JPH0584106B2 - - Google Patents

Description

[Detailed Description of the Invention] [Field of Application of the Invention] The present invention relates to a signal processing circuit for a video camera, and in particular to a detection circuit that generates control voltages for an automatic iris (auto iris) circuit and an automatic gain control (AGC) circuit. .

[Background of the invention]

Conventionally, video cameras have generally been equipped with an auto-iris circuit or an AGC circuit to optimize the amount of incident light or signal for the dynamic range of the circuit. The voltages that control each of the above circuits are obtained by detecting the video (luminance) signal and rectifying and smoothing (detecting) it, but if necessary, an average detected voltage can be obtained depending on the video content to improve the image quality. The brightness is controlled to an average brightness, the peak detection voltage corresponding to high brightness is obtained to optimize the high brightness area, or a mixed voltage of both is obtained. Figure 1 shows an example of a conventional one-time configuration.
A conventional example will be explained below using FIGS. 1 and 2.
In FIG. 1, 1 is a lens device and 2 is an iris mechanism. Reference numeral 3 denotes an image pickup device which outputs a video signal, which is amplified by a preamplifier 4, and then subjected to various signal processing such as clamping, addition of a basic component, and blanking by a signal processing circuit 5. Thereafter, the AGC circuit 6 optimizes the signal level and outputs it to the terminal 7. After that, although not shown, various camera signal processing is performed separately. Of the video signals obtained at terminal 7, a signal corresponding to approximately the center of the screen is extracted by a gate pulse applied to terminal 8 of gate circuit 9, and the extracted signal is sent to a peak value detection circuit. 10, each detected by the average value detection circuit 11, and the addition circuit 12
for the iris by adding in the desired ratio according to
Obtains control voltage for AGC. The control voltage obtained by detection is amplified by the amplifier circuit 13 and applied to the AGC and iris switching circuit 14. When the amount of incident light is relatively small, this switching circuit 14 first applies a control voltage to the AGC circuit 6 such that the gain attenuates from the maximum gain state as the amount of incident light increases, and after completing the predetermined gain control, When the amount of light incident on the imaging device 3 increases, a control voltage is applied to the iris drive circuit 15 to gradually close the iris mechanism 2 from the open state, thereby controlling the amount of light incident on the image sensor 3. The level of the video signal obtained at terminal 7 as explained above is
The amount of incident light is controlled to a predetermined value over a wide range.

The role of the gate circuit 9 will be explained below. In general, video camera photographers position the subject they want to photograph in the center of the screen. For example, if they want to photograph a person, they position the person's face in the center of the screen as shown in Figure 2. When operating a camera, the contrast ratio between various subjects within the imaging screen is often uniform, so there is nothing unnatural about detecting video signals over the entire area of the imaging screen and controlling the iris and AGC. do not have. However, as shown in Figure 2, when the sky is photographed in the background of a person, for example in the upper part A of the screen, or in a backlit situation, or when a light source such as a fluorescent lamp is photographed directly behind the person indoors, etc. Photographing situations often occur in which the contrast ratio of the object to be photographed is relatively large. In the case of such an influence situation, in the conventional control method, the video signal level is controlled so as to optimize the amplitude corresponding to the background pattern A, which is dominant in the video level, as shown in Figure 3a. The subject I originally wanted to photograph is as shown in section B.
Compared to the rated level, the image becomes black and dull, making the photographed image extremely unnatural. If a gate circuit 9 is provided here to detect the video signal at the center B of the screen as shown by the broken line in FIG.
As shown in B', even under the above situation, the video signal is optimized for the subject to be photographed. In this case, the high brightness area A' in the background that exceeds the rated level is removed by the white peak clipping process of the camera signal processing circuit installed after terminal 7.
Clipped by ~130%. Clipping the high-brightness area is a normal signal processing means in video cameras, and since the image is originally high-brightness (white image), the captured image does not seem very unnatural. Therefore, as described above, the gate circuit 9 is quite effective for photographing purposes. However, as shown in part C of Fig. 3c, the subject D to be photographed
If the contrast ratio between the two areas is abnormally large, the image sensor will have insufficient beam current in the highlighted areas, causing a phenomenon called comet tail (white trailing when the camera pans) and blooming, resulting in image deterioration. The degree of this image quality deterioration increases as the highlight becomes more prominent. As a countermeasure to this problem, the beam current automatically follows the highlighted area, and ABO (Automatic Beam
Although it is possible to solve this problem using a circuit called a "optimizer", the scale and design of the circuit are not so easy, and a considerable amount of consideration and expense must be invested. Therefore, it cannot be used in inexpensive home video cameras. Further, even if an ABO circuit is adopted, it is impossible to completely suppress the above phenomenon considering the actually existing contrast ratio.

[Purpose of the invention]

An object of the present invention is to provide a control device for iris control or AGC control that improves the above-mentioned drawbacks of the prior art and is less likely to cause comet tail or blooming.

[Summary of the invention]

In order to achieve the above object, the present invention provides a signal whose peak value exceeds a predetermined level (Vth in FIG. The iris AGC control voltage is obtained by adding an appropriate amount of this detection signal to the conventional detection control voltage described above. In other words, even for abnormally large highlight signals around the screen (outside the broken line in Figure 2), for pictures that exceed a predetermined level Vth, the iris and AGC are controlled to some extent, and the signal shown in Figure 3 is As shown in parts C and D', as shown in part C and part D', an abnormally large comment table is created for the highlight part C, and the image part D' to be photographed is made black (dark) to the extent that blooming does not occur. ) The level of the video signal is appropriately controlled to the extent that it does not become excessively high, thereby obtaining a good captured image. This predetermined level Vth is set to an appropriate value larger than the rated level, but it is convenient to choose, for example, about twice the rated level.

[Embodiments of the invention]

An embodiment of the configuration of a detection circuit according to the present invention will be described below with reference to FIG.

In this figure, circuit blocks with the same symbols as in FIG. 1 indicate the same functions and operations. In Figure 4
The video signal obtained at the output terminal 7 of the AGC circuit 6 is guided to various camera signal processing circuits 19 such as a white peak clipping circuit, a γ correction circuit, etc., which follow the subsequent stage, and detects the signal at the center of the screen (or Signals outside the center of the screen may be weighted by clipping them at a predetermined level, etc.) and are applied to a gate circuit 9 and a highlight signal detection circuit 16 that detects only signals exceeding the predetermined level. ,
Each signal is detected. The output signal of the gate circuit 9 is sent to a peak detection circuit 10 and an average value detection circuit 1.
1, the output signal of the highlight signal detection circuit 16 is subjected to peak detection and average value detection, respectively, by the peak detection circuit 17,
The signals are added and mixed at an appropriate ratio and output as a control voltage to the output of the adder circuit 20. According to this circuit configuration, when the contrast ratio between various subjects in the shooting screen is almost uniform over the entire screen, or when the video signal is below a predetermined highlight detection level (Vth), , the iris circuit and the AGC are controlled only by the control voltage obtained based on the video signal at the center of the screen extracted by the gate circuit 9.
The circuit is controlled to optimize the video signal level for most shooting situations. As mentioned above, if abnormal highlights occur in the peripheral area of the screen, set the level (Vth) to an appropriate value, for example, by detecting a highlight signal that is at least twice the rated level. The applied voltage can also be controlled at a predetermined ratio, suppressing comet tails and blooming to obtain an optimal image. Note that although the above explanation is for the case where the gate circuit 9 is provided,
Even if the gate circuit 9 is omitted, the effects of the present invention can be obtained to some extent.

Next, a specific example of a circuit according to the present invention is shown in FIG. 9, 11, 20, 10, indicated by broken lines in this figure
Each block 17 and 16 corresponds to the portion indicated by the same reference numeral in FIG. Transistors Q ₁ , Q ₂ , Q ₃ ,
A gate circuit is configured by the _Q4 diode D ₁ and resistors R ₁ and R ₂ . A positive gate pulse is applied to the base of the transistor _Q2 , and the level is high (H) for a period corresponding to the center of the screen. In other words, at H level, H > E _R with respect to the base bias voltage E _R of transistor Q ₁ , and transistor Q ₂ is ON.
As a result, transistor _Q3 is turned off. Therefore, during the H level period, the video signal input to the base of the transistor _Q4 is directly generated across the resistor _R2 . Next, during the period corresponding to the area outside the center of the screen, a low (L) level is applied to terminal 8.
E _R >L, transistor _Q1 turns on and _Q2 turns off
do. Therefore, the level shifting diode
A voltage E _R is level-shifted by D ₁ and applied to the base of the transistor Q ₃ . Here terminal 8
If the level of the video signal obtained is higher than E _R - E _B (black level), transistor _Q4 turns OFF,
Signal levels above this are clipped and E _R −
For a relatively small amplitude level signal below E _B , a video signal of the same level is generated across the resistor R ₂ . In other words, arbitrary weighting can be performed by setting the level of _ER . The video signal generated across the resistor _R2 is applied to the base of the transistor _Q5 , and is smoothed by the transistor _Q5 , resistors _R3 , _R7 , and capacitor _C1 , and an average detected voltage is generated across _C1. . Furthermore, the video signal generated across the resistor R ₂ is also applied to the base of the transistor Q ₆ , and the peak value that follows the peak value of the signal applied to the base of the transistor Q ₆ is generated by the transistor Q ₆ and the capacitor C ₂ . A value detection voltage is generated across capacitor _C2 . On the other hand, the video signal obtained at terminal 7 is divided into predetermined levels by resistors R ₅ and R ₆ and then applied to the base of transistor Q ₇ .
The peak is detected in the same way by Q ₇ and C ₂ . That is, depending on the division ratio of the resistors R ₅ and R ₆ , it is possible to detect and detect a highlight signal higher than an arbitrarily determined level. By setting the resistor _R4 , each of the detected voltages described above can be added at an arbitrary ratio, and a control voltage having the characteristics of the present invention can be obtained at the terminal 21. As can be seen from this embodiment, the highlight signal detection section, which is the main part of the present invention, consists of transistors Q ₇ ,
It is extremely simple and can be realized using only resistors R ₅ and R ₆ .
It goes without saying that a parabolic waveform voltage or the like may be applied to the gate voltage application terminal 8 of the gate circuit 9 for weighting.

〔Effect of the invention〕

As explained above, according to the present invention, a highlight signal exceeding a predetermined level is detected even under a relatively special shooting situation where a large highlight signal is generated in the background, and the detected voltage is controlled using the conventional control method. By simply adding it to the signal, it is possible to control the signal in the highlighted area by an appropriate amount, suppressing abnormal phenomena such as blooming and comet tails to an unnoticeable level, and greatly improving the overall quality of the screen.
Further, by adopting the present invention, there are economical effects such as the ability to eliminate the ABO circuit or reduce the capacity of the ABO circuit, and if the ABO circuit is used in combination, an even greater effect of improving image quality can be obtained.

[Brief explanation of drawings]

Figure 1 shows the iris of a conventional video camera.
A block diagram showing the AGC control system, Fig. 2 is an explanatory diagram showing an example of a photographed image, Fig. 3 is an iris control system,
Waveform diagram for explaining the operation of AGC control, Part 4
The figure is a block diagram showing an embodiment of the iris and AGC detection circuit according to the present invention, and FIG. 5 is a circuit diagram showing a specific circuit of the main part of the embodiment of the present invention. 1... Lens system, 2... Iris mechanism, 3...
Image sensor, 4...Preamplifier, 5...Signal processing circuit, 6...AGC circuit, 8...Gate signal application terminal, 9...Gate circuit, 10...Peak value detection circuit, 11...Average value detection circuit , 12...Addition circuit, 13...Amplification circuit, 14...AGC, iris control voltage switching circuit, 15...Iris drive circuit, 16...Highlight signal detection circuit, 17...
Peak detection circuit, 20...addition circuit, 19...camera signal processing circuit.

Claims (1)

[Claims] 1. A video camera configured to convert light incident through an iris into a video signal using an image sensor, and to perform signal processing on the obtained video signal using a video signal processing circuit, Extracting a video signal corresponding to the central part of the photographed screen from the video signal, detecting a peak value, an average value, or both of the extracted video signal,
a first detection means for generating a first control signal according to the obtained value; and a first detection means for generating a first control signal according to the obtained value; a second detection means for extracting a signal, detecting a peak value of the extracted video signal, and generating a second control signal according to the obtained value; The first
mixing means for mixing a predetermined amount of the second control signal generated by the second generation means with the control signal; a control circuit for controlling at least one of the aperture of the video signal processing circuit and the gain of the video signal processing circuit. 2. The video camera control circuit according to claim 1, wherein the first detection means extracts a video signal corresponding to a central portion of the photographic screen from among the video signals, and extracts a video signal corresponding to a central portion of the photographic screen from among the video signals. The video signals corresponding to the part are extracted by clipping the video signals below a predetermined value as they are, and clipping the other video signals at the predetermined value, and extracting the peak value, average value, or both of the extracted video signals. 1. A control circuit for a video camera, characterized in that the control circuit detects a signal and generates a first control signal according to the obtained value.