value of D is calculated and compared with the value of Th _D. When “| D |> Th _D ”, the central part average luminance data I ₁ and the whole part average luminance data I ₂ are largely different, and there is a high probability that the main subject is located at the central part of the screen. In consideration of this, when exposure control is performed in the average photometry mode as it is, the exposure amount of the main subject often becomes inappropriate. Therefore, "| D |>
When it is determined that Th _D ", together with the flag F _C is set indicating the transition to the center-weighted metering mode,
The exposure correction signal corresponding to the central average luminance data I ₁ is MP
A driving speed control signal output from the U46 via the D / A converter 58 and increasing the operation speed is output from the MPU 46 to the D / A converter 58.
Output via the / A converter 60. In addition, “| D | ≦ T
as it is maintained at the average metering mode at the time of the h _D ".

After the exposure / gain control is performed in this manner,
ΔT is the time after which division photometry performed again, relative to the total portion average luminance data I ₂₀ and the central portion average luminance data I ₁₀ obtained in the previous segment metering, new central portion average brightness as shown in FIG. 5 data I ₁₁ and the entire portion average luminance data I ₂₁ is detected. Then, from the absolute value of the difference between the respective luminance data, K
Value of ₁ and K ₂ are determined. The values of K ₁ and K ₂ are compared with respective threshold values Th ₁ and Th ₂ and classified according to Table 1 described above. Further, the ΔT time is set to CCD1.
4 can be made to coincide with the field readout cycle. In this case, it is not necessary to provide a separate timer for measuring ΔT.

When the values of K ₁ and K ₂ are obtained in this way,
Before and after the elapse of the ΔT time, the state of the shooting scene is estimated. Under the center-weighted metering mode, when the condition, that is, both the center average luminance data I ₁ and the whole-part average luminance data I ₂ greatly changed, the shooting scene was switched by, for example, panning a video camera greatly. Therefore, after canceling the center-weighted metering mode, the exposure / gain control is restarted in the average metering mode for a new scene.

In the case where only the central portion average luminance data I ₁ largely changes, the entire portion average luminance data I ₂ does not change much and | D ₁ |> | D ₀ | a scene as the main subject has moved to the center of the screen at the time, high-speed exposure-gain control on the basis of the central portion average luminance data I ₁₁ that is newly detected while the center-weighted metering mode in this case Let me do it. still,
Central part average luminance data I ₁ , whole part average luminance data I ₂
If both of them hardly change, it means that the same scene with the same luminance distribution is being shot continuously, so that the center-weighted metering mode is continued and the exposure / gain control is performed at high speed. You should let it.

In conditions other than the condition A and the condition in the center-weighted metering mode or the conditions in the average metering mode, whether the main subject is moving in the subject screen shown in FIG. By panning the video camera in accordance with the movement of the subject, it is estimated that the scene is a scene in which only the luminance changes in the background. In such a case, the MPU 46 outputs a drive speed control signal for reducing the operation speed of the exposure / gain control via the D / A converter 60. Thereby, the iris drive circuit 32 and A
The GC control circuit 36 operates slowly, and the exposure amount of the main subject does not change rapidly.

As described above, when continuity is recognized in the photographic scene, the operation speed of the exposure / gain control is set to be lower than the exposure / gain control at a high speed following the change in the luminance of the background. The exposure amount is stabilized.
Of course, by performing such control, background depiction is sacrificed to some extent, but in a continuous scene, maintaining a stable exposure of the main subject provides a natural-looking image during image reproduction.

The determination of the continuity of the photographic scene is not limited to the above embodiment, but may be performed based on a spatial frequency component in the luminance signal. For example, when focusing is performed, a BFP having a pass bandwidth of 600 KHz to 2.4 MHz, which is a spatial frequency component of a general subject.
(Band pass filter) is used in place of the LPF 24 in FIG. 1, and the continuity of the photographed scene can be determined based on the output data of the screen center integration circuit 40 at this time. That is, the auto-focusing device mounted on the video camera sets an area close to the area surrounded by the broken line in the center of the screen shown in FIG. 2 as the distance measurement area, and the photographing lens 10 focuses on this area. Is going. Since the BPF passes through the spatial frequency components that increase during focusing, the output data from the center-of-screen integrating circuit 40 is the integral value of the spatial frequency components in the focused area.

The integrated value of the spatial frequency component is compared before and after the ΔT time as described above, and when there is a large change, it can be estimated that the object to be measured, that is, the main subject has changed. Therefore, in this case, the operating speed of the exposure / gain control may be increased, and when the integrated value of the spatial frequency component does not change much, the operating speed of the exposure / gain control may be reduced.

If the angle of view of the photographing lens 10 is θ, the size of the main subject in one direction in the subject screen is L, and the distance to the main embodiment is X, L∝1 / (tan θ · X ) There is a relationship. Therefore, this value of L is used as the amount of change, and ΔT
It is also possible to presume that a scene change has occurred when the change has changed beyond a predetermined range before and after the time.

Incidentally, even if the video camera itself maintains the same posture, the brightness distribution in the screen may be greatly changed by zooming. In such a case, it is natural that the scene is substantially switched.
It is preferable to increase the operation speed of the exposure / gain control. In this case, the switching of the scene can be determined by detecting a temporal change amount of the zoom position of the photographing lens 10.

The exposure control device for a video camera according to the present invention can be applied not only to a video movie camera but also to an electronic still video camera.

[0041]

As described above, according to the exposure control apparatus for a video camera according to the present invention, only when the brightness of the subject is less than the predetermined brightness, two independent iris control systems and AGC systems are provided. Since the control system is operated simultaneously and only the iris control means is operated when the brightness of the subject is equal to or higher than a predetermined brightness, the iris and the AGC
There is an advantage that a natural image can be obtained with almost no overshoot of a change in brightness caused by the double operation.

[Brief description of the drawings]

FIG. 1 is a block diagram of a video camera including an embodiment of an exposure control device for a video camera according to the present invention.

FIG. 2 is an explanatory diagram illustrating an example of a subject screen.

FIG. 3 is a graph showing a range of an exposure correction amount with respect to the brightness of a subject.

FIG. 4 is a graph showing switching points between iris characteristics and AGC control.

FIG. 5 is a graph showing an example of a change in photometric value in each photometric area.

FIG. 6 is a flowchart used to explain the operation of the embodiment shown in FIG. 1;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 ... Photographing lens 12 ... Iris 14 ... Image sensor (CCD) 18 ... Preamplifier 20 ... AGC amplifier 22, 28 ... Detection circuit 26 ... Video signal processing circuit 30, 34 ... Integration circuit 32 ... Iris drive circuit 36 ... AGC Control circuit 40 ... Screen central part integration circuit 42 ... Screen whole integration circuit 44 ... Window signal generation circuit 46 ... MPU

Claims (1)

(57) [Claims] 1. An image pickup device converts object light incident on an image pickup device via a photographing lens and an iris into an electric signal, and outputs the electric signal to a video signal processing circuit via a preamplifier and an AGC amplifier. The video signal processing circuit appropriately performs signal processing to obtain a required video signal. An electric signal output from the preamplifier is input as a detection signal for iris, and the detection for iris is performed. An iris control unit for controlling the iris based on a signal; an electric signal output from the AGC amplifier being input as an AGC detection signal; and controlling a gain of the AGC amplifier based on the AGC detection signal. and AGC means, a detecting means for detecting the brightness of the subject, the brightness of the subject detected by the detecting means the
Above a certain brightness that can be controlled only by iris
In the case of, the gain of the AGC amplifier is fixed at a constant value.
Means for stopping the gain control in the AGC means so as to be performed .