JPH10285432A - Clamp device for video signal - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable a clamp device having high accuracy and fast responsiveness not to change the dynamic range of a digital video signal. SOLUTION: An output signal from an image-pickup element 1 is clamped by an analog clamp circuit 4 via a preamplifier 2 and a gain control circuit 3 and converted into a digital video signal at an A/D converter 6, through a pre-knee circuit 5 that compresses a signal with a high dynamic range into a signal with a dynamic range which is possible for digital processing. Next, data averaging of a block level of the digital video signal for a clamp period are compared with digital reference level data at a level comparator circuit 10, a clamp reference value is fine-adjusted at an adder 8 that receives a correction signal, resulting from converting the comparison result into an analog signal at a D/A converter 9 and an output of the adder 8 is given to the analog clamp circuit 4.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video signal clamping device used in a video camera or the like, and particularly suitable for an A / D converter.

[0002]

2. Description of the Related Art With the advancement of digital signal processing technology in video cameras, video signal processing circuits are increasingly replacing analog circuits with digital circuits. In addition, performance and functions have been improved.

The performance of the A / D converter is one of the important characteristics for stabilizing the image quality of a video camera including a digital signal processing circuit. Among such circuits, a clamp circuit for stabilizing the black level of a video signal is also a major factor.

For example, high-performance color video cameras of the R, G, and B systems amplify the output of an image pickup device such as a CCD with a preamplifier, and then provide a gain control circuit and a video signal processing circuit such as detail processing and gamma processing. After that, a color video signal such as NTSC is output by an encoder.

In such a high-performance camera, digitization of detail processing, gamma processing, etc., has already been performed according to the number of quantizations of the A / D converter, but the dynamic range of a video signal such as a gain control circuit portion has been reduced. Necessary parts are constituted by analog circuits.

The clamp circuit in the A / D converter is
By the gain adjustment and the white balance adjustment performed by the gain control circuit, the black level, which is the reference of the video signal, fluctuates, and the black balance is shifted so that the image quality does not deteriorate.

FIGS. 2 to 4 are block diagrams of a conventional clamp circuit in an A / D converter of a commonly used video camera.

FIG. 2 shows a conventional method of performing analog clamping before the A / D converter 6. The imaging device 1 is a photoelectric conversion device such as a CCD. The preamplifier 2 is a circuit that detects and amplifies the output of the image sensor 1. Gain control circuit 3
Is a circuit for adjusting the gain and white balance of the camera. The clamp circuit 4 is an analog clamp circuit that clamps a black portion of the video signal to a clamp reference value in accordance with an input range of the A / D converter 6 by a clamp pulse. The A / D converter 6 is an A / D converter that converts an analog signal into a digital signal. The digital process circuit 7 is a digital video signal processing circuit that performs detail processing, gamma processing, and the like. The microcomputer 12 is a microcomputer that determines the state of the camera and outputs control signals for white balance adjustment and gain adjustment to the gain control circuit 3.

FIG. 3 shows a method of comparing digital data after A / D conversion with a digital clamp reference value and adding the comparison result to an analog video signal. The comparison circuit 15 compares the data of the black portion of the video signal in the output signal of the A / D converter 6 with a digital reference value indicating a level to be processed as black by the digital process circuit 7 for a period of a clamp pulse, and compares the data with one bit. This is a circuit that outputs an error signal. The integrating circuit 14 is a smoothing circuit that integrates the error signal into an analog signal in order to reduce the influence of noise. The integrated analog signal is added to the analog video signal by the adder 13, and then A / D conversion is performed.

FIG. 4 shows a digital clamp circuit for processing using digital data after A / D conversion. Comparison circuit 1
In 8, the data of the black portion of the video signal output from the A / D converter 6 is compared with a digital reference value indicating a level to be processed as black by the digital process circuit 7 for a clamp pulse period. Output with data. The time constant circuit 17 is a smoothing circuit for preventing the output of the comparison circuit 18 from suddenly largely changing or appearing as noise on the screen due to the influence of noise in the black portion of the video signal. Then, the output data of the time constant circuit 17 is added by the adding circuit 16 to stabilize the black portion of the video signal.

[0011]

However, the clamp circuit according to the prior art described above may not satisfy any of accuracy, responsiveness, a clamp range, a dynamic range of a video signal, and the like. For example, in the case of the system shown in FIG. 2, since the analog clamp system is used, the accuracy and temperature stability of the analog circuit are inferior to those of the digital system. Although the accuracy of the method of FIG. 3 is very high because comparison is performed using digital data, it is necessary to increase the time constant of the integrating circuit 14 as a smoothing circuit in order to add digital data to an analog circuit such as the adder 13. Therefore, the responsiveness at the time of gain adjustment or power-on is poor. Also, in the method of FIG.
Since the clamp is not performed in front of the converter 6, if the input black level fluctuation is large with respect to the input range of the A / D converter 6,
The dynamic range of the video signal changes, especially R, G,
In the high quality camera of the B system, overexposure or color misregistration may occur at a high level.

As a method for improving the accuracy, the clamping range, and the like, a clamp circuit combining an analog clamp circuit and a digital clamp circuit (Japanese Patent Laid-Open No. 4-336766).
However, since the digital clamp method similar to that of FIG. 2 is used, the dynamic range of the video signal fluctuates.

A video signal clamping device according to the present invention comprises:
Based on the analog clamp circuit with good responsiveness, A / D
The digital data after the converter is compared with the digital reference value,
An object of the present invention is to solve the above problem by feeding back the difference signal to an analog clamp circuit.

[0014]

According to the present invention, in order to achieve the above object, an analog clamp means for clamping a black level portion of an input analog video signal with a corrected clamp reference signal; A / D conversion means for converting the analog video signal clamped by the clamp means into a digital video signal, and comparing a black portion of the digital video signal obtained from the A / D conversion means with a predetermined digital reference value; Comparing means for outputting the result of the comparison as an analog correction signal; and correcting the predetermined clamp reference value with the correction signal obtained from the comparison means, and using the corrected value as the corrected reference signal. And correction means for providing

Further, in the present invention, there is provided averaging means for averaging a black level portion of the digital video signal obtained from the A / D conversion means for a predetermined period and giving the averaged value to the comparison means. You may.

Further, in the present invention, compression means for compressing the analog video signal clamped by the analog clamp means into a predetermined dynamic range may be provided.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Next, an embodiment of the present invention will be described with reference to FIG. FIG. 1 shows an embodiment in which the present invention is applied to a video camera. In the figure, an imaging device 1 is a photoelectric conversion device such as a CCD. Preamplifier 2
Is a circuit for detecting and amplifying the output of the image sensor. The gain control circuit 3 is a circuit that performs gain adjustment and white balance adjustment of the camera. The clamp circuit 4
The black portion of the analog video signal is changed to A /
This is an analog clamp circuit that clamps to a corrected clamp reference value described later according to the input range of the D converter 6. Also, the penny circuit 5 is a circuit for compressing the gain of a video signal above a certain set level to less than 1 in order to keep the dynamic range used for signal processing of the camera without overexposure, out of the large dynamic range of the clamped video signal. It is. The A / D converter 6 converts the compressed analog video signal into a digital video signal.
/ D converter. The digital process circuit 7 is a video signal processing circuit that performs digital video signal detail processing, gamma processing, and the like.

The averaging circuit 11 is a circuit for averaging the number of black portion clocks of the digital video signal from the A / D converter 6 for a predetermined period, for example, in one field. The addition can reduce an error due to random noise or the like. The level comparison circuit 10 compares data obtained by averaging the black portion of the digital video signal by the averaging circuit 11 with a digital reference value indicating a black level to be used as a reference by the digital process circuit 7, and compares the difference data. It is output as correction data of about 3 bits. The number of bits is determined in consideration of how much an error generated in the analog clamp circuit 4, the penny circuit 5, the A / D converter 6, and the like is the quantization number after A / D conversion. The D / A converter 9 converts the correction data into an analog correction signal and outputs it. The microcomputer 12 is a microcomputer that determines the state of the camera and outputs a control signal for white balance adjustment and gain adjustment.

The adder 8 adds the correction signal from the D / A converter 9 to the clamp reference value, and supplies the added output to the analog clamp circuit 4 as a corrected clamp reference value. Thus, the variation / error of the black reference level due to the error / temperature variation of each circuit from the clamp of the analog video signal to the A / D conversion, such as the analog clamp circuit 4, the plenary circuit 5, and the A / D converter 6, are performed. So that it absorbs.

Normally, the analog clamp circuit 4 has a D / A
If the output of the converter 9 is not added, errors and fluctuations in the black level of the digital signal will occur due to errors and temperature fluctuations in the analog clamp circuit 4 and the circuit until the video signal thereafter is digitized. Also, if the camera has high quality R,
In the case of a G or B type color camera, the black balance is deviated and the image quality is greatly deteriorated.

In this embodiment, the data obtained by averaging the black portion of the output of the A / D converter 6 by the averaging circuit 11 and the digital reference value indicating the black reference level by the digital process circuit 7 are used as the level. The comparison is performed by the comparison circuit 10. As a result of the comparison, when the output of the averaging circuit 11 is larger than the digital reference value, correction data for controlling the reference signal of the analog clamp circuit 4 in a lowering direction is output from the level comparison circuit 10, and the D / A After conversion into an analog correction signal by the converter 9, the adder 8 corrects the clamp reference value of the analog clamp circuit 4 so as to lower the reference value.

When the power is turned on or when the gain of the gain control circuit 3 suddenly changes largely due to white balance adjustment or gain adjustment and the black level reference greatly changes, the response speed of the analog clamp circuit 4 is high. The clamping can be performed in a short time with a small error. Thereafter, clamping can be performed with high precision by the level comparison circuit 10.

[0023]

As described above, according to the present invention, A
The video signal clamping device used for the / D conversion section is based on an analog clamp circuit having good responsiveness, and the digital video signal after the A / D converter is compared with a digital reference value.
Since the comparison result is fed back to the analog clamp circuit, it is possible to correct insufficient accuracy in the analog clamp circuit while utilizing the high-speed response of the analog clamp circuit, and to perform A / D conversion from the clamp. , The fluctuation of the black level based on the error of the circuit, the fluctuation of the temperature, and the like, can be reduced to obtain high image quality.

In particular, by applying the present invention to an apparatus requiring high image quality such as an R, G, B type color video camera, accuracy, responsiveness, clamping range, dynamic range of video signal, etc. are satisfied. A clamping device can be realized.

Further, by averaging the black level portion of the digital video signal after the A / D conversion and comparing it with the digital reference value, the influence of noise and the like can be reduced.

Further, the digital video signal having a wide dynamic range after clamping is compressed so that it can be processed by the digital processing circuit at the subsequent stage, so that the black level caused by an error of the pliny circuit as a compression means or the like is reduced. Can be suppressed.

[Brief description of the drawings]

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

FIG. 2 is a block diagram illustrating an example of a conventional clamp circuit.

FIG. 3 is a block diagram showing another example of a conventional clamp circuit.

FIG. 4 is a block diagram showing still another example of the conventional clamp circuit.

[Explanation of symbols]

 Reference Signs List 4 analog clamp circuit 5 penny circuit 6 A / D converter 8 adder 9 D / A converter 10 level comparison circuit 11 averaging circuit

Claims (3)

[Claims] 1. An analog clamp means for clamping a black level portion of an input analog video signal with a corrected clamp reference signal; and A for converting the analog video signal clamped by the analog clamp means into a digital video signal. A / D conversion means, a comparison means for comparing a black level portion of the digital video signal obtained from the A / D conversion means with a predetermined digital reference value, and outputting the comparison result as an analog correction signal; A video signal clamping device comprising: a correction unit that corrects a predetermined clamp reference value with the correction signal obtained from the comparison unit and supplies the corrected value to the analog clamp unit as the corrected reference signal. 2. A black level portion of the digital video signal obtained from the A / D conversion means is averaged for a predetermined period,
2. The video signal clamping device according to claim 1, further comprising an averaging means for giving an averaged value to said comparison means. 3. The apparatus according to claim 1, further comprising compression means for compressing said analog video signal clamped by said analog clamp means into a predetermined dynamic range.
A video signal clamping device as described in the above.