KR20070025136A - Method for compensating digital gain in cmos image sensor - Google Patents

Abstract

A method for compensating a digital gain in a CMOS image sensor is provided to actively compensate a signal level by using gain values of a variable amplifier and a color variable amplifier, thereby obtaining a reliable signal value for image processing. A method for compensating a digital gain in a CMOS(Complementary Metal Oxide Semiconductor) image sensor comprises the following steps of: detecting a gain value of an analog variable amplifier and a gain value of a color variable amplifier; calculating a gain value which will be used in a digital gain compensator after reading the gain values of the analog variable amplifier and the color variable amplifier according as the analog variable amplifier or the color variable amplifier shows a gain value below one time; and enabling the digital gain compensator to perform differential gain compensation for a luminance value and a color value by using the calculated gain value.

Description

Digital gain compensation method of CMOS image sensor {METHOD FOR COMPENSATING DIGITAL GAIN IN CMOS IMAGE SENSOR}

1 is a block diagram illustrating a CMOS image sensor.

2 is a block diagram of a CMOS image sensor in accordance with an embodiment of the present invention.

3 shows an example of signal level compensation according to gain values of a variable amplifier and a color variable amplifier.

The present invention relates to a CMOS image sensor related technology, and more particularly, to a digital gain compensation method of a CMOS image sensor.

An image sensor refers to a device for taking an image by using a property of a semiconductor in response to light. In other words, the pixel senses the brightness and wavelength of different light emitted from each subject and reads it as an electric value. It is the role of the image sensor to make this electrical value (code value) a level capable of signal processing.

The current image sensor market is divided into charge coupled device (CCD) and CMOS image sensors.

The CCD is an integrated light emitting diode that receives light and converts it into an electrical signal. A rectangular, 1 / 3-inch 1 / 2.7-inch 1 / 2-inch 2 / 3-inch CCD lens is commonly used. The CCD uses a method of converting the intensity of light into the amount of charge and making it accumulate in electrons. Therefore, in the CCD image sensor, the pixel, which is a constituent particle of the image, receives light and is changed into a flow of electrons (charge) by a special semiconductor. The CCD semiconductor then sends this charge to the modulator to convert it into a normal electrical signal. This signal is decoded by the electric circuit to synthesize the image.

On the other hand, CMOS image sensors have a modulator attached to each pixel that converts charge into an electrical signal. Because of this, only necessary data can be read and the decoding speed is faster than that of CCD. In addition, in CCD image sensors, the transfer of charge requires the power of a number of special voltages not used in conventional semiconductors, but CMOS image sensors do not. Only one type of voltage is required. Thus, CMOS image sensors operating at a single voltage can lower power consumption compared to CCDs. It also has the advantage of miniaturizing the entire system because no IC is needed to control the various voltages. In addition, the merit of the CMOS image sensor is particularly noticeable in terms of miniaturization and the possibility of one chip with a peripheral circuit. Because CCD image sensors require special high voltages, they cannot be attached to silicon wafers, such as peripheral chips, which are responsible for processing the captured images. On the other hand, a CMOS image sensor that uses only the same level of voltage as a conventional semiconductor can be integrated onto a single silicon wafer along with surrounding signal processing circuitry. For example, a circuit for converting image data captured by a CMOS image sensor into a digital signal, a memory for storing data, and a micro arithmetic processing device for controlling data output or the entire device can be integrated in one chip. . This enables chip miniaturization and cost down simultaneously.

Like this, CMOS image sensor is composed of mass-produced CMOS switch, so the circuit integration is high, and it can be integrated with peripheral IC to add various functions, and power consumption is about 10% of CCD and the price is low. ), It is widely applied to PC camcorders and mobile communication terminals.

1 is a block diagram illustrating a CMOS image sensor.

Referring to FIG. 1, the CMOS image sensor is largely comprised of a pixel array unit 10, an analog / digital converting unit 11, a pixel line memory unit 12, an image signal processing unit 13, a control and an external system. It consists of an interface unit 14.

The control and external system interface unit 14 controls the overall operation of the CMOS image sensor using a finite state machine (FSM), and serves as an interface with an external system. In addition, it is provided with a batch register, which allows programming of matters related to various internal operations, and controls the operation of the entire chip according to this program information.

The pixel array unit 10 includes unit pixels arranged in horizontally N and vertically M (N × M) patterns for maximizing the response of light. The pixel array unit 10 detects information about an image from the outside. It is an integral part of the overall sensor.

The analog / digital converter 11 converts the analog voltage sensed by each pixel of the pixel array unit 10 into a digital value so that the digital system can process it.

The pixel line memory unit 12 stores digital values of the pixels converted by the analog / digital converter 11 and is composed of several lines to perform various functions of the image signal processor of the rear stage.

The image signal processor 13 may perform color interpolation, color correction, gamma correction, auto white balance correction, and auto exposure depending on the function of the image sensor. Various functions are performed to improve the performance of the image sensor based on the output values of the pixels stored in the pixel line memory unit 12 such as correction.

Meanwhile, in the CMOS image sensor including the automatic exposure compensation device and the automatic white compensation device in the image signal processor 13, the gain adjustment values of the variable amplifier and the color correction variable amplifier move dynamically according to various external environmental conditions. Under certain conditions where bright light is input, the RGB saturation range of the signal may vary, resulting in poor image quality such as whites not appearing white.

That is, the conventional CMOS image sensor performs image correction by applying a sub-gain value of 1 or less, since the output image is saturated when the variable amplifier gain value of the automatic exposure compensation apparatus is high in a bright light input environment. do. In addition, the digital gain compensator compensates for the reduced signal level in the digital stage based on the variable amplifier gain ratio to compensate for the dynamic range due to the gain reduction in the exposure compensation.

However, since the exposure compensation and the color correction are divided into separate areas, the correction is performed, so that the analog variable amplifier and the color variable amplifier operate in different spaces. Therefore, under a specific condition where bright light is input, the color variable amplification unit is applied with a gain value of 1 or less, and the digital gain compensator compensates for the gain value reduced in the variable amplification, while the color signal is reduced to secure the signal level for the color signal. it's difficult. This causes problems such as chroma saturation.

The present invention has been proposed to solve the above problems of the prior art, and provides a digital gain compensation method of a CMOS image sensor that can secure a signal level for a color signal in an environment where bright light is input. have.

According to an aspect of the present invention for achieving the above technical problem, detecting the gain value of the analog variable amplifier and the gain value of the color variable amplifier; Reading gains of the analog variable amplifier and the color variable amplifier as the analog variable amplifier or the color variable amplifier exhibits gain values of 1 times or less and calculating a gain value to be used in the digital gain compensator; And performing gain compensation on the luminance value and the color value in the digital gain compensator differently using the calculated gain value.

The present invention compensates the dynamic range of a signal through signal compensation in a digital stage in which the level of the signal varies depending on the analog variable amplifier and the color variable amplifier, thereby preventing color distortion. As analogue variable and color-variable amplifiers change in different areas due to changes in the external environment, the ADC's dynamic range reduction can be compensated at the digital stage behind the ADC without compensating at the analog stage. Accordingly, the present invention detects the gain of the analog variable amplifier and the gain of the color variable amplifier of the CMOS image sensor so that the luminance and the color can be actively corrected in the case where a single amplifier shows a gain of less than 1 times. Designed. Therefore, the compensation of the same gain compensation value for the brightness and the color signal only by compensation of the variable amplifier can prevent the chromatic drift due to the amplification factor less than 1 generated in the color variable amplification unit, and the variable amplification unit sensitive to external conditions or The digital gain compensation unit compensates for the stable signal range width rather than the color variable amplification unit.

Hereinafter, preferred embodiments of the present invention will be introduced in order to enable those skilled in the art to more easily carry out the present invention.

In the image sensor, the luminance component is the most sensitive component of the human eye. The luminance value can be adjusted by adjusting the condensing time and adjusting the variable amplification unit. For example, even though the same condensing time is given, the luminance values are different depending on the brightness around the image sensor. Among the methods of adjusting the luminance value, the method of adjusting the condensing time can suitably adjust the luminance component, but when the condensing time does not become an integer multiple of the period of the ambient light source, the CMOS image sensor processes the line method. The problem arises that it will cause flicker.

On the other hand, in the case of adjusting the luminance value by adjusting the variable amplifier, the banding noise can be minimized, but there is a problem of amplifying even the noise from the pixel. Therefore, the automatic exposure compensation unit adjusts the average latitude information and the target latitude value of the image from the outside. In comparison, the appropriate brightness control method is selected.

In the case of color correction, when the light source of each case of sunlight, fluorescent lamp and incandescent lamp is irradiated on the same white paper, the spectral distribution curve characteristics of the light source make natural color in the sunlight, blue in fluorescent lamp, and red in incandescent lamp. At this time, human recognizes color by correcting color difference caused by different light sources, but when photographing an object with an image sensor, it inputs the difference of color represented by actual light source as it is. When taken with a sensor, different color temperatures are represented. At this time, the image sensor automatically controls the size of the color component input according to the light source, such as a human, automatically. Therefore, the autoexposure corrector and the auto white corrector operate on different areas.

2 is a block diagram of a CMOS image sensor according to an embodiment of the present invention.

Referring to FIG. 2, when the operating environment of the image sensor moves from a dark environment to a bright environment, if the average luminance is greater than the target luminance value, first, the gain value of the variable amplification unit is calculated by the step value calculated in the negative direction. Decrease. At this time, if the average brightness is greater than the target latitude even though bright light continues to decrease and the gain value of the variable amplifier is reduced, the banding lock is released and the focusing time is reduced. After that, if the gain of the variable amplifier continues to decrease and becomes smaller than the banding gain, the gain of the analog variable amplifier acts as a sub-gain on the incoming input signal, resulting in a gain less than one. As such, the automatic color compensator operates with the signal value coming from the pixel array fixed at a gain less than 1 of the variable amplifier. The automatic color corrector adjusts the color variable amplification unit using the saturation average value obtained from the automatic color compensator.

3 is a diagram illustrating an example of signal level compensation according to gain values of a variable amplifier and a color variable amplifier.

Referring to FIG. 3, when the color variable amplifier and the gain value of 1 or less are applied when the correction value calculated by the automatic exposure compensation device adjusts the gain value of the variable amplifier and the gain value of 1 or less is applied, the existing digital gain. Since the compensator calculates the correction value of the digital gain compensator only by the gain value of the variable amplifier, there is no compensation method for applying a gain value of 1 or less in the color amplifier.

To solve this problem, if the gain of the color variable amplification unit is smaller than the reference target gain in consideration of the saturation compensation reference value, the gain of the variable amplifier and the color variable amplifier is read and the gain to be used in the digital gain compensator using the two gain values. Calculate the value. Using the calculated gain value, the digital gain compensator base compensates the luminance value and the color value differently.

Therefore, by applying the present invention, the active digital gain compensator is applied to prevent color distortion due to environmental changes, thereby keeping the maximum signal range width constant, thereby solving color distortion caused by the signal range reduction, thereby greatly improving image color reproduction. Contribute.

Although the technical idea of the present invention has been described in detail according to the above preferred embodiment, it should be noted that the above-described embodiment is for the purpose of description and not of limitation. In addition, those skilled in the art will understand that various embodiments are possible within the scope of the technical idea of the present invention.

When the image sensor is exposed to various environments, the dynamic range is not guaranteed due to the reduction of the gain value of the variable amplifier and the color variable amplifier controlled by the automatic exposure compensation device and the automatic white compensation device. The output was as follows to cause color distortion. The present invention has a great effect to secure a reliable signal value for image processing by actively compensating the signal level using the gain value of the variable amplifier and the color variable amplifier.

Claims (1)

Detecting a gain value of the analog variable amplifier and a gain value of the color variable amplifier; Calculating gains to be used in a digital gain compensator by reading gains of the analog variable amplifier and the color variable amplifier as the analog variable amplifier or the color variable amplifier exhibits gain values of 1 times or less; And Performing gain compensation on the luminance value and the color value differently in the digital gain compensator using the calculated gain value Digital gain compensation method of the CMOS image sensor comprising a.

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