JP2667721B2 - Video camera - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video camera, and more particularly, to an improvement in backlight correction of a video camera.

[Prior Art] FIG. 3 is an example of a circuit configuration of a video camera.

In FIG. 1, image light from a subject (not shown) passes through an imaging lens 1 and an iris 2 to an imaging device 3, for example,
It is supplied to the CCD solid-state imaging device. The output signal of the image pickup device 3 is supplied to a process circuit 5 after automatic gain control is performed by an AGC circuit 4, and the luminance signal Y and the color difference signal (red color difference signal RY, blue color difference signal BY) is output.

The luminance signal Y output from the process circuit 5 is supplied to the gamma correction circuit 6. In this case, as shown in FIG. 4, if the gamma value γ1 of the image sensor 3 is 1, the gamma value γ3 of the cathode ray tube is 2.2, and the gamma value γ2 of the gamma correction circuit 6 is 1/2. The correction is made to be 0.45.

As a result, the total gamma value γ becomes γ1, γ2, γ3 ≒ 1.
Is corrected so that the input light to the image sensor 3 and the output light of the CRT correspond linearly (see broken line a in FIG. 5). In FIG. 5, the dashed line b indicates the gamma characteristic (γ3 = 2.2) of the CRT, and the solid line c
Indicates the gamma correction characteristic (γ2 = 0.45) of the video camera.

The luminance signal Y output from the gamma correction circuit 6 is supplied to an encoder 8 via a gain amplifier 7. Also,
The color difference signals RY and BY output from the process circuit 5 are supplied to the encoder 8.

In the encoder 8, a synchronization signal is added to the luminance signal Y, and a burst signal is added and color modulation is performed to the color difference signals RY and BY to perform the carrier chrominance signal C.
Are formed, and the luminance signal Y and the carrier chrominance signal C
Are added to form a color video signal SV.

The color video signal SV formed by the encoder 8 is supplied to an output terminal 9 derived from the encoder 8.

By the way, the opening of the iris 2 is automatically controlled according to the light amount level of the subject, even if not described above. For this reason, when capturing an object with a high contrast (high luminance around the dark part and a large light amount level), the contrast in the dark part cannot be obtained, and so-called blackout occurs.

Backlight correction is performed to prevent such blackouts. At the time of backlight correction, a control signal SBL is supplied from the terminal 10 to the iris 2 and the AGC circuit 4, and control is performed so that the iris 2 opens and the gain of the AGC circuit 4 increases. As a result, the level of the luminance signal Y is increased, so that it is possible to obtain a sufficient contrast in the dark portion and avoid the blackout condition.

[Problems to be Solved by the Invention] However, as described above, the opening of the iris 2 and the AGC
According to the circuit that controls the gain of the circuit 4 to increase, not only the low-level portion of the luminance signal Y corresponding to the dark portion but also the high-level portion increases at a similar rate, so that the high-level portion is saturated. As a result, so-called overexposure often occurred.

Therefore, in the present invention, it is possible to perform backlight correction while suppressing the occurrence of overexposure.

[Means for Solving the Problems] A video camera according to the present invention comprises: a process circuit for processing an output signal of an image sensor to form a first video signal;
A clip circuit that clips the first video signal at a predetermined clip level, a gain control amplifier that controls the level of the output signal of the clip circuit, and a second video signal that is obtained by adding the output signal of the gain control amplifier to the first video signal. An adder for obtaining two video signals is provided, and the gain of the gain control up is set to 0 at the time of normal imaging, and is set to be larger than 0 at the time of backlight correction according to the degree of correction.

[Operation] At the time of backlight correction, the gain of the gain control amplifier becomes larger than 0, and the output signal of the clipping circuit is supplied to the adder via the gain control amplifier. It is increased from the signal. In this case, when the level of the first video signal is low, the first video signal is supplied to the adder via the gain control amplifier without being clipped by the clipping circuit, and the rate of increase is large. On the other hand, when the level of the first video signal is large, the first
Is supplied to the adder via the gain control amplifier after being clipped by the clipping circuit, and the rate of increase is small.

Therefore, when the level of the first video signal is low, the rate of increase in the level of the second video signal relative to the first video signal is large. Can be taken sufficiently, and it is possible to avoid a blackout condition as in the conventional backlight correction. Further, when the level of the first video signal is large, the rate of increase in the level of the second video signal relative to the first video signal is small. It is possible to reduce the occurrence.

Embodiment An embodiment of the present invention will be described below with reference to FIG. In FIG. 1, parts corresponding to those in FIG. 3 are denoted by the same reference numerals, and detailed description thereof will be omitted.

In this example, the luminance signal Y output from the process circuit 5 is supplied to the adder 11.

Further, the luminance signal Y is supplied to the peak clipping circuit 12 and subjected to clip processing.

For example, when the luminance signal Y supplied to the peak clipping circuit 12 is as shown in FIG.
12 outputs a cut signal of a portion higher than the clip level VCL as shown in FIG. In this case, the clip level VCL can be set arbitrarily, but is set to, for example, about 10% of the maximum level of the luminance signal Y.

The output signal of the clip circuit 12 is supplied to the adder 11 via the gain control amplifier 13. Then, the output signal of the adder 11 is supplied to the gamma correction circuit 6.

The amplifier 13 is supplied with a gain control signal SGC, and the gain is controlled. In this case, the gain of the amplifier 13 is set to 0 when no backlight correction is performed (at the time of normal imaging). On the other hand, when the backlight correction is performed (at the time of backlight correction), the gain is set to 0 according to the degree of correction. Be made larger.

This embodiment is configured as described above, and the signal of the luminance signal Y supplied to the gamma correction circuit 6 does not increase because the signal is not supplied from the amplifier 13 to the adder 11 during normal imaging. For example, when the luminance signal Y output from the process circuit 5 is as shown in FIG. 2A, the signal supplied to the gamma correction circuit 6 is as shown by a broken line in FIG.
The output signal of the gamma correction circuit 6 is as shown by a broken line in FIG.

At the time of backlight correction, the adder 11
Since the signal (shown in FIG. C) is supplied, the level of the luminance signal Y supplied to the gamma correction circuit 6 increases. For example, the luminance signal Y output from the process circuit 5
However, when it is as shown in FIG. 4A, the signal supplied to the gamma correction circuit 6 is as shown by a solid line in FIG. 4D, and the output signal of the gamma correction circuit 6 is as shown by a solid line in FIG. Become.

Thus, according to this example, at the time of backlight correction,
Since the output signal of the amplifier 13 is added to the luminance signal Y by the adder 11, the level of the luminance signal Y increases. for that reason,
When capturing an image of a subject having a high contrast, the contrast of a dark portion can be sufficiently obtained, and a blackout condition can be avoided as in the conventional backlight correction.

In this case, when the level of the luminance signal Y is low as in a dark portion of the screen, the luminance signal Y is added to the original luminance signal Y via the amplifier 13 without being clipped by the clipping circuit 12. On the other hand, when the level of the luminance signal Y is large, the luminance signal Y is clipped by the clip circuit 12 and then added to the original luminance signal Y via the amplifier 13. Therefore, the rate of increase in the portion where the level of the luminance signal Y is large is smaller than the rate of increase in the portion where the level of the luminance signal Y is small, so that overexposure occurs when backlight correction is performed as compared with the conventional backlight correction. Can be reduced.

[Effects of the Invention] As described above, according to the present invention, when performing backlight correction, the rate of increase in a portion where the level of a video signal is high is smaller than the rate of increase in a portion where the level of a video signal is low. Therefore, the occurrence of overexposure can be reduced as compared with the conventional backlight correction, and the image quality at the time of backlight correction can be improved.

[Brief description of the drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, FIG. 2 is an explanatory diagram of its operation, FIG. 3 is a block diagram of a conventional example, and FIGS. 4 and 5 are explanatory diagrams of gamma correction. 2 ... Iris 3 ... Imaging element 4 ... AGC circuit 5 ... Process circuit 6 ... Gamma correction circuit 7 ... Encoder 9 ... Output terminal 11 ... Adder 12 ... Peak clip circuit 13 ... Gain control Amplifier

Claims (1)

(57) [Claims] 1. A process circuit for processing an output signal of an image sensor to form a first video signal, a clip circuit for clipping the first video signal at a predetermined clip level, and an output signal of the clip circuit A gain control amplifier that controls the level of the first video signal and an adder that adds the output signal of the gain control amplifier to the first video signal to obtain a second video signal. A video camera, which is set to 0 at the time of imaging and is set to be larger than 0 at the time of backlight correction according to the degree of correction.