JP3289293B2 - Clamp circuit - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a clamp circuit for reproducing a DC component of an image signal.

[0002]

2. Description of the Related Art A clamp circuit is a circuit for determining a reference voltage serving as a reference when performing so-called gamma correction, white clip, knee, DC reproduction, etc., and includes a so-called peak value clamp type (peak clamp circuit) and a synchronous clamp. The shape is known, and the synchronous clamp type has a large clamping effect,
It is used in many video equipment.

Here, a brief description will be given of a feedback-type synchronous clamp type clamp circuit widely used in video cameras. This clamp circuit is a CDS that performs so-called correlated double sampling of an image signal from an image sensor.
(Correlated Double Sampling) circuit, which is connected to the subsequent stage, and covers the upper and lower or left and right of the imaging area of an imaging element, for example, a CCD image sensor (hereinafter referred to as CCD) 40 with a black mask 41 as shown in FIG. After light is prevented from being incident and the image signal corresponding to the portion covered with the black mask 41 discretely supplied through the CDS circuit is sampled and held periodically, the so-called optical black level obtained by sample holding is obtained. (Hereinafter referred to as an optical black level) is subtracted from an image signal corresponding to a portion of the CCD 40 not covered by the black mask 41 to clamp the image signal. When the upper and lower portions of the imaging area of the CCD are covered with a black mask, an optical black level is obtained during a so-called vertical retrace period.
An optical black level can be obtained during the horizontal retrace period. Normally, an optical black level obtained for each line is used.

By the way, in such a clamp circuit,
If the time constant of the sample and hold circuit provided in the clamp circuit is shortened, the waveform response of the image signal, that is, the frequency characteristic, improves, but if the time constant is too short, a different optical black level is sampled for each line. Hold and clamp, and as a result,
Horizontal streak noise was generated on the screen, which was one of the causes of image quality degradation. Therefore, in the conventional clamp circuit, the time constant of the sample-and-hold circuit has to be increased to some extent in consideration of the balance between the waveform responsiveness of the imaging signal and the image quality.

[0005]

In such a conventional clamp circuit, for example, as shown in FIG.
When excessive (large amount of) light is incident on the CCD, for example, when light from 0 is directly incident on the video camera 51, the light receiving section (photoelectric conversion section) of the CCD overflows the charge as shown in FIG. (A light receiving portion that has overflowed is indicated by oblique lines falling to the right), and electric charges are also accumulated in a part of the light receiving portion 42 covered by the black mask 41. As a result, the optical black level corresponding to the light receiving section 42 becomes inappropriate (erroneous). By the way, if the time constant of the sample-and-hold circuit is made longer as described above, this erroneous optical black level is sampled and held, and its effect continues for a long time until a properly clamped imaging signal is obtained (restored). There was a problem that it took time. That is, in the conventional clamp circuit, the user has long seen an image with poor image quality based on the image pickup signal clamped at the abnormal optical black level.

If the time constant of the sample-and-hold circuit is made longer, there is also a problem that the clamping is normally performed after the power of the video camera is turned on, and it takes a long time to obtain a good quality image.

The present invention has been made in view of such circumstances, and, for example, when excessive light is incident on an image pickup device as compared with a conventional clamp circuit, an abnormal optical due to the excessive incident light. It is an object of the present invention to provide a clamp circuit capable of shortening the time required for a user to view an image with poor image quality, such as being able to restore a black level in a short time.

[0008]

According to the present invention, there is provided first sample and hold means for sampling and holding an optical black level discretely included in an image pickup signal, and periodically holding the optical black level by the first sample and hold means. Adding means for subtracting the optical black level from the image signal and clamping the image signal. The first sample-and-hold means converts the optical black level contained in the image signal output from the adding means to an optical signal. A feedback-type synchronous clamp-type clamp circuit that samples and holds and clamps by the adding means, wherein the second circuit samples and holds an optical black level discretely included in an image signal input to the adding means. A sample and hold means, and a detection means for detecting an abnormality in the optical black level periodically held by the second sample and hold means. If, on the basis of the output of the detection means,
And control means for controlling the first sample and hold means to stop the sample and hold operation when the optical black level is abnormal.

[0009]

According to the present invention, an optical black level included in an input image signal is sampled and held by a second sample-and-hold means in a feedback-type synchronous clamp-type clamp circuit, so that the optical black level abnormality is detected. If the optical black level is detected by the detecting means and the optical black level is abnormal, the sample and hold operation of the first sample and hold means for sampling and holding the optical black level included in the clamped image signal output from the adding means is performed. It is controlled by the control means to stop.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of a clamp circuit according to the present invention will be described below with reference to the drawings. In this embodiment, the clamp circuit according to the present invention is applied to a video camera.
FIG. 1 is a block diagram showing a circuit configuration of a main part of the video camera.

Next, an embodiment of the clamp circuit according to the present invention will be described. The clamp circuit according to the present invention is applied to a video camera, and FIG. 1 is a block diagram showing a circuit configuration of a main part of the video camera.

The main part of the video camera of this embodiment is shown in FIG.
As shown in the figure, C for converting light from a subject into an imaging signal
A CD image sensor (hereinafter referred to as CCD) 11 and the C
It comprises a CDS circuit 12 for performing so-called correlated double sampling of an image signal from the CD 11, and a clamp circuit 30 for clamping the image signal from the CDS circuit 12.

The main part of this video camera performs correlated double sampling of an image signal from the CCD 11 obtained by photographing a subject, and then discretely converts the image signal into, for example, a horizontal scanning period (hereinafter referred to as one line). An image signal is clamped on the basis of a so-called optical black level (hereinafter referred to as an optical black level) included in each image, and the clamped image signal is supplied from the terminal 13 to, for example, a so-called white clip circuit (not shown). It has become.

More specifically, as shown in FIG. 1, the clamp circuit 30 subtracts an optical black level from the image signal from the CDS circuit 12 to clamp the image signal, and an adder 31 for clamping the image signal. An amplifier 32 for amplifying the output of the adder 31, a first sample-and-hold circuit 33 for sampling and holding the output of the amplifier 32 at a predetermined period by a clamp pulse supplied through a terminal 14, and the CDS circuit 12 From the terminal 1
5 that performs sampling and holding with a sampling clock having a phase earlier than that of the clamp pulse supplied through
, A reference power supply 34 for generating a reference voltage, and comparing the optical black level sampled and held by the second sample and hold circuit 36 with the reference voltage from the reference power supply 34 to obtain an optical black level. A first sample-and-hold circuit 3 based on the output of the comparator 35 when the optical black level is abnormal.
And a gate circuit 37 for gating the clamp pulse supplied via the terminal 14 in order to stop the sample-hold operation of No. 3.

Then, as described in the background art, the clamp circuit 30 is provided by the first sample-and-hold circuit 33 having a predetermined time constant having a certain length in order to prevent generation of horizontal streak noise. , Sample and hold the optical black level included in the imaging signal from the CDS circuit 12 for each line, for example, and subtract the optical black level from the imaging signal from the CDS circuit 12 to clamp the imaging signal. Detects optical black level abnormalities caused by excessive (large amount of) light from the flash during shooting, and when the optical black level becomes abnormal,
The sample and hold operation of the first sample and hold circuit 33 is stopped.

That is, as shown in FIG. 1, the first sample-and-hold circuit 33 includes a switch 3 for sampling and holding the image signal from the amplifier 32.
3a, a capacitor 33b, and an integrator 33c for integrating the optical black level sampled and held by the capacitor 33b.

The first sample-and-hold circuit 33 covers, for example, the left and right sides of the CCD 11 with a black mask as described in the related art (shown in FIG. 3), and receives light from the light-receiving section covered with the black mask. The optical black level corresponding to the image pickup signal is sampled and held based on the clamp pulse supplied via the terminal 14, and when the optical black level is abnormal, the first control signal from the comparator 35 is used. The clamp pulse supplied to the sample hold circuit 33 is stopped.

[0018] Specifically, the integrator 33c is also shown in FIG. 1, a differential amplifier 33c _1, and a capacitor 33c ₂ negative feedback to the differential amplifier 33c _1, the differential amplifier 33c is a resistor 33c ₃ Metropolitan connected between the _first negative input terminal and ground, also, the second sample-and-hold circuit 36, the switch 36a for sampling and holding the image pickup signal from the CDS circuit 12
And a capacitor 36b.

The second sample-and-hold circuit 36 receives the image signal (shown in FIG. 2A) from the CDS circuit 12 as shown in FIG. 2B), the comparator 35 compares the optical black level held by the capacitor 36b with a reference voltage from the reference power supply 34, thereby covering the black mask with, for example, as shown in FIG. 2C. At times t ₁ , t ₃ , and t ₄ at which an optical black level corresponding to the image pickup signal of the light receiving unit of the CCD 11 obtained normally is supplied to the gate circuit 37 during the period until the next clamp pulse, for example, a low level. and flows charge in the light receiving portion by excessive incident light at time t ₂ is not normal optical black level is obtained And supplies a signal as a next time to a clamp pulse (between times t _{2 ~} time t ₃₎ Hi-Lo level to the gate circuit 37.

The gate circuit 37 gates the clamp pulse supplied via the terminal 14 according to the signal from the comparator 35, and for example, as shown in FIG.
When the signal from is low, the clamp pulse passes through the first sample and hold circuit 33 (the clamp pulse that has disappeared due to gating is indicated by a broken line). The first sample-and-hold circuit 33 samples and holds the imaging signal from the amplifier 32 with the gated clamp pulse. As a result, when an abnormality of the optical black level is detected, the sample and hold operation of the first sample and hold circuit 33 is stopped, so that the abnormal optical black level can be prevented from being held, and the abnormal optical black level can be maintained. It is possible to prevent the image quality from deteriorating.

[0021]

As is apparent from the above description, according to the present invention, in the synchronous clamp type clamp circuit of the feedback system, the optical black level included in the input image signal is sampled by the second sample and hold means. The optical black level abnormality is detected by the detecting means, and when the optical black level is abnormal, the optical black level included in the clamped image signal output from the adding means is sampled and held. Since the control means controls the sample and hold operation of the first sample and hold means to be stopped, it is possible to prevent the abnormal optical black level from being held, thereby preventing the image quality from being deteriorated due to the abnormal optical black level. can do.

[Brief description of the drawings]

FIG. 1 is a block diagram showing a circuit configuration of a main part of a video camera to which a clamp circuit according to the present invention is applied.

FIG. 2 is a time chart for explaining the operation of the video camera.

FIG. 3 is a view showing a light receiving surface for explaining an overflow of charge of the CCD.

FIG. 4 is a diagram showing a relationship between a flash and a video camera.

[Explanation of symbols]

 Reference Signs List 30 clamp circuit 31 adder 33, 36 first and second sample-hold circuit 35 comparator 37 gate circuit

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) H04N 5/16 H04N 5/18

Claims (1)

(57) [Claims] 1. A first sample-and-hold means for sampling and holding an optical black level discretely included in an image pickup signal, and an optical black level periodically held by the first sample-and-hold means to obtain an image signal. And an adder for clamping the imaging signal, and sampling and holding the optical black level included in the imaging signal output from the adding means by the first sample and hold means. A synchronous clamp type clamp circuit of a feedback system for clamping by the following: a second sample and hold unit that samples and holds an optical black level discretely included in an image signal input to the addition unit; Detecting means for detecting an abnormality of the optical black level periodically held by the sample holding means, and an output of the detecting means. And a control means for controlling a stop of the sample and hold operation of the first sample and hold means when the optical black level is abnormal.