JP2579299B2 - Clamp circuit for electronic endoscope device - 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 an electronic endoscope apparatus, and more particularly to a clamp circuit for processing a video signal obtained by a solid-state image sensor so that a black level of the video signal becomes constant.

[0002]

2. Description of the Related Art In an electronic endoscope apparatus, an electronic endoscope (scope) is inserted into a body to be observed, for example, in a body cavity, and a CCD as a solid-state image pickup device provided at the tip of the electronic scope.
(Charge Coupled Device) or the like to image the inside of the object to be observed, thereby displaying an image of the inside of the object to be observed in color on a monitor.

FIG. 4 (a) shows a circuit configuration of this type of apparatus. In the figure, an amplifier 2 and a clamp circuit 3 are connected to a CCD 1, and a video signal obtained by the CCD 1 is output by the amplifier 2. R (red), G (green), B (blue)
After being inverted and amplified with a different gain for each time, the clamp circuit 3 performs a clamp (DC reproduction) process by the clamp pulse CP1 so that the black (black) level of each RGB video signal is equal to a constant reference value. That is, the above C
Due to the spectral sensitivity characteristics of a solid-state imaging device such as a CD1 and the optical characteristics of an RGB color filter in the field sequential method, RGB
There is a difference in the amplitude of each video signal. In order to eliminate the difference in the signal level, the video signal is amplified with different gains and the video signal is clamped.

The above-mentioned clamp circuit 3 has a video signal 1
A blanking circuit 4 for adjusting a blank portion between horizontal scans to a black level is connected, and FIG. 5 shows signal processing in the blanking circuit 4. In the video signal shown in FIG. 5A, the optically black portion 100 has already been clamped by the clamp circuit 3, and thereafter the blanking circuit 4 performs one horizontal scanning period (1).
Immediately after the optical black portion 100 in H), a blank portion 101 of the scan is formed. In the blanking circuit 4, the blank portion 101 is clamped based on the clamp pulse CP2 in FIG.
As shown in FIG.
Blanking level insertion or level adjustment is performed so that

A gamma (γ) correction circuit 5 is connected to the blanking circuit 4 (FIG. 4A). The gamma correction circuit 5 controls the amount of light input to the CCD 1 to a monitor. Correction is performed so that the electrical output has a fixed relationship. That is, as shown in FIG. 4B, since the output characteristic of the TV monitor becomes non-linear as shown by the curve 300, the input signal is corrected by the inverse characteristic of the curve 301 to obtain the linear input of the video signal. Output characteristics are ensured, so that image information obtained by the CCD 1 can be reproduced on a monitor in a faithful color.

[0006]

However, in the clamp circuit of the above-mentioned electronic endoscope apparatus, the blanking circuit 4 itself performs a clamp operation separately from the clamp circuit 3 so as to match a preset reference voltage. Figure 5
As shown in (c), the blank portion 1 of the video signal
01 has a problem that the black level does not completely match the reference line 200 to be reproduced.

In the gamma correction circuit 5,
As described above, the video signal is amplified by the gain of the curve 301 in FIG. 4B, and the subsequent circuit also performs the clamping process and the like. The operation is performed with reference to the sample and hold voltage (black level) at the time of the operation. However, in this case, since the sample and hold voltage becomes the black level value detected by the relatively narrow clamp pulse CP1, there is a problem that the S / N ratio of the video signal is reduced due to fluctuations for each scanning line. That is, an accurate value can be obtained by detecting a black level voltage of a video signal serving as a reference for gamma correction processing or the like with a wide-width pulse (clamp pulse), and the optical black level shown in FIG. Part 100
In addition, it is only necessary to detect a wide range of black levels including the blank portion 101. However,
Conventionally, as described above, since the blank portion 101 is not completely aligned with the black level, the black level value of this portion cannot be used.

SUMMARY OF THE INVENTION The present invention has been made in consideration of the above problems, and has as its object to make a blank portion of a video signal completely coincide with a black level of an optical black portion, thereby improving an SN ratio in subsequent signal processing. An object of the present invention is to provide a clamp circuit for an electronic endoscope device that can be operated.

[0009]

In order to achieve the above object, a clamp circuit according to the present invention provides an electronic endoscope apparatus for performing a blanking process and a clamp process on a video signal obtained by a solid-state imaging device. A feedback loop connected before and after the blanking circuit for performing the blanking process in the clamp circuit for sampling and holding the optical black portion of the video signal output from the blanking circuit, And a feedback loop connected to form a double loop with the first clamp circuit, wherein the first clamp circuit clamps the video signal so that the video signal is equal to the reference voltage. The output during blanking operation is sampled and held, and this sample and hold voltage is equal to the reference voltage. Kunar so is characterized by providing a second clamp circuit for clamping a video signal, to the.

[0010]

According to the above arrangement, the video signal is inputted to the blanking circuit after being amplified, and unnecessary signal components during scanning are removed by the blanking circuit. , And to the first and second feedback clamp circuits. Then, in the first clamp circuit, the black level is sampled and held by the first clamp pulse from the optical black (optical black) portion of the video signal, and the black level is clamped so as to be equal to the reference voltage. The video signal is sampled and held by the second clamp circuit at the level of the blank portion, and is clamped so that the level of the blank portion matches the reference voltage. Therefore, the levels of the optical black portion and the blank portion of the video signal match the reference voltage.

[0011]

FIG. 1 shows a configuration of a clamp circuit for an electronic endoscope apparatus according to a first embodiment. As shown in FIG.
The amplifier 10 to which the video signal is input is a variable amplifier for the G (red) and B (blue) channels together with the switching circuit 11. ENE. (Enable) signal,
B. Amplified with different gains depending on the ENE. Signal. A gamma (γ) correction circuit 14 is connected to the amplifier 10 via a mixer 12. A switch S 1 for performing a switching operation by a composite blanking pulse (C.BLK) and an amplifier are provided downstream of the gamma correction circuit 14. 15 is provided.

Then, a loop is formed to feed back the mixer 12 from the subsequent stage of the amplifier 15, and a first loop is formed in this feedback loop.
And a loop is formed to feed back from the subsequent stage of the amplifier 15 to the input terminal b of the switch S1,
A second clamp circuit 20 is provided in this feedback loop. Therefore, the first and second feedback clamp circuits 18 and 20 form a double feedback loop with the blanking circuit 16 interposed therebetween.

FIG. 2 shows a detailed circuit configuration of the circuit shown in FIG. 1. In FIG. 2, a mixer 12 includes resistors R1, R
2 and a capacitor C1, and the gamma correction circuit 14 is a well-known circuit as shown in FIG.
Is a switch S1, an emitter-follower transistor TR1, a feedback output transistor TR2,
It comprises a transistor TR3 for outputting to the processing circuit of the next stage. The first clamp circuit 18 includes a sample-and-hold circuit including a field-effect transistor TR4 for performing a switching action and a capacitor C2, and performs a sampling operation by applying a clamp pulse CP1 to the gate terminal of the transistor TR4. The voltage of the optical black part
Holds at 2. An operational amplifier 23 as a comparator is provided at the subsequent stage of the sample-and-hold circuit via an operational amplifier 22. The (positive input terminal of) the operational amplifier 23 is supplied with a reference voltage from a variable voltage source VR. The operational amplifier 23 has a capacitor C3,
Also functions as a low-pass filter. According to the first clamp circuit 18, the voltage level of the optical black portion of the video signal output from the blanking circuit 16 is sampled and held by the clamp pulse CP1, and this signal is compared with the reference voltage. By being added to the mixer 12, the optical black portion is aligned with the black level (reference voltage).

On the other hand, the second clamp circuit 20 has a configuration similar to that of the first clamp circuit 18, has a sample and hold circuit including a transistor (FET) TR5 and a capacitor C4, and has a clamp at the gate terminal of the transistor TR5. A pulse CP2 is applied, whereby the voltage of the blank portion sampled is held by the capacitor C4. An operational amplifier 24 and an operational amplifier 25 as a comparator are provided at the subsequent stage of the sample and hold circuit, and the operational amplifier 25 is supplied with the same reference voltage from the variable voltage source VR as in the case of the clamp circuit 18.
A parallel circuit of a resistor R3 and a capacitor C6 is inserted between the operational amplifier 25 and the switch S1.
The responsiveness of the blanking operation by switching the switch S1 is improved by the capacitor C6. According to the second clamp circuit 20, the voltage level of the blank portion of the video signal input from the amplifier 15 is sampled and held by the clamp pulse CP2, and this voltage is compared with the reference voltage. When the output is fed back via the terminal b, the blank portion is aligned with the black level (reference voltage).

The embodiment has the above configuration.
The operation will be described based on FIG. First, a video signal obtained by the CCD (1), which is a solid-state imaging device, is
After the inversion amplification in the mixer 12, the gamma correction circuit 1
4 to the blanking circuit 16. In the blanking circuit 16, the video signal shown in FIG. 5A is blanked by the C.BLK signal shown in FIG. 5B, and the switching operation of the switch S1 based on the C.BLK signal is performed. As a result, a blank portion 101 is formed in the video signal as shown in FIG. Then, this video signal is supplied to the first clamp circuit 18.
Is supplied to the second clamp circuit 20, and is supplied to the first clamp circuit 18 in FIG.
The optical black portion is sampled and held by the first clamp pulse CP1 shown in FIG. This sample and hold voltage is compared with a reference voltage set by the variable voltage source VR by an operational amplifier 23 as a comparator,
The difference voltage is output to the mixer 12, and the mixer 12 adds the difference voltage to the video signal.
Therefore, when the sample and hold voltage is lower than the reference voltage, the voltage of the video signal is increased, and when it is higher, the voltage of the video signal is reduced. As a result, as shown in FIG. The optical black portion 100 of the video signal for each horizontal scanning period is aligned with the reference voltage level 200 which is the black level.

On the other hand, in the second clamp circuit 20,
The blank portion 101 is sampled and held by the clamp pulse CP2 shown in FIG. This sample-hold voltage is compared with a reference voltage set by the variable voltage source VR by an operational amplifier 25 as a comparator, and the difference voltage is added to the video signal via the terminal b of the switch S1. Therefore, when the sample and hold voltage is lower than the reference voltage, the voltage of the video signal is increased, and when it is higher, the voltage of the video signal is reduced. As a result, as shown in FIG. Then, the blank portion 101 of the video signal is aligned with the reference voltage level 200 which is the black level.

As described above, in the embodiment, the optical black portion 100 is controlled by the first clamp circuit 18.
The blank portion 101 is DC-reproduced to a black level (reference voltage 200) by the second clamp circuit 20 and both are simultaneously processed by the double clamp circuit, so that the optical black portion 100 and the blank portion 101 can be precisely black level. Will match.

In this case, the processing after the gamma correction processing and the processing after the blanking circuit 16 are set not in the first clamp pulse CP1 having a narrow width as in the related art but in an area including the blank portion 101. Can be processed using a wide clamp pulse.
Can be processed based on the black level value obtained by using the clamp pulse CP2. When processing is performed with this wide clamp pulse, even if there is noise in the black level of the video signal, it can be averaged, and the accurate black level value is used as a reference for amplification processing for gamma correction or a reference for subsequent clamp processing and the like. It is possible to improve the S / N ratio of the video signal.

In the above embodiment, the gamma correction circuit 14 is arranged in the feedback loop of the first and second clamp circuits. However, this gamma correction circuit 14 is arranged and connected at the subsequent stage of the blanking circuit 16. You may do so.

[0020]

As described above, according to the present invention,
A double feedback loop is connected before and after the blanking circuit that performs the blanking process, and one feedback loop samples and holds the optical black portion of the video signal output from the blanking circuit. Is provided with a first clamp circuit for clamping a video signal so that the voltage becomes equal to the reference voltage, and the other feedback loop samples and holds the output during the blanking operation of the blanking circuit. Since the second clamp circuit for clamping the voltage to be equal to the voltage is provided, the blank portion of the video signal can be accurately matched with the black level. As a result, in the gamma correction or the video signal processing thereafter, there is an advantage that the processing can be performed with a clamp pulse having a wide width, and the signal processing can be favorably performed with the SN ratio improved.

The use of a double clamp circuit also has the advantage that the circuit configuration can be simplified and simplified.

[Brief description of the drawings]

FIG. 1 is a circuit block diagram illustrating a configuration of a clamp circuit for an electronic endoscope apparatus according to an embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific and detailed configuration of a circuit according to an embodiment.

FIG. 3 is a waveform chart for explaining the operation of the embodiment circuit.

4A and 4B are diagrams illustrating a conventional electronic endoscope apparatus, wherein FIG. 4A is a circuit block diagram showing a configuration inside the apparatus, and FIG.
FIG. 3 is a diagram for explaining gamma correction.

FIG. 5 is a waveform diagram showing a video signal processed by a conventional circuit.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... CCD, 2,10,15 ... Amplifier, 5,14 ... Gamma correction circuit, 12 ... Mixer, 16 ... Blanking circuit, 18 ... First clamp circuit, 20 ... Second clamp circuit, S1 ... Switch , R1, R3 ... resistance, C1-C7 ... capacitor, TR1-TR5 ... transistor.

Claims (1)

(57) [Claims] 1. A clamp circuit for an electronic endoscope apparatus that performs a blanking process and a clamp process on a video signal obtained by a solid-state imaging device, and is connected to a stage before and after the blanking circuit that performs the blanking process. A first clamp circuit that samples and holds an optical black portion of a video signal output from the blanking circuit, and clamps the video signal so that the sample / hold voltage becomes equal to a reference voltage; A feedback loop connected to form a double loop together with the first clamp circuit, which samples and holds an output of the blanking circuit during a blanking operation; A second clamp circuit for clamping the video signal so as to be equal. And a clamp circuit for an electronic endoscope device.