JPH04291590A - Clamp circuit for electronic endoscope device - Google Patents

Abstract

PURPOSE:To improve an S/N and to obtain clear images by eliminating the dark currents of chrominance signals having a high amplification factor in video signals or the influence of random noise by the clamp circuit. CONSTITUTION:After amplifying RGB video signals obtained at a CCD 1, the above-mentioned video signals are converted to digital signals by an A/D converter 5, these digital signals are temporaroly stored in a memory, and the images in an object to be observed are displayed. At such an electronic endoscope apparatus, a feedback clamp circuit 10 is connected just before the above- mentioned A/D converter 5 so as to enable operations while switching the clamp voltage of the chrominance signal having the high amplification factor in the above-mentioned RGB video signals to a voltage different from the clamp voltage of the other chrominance signal, and the black levels of the respective RGB video signals are made coincident to a reference line.

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, and more particularly to a clamp circuit for processing a video signal obtained by a solid-state image pickup device so that the black level is constant.

0002

2. Description of the Related Art An electronic endoscope device has an electronic endoscope (electronic scope) inserted into a body to be observed, such as a body cavity.
CD (Charge Coupled Device)
By capturing an image of the inside of the body to be observed, etc., the image inside the body to be observed is displayed in color on a monitor.

The circuit configuration of this type of device is shown in FIG. 4. In FIG. 4, a signal processing circuit 2 is connected to the CCD 1. Amplification processing is performed using different gains for each of the RGB video signals (red), G (green), and B (blue), and clamp processing is performed so that the black level of each RGB video signal is aligned with a constant reference line. In other words, there is a difference in the amplitude of each RGB video signal due to the spectral sensitivity characteristics of the solid-state image sensor such as the CCD1 mentioned above and the optical characteristics of the RGB color filter in the frame sequential method. The video signal is amplified (inverted amplification) using the gain, and the video signal is reproduced with direct current. A gamma (γ) correction circuit 3 is connected to this signal processing circuit 2, and in this gamma correction circuit 3, the electrical output to the monitor has a constant relationship with the amount of light input to the CCD 1. Such corrections are made.

A blanking circuit 4 is connected to the gamma correction circuit 3, and the blanking circuit 4 performs processing to match the blank portion between horizontal scans to the black level, which is a reference line. A memory 6 and a D/A converter 7 are connected to this blanking circuit 4 via an A/D converter 5, and a monitor (not shown) is externally connected to this D/A converter 7. Therefore, the video signal digitally converted by the A/D converter 5 is temporarily stored in the memory 6, and then read out at a predetermined speed, so that the image inside the object to be observed is displayed in color on the monitor. .

[0005]

[Problems to be Solved by the Invention] In the electronic endoscope device, as mentioned above, there are differences in RGB signal levels depending on the sensitivity and other optical characteristics of the solid-state image sensor such as the CCD 1, so the signal processing circuit In No. 2, amplification processing is performed with different gains. However, especially in CCD1, the sensitivity of blue (B) is extremely low, and the gain is set larger than that of other colors.This increases the influence of dark current and random noise, and after clamping, However, there was a problem that the black level was not aligned with the reference line. That is, for example, frame transfer type C
In CD1, polysilicon with low blue sensitivity is used in the light receiving section, and the level of the blue (B) signal is lower than that of other red (R) and green (G) signals. Therefore, the amplification gain of the B signal is about four times larger than that of the R signal, which increases low-level dark current and random noise, resulting in a significant deterioration of the S/N ratio.

FIG. 5 shows the output state of the video signal (vertical scanning period) from the blanking circuit 4 of FIG. 4, and as shown, the R signal and the G signal have a black level of 10.
0 is almost aligned with the reference line 101. However, in the case of the B signal, the black level 100 exists at a position slightly away from the reference line 101 as shown in the figure, and although the black levels of all video signals appear to match by the clamp operation,
The true black level of the signal will not be aligned with the reference line. Therefore, if you look carefully at the image displayed on the screen, it will appear bluish or foggy and unclear.

The present invention has been made in view of the above problems, and its purpose is to eliminate the effects of dark current and random noise of highly amplified color signals, improve the S/N ratio, and obtain clear images. An object of the present invention is to provide a clamp circuit for an electronic endoscope device that can perform the following functions.

[0008]

[Means for Solving the Problems] In order to achieve the above object, the present invention amplifies an RGB video signal obtained by a solid-state image sensor, and then converts the video signal into an A/D converter.
In an electronic endoscope device that converts the digital signal into a digital signal using a converter and temporarily stores this digital signal in a memory to display an image inside the object, the RGB video signal is converted into a digital signal immediately before the A/D converter. The present invention is characterized in that a clamp circuit is connected which operates by switching the clamp voltage of a highly amplified color signal to a voltage different from the clamp voltage of other color signals so that each black level of RGB coincides with each other.

[0009]

[Operation] According to the above configuration, the video signal is subjected to DC reproduction in the clamp circuit, but for example, the R (red) and G (green) signals in this circuit are processed by a predetermined clamp as in the conventional case. At the same time as the clamp operation is performed by voltage, B (blue)
In the case of a signal, the clamping operation is performed by switching to a different voltage, for example, a clamping voltage slightly lower than the above-mentioned clamping voltage. According to this clamp voltage of the B signal, changes in the black level due to dark current etc. that occur when the B signal is amplified are corrected, and the black level of all color video signals matches the reference line. Become.

0010

[Embodiment] Fig. 1 shows the configuration of a clamp circuit for an electronic endoscope device according to an embodiment. A gamma correction circuit 3 that performs correction, a blanking circuit 4 that fills in signals in blank portions between horizontal scans, and an A/D converter 5 that converts analog video signals into digital signals are provided. Further, a signal processing circuit 8 is provided after the CCD 1, and in this embodiment, each RGB video signal is extracted and amplified (inverted amplification) by each RGB enable (ENE.) signal. In addition to the processing, clamp processing is also performed to match the black levels.

A feedback clamp circuit 10 is provided immediately before the A/D converter 5, and this feedback clamp circuit 10 includes an adder 11, an amplifier 12 as a buffer, and an optical black pulse (O
It has a sample hold (S/H) circuit 13 and a comparator 14 to which BP) is input as a sample pulse. In other words, although the above-mentioned signal processing circuit 8 performs clamp processing, the R signal and G signal are Since the amplification degree of the B signal is increased in comparison, dark current, random noise, etc. are amplified, and there is a problem that the position of the true black level is not completely aligned. Therefore, in the embodiment, the above feedback clamp circuit 10 is used to match the black level of each RGB signal to a reference value.

Such a feedback clamp circuit 1
In the comparator 14 of 0, the reference voltage for comparison is set.
Two voltage sources 15 and 16 are provided, and one voltage source 15 sets a reference voltage V1 that serves as a clamp voltage for the R signal and the G signal, and the other voltage source 16 sets a reference voltage VB that serves as a clamp voltage for the B signal. ing. A B-ENE. A switch 17 that performs a switching operation based on a signal is inserted, and this switch 17 switches between the voltage sources 15 and 16.

FIG. 2 shows the sample hold circuit 13
An explanation of the OBP supplied in the figure is shown, and the video signal obtained by the horizontal synchronization signal in figure (a) is a signal as shown in figure (b). This video signal includes blanking 200 every horizontal synchronization period, but a light shielding portion is set at the end of the CCD 1 so that an optically black portion 201 exists immediately before the blanking 200. be done. Therefore, the black level (voltage) can be extracted by sampling and holding this black portion 201 using an optical black pulse (OBP) shown in FIG. 3(c).

The embodiment has the above configuration, and is shown below in FIG.
The effect will be explained with reference to. When the video signal obtained by the CCD 1 in FIG. 1 is supplied to the signal processing circuit 8, video signals of each color are extracted by the RGB enable signals shown in FIGS. 3(b) to 3(d). It is amplified (inverted amplification) with different gains, and clamped so that its black level matches the reference value. The output of the signal processing circuit 8 will be supplied to the feedback clamp circuit 10 after being subjected to gamma correction and blanking processing, but the output signal of the blanking circuit 4 is shown in FIG. The video signal (one vertical scanning period) is as shown in FIG. Since the B (blue) signal has low sensitivity, the amplification gain in the signal processing circuit 8 is R (red).
Since there is a difference of about four times compared to the signal, the true black level 100 of the B signal deviates from the reference line 101 as shown in the figure.

The output of the blanking circuit 4 is sent to the sample hold circuit 1 in the feedback clamp circuit 10.
The sample and hold circuit 13 detects the DC level of the video signal based on the input OBP, thereby determining the black level (voltage) of the video signal.
is taken out as a sample and hold voltage. and,
This sample and hold voltage is compared by a comparator 14, and the difference between it and the reference voltage set in the comparator 14 is determined by an adder 11.
Output to. In this case, when the video signal is an R (red) signal or a G (green) signal, the switch 17 is connected to the voltage source 15 side as shown in the figure, and the voltage V1 is set as the reference voltage of the comparator 14. . Therefore, in the case of R and G signals, the comparator 14 outputs the difference from the reference voltage V1, so this difference signal is added to the adder 11, and if the sample and hold voltage is higher than the voltage V1, blanking is performed. The output of the circuit 4 operates so that it becomes low, and vice versa, so that the black level becomes the voltage V1 as shown in FIG. 3(e).
They will be all together.

On the other hand, when the B (blue) signal is present, the B-ENE.
Based on the signal, the switch 17 is switched to another voltage source 16, and the voltage VB is set as the reference voltage of the comparator 14. Therefore, in the case of the B signal, the comparator 14 compares the sample-and-hold voltage with the voltage VB, and when the sample-and-hold voltage is higher than the voltage VB, the output of the blanking circuit 4 is lowered. In this case, the true black level of the B signal becomes equal to the voltage V1 as shown in FIG. 3(e).

That is, as shown in the figure, components of dark current and random noise are dropped between the reference voltages V1 and VB, and as a result, all RGB video signals are processed with improved S/N ratio. The black levels match. Further, in the embodiment, by using a feedback clamp circuit, it is possible to satisfactorily match the black levels of each color of the video signal, which were not evened up by the clamping operation of the signal processing circuit 8. This video signal is then digitally converted by the A/D converter 5, and the black level is determined by LSB (
The maximum value of the video signal is converted to the MSB (minimum conversion bit), which is temporarily stored in memory, and then read out in accordance with the scanning speed of the monitor. As a result, a clear image is displayed on the monitor, and the inside of the object to be observed can be observed satisfactorily. In addition, in the above,
Although the amplifier 12 functions as a buffer, it is also possible to perform predetermined amplification for adjustment or to take over the amplification processing performed by the signal processing circuit 8.

Although the feedback clamp circuit 10 is used in the above embodiment, the present invention can be adapted to use a commonly used clamp circuit to switch the clamp voltage of a color signal having a large amplification factor. In addition, in the example, B
(Blue) The clamp voltage of the signal is switched, but this switching is determined by the characteristics of the image sensor, and the G (green) signal is switched.
When the signal amplification factor becomes high, the G signal is to be switched, and it is also possible to switch to two types of color signals.

[0019]

[Effects of the Invention] As explained above, according to the present invention,
Immediately before the A/D converter of the electronic endoscope device, connect a clamp circuit that operates by switching the clamp voltage of the highly amplified color signal in the RGB video signal to a voltage different from the clamp voltage of other color signals. This reduces S caused by the effects of dark current and random noise in highly amplified color signals.
It is possible to prevent the deterioration of the N ratio and to match the true black level with the reference line, making it possible to display a clear image on the monitor.

[Brief explanation of the drawing]

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

FIG. 2 is a waveform diagram for explaining the operation of the sample and hold circuit of the embodiment.

FIG. 3 is a waveform diagram for explaining the operation of the example circuit.

FIG. 4 is a circuit block diagram showing the configuration of a conventional electronic endoscope device.

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

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1... CCD, 2,8... Signal processing circuit, 3... Gamma correction circuit, 5... A/D converter, 10... Feedback clamp circuit, 11
...Adder, 13...Sample and hold circuit, 14...
Comparator, 15, 16... Voltage source.

Claims (1)

[Claims] Claim 1: After amplifying an RGB video signal obtained by a solid-state image sensor, the video signal is converted to an A/D converter.
In an electronic endoscope device that converts the digital signal into a digital signal using a converter and temporarily stores this digital signal in a memory to display an image inside the object, the RGB video signal is converted into a digital signal immediately before the A/D converter. An electronic endoscope is connected to a clamp circuit that operates by switching the clamp voltage of a highly amplified color signal to a voltage different from the clamp voltage of other color signals so that each black level of RGB matches each other. Clamp circuit for equipment.