JPS63126371A - Automatic sensitivity control circuit for video camera - Google Patents

Abstract

PURPOSE:To instantaneously obtain a stable output video and to reduce a consuming power by charging a capacitor instantaneously by an accelerating charge circuit after a power source is charged, instantaneously responding to a detection signal to operate a gain control circuit and turning off the accelerating charge circuit at the time of reaching a steady state. CONSTITUTION:When the power source of a video camera which is not shown is charged, a DC voltage is supplied to the power source terminal 23b of the accelerating charge circuit 23, a charge current is passed to an earth line through the capacitor 23c and a resistance 23d and the base of a transistor 23a connected to the resistance 23d is biased. Accordingly, the transistor 23a is turned on and the current is passed through a collector/emitter from the terminal 23b and the capacitor 23c is charged. At this time, a charging voltage is made higher than the detection signal through a diode 22, it is fed back to the gain control circuit 21, so that the output video signal is started from a gain suppressed state. Thereafter, when the charging is completed, the voltage of the resistance 23d is decreased and the transistor 23a is turned off.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an automatic sensitivity control circuit for a video camera that automatically controls the video signal level in the video camera.

[Conventional technology]

FIG. 2 is a block diagram showing the configuration of a conventional general video camera. In FIG. 2, a lens 1 is configured to form an optical image on an image sensor 2, and this image sensor 2
is driven by a drive circuit 3. A process circuit 4 is connected to the output terminal of the image sensor 2, and an automatic sensitivity control circuit 5 (hereinafter referred to as
(referred to as an AGC circuit) is provided.

The output of the AGC circuit 5 is output to a luminance signal processing circuit 6 and a color signal processing circuit 7, and then output as a luminance signal and a color signal, respectively.

Further, 8 is an auto iris circuit branch-connected from the process circuit 4, and its output is the aperture vl provided in the lens 1.
connected to a.

FIG. 3 is a diagram showing details of the AGC circuit 5 of FIG. 2. In the figure, 11 is a gain control circuit whose input side is connected to the output of the process circuit 4, and whose output side is connected to the luminance signal processing circuit 6 and the color signal processing circuit 7, and to the input side of the detection circuit 120. connected to the detection circuit 12
The output side of is connected to the input side of the gain control circuit 11 in a feedback manner.

The detection circuit 12 includes resistors 12b, 12d, and a diode 12a.
1 capacitor 12c, the gain control circuit 1
1 output is via diode 12a and resistor 12bt-,
It is grounded through a parallel circuit of a charging capacitor 12c and a discharging resistor 12d, and outputs to the luminance signal processing circuit 6 and color signal processing circuit 7.

Next, the operation will be explained. First, in FIG. 2, light incident from a lens 1 forms an optical image on the imaging surface of an image sensor 2, and the optical image is converted into an electrical signal by the image sensor 2 driven by a drive circuit 3. Process circuit 4
sent to.

The signal that has passed through the process circuit 4 is subjected to gain control in the AGC circuit 5, and then separated into a luminance signal component and a color signal gray component by a filter or the like. and is output as a color signal.

The auto iris circuit 8 detects the signal level of the process circuit 4 and adjusts the range/e1 so that the signal level is constant.
The aperture 1a is controlled.

Here, the AGC circuit 5 will be explained in more detail with reference to FIG. The signal from the process circuit 4 is sent to the AGC circuit 5
The output signal is outputted to the next stage through the gain control circuit 11't- in the detector, but since this output signal is fed back to the input side of the gain control circuit 11 via the detection circuit 12, the magnitude of the detected signal is The gain of the gain control circuit 11 is varied by , so that the output signal is always constant.

Therefore, within the variable range of the gain control circuit 11, a stable output signal can be obtained even if the input signal fluctuates.

Next, the inside of the detection circuit 12 will be explained in more detail. The diode 12a is for detecting the output signal of the gain control circuit 11. Resistor 12b and capacitor 12
8 is formed by a charging time constant circuit, and the capacitor 12C and resistor 12d form a discharging time constant circuit.

The detection signal from the diode 12a is fed back to the input side of the gain control circuit 11 via these time constant circuits.

By the way, these time constant circuits are used to make changes in the image gradual. That is, in the absence of these time constant circuits, the fluctuation of the detected signal from the diode 12a itself is input to the gain control circuit 11 in real time, so the gain changes each time in response to subtle changes in the input signal. As a result, the image appears to flicker or suddenly change, making the screen extremely difficult to view.

However, by providing a time constant circuit, these changes can be made more gradual. That is, when the detected signal decreases, it is discharged with a time constant of (capacitor 12c x resistor 12d), so that it can be fed back to the input side of the gain control circuit 11 with a gradual change, and when the detected signal becomes high (resistor 12b x By charging with the time constant of the capacitor 12c), gradual feedback can be achieved as well.

As a result, even when the camera is suddenly pointed from a bright subject to a dark subject, for example, the gain can be increased gradually so as not to make the viewer feel uncomfortable.

Note that the AG is used to control the video signal level.
In addition to the C circuit 5, there is an auto iris circuit 8, which functions to control the aperture 1a of the lens 1 to keep the amount of light incident on the image sensor 2 constant.

Therefore, within the range in which the auto iris circuit 8 operates, the output signal level from the image sensor 2 is controlled to be constant, so the input signal to the AGC circuit 5 also does not fluctuate.
No gain change occurs either.

Therefore, the gain of the AGC circuit 5 changes and starts controlling the output video signal when photographing a dark subject where the amount of light incident on the image sensor 2 is less than a predetermined value even if the aperture 1a of the lens 1 is opened. It's time to take a picture.

In this way, when the subject is bright, the auto iris circuit 8 controls the video signal, and when it is still dark, the AGC circuit 5 controls the video signal.

[Problem that the invention seeks to solve]

Since the conventional AGC circuit is configured and operates as described above, the capacitor 12c is in a discharged state at the first stage when the power of the camera is turned on. During charging with the time constant 12c), the feedback signal to the gain control circuit 11 has a charging characteristic that gradually increases from a value lower than a predetermined value.

Therefore, during this charging period, the gain of the gain control circuit 11 is in a high state, and if the subject is bright, a saturation phenomenon can be seen in the image, albeit for a relatively short time, giving an unnatural feeling to the viewer. 90 Problems This invention has been made to solve these problems. During normal shooting, the changes occur slowly like in the conventional device, and only when the power is turned on, it operates instantaneously, eliminating the problem of image start-up. The purpose is to obtain an automatic sensitivity control circuit for a video camera that eliminates naturalness.

[Next method to solve the problem]

The automatic sensitivity control circuit for a video camera according to the present invention has a switch circuit that operates only when the power is turned on for the capacitor of the time constant circuit of the detection circuit, and an accelerated charging circuit that instantly completes charging of the capacitor. It was established.

[For production]

In this invention, the accelerating charging circuit charges the capacitor instantaneously after power is turned on, responds to the detection signal instantaneously, and the gain control circuit also operates instantaneously, and when the power is turned on and the steady state is reached. When the switch circuit of the accelerated charging circuit is turned off, the conventional time constant circuit operates and the video signal changes slowly.

〔Example〕

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of an automatic sensitivity control circuit for a video camera according to the present invention will be described below with reference to the drawings. FIG. 1 is a circuit diagram of one embodiment. In FIG. 1, 5A corresponds to the conventional AGC circuit 5 shown in FIG.

In FIG. 1, gain control circuit 21. Diode 22!1 and resistor 2 in the TA liquid circuit 22 and the detection circuit
The configuration of the resistor 22d for discharging the 2b1 capacitor 22C1 is exactly the same as the conventional AGC circuit explained in FIG.

Further, 23 indicates an accelerated charging circuit, and a transistor 23a
The collector is connected to the power supply terminal 23b, and the emitter is connected to the input terminal of the gain control circuit 210 and the connection point between the capacitor 22c and the resistor 22b of the detection circuit 22.

The pace of the transistor 23a is connected to the connection point between the capacitor 23c and the resistor 23d. This capacitor 2
3c and resistor 23d are connected in series between power supply terminal 23b and ground.

Next, the operation will be explained. When the video camera (not shown) is powered on, the power terminal 23 of the accelerated charging circuit 23
Since the blCDC voltage is supplied, a charging current flows to the ground line through the capacitor 23C and the resistor 23dtl, so the transistor 23a connected to the resistor 23d
The base of is biased.

Therefore, the transistor 23a is turned on, and current flows from the power supply terminal 23b through the collector and emitter 'fc, and the capacitor 22c is charged.

This charging voltage is higher than the detection signal passed through the ninth diode 22, which has a high value almost close to the power supply voltage, and is fed to the gain control circuit 21, so the gain control circuit 21 works in the direction of suppressing the gain. work. therefore,
The output video signal starts from a state where the gain is suppressed.

When the charging power of the capacitor 23c of the accelerated charging circuit 23 is completed, the charging power stops flowing, and the voltage of the resistor 23d also decreases. When this voltage becomes lower than the detection signal level, the transistor 23a becomes reverse biased and turns off.

That is, since the accelerated charging circuit 23 is turned off, the detection circuit 22 operates in exactly the same way as the conventional device.
This is the steady state of operation.

Therefore, since the accelerated charging circuit 23 operates only during the charging period to the capacitor 23c, the capacitor 23c
By selecting an appropriate value for the optical time constant circuit consisting of C and resistor 23d, it is possible to instantaneously transition from a gain-suppressed image to a stable steady image. No unnatural images appear when the gain increases to the maximum.

In addition to the AGC circuit described above, auto iris circuits, auto white balance circuits, etc. are required to respond quickly when the power is turned on and to operate slowly during normal operation, as explained above. The same BK acceleration charging circuit 23' can also be applied.

〔Effect of the invention〕

As explained above, in this invention, the capacitor is charged by the accelerated charging circuit instantly after the '1lic power is turned on, the response to one detection word is instantaneously performed, the gain control circuit is instantaneously activated, and the power is turned on. The switch element of the accelerated charging circuit is turned off when the steady state is reached.
The output video can be stabilized instantly by turning on the video camera's power switch or power saving switch. This is also effective in designing the system to reduce power consumption.

Additionally, the cost of additional circuitry is extremely low since it can be achieved with a small number of combinations of fairly standard components such as transistors, capacitors, and resistors.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a circuit diagram of an embodiment of the automatic sensitivity control circuit of a video camera according to the present invention, FIG. 2 is a block diagram showing the configuration of a conventional video camera, and FIG. FIG. 2 is a circuit diagram showing the internal configuration of an AGC circuit in the video camera shown in the figure. 5A... AGC circuit, 21... Gain control circuit, 22
...Detection circuit, 22a...Diode, 22b, 2
2d. 23d...Resistor, 22c, 23c...Capacitor,
27... Accelerated charging circuit, 23a... Transistor,
23b...Power terminal. In addition, the same reference numerals in the figures indicate corresponding parts.

Claims (1)

[Claims] A gain control circuit that controls the gain of the video signal obtained by the image sensor of the video camera to keep the level of the video signal constant; A detection circuit including a specific circuit for smoothing changes and feeding back to the gain control circuit, and an acceleration charging circuit for accelerating charging to the time constant circuit in order to stabilize the image in a short time when the power is turned on. An automatic sensitivity control circuit for video cameras featuring: