JPH04329776A - Video camera with reception sensitivity control function - Google Patents

Abstract

PURPOSE:To prevent deterioration in the S/N by setting a switching circuit ON at low illuminance so as to reduce an output of a detection circuit thereby controlling a contour correction of a luminance signal system to be outputted from a video signal processing unit and a gain of a chroma signal system. CONSTITUTION:The video camera is provided with a sensitivity increase circuit amplifying a video signal subject to photoelectric conversion in response to a luminous quantity detection signal representing a low illuminance from a luminous quantity detection circuit, an AGC device 8 adjusting the automatic gain of the video signal amplified by the sensitivity increase circuit, a detection circuit 10 detecting an output of the AGC device and a switching circuit 15 coupled between an output of the detection circuit and a ground potential point and switched in response to the luminous quantity detection signal and representing low illuminance.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] This invention relates to an optical system that photoelectrically converts a subject image formed on an imaging surface, and a composite video signal that is a mixture of a luminance signal and a chroma signal by processing the photoelectrically converted signal. The present invention relates to a video camera with a receiving sensitivity control function including a video signal generating device, and particularly relates to improving control of receiving sensitivity at low illuminance.

0002

2. Description of the Related Art When photographing a dark scene with a video camera, the amount of incident light decreases, so it is necessary to amplify the signal from the image pickup device. Such amplification operation is generally called sensitivity increase.

FIG. 2 is a block diagram of a conventional video camera having a sensitivity increasing function. Referring to FIG. 2, this video camera includes an optical system 1 for condensing incident light from a subject and forming an optical image of the subject on a predetermined imaging plane; An aperture 2 for regulating the amount of incident light, an image sensor 3 placed on the imaging plane of the optical system 1 and converting an optical image into a video signal through photoelectric conversion, and noise removed from the output of the image sensor 3. a sampling circuit 4 for regenerating the output of the sampling circuit 4, a clamp circuit 5 for regenerating the DC output of the sampling circuit 4, and an automatic aperture control circuit 6 for controlling the aperture ratio of the aperture 2 according to the signal level output from the clamp circuit 5. , an AGC (automatic gain control) device 8 that keeps the video signal output from the clamp circuit 5 at an optimal amplitude, and a video signal processing device that processes the video signal output from the AGC device 8 and outputs a luminance signal Y and a chroma signal C. device 9, a mixing circuit 11 that mixes the luminance signal Y and chroma signal C output from the video signal processing device 9 and outputs a composite video signal, a light amount detection circuit 12 that detects the illuminance level of the subject, and a sensitivity increaser. The sensitivity up signal generation circuit 13 generates a signal to control the sensitivity up circuit 7 using the switch SW1, the output of the light amount detection circuit 12, and the output of the sensitivity up switch SW1, and the video signal level output from the AGC circuit 8. and a detection circuit 10 for detecting.

FIG. 3 is a block diagram showing details of the AGC device 8. Referring to the figure, the AGC device 8 includes an AGC
It includes an amplifier 8a, an AGC amplifier control circuit 8b, a gain error detection circuit 8d, a reference voltage generation circuit 8e, and a maximum gain control circuit 8c. The AGC amplifier control circuit 8b is
A DC voltage is applied to the GC amplifier 8a to control the amplification gain of the AGC amplifier 8a. The AGC amplifier 8a has an input connected to the output of the sensitivity increasing circuit 7, and its amplification gain is equal to the AGC amplifier 8a.
It is changed by the DC voltage from the amplifier control circuit 8b. The reference voltage generation circuit 8e generates a reference voltage for comparison, and applies the generated reference voltage to the gain error detection circuit 8d.
give to The gain error detection circuit 8d is an AGC amplifier 8.
The input is connected to the output of a, and the AGC amplifier control circuit 8
The output is connected to the input of the AGC amplifier 8a, and detects an error in the optimum signal level for detecting the signal level output from the AGC amplifier 8a, and the detected error is sent to the AGC amplifier control circuit 8b.
give to The maximum gain control circuit 8c is connected to the AGC amplifier control circuit 8b, and controls the AGC amplifier control circuit 8 so that the gain of the AGC amplifier 8a does not exceed a certain amplification gain.
Controls the output of b.

FIG. 34 is a block diagram showing details of the video signal processing device 9. Referring to the figure, this video signal processing device 9 includes a pre-stage luminance signal processing circuit 9a, an outline correction circuit 9b, an outline correction amount control circuit 9c, a post-stage luminance signal processing circuit 9d, and a color signal processing circuit 9a. It includes a circuit 9e, a color gain control circuit 9f, and a low luminance suppression signal generation circuit 9g. The pre-luminance signal processing circuit 9a removes carriers from the video signal output from the AGC circuit 8, and further performs gamma correction,
Performs processing on luminance signals such as knee correction. The contour correction circuit 9b performs correction to sharpen the image from the output signal of the pre-luminance signal processing circuit 9a. The contour correction amount control circuit 9c provides the amount to be corrected in the contour correction circuit 9b. Post-stage luminance system signal processing circuit 9d
are the output of the previous stage luminance system signal processing circuit 9a and the contour correction circuit 9
The input is connected to the output of b, and processes necessary for generating a composite video signal, such as white clipping, pedestal correction, and synchronization signal addition, are performed on the input video signal. The color signal processing circuit 9e has an input connected to the output of the AGC circuit 8, separates and extracts the color signal from the video signal,
This extracted signal is subjected to gamma, white balance adjustment, matrix conversion processing, etc. to generate a color difference signal. The color difference signals generated in this manner are carrier-balanced and subjected to encoding processing and bursting. The color gain control circuit 9f has an input connected to the output of the color signal processing circuit 9a, controls the gain of the video signal, and outputs a chroma signal. The low luminance suppression signal generation circuit 9g has an input connected to the output of the detection circuit 10, and generates a signal for controlling the contour correction amount control circuit 9c and the color system detection control circuit 9f.

The operation of the conventional video camera shown in FIGS. 2 to 4 will be explained. First, incident light from a subject is imaged by the optical system 1 onto a predetermined imaging plane. At this time,
The angle of view and amount of light of the incident light are regulated to be optimal by the optical system 1 and the aperture 2. The image sensor 3 converts the formed subject image into a video signal through photoelectric conversion. The sampling circuit 4 removes noise from the video signal output from the image sensor 3. The output signal of the sampling circuit 4 is DC-regenerated by the clamp circuit 5 and given to the sensitivity up circuit 7, and the output of the sensitivity up circuit 7 is further outputted to the AG.
C device 8. The sensitivity increase circuit 7 operates to increase the level of the output signal S2 to the AGC device 8 only when the output S6 of the sensitivity increase signal generation circuit 13, which will be described later, is at a high level.

The output of the clamp circuit 5 is also applied to an automatic aperture control circuit 6. The automatic aperture control circuit 6 controls the aperture ratio of the aperture 2 so that the level of the video signal output from the clamp circuit 5 falls within a predetermined range. AGC device 1
performs automatic gain control to maintain the optimum amplitude of the video signal. The AGC amplifier 8a receives the input video signal S in response to the DC voltage applied from the AGC control circuit 8b.
2 is amplified, and this amplified video signal 3 is provided to a video signal processing device 9 and a detection circuit 10. The output S3 of the AGC amplifier 8a is also given to a gain error detection circuit 8d. The gain error detection circuit 8d detects the output S of the AGC amplifier 8a.
The DC voltage obtained by smooth detection of the reference voltage 3 is compared with the reference voltage provided from the reference voltage generation circuit 8e, and the comparison result is amplified and provided to the AGC amplifier control circuit 8b. That is, the output of the AGC amplifier 8a is fed back.

The AGC amplifier control circuit 8b is controlled to increase its amplification gain when the amplitude of the output signal S3 of the AGC amplifier 8a is reduced relative to the AGC amplifier 8a. Conversely, the output signal S3 of the AGC amplifier 8a
When the amplitude of the signal increases, the amplification gain is controlled to decrease. The AGC amplifier control circuit 8b is an AGC amplifier control circuit 8b.
As long as the level of the output signal S3 of the amplifier 8a is within a predetermined range, the amplification gain of the AGC amplifier 8a continues to be controlled in response to the DC voltage applied from the gain error detection circuit 8d.

The maximum gain control circuit 8c has the function of limiting the continuous control operation by the AGC control circuit 8b.
The operation of the AGC amplifier control circuit 8b is restricted so that the amplification gain of the GC amplifier 8a does not exceed a certain value. This is the S/N of the output signal S3 of the AGC amplifier 8a when the gain of the AGC amplifier 8a is increased without limit.
This is because there is a risk that the temperature will deteriorate to a level that cannot be tolerated in practice.

Video signal S3 output from AGC device 8
is applied to the front-stage luminance signal processing circuit 9a, and after being processed by carrier removal, gamma correction, knee correction, etc., is applied to the contour correction circuit 9b and the rear-stage luminance signal processing circuit 9d. A contour correction signal is obtained by the contour correction circuit 9b from the video signal output from the front-stage luminance signal processing circuit 9a, and is provided to the rear-stage luminance signal processing circuit 9d. The rear-stage brightness signal processing circuit 9d performs contour correction on the output signal from the front-stage brightness signal processing circuit 9a using the contour correction signal from the contour correction circuit 9b. The subsequent luminance signal processing circuit 9d further performs processes such as white clipping, pedestal correction, and synchronization signal addition on the contour-corrected signal to generate a luminance signal. This generated luminance signal Y is given to the mixing circuit 11.

Video signal S3 output from AGC device 8
The signal is also supplied to the color signal processing circuit 9e, where processing such as color separation processing, gamma correction, white balance adjustment, matrix, and color difference signal generation is performed. Further, the color difference signal is subjected to processing such as carrier balance, encoding processing, and burst load, and then processed by the color system gain control circuit 9.
It is output to f. The color gain control circuit 9f controls the gain of the color signal and outputs it to the mixing circuit 11 as a chroma signal.

Video signal S3 output from AGC device 8
is also provided to the detection circuit 10. Video signal S3
is smoothed and converted into a DC voltage by the detection circuit 10, and then applied as a DC voltage E to the low brightness suppression signal generation circuit 9g. The low-luminance suppression signal generation circuit 9g provides the low-luminance suppression signal S7 to the contour correction control circuit 9c and the color system gain control circuit 9f.

[0013] Generally, when the subject is under low illuminance, the S/N of the video signal deteriorates. In order to prevent this S/N deterioration, the following steps are taken. That is, in the case of low illuminance, the level of the output S3 of the AGC device 8 decreases, so that the DC voltage E generated by the detection circuit 10 also decreases. In response to this decrease in DC voltage E, the low brightness suppression signal generation circuit 9g generates a low brightness suppression signal S7 to lower the amount of contour correction and reduce noise in the brightness signal system. In this way, the color system gain can be lowered, chroma signal noise can be reduced, and S/N deterioration can be prevented.

The sensitivity up signal generation circuit 13 provides a sensitivity up signal S6 to the sensitivity up circuit 7 in the following manner.

FIG. 5 is a characteristic diagram of the video signal S1 from the clamp circuit 5 with respect to the illuminance of the object. In the figure,
The vertical axis is the output level of the video signal S1, and the horizontal axis is the illuminance e.

(1) When the illuminance e is in the range e≧e0, the automatic aperture circuit 6 controls the aperture ratio of the aperture 2,
The output S1 of the clamp circuit 5 is kept constant.

(2) When the illuminance e is e<e0, the aperture 2 is in an open state, and the level of the video signal S1 is proportional to the illuminance e.

(3) When the illuminance e is e1 < e,
The light amount detection circuit 12 outputs a low level light amount signal S4.

(4) When the illuminance e is e<e1, the light amount detection circuit 12 outputs a high level light amount detection signal S4. The sensitivity up switch SW1 has a high level when it is on and a low level when it is off. The sensitivity up signal generation circuit 13 generates a light amount detection signal S4 and an output signal S5 of the sensitivity up switch SW1.
is input, and only when the light amount detection signal S4 is at a high level, it receives the output S5 of the sensitivity up switch SW1 and outputs the sensitivity up signal S6.

That is, if the sensitivity up switch SW1 is set to high level when the light amount detection signal S4 is at a high level, a high level sensitivity up signal S6 is output from the sensitivity up signal generating circuit 13, and the sensitivity up circuit is activated. 7 to gain up state. In response to this,
The level of the input signal S2 to the AGC device 8 increases. Once the sensitivity up signal S6 is set to high level, even if the light amount detection signal S4 becomes low level,
It is meant to remain at a high level. This is because if the sensitivity increase is turned on and off every time the illuminance changes, the video signal will become unstable due to the time constant of the AGC device 8, and the screen will become unnatural.

FIG. 6 is a diagram for explaining the sensitivity increasing operation. FIG. 6(a) shows the characteristics of the illuminance and the output S2 of the sensitivity increasing circuit 7, where the broken line is when the sensitivity is not increased and the solid line is when the sensitivity is increased. Figure 6
(b) shows the characteristics of the AGC device 8. FIG. 6(c) is a diagram showing the relationship between the video signal S3 output from the AGC device and the illuminance, and is a combination of FIG. 6(a) and FIG. 4(b).

By the way, S of the video signal at low illuminance
/N deteriorates for the following reasons. That is, the video signal S1 input to the sensitivity increase circuit 7 always contains a constant level of random high frequency signals that have no correlation with the amount of incident light. This noise mainly occurs in the image sensor 3. On the other hand, the level of the video signal is
depends on the amount of incident light. Therefore, when the subject illuminance decreases, the video signal level S1 decreases, while the noise maintains a constant level, so the S/N of the video signal S1 input to the sensitivity increase circuit 7 deteriorates. . Therefore, the S/N of the video signal S2 with increased sensitivity is
will get even worse.

In order to prevent this S/N deterioration, the DC voltage E detected by the detection circuit is transmitted to the low brightness suppression signal generation circuit 9.
give to g. The low-luminance suppression signal generation circuit 9g supplies the low-luminance suppression signal S7 to the contour correction control circuit 9c and the color system gain control circuit 9f. The contour correction control circuit 9c and the color system gain control circuit 9f operate to lower the contour correction amount and the color system gain, respectively, when the low luminance suppression signal S7 decreases. By performing such an operation, luminance signal system noise and chroma signal system noise at low illuminance are reduced, and deterioration of the S/N ratio is prevented.

[0024]

[Problems to be Solved by the Invention] However, in the sensitivity-up state, the level of the input signal S3 to the detection circuit 10 increases, so the DC voltage E increases and the low brightness suppression signal S7
It also goes up. Therefore, while the S/N of the video signal whose sensitivity has been increased will deteriorate, the amount of suppression of the low-luminance suppression signal will decrease, and the S/N of the video signal will deteriorate significantly. This will result in As a result, a sufficient amount of sensitivity increase cannot be obtained.

Therefore, an object of the present invention is to provide a video camera with a reception sensitivity control function that can increase sensitivity without extremely deteriorating the S/N in a low-light area by performing a low-luminance suppression effect even when sensitivity is increased. The goal is to provide the following.

[0026]

[Means for Solving the Problems] To achieve the above object, the present invention includes an optical system that photoelectrically converts a subject image formed on an imaging surface, and a luminance signal and chroma signal that processes the photoelectrically converted signal. This video camera includes a video signal processing device that generates a composite video signal mixed with a video signal, and has the following features. That is, means for detecting a decrease in the amount of incident light based on a comparison between the photoelectrically converted signal and a predetermined level and generating an incident light amount decrease detection signal; and in response to the generated incident light amount decrease detection signal, receiving sensitivity increasing means for increasing the receiving sensitivity by converting the photoelectrically converted signal; automatic gain adjusting means to which the signal amplified by the receiving sensitivity increasing means is applied; and detection for detecting the output of the automatic gain adjusting means. switching means coupled between the output of the detection means and ground potential and configured to switch in response to the detection signal for detecting a decrease in the amount of incident light; noise suppressing means for suppressing the noise level of the video signal processing device.

[0027]

According to the present invention, in the case of low illuminance, a detection signal for detecting a decrease in the amount of incident light is generated, and the reception sensitivity increasing means is activated by the generated detection signal for decreasing the amount of incident light. In addition, the switching means switches in response to the incident light amount reduction detection signal to reduce the output level of the detection means. Therefore, the video signal processing device can perform a low-luminance suppression operation that reduces luminance signal thread noise and color signal thread noise even during low illuminance. As a result, deterioration of the S/N of the video signal can be prevented.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a block diagram of a video camera with a receiving sensitivity control function according to the present invention. Referring to the figure, this video camera differs from the video camera shown in FIG. A resistor R whose one end is connected to the output of the detection circuit 10
2, and is coupled between the other end of the resistor R2 and the ground potential, and A
A switching circuit 15 that switches in response to the output from the ND gate 14 is provided. Other than these, the structure is similar to the conventional video camera shown in FIGS. 2 to 4, and similar circuits are denoted by the same reference numerals.
The explanation will be omitted as appropriate.

Next, the operation of the video camera with reception sensitivity control function shown in FIG. 1 will be explained. First, when the illuminance e becomes e<e1 as shown in FIG. 5, the light amount detection signal S4 output from the light amount detection circuit 12 becomes high level.
At this time, when the sensitivity up switch SW1 is turned on,
The sensitivity up signal generation circuit 13 generates a high level sensitivity up signal S6.

The sensitivity up circuit 7 amplifies the input signal S1 in response to the high level sensitivity up signal S6,
Output the amplified video signal S2. Sensitivity increase circuit 7
The video signal S2 outputted from the AGC device 8 is amplified by the AGC device 8, and then given to the detection circuit 10 and the video signal processing device 9. The signal processing up to this point is the same as in the conventional example.

Then, a high level light amount detection signal S4
In response to the high level sensitivity up signal S6, A
The ND gate 14 outputs a high level signal to turn on the switching circuit 15. Switching circuit 15
When turned on, the DC voltage E generated from the detection circuit 10 drops to approximately the ground potential.

In response to the reduced DC voltage E, the low luminance suppression signal generating circuit 9g (see FIG. 4)
The level of the low brightness suppression signal S7 is reduced. Therefore, the gain of the color system gain control circuit 9f decreases, and
The amount of control generated by the contour correction amount control circuit is reduced, noise in the luminance signal system is reduced, and noise in the chroma signal system is also reduced. As a result, S
/N can be prevented from deteriorating.

[0033]

According to the invention described above, when the illuminance is low, the gain of the video signal whose sensitivity has been increased by the receiving sensitivity increasing means is adjusted by the automatic gain adjusting means. By processing this gain-adjusted video signal by a video signal processing device, it is possible to increase the sensitivity of the luminance signal and chroma signal. The switching means can reduce noise in the luminance signal system and noise in the chroma signal output from the video signal processing device by reducing the output of the detection means in response to the detection signal for decreasing the amount of incident light. can. Therefore, it is possible to obtain the effect that the sensitivity can be increased without deteriorating the S/N ratio at low illuminance.

[Brief explanation of drawings]

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

FIG. 2 is a block diagram showing a conventional video camera.

FIG. 3 is a block diagram showing details of the AGC device of the video camera.

FIG. 4 is a block diagram showing details of a video signal processing device of a video camera.

FIG. 5 is an output characteristic diagram of the clamp circuit.

FIG. 6 is a characteristic diagram for explaining a sensitivity increase operation.

[Explanation of symbols]

5 is a clamp circuit, 6 is an automatic aperture adjustment circuit, 7 is a sensitivity increase circuit, 8 is an AGC device, 9 is a video signal processing device, 9
10 is a detection circuit, 11 is a mixing circuit, 12 is a light amount detection circuit, 13 is a sensitivity increase signal generation circuit 14 is an AND gate, and 15 is a switching circuit.

Claims (1)

[Claims] 1. An optical system that photoelectrically converts a subject image formed on an imaging surface, and a video signal processing device that processes the photoelectrically converted signal to generate a composite video signal that is a mixture of a luminance signal and a chroma signal. a video camera comprising: means for detecting a decrease in the amount of incident light based on a comparison between the photoelectrically converted signal and a predetermined level and generating an incident light amount decrease detection signal; and the generated incident light amount decrease detection signal. a receiving sensitivity increasing means for amplifying the photoelectrically converted signal and increasing the receiving sensitivity in response to the signal, an automatic gain adjusting means to which the signal amplified by the receiving sensitivity increasing means is applied, and the automatic gain adjusting means a detection means for detecting the output of the detection means; a switching means coupled between the output of the detection means and a ground potential and switching in response to the detection signal for detecting a decrease in the amount of incident light; A video camera with a receiving sensitivity control function, further comprising a noise suppressing means for suppressing a noise level of the video signal processing device during low illumination.