JP2004328554A - Video signal clamp circuit - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a video signal clamp circuit with which a stable image can be speedily outputted regardless of a gain value in video signal processing by controlling a gain of a video signal while using a variable gain amplifier, and of which circuit scale is smaller than a conventional circuit. <P>SOLUTION: A gain value is set in a variable gain differential amplifier 2 and at the same time, a D/A setting value corresponding to the gain value is found by a correcting value output means 8 and inputted to the variable gain differential amplifier 2 as a subtracted value for black level correction. Namely, while monitoring the gain value set in the variable gain differential amplifier 2, the black level correcting value is controlled while being interlocked therewith. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to a clamp circuit used in a circuit that converts an analog video signal output from a solid-state imaging device such as a CCD or a MOS sensor into a digital signal and generates an output video signal.
[0002]
[Prior art]
In a device that handles a video signal from a solid-state imaging device, a video signal clamp circuit (called an OB clamp circuit) for fixing a black level is used. Normally, a black level of a video signal is fixed using a light-shielding portion (optical black, hereinafter referred to as OB) provided on an imaging surface of a solid-state imaging device as a reference of the black level.
[0003]
(Patent Document 1) proposes a clamp circuit capable of controlling a time constant according to a situation. The configuration is shown in FIG.
In the configuration of FIG. 3, an input analog video signal from an image sensor (not shown) is input to a differential amplifier 102 via an analog video input 101 and amplified. The output signal of the differential amplifier 102 is input to the differential amplifier 104 via the variable gain amplifier 103. The variable gain amplifier 103 can change the gain according to a control signal input from the outside.
[0004]
The output of the differential amplifier 104 is digitally converted to an output digital video signal via an A / D converter 105 and output to a digital video signal processing unit (not shown) via a digital video output 106.
[0005]
The output of the A / D converter 105 is subtracted from the reference value input from the reference value input terminal 108 in the subtracter 107, and an error from the reference value that is originally desired to be clamped is output.
[0006]
The output of the subtracter 107 is supplied to the averaging circuit 110, and a value obtained by averaging the error from the reference level for several pixels is output. At this time, an average error from the reference values of the OB portion and the non-signal portion of the video signal is output by the control signal from the input 109.
[0007]
The output of the averaging circuit 110 is clipped by a clipping circuit 111, and the output of the clipping circuit 110 is input to an adder 113 by a vertical integration circuit 112 with an error level averaged over a plurality of horizontal scanning line periods. The output of the D-type flip flip 114, which is the feedback DC level, is added to obtain the next feedback DC level. Then, the analog signal is converted into a new feedback DC level by the D / A converter 115 as a new feedback DC level by the clock HCLK2 in the next horizontal scanning line, and the analog signal level is input to the differential amplifier 104. That is, feedback is applied to the differential amplifier 104 by comparing the level of the OB section with the reference value.
[0008]
The output of the clip circuit 111 is multiplied by a gain k by a multiplier 116 and then input to a one-field delay circuit 118 via an adder 117. The output of the one-field delay circuit 118 is input to the adder 117, added to the current feedback DC level, converted to an analog signal level by the D / A converter 119, and input to the differential amplifier 102. That is, feedback is applied to the differential amplifier 102 by comparing the level of the non-signal portion with the reference value.
[0009]
[Patent Document 1]
JP 2000-224440 A
[Problems to be solved by the invention]
In general, when imaging a dark place or an image of a bright place, the gain of a video signal is controlled by changing the gain of a variable gain amplifier provided in series with a video signal circuit.
[0011]
In this case, when the gain is changed, the DC current component of the video signal always fluctuates. Therefore, in order to obtain an output video signal by converting the digital signal by an A / D converter in the subsequent stage of the variable gain amplifier. It is necessary to put the output signal of the variable gain amplifier in the effective range of the digital conversion of the A / D converter.
[0012]
In the circuit configuration of Japanese Patent Application Laid-Open No. 2000-224440, when the fluctuation of the gain of the variable gain amplifier 103 is small, the difference between the OB level and the reference level is small, and the OB level can be stabilized.
[0013]
However, today, the number of cameras mounted on cellular phones and the like has increased, and there has been a demand for easily capturing images in dark places. In this case, it is necessary to increase the gain of the variable gain amplifier 103 more than ever.
[0014]
In general, when the gain of the variable gain amplifier is increased, the fluctuation of the output DC component of the variable gain amplifier increases, which may exceed the effective range of the digital conversion of the A / D converter.
[0015]
FIG. 4B shows an example of a normal output video when the analog video signal output from the variable gain amplifier falls within the effective range of the digital conversion of the A / D converter. In this case, a normal image can be obtained like the output image described for reference.
[0016]
On the other hand, when the analog video signal output from the variable gain amplifier deviates from the effective range of the digital conversion of the A / D converter to a high level as shown in FIG. When the image is whitened, and conversely, deviates to a low level as shown in FIG. 4 (c), the output image is blackened, and neither of the cases of FIGS. 4 (a) and 4 (c) is established as an image. .
[0017]
In the configuration of Japanese Patent Application Laid-Open No. 2000-224440, when the OB level deviates significantly from the reference value, the convergence speed can be increased by increasing the clip value of the clip circuit 111, but the feedback method is employed. Therefore, at least two or more feedbacks are required. This will be specifically described with reference to FIG.
[0018]
FIG. 5 shows a state of correction in a case where an image is deviated to a level higher than the reference value and white floating occurs.
In this case, as shown in FIG. 5 (a), a once-whitened video appears, and then correction is performed as shown in FIG. 5 (b) so as to approach the target value. However, since the correction is still out of the target value, as shown in FIG. 5C, further correction is performed to reach the target value and a normal image is obtained.
[0019]
That is, when a large gain is set in the variable gain amplifier 103, the image may be blackened or whitened out beyond the effective range of the A / D converter 105, and a video may not be established. There is a problem that a normal image cannot be obtained.
[0020]
Therefore, an object of the present invention is to provide a video signal clamp circuit that can quickly output a stable image regardless of the gain value set in the variable gain amplifier 103 and has a smaller circuit size than the conventional one.
[0021]
[Means for Solving the Problems]
The video signal clamping circuit according to the present invention is characterized in that the black level correction value is controlled in conjunction with the gain value set in the variable gain amplifier while monitoring the gain value.
[0022]
A video signal clamping circuit according to the present invention includes a differential amplifier capable of amplifying and outputting an input analog video signal and controlling the gain thereof by a gain setting value from an external input terminal. An A / D converter for converting the A / D conversion into a black level correction value and a correction value according to a gain set value from the external input terminal to the differential amplifier. The digital video signal is output from the output side of the device.
[0023]
According to this configuration, at the same time as setting the gain value, the set value of the D / A converter according to the gain value is set by the correction value output means. Therefore, the gain value is set regardless of the gain value set in the differential amplifier. The output of the differential amplifier can be kept within the effective range of the A / D converter, and a stable image can be obtained quickly.
[0024]
Specifically, a correction value output means, a computing unit that substitutes a gain setting value for setting the gain of the differential amplifier into a calculation formula and outputs analog data having a fixed relationship with the gain setting value, When the D / A converter is configured to convert the analog data output from the arithmetic unit into a digital signal and output the digital data to the differential amplifier as a black level correction value, the OB level of the signal output from the differential amplifier becomes A calculation formula capable of calculating a set value for generating an output DC from the D / A converter, which does not deviate from the reference value, is used.
[0025]
Further, a differential amplifier capable of amplifying and outputting an input analog video signal and controlling its gain by a gain set value from an external input terminal, and an A / D converter for converting an output of the differential amplifier into a digital signal A averaging circuit for averaging the level of the OB portion of the output of the A / D converter to output an OB average value; and calculating a difference value between the OB average value from the averaging circuit and an externally input OB reference value. A subtractor that outputs the difference value and a correction value according to a gain set value from the external input terminal to the differential amplifier as a black level correction value, wherein the A / D conversion is performed. The digital video signal is output from the output side of the device.
[0026]
According to this configuration, it is possible to absorb a variation in the output from the differential amplifier due to a manufacturing variation of the image pickup device and a temperature change during the operation of the system, so that a more stable image can be output. In addition, by being able to absorb the variation of the imaging element, the range of guaranteed characteristics of a product using the video signal clamp circuit can be expanded, and an improvement in yield can be expected.
[0027]
Specifically, the correction value output means has a fixed relationship with the gain setting value by substituting the gain setting value for setting the gain of the differential amplifier and the difference value output from the subtractor into a calculation formula. An arithmetic unit for outputting analog data, and a D / A converter for converting the analog data output from the arithmetic unit into a digital signal and outputting the digital data to the differential amplifier as a black level correction value.
[0028]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to FIGS.
(Embodiment 1)
FIG. 1 shows a video signal clamping circuit according to the first embodiment of the present invention.
[0029]
In the configuration of FIG. 1, 1 is a video signal input terminal, 2 is a variable gain differential amplifier, 3 is an A / D converter, 4 is a digital video output terminal, 5 is an external input terminal, 6 is a computing unit, and 7 is D / A converter. In this embodiment, the arithmetic unit 6 and the D / A converter 7 constitute a correction value output unit 8.
[0030]
The input analog video signal 11 input from the video signal input terminal 1 is composed of an effective signal portion including a video signal and a blanking period not including a video signal. The blanking period includes an OB section serving as a black level reference and a non-signal section. Since the reference of the black level of the video equipment is the OB section, it is necessary to keep the level of the OB section constant. Further, it is usual that the gain of the video signal is controlled by a variable gain amplifier using a variable gain in the video signal processing. At this time, the DC current component of the video signal fluctuates. The analog video signal 12 output from the variable gain differential amplifier 2 is input to the A / D converter 3. It is necessary to control the DC current component to a constant level in order to input an appropriate input range of the A / D converter 3. The purpose of the clamp circuit is to perform these two controls.
[0031]
The input analog video signal 11 is input to a non-inverting input terminal (+) of the variable gain differential amplifier 2 and is amplified by a gain specified by a predetermined gain setting value 16 input from the external input terminal 5. The video signal 12 is input to the A / D converter 3. Here, a DC voltage 15 described later is applied to the inverting input terminal (−) of the variable gain differential amplifier 2, and is differentially amplified with the input analog video signal 11.
[0032]
The digital video signal 13 digitally converted by the A / D converter 3 is supplied from a digital video output terminal 4 to a digital signal processing circuit (not shown).
The gain set value 16 from the external input terminal 5 is input to the arithmetic unit 6, an operation according to this value is performed, and the D / A set value 14 to the D / A converter 7 is output.
[0033]
Here, the calculation in the calculator 6 will be described.
It is assumed that the arithmetic expression is a linear linear expression y = Ax + B. Here, y is a set value to the D / A converter 7, x is a gain set value 16 from the external input terminal 5, and A and B are constants. As for the constants A and B, gain values in a range defined as a gain set value 16 are input in consideration of the characteristics of the variable gain differential amplifier 2, the A / D converter 3, and the D / A converter 7. Is determined to calculate the D / A set value 14 for the D / A converter 7 so that the level of the OB section becomes the target value.
[0034]
The D / A set value 14 is digitally converted by the D / A converter 7 to become a DC voltage 15 and is input to the inverting input terminal (−) of the variable gain differential amplifier 2.
That is, a unique value corresponding to the gain setting value 16 is calculated by calculation, set in the D / A converter 7, and the DC voltage 15 is determined, thereby fixing the OB level.
[0035]
At this time, it is desirable to configure so as to set the gain setting value 16 in the variable gain differential amplifier 2 and at the same time to set the D / A setting value 14 in the D / A converter 7.
[0036]
A correction value output unit 8 that outputs a correction value according to a gain setting value from the external input terminal 5 to the variable gain differential amplifier 2 as a black level correction value is provided by a gain that sets the gain of the variable gain differential amplifier 2. A computing unit 6 that substitutes the set value 16 into a calculation formula and outputs analog data having a fixed relationship with the gain set value 16, and a variable gain differential amplifier 2 that converts the analog data output from the computing unit 6 into digital data. And a D / A converter 7 that outputs a black level correction value. The correction value output means 8 writes data corresponding to a gain setting value 16 for setting the gain of the variable gain differential amplifier 2. It is also possible to constitute a storage means for reading out and outputting a black level correction value according to the gain setting value 16 from time to time.
[0037]
(Embodiment 2)
FIG. 2 shows a video signal clamp circuit according to the second embodiment.
The difference from the first embodiment shown in FIG. 1 lies only in the configuration of the correction value output means 8. The same components will be described with the same reference numerals as in the first embodiment.
[0038]
In the configuration of FIG. 2, 1 is a video signal input terminal, 2 is a variable gain differential amplifier, 3 is an A / D converter, 4 is a digital video output terminal, 45 is an averaging circuit, 46 is an OB reference value input terminal, 47 Is a subtractor, 48 is an external input terminal, and 49 is a calculator. In this embodiment, the computing unit 49 and the D / A converter 7 constitute the correction value output means 8.
[0039]
The input analog video signal 11 is input to the non-inverting input terminal (+) of the variable gain differential amplifier 2 and is amplified by a gain specified by a predetermined gain setting value 16 input from the external input terminal 5. 12 is input to the A / D converter 3. Here, a DC voltage 58 described later is applied to the inverting input terminal (−) of the variable gain differential amplifier 2 to be differentially amplified with the input analog video signal 11.
[0040]
The digital video signal 13 digitally converted by the A / D converter 3 is supplied from a digital video output terminal 4 to a digital signal processing circuit (not shown), and is also input to an averaging circuit 45.
[0041]
The averaging circuit 45 averages the level of the OB section and outputs an OB average value. The OB average value is subtracted by the subtractor 47 from the OB reference value 54 input from the reference value input terminal 46, and a difference value 55 from the OB reference value 54 is output.
[0042]
The computing unit 49 receives the gain setting value 16 and the difference value 55 from the external input terminal 48 as inputs, performs an operation according to the values, and outputs a D / A setting value 57 to the D / A converter 7.
[0043]
Here, the calculation in the calculator 49 will be described.
It is assumed that the arithmetic expression is a linear linear expression y = Ax + B. Here, y is a set value to the D / A converter 7, x is a gain set value 16 from the external input terminal 5, and A and B are variables. In consideration of the characteristics of the variable gain differential amplifier 2, the A / D converter 3, and the D / A converter 7, the variables A and B are set such that the level of the OB section becomes the OB reference value 54 with respect to the gain setting value 16. It is necessary to determine the initial value so as to calculate the D / A set value 57 of the D / A converter 7 as follows. When the difference value 55 takes a positive value, the variables A and B are reduced, and when the difference value 55 takes a negative value, the variables A and B are increased, and the linear linear expression is dynamically corrected.
[0044]
The D / A set value 57 from the arithmetic unit 49 becomes a DC voltage 58 by the D / A converter 7 and is input to the inverting input terminal (−) of the variable gain differential amplifier 2.
That is, the OB level is fixed by calculating a unique value corresponding to the gain setting value 16 by an arithmetic expression, setting the value in the D / A converter 7, and determining the DC voltage 58. Further, by dynamically correcting the arithmetic expression, it is possible to provide a clamp circuit that responds to variations in the characteristics of the image sensor and changes in temperature.
[0045]
At this time, it is desirable to set the D / A setting value 57 in the D / A converter 7 at the same time as setting the gain setting value 16 in the variable gain differential amplifier 2.
The correction value output means 8 for outputting the difference value 55 and a correction value according to the gain set value from the external input terminal 5 to the variable gain differential amplifier 2 as a black level correction value is provided by the gain of the variable gain differential amplifier 2. And a difference value 55 output from the subtractor 47 are substituted into a calculation formula to output analog data having a fixed relationship with the gain setting value 16. The D / A converter 7 converts the output analog data into digital data and outputs it to the variable gain differential amplifier 2 as a black level correction value. A storage means in which data corresponding to the gain setting value 16 for setting the gain and the difference value 55 output from the subtractor 47 is written, is stored in accordance with the gain setting value 16 and the difference value 55 from time to time. It may be configured to output the black level correction value is read out from 憶 means.
[0046]
【The invention's effect】
As described above, according to the present invention, the output of the differential amplifier falls within the effective range of the A / D converter in conjunction with the gain value set in the differential amplifier that amplifies the input analog video signal. And a stable image can be obtained quickly.
[0047]
Further, it is possible to absorb fluctuations in the output from the differential amplifier due to manufacturing variations of the image pickup device and temperature changes during system operation, so that a more stable image can be output. In addition, by being able to absorb the variation of the imaging element, the range of guaranteeing the characteristics of the product can be expanded, and the yield rate can be improved.
[Brief description of the drawings]
FIG. 1 is a block diagram of a video signal clamp circuit according to a first embodiment of the present invention; FIG. 2 is a block diagram of a video signal clamp circuit according to a second embodiment of the present invention; FIG. FIG. 4 is a block diagram of a clamp circuit. FIG. 4 is an explanatory diagram for explaining a problem. FIG. 5 is an explanatory diagram of a correction process until a normal image is obtained when whitening occurs.
Reference Signs List 1 video signal input terminal 2 variable gain differential amplifier 3 A / D converter 4 digital video output terminal 5 external input terminal 6 arithmetic unit 7 D / A converter 8 correction value output means 11 input analog video signal 12 analog video signal 14 D / A setting value 16 Gain setting value 45 Averaging circuit 46 OB reference value input terminal 47 Subtractor 48 External input terminal 49 Operation unit

Claims (6)

A differential amplifier that amplifies and outputs an input analog video signal and controls the gain by a gain setting value from an external input terminal;
An A / D converter for converting the output of the differential amplifier into digital;
Correction value output means for outputting a correction value according to a gain setting value from the external input terminal to the differential amplifier as a black level correction value, and outputting a digital video signal from the output side of the A / D converter Video signal clamp circuit. Correction value output means,
An arithmetic unit that outputs analog data having a fixed relationship with the gain setting value by substituting the gain setting value for setting the gain of the differential amplifier into a calculation formula;
2. The video signal clamp circuit according to claim 1, further comprising a D / A converter that converts the analog data output from the arithmetic unit into a digital signal and outputs the digital data to the differential amplifier as a black level correction value. Correction value output means,
2. The video signal according to claim 1, wherein said video signal is constituted by storage means for writing data corresponding to a gain setting value for setting a gain of the differential amplifier, and for reading and outputting a black level correction value according to the gain setting value at times. Clamp circuit. A differential amplifier that amplifies and outputs an input analog video signal and controls the gain by a gain setting value from an external input terminal;
An A / D converter for converting the output of the differential amplifier into digital;
An averaging circuit for averaging the level of the OB portion of the output of the A / D converter and outputting an OB average value;
A subtractor that outputs a difference value between the OB average value from the averaging circuit and an OB reference value input from outside;
Correction value output means for outputting the difference value and a correction value according to a gain set value from the external input terminal to the differential amplifier as a black level correction value, wherein a digital value is output from the output side of the A / D converter. Video signal clamp circuit that outputs video signals. Correction value output means,
An arithmetic unit that outputs analog data having a fixed relationship with the gain setting value by substituting a gain setting value for setting the gain of the differential amplifier and a difference value output from the subtractor into a calculation formula;
5. The video signal clamping circuit according to claim 4, further comprising a D / A converter that converts the analog data output from said arithmetic unit into a digital signal and outputs the digital data as a black level correction value to said differential amplifier. Correction value output means,
Data corresponding to a gain setting value for setting the gain of the differential amplifier and a difference value output from the subtractor are written, and are read out according to the gain setting value and the difference value from time to time to output a black level correction value. 5. The video signal clamping circuit according to claim 4, wherein said video signal clamping circuit comprises a storage means for performing the operation.

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