CN110708481B - Fixed pattern noise removing method based on difference reset - Google Patents
Fixed pattern noise removing method based on difference reset Download PDFInfo
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Abstract
The invention discloses a method for removing fixed pattern noise based on difference reset, which comprises the following steps: s01: carrying out single-frame segmented exposure on the pixel array; s02: the method for reading signals of the pixel array specifically comprises the following steps: s021: carrying out soft reset: setting a reset signal in a pixel unit as an intermediate voltage, and reading a differential reset signal; s022: and (3) carrying out hard reset: setting a reset signal in a pixel unit to a high voltage; s023: opening the transmission MOS tube to enable an exposure signal in the photodiode to be transmitted to the suspended diffusion region, and reading a differential pixel transmission signal; s03: and subtracting the difference pixel transmission signal and the difference reset signal to obtain an exposure signal with the fixed pattern noise removed. The invention provides a fixed pattern noise removing method based on difference reset, which eliminates the fixed pattern noise in an image by introducing the fixed pattern noise identical to an exposure signal when a reset signal is read and by subtraction processing.
Description
Technical Field
The invention relates to the field of image sensors, in particular to a method for removing fixed pattern noise based on difference reset.
Background
The CMOS image sensor is widely used in daily life and processes due to its advantages of high integration, low power consumption, low cost, and the like. For a CMOS image sensor, both high dynamic range and high frame rate are important indicators for measuring the CMOS image sensor. For a CMOS image sensor with a small dynamic range, if a small signal needs to be captured, a long exposure time needs to be adopted, but the large signal is saturated, and large signal data cannot be obtained; on the contrary, if a large signal is captured, the exposure time needs to be shortened, but the small signal cannot be distinguished, and effective data of the small signal is obtained. High dynamic range is therefore an important indicator of CMOS image sensors.
At present, the common methods for improving the dynamic range mainly comprise three methods of multi-frame exposure fusion, different conversion gains and segmented exposure, but the multi-frame exposure fusion needs to obtain a large and small complete signal of one image through multi-frame exposure data, and the frame rate needs to be sacrificed; the gain conversion method is to convert the two frames with different gains to obtain an image, or to read data with different conversion gains for each pixel (also called pixel unit) in one frame, and the two methods can also reduce the frame rate by half. Both approaches of multi-frame exposure fusion and different conversion gains require additional ISP (digital signal processor) algorithmic processing. Redundant signals in the large signals overflow to a power supply in a single frame in a segmented exposure mode, so that the originally saturated large signals are not saturated, and the small signals are not influenced.
However, a light sensitive curve in a single-frame segmented exposure HDR (High-Dynamic Range) mode may form a knee point, and the knee point of each pixel is different due to different threshold voltages of segmented exposure control tubes of each pixel, that is, a knee point difference is formed; so that FPN (Fixed Pattern Noise) of the image increases. One of the prior art is to eliminate the inflection point difference of each line of output signals by pre-exposing each line and then collecting the reference output signals containing the inflection point difference, thereby achieving the effect of optimizing the FPN. However, this approach to eliminating FPN doubles the processing time per line, reducing the image sensor frame rate.
Disclosure of Invention
The invention aims to provide a fixed pattern noise removing method based on difference reset, which eliminates the fixed pattern noise in an image through subtraction processing by introducing the fixed pattern noise identical to an exposure signal when a reset signal is read.
In order to achieve the purpose, the invention adopts the following technical scheme: a method for removing fixed pattern noise based on difference reset comprises the following steps:
s01: carrying out single-frame segmented exposure on the pixel array, wherein the single-frame segmented exposure comprises low-full-well exposure and high-full-well exposure which are respectively carried out in the same frame; the pixel array comprises a pixel unit, the pixel unit comprises a transmission MOS tube, a photodiode, a reset MOS tube and a suspension diffusion region, the photodiode is connected with the suspension diffusion region through the transmission MOS tube, and the suspension diffusion region is also connected with a reset signal through the reset MOS tube;
s02: the method for reading signals of the pixel array specifically comprises the following steps:
s021: carrying out soft reset: setting the reset signal in the pixel unit to be an intermediate voltage which is less than the power voltage and greater than the grounding voltage, and reading the differential reset signal;
s022: and (3) carrying out hard reset: setting a reset signal in a pixel unit to be higher than or equal to the sum of a power supply voltage and a threshold voltage of a reset MOS tube;
s023: opening the transmission MOS tube to enable an exposure signal in the photodiode to be transmitted to the suspended diffusion region, and reading a differential pixel transmission signal;
s03: and subtracting the difference pixel transmission signal and the difference reset signal to obtain an exposure signal with the fixed pattern noise removed.
Further, in the step S01, the pixel unit is controlled by the reset MOS transistor to perform single-frame segmented exposure.
Further, the reset MOS tube controls the pixel unit to perform exposure with low full well volume first, and then perform exposure with high full well volume.
Further, the threshold voltages of the reset MOS transistors of different pixel units in the pixel array are different, so that the total charges accumulated in the same frame of exposure of different pixel units are different, and the total charges accumulated in the exposure of a pixel unit are positively correlated to the threshold voltage of the reset MOS transistor in the pixel unit.
Further, in step S01, a single frame of a low full-well exposure and B high full-well exposure is performed on the pixel array, where a and B are positive integers greater than 0, the full-well charge amounts corresponding to the a low full-well exposure are different, and the full-well charge amounts corresponding to the B high full-well exposure are different.
Further, the reset MOS transistors of the pixel units in the pixel array have different threshold voltages, the voltages at the source and the drain of the reset MOS transistor in each pixel unit in the step S021 are not equal, the reset voltages of the floating diffusion regions are not equal, and the reset voltage of the floating diffusion region in the pixel unit is negatively related to the threshold voltage of the reset MOS transistor in the pixel unit.
Furthermore, in step S022, the voltages of the source and the drain of the reset MOS transistor in each pixel unit are equal, the reset voltages of the floating diffusion regions are also equal, and the reset voltage of the floating diffusion region of each pixel unit is equal to the power voltage.
Further, the differential pixel transmission signal of each pixel unit in step S023 is inversely related to the threshold voltage of the reset MOS transistor in the pixel unit.
Further, when the hard reset is performed in step S022, the reset signal reading is not performed.
The invention has the beneficial effects that: in the complete signal reading period after one exposure, the invention enables the suspended diffusion region to carry out two times of resetting and two times of signal reading. The two times of resetting are respectively soft resetting and hard resetting, the difference reset signal generated by the soft resetting is an effective reset signal read by the image sensor, the hard resetting is only to ensure that the electric potentials of the suspended diffusion regions before the pixel signal transmission are equal, the image sensor does not read the reset signal at the moment, and finally the difference pixel transmission signal read after the pixel signal transmission contains the difference of the electric charge quantity generated in the exposure stage. Because the difference reset signal and the difference pixel transmission signal which are read twice are in negative correlation with the threshold voltage of the reset MOS tube, the exposure signal from which the fixed mode noise is removed is obtained by subtracting the difference reset signal and the difference pixel transmission signal. The invention introduces the same difference with the exposure signal in the signal reading process, and simply and effectively eliminates the fixed mode noise in the image through subtraction processing, and the frame rate of the image sensor can not be reduced.
Drawings
FIG. 1 is a prior art HDR image sensor duty cycle based on single frame segmented exposure;
FIG. 2 is a timing diagram of a conventional HDR image sensor based on the change of the full well charge of a 4T pixel cell;
FIG. 3 is a schematic diagram of a 4T pixel unit structure;
FIG. 4 is a schematic diagram of the operation mode of the HDR FPN optimization based on differential reset according to the present invention;
FIG. 5 is a schematic diagram of the working timing sequence of HDR FPN optimization based on differential reset of 4T pixel units according to the present invention;
FIG. 6 is a diagram of the change of potential barrier during the exposure process of low full well based on 4T pixel unit according to the present invention;
FIG. 7 is a diagram illustrating the variation of potential barrier during the exposure process based on the high well-filling amount of 4T pixel unit;
FIG. 8 is a diagram illustrating the potential barrier variation during the soft reset process based on the 4T pixel unit according to the present invention;
FIG. 9 is a diagram illustrating the variation of potential barrier during the hard reset light process based on 4T pixel unit according to the present invention;
FIG. 10 is a diagram illustrating the variation of potential barrier during the reading process of the differential pixel transmission signal based on the 4T pixel unit.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, embodiments of the present invention are described in detail below with reference to the accompanying drawings.
Referring to fig. 1-3, fig. 1 is a schematic diagram of a conventional HDR image sensor operating mode based on one-frame segmented exposure, in which a pixel undergoes two-stage exposure within one frame time: the exposure signal is READ and the exposure signal is READ through two signal readings after two exposures are finished, the two signal readings comprise reset signal reading and exposure signal reading, soft reset is not carried out in the prior art, only a hard reset signal without inflection point difference is READ, and due to the fact that the exposure signal contains the inflection point difference, the exposure signal with difference and the hard reset signal without inflection point difference are subtracted in the prior art, the inflection point difference cannot be removed, and the final image FPN is large. In the low-full-well-quantity exposure stage, due to the difference of the threshold voltages Vth of the MOS tubes of different pixel units for controlling the change of the full-well charge quantity, the inflection points on the final signal output curve of each pixel are different, so that the FPN of the final image is larger. Taking the conventional 4T pixel unit in fig. 3 as an example, as can be seen from the timing diagram in fig. 2, in the 4T pixel unit in the prior art, the transmission MOS transistor TX is generally used to control the barrier height between the photodiode in the pixel unit and the power supply, so as to implement the low-well-fill exposure and the high-well-fill exposure. The inflection point of each pixel is different due to the different threshold voltages of the segmented exposure control tubes of each pixel, namely inflection point difference is formed; causing the FPN of the image to increase.
As shown in fig. 4, the present invention provides a method for removing fixed pattern noise based on difference reset, which comprises the following steps:
s01: carrying out single-frame segmented exposure on the pixel array, wherein the single-frame segmented exposure comprises low-full-well exposure and high-full-well exposure which are respectively carried out in the same frame; the pixel array comprises a pixel unit, the pixel unit comprises a transmission MOS tube, a photodiode, a reset MOS tube and a suspension diffusion region, the photodiode is connected with the suspension diffusion region through the transmission MOS tube, and the suspension diffusion region is also connected with a reset signal through the reset MOS tube.
The method provided by the invention is suitable for the pixel unit which can be controlled by the reset MOS tube, the pixel unit is controlled by the reset MOS tube to carry out low full-well exposure EXP _ LFWC and high full-well exposure EXP _ HFWC, namely, the low full-well exposure and the high full-well exposure are realized by adjusting the relevant time sequence and voltage of the reset MOS tube, therefore, the difference of exposure signals is caused by the reset MOS tube, and the FPN can be eliminated after the difference of the reset MOS tube is introduced in the soft reset in the reset stage. In the present invention, the low well charge exposure refers to the low well charge exposure, and the high well charge exposure refers to the high well charge exposure. All pixel units in the pixel array are exposed and accumulated with charges in a segmented exposure stage, and the sequence of the segmented exposure is to expose with low full trap quantity and then expose with high full trap quantity.
During the segmented exposure, the full trap charge quantity of different pixels in the pixel array is related to the threshold voltage of the reset MOS tube. Under the same control condition, the threshold voltages of the reset MOS tubes of different pixel units are different, so that the full-well charge quantities of different pixel units are different, and therefore, the total charge quantities accumulated in different pixel units within the same total exposure time are also different, namely, the total charge quantities accumulated in different pixel units are related to the threshold voltage of the reset MOS tubes within the same exposure time, and the accumulated exposed total charge is positively related to the threshold voltage of the reset MOS tubes.
The segmented exposure can also be multiple segmented exposure which is more than 2 times, but the full trap charge amount of each segmented exposure is different, and the segmented times in the single-frame exposure do not influence the method. The pixel array can be subjected to single-frame exposure with the low well-filling amount A times and exposure with the high well-filling amount B times, wherein A and B are positive integers larger than 0, the well-filling charge amounts corresponding to the exposure with the low well-filling amount A times are different, and the well-filling charge amounts corresponding to the exposure with the high well-filling amount B times are different.
S02: the method for reading the signals of the pixel array specifically comprises the following steps:
s021: carrying out soft reset: the reset signal in the pixel unit is set to be an intermediate voltage which is smaller than the power supply voltage and larger than the grounding voltage, and the difference reset signal Vrst is read, wherein the difference reset signal Vrst at the moment comprises an inflection point.
In this step, the reset signal is at an intermediate voltage, the intermediate voltage refers to a voltage value smaller than the power voltage and larger than the ground voltage, at this time, voltages at the source and the drain of the reset MOS transistor are not equal, and the reset voltage of the floating diffusion region connected to the reset MOS transistor is related to the threshold voltage of the reset MOS transistor in the pixel unit. Due to process deviation, threshold voltages of reset MOS transistors of different pixel units are different, and then reset voltage values of floating diffusion regions in different pixel units are different, and reset signals of the pixel units read by the image sensor are also different. And the higher the threshold voltage of the reset MOS tube is, the lower the reset voltage value of the floating diffusion region in the corresponding pixel unit is, and the smaller the reset signal voltage value in the corresponding pixel unit read by the image sensor at the moment is, the reset voltage of the floating diffusion region in the pixel unit is inversely related to the threshold voltage of the reset MOS tube in the pixel unit.
S022: and (3) carrying out hard reset: the reset signal in the pixel unit is set to a high voltage equal to or higher than the power supply voltage. The hard reset is only for equalizing the potentials of the floating diffusion regions before the pixel signal is transmitted, and the image sensor does not read the reset signal at this time.
In the step, the reset signal is a high voltage, where the high voltage is not less than the sum of the power voltage connected to the reset MOS transistor and the threshold voltage of the reset MOS transistor, and when the reset is hard, the voltage values at the source and drain ends of the reset MOS transistor are equal, that is, the voltage value of the floating diffusion region is equal to the voltage value of the power connected to the reset MOS transistor, and it can be considered that the voltages of the floating diffusion regions of all the pixel units are equal to the power voltage.
S023: and opening the transfer MOS tube to transfer the exposure signal in the photodiode to the floating diffusion region, and reading a difference pixel transfer signal Vsig, wherein the difference pixel transfer signal Vsig at the moment contains an inflection point.
In this step, after the transfer MOS transistor between the photodiode and the floating diffusion region is turned on, the charge generated by the exposure of the photodiode is transferred to the floating diffusion region, so that the voltage of the floating diffusion region is reduced, and the reduced voltage is determined by the amount of charge generated by the photodiode during the exposure. Since the floating diffusion region is hard reset before the exposure signal is transmitted, the voltage of the floating diffusion region is reduced after the exposure signal is transmitted, and the magnitude of the reduced voltage signal is determined only by the exposure charge amount. For the HDR image sensor performing the segmented exposure based on the reset MOS transistor, the charge generated by the exposure is related to the threshold voltage of the reset MOS transistor, and due to the process deviation, the threshold voltages of the reset MOS transistors of different pixel units are different, so that the charge amount generated by the exposure in different pixel units is different under the same light intensity and exposure time. The larger the threshold voltage of the reset MOS tube is, the more the corresponding pixel unit can generate the electric charge in the whole exposure stage, and the smaller the voltage value of the read differential pixel transmission signal is, namely, the differential pixel transmission signal is in negative correlation with the threshold voltage of the reset MOS tube in the corresponding pixel unit.
S03: and subtracting the difference pixel transmission signal and the difference reset signal to obtain an exposure signal Vpf with the fixed pattern noise removed, wherein the exposure signal Vpf does not contain an inflection point.
The difference pixel transfer signal read after the final pixel signal transfer contains the difference in the amount of charge generated during the exposure phase. Because the difference reset signal and the difference pixel transmission signal which are read twice are in negative correlation with the threshold voltage of the reset MOS tube, the exposure signal from which the fixed mode noise is removed is obtained by subtracting the difference reset signal and the difference pixel transmission signal.
The method of the present invention is further explained below with reference to fig. 5-10 by using 4T pixel units, and the method of the present invention can be applied to all pixel units, and is not limited to 4T pixel units.
As shown in fig. 5, the single-frame segment exposure of the present invention employs the low-well-fill exposure and the high-well-fill exposure in the prior art, and the exposure period is the same as that of the conventional HDR image sensor based on the adjustment of the well-fill charge amount.
As shown in fig. 5 and 6, the exposure of the low full-well quantity is realized by reducing the voltage of the reset MOS transistor RST to an intermediate voltage, and the schematic diagram of the charge barrier in the pixel unit is shown in fig. 6 during the exposure of the low full-well quantity. Due to the difference in threshold voltage of the reset MOS transistor RST, the full-well barrier in the low-full-well exposure period in different pixel units is different, and the maximum charge amount that can be stored at the photodiode PD in the exposure period is different.
As shown in fig. 5 and 7, when the pixel array enters the high full-well exposure period, the gate voltage of the transfer MOS transistor TX decreases, the transfer MOS transistor TX turns off, and the photodiode PD continues to accumulate charges, as shown in fig. 7. The charges sensed by each pixel unit at this stage are the same regardless of the pixel unit photo-induced noise, but the total charges at the photodiode PD are different due to the different charges at the low full-well exposure stage.
As shown in fig. 5, when the pixel array enters the signal reading READ phase after exposure, there are two signal readings per pixel unit: a difference reset signal Vrst and a difference pixel transfer signal Vsig.
As shown in fig. 5 and 8, in the soft reset phase, differential reset signal reading is performed, the voltage of the reset MOS transistor RST is an intermediate voltage value, the reset voltage of the floating diffusion region FD in each pixel unit is different, the barrier of the floating diffusion region FD in each pixel unit is different, and the differential reset signal Vrst of different pixel units is read.
As shown in fig. 5 and 9, after the differential reset signal is read, the reset MOS transistor RST is pulled high, at this time, the FD point is reset hard, and the FD point barriers of the floating diffusion regions of all the pixel units in the pixel array are the same as the power supply, as shown in fig. 9, that is, the FD point barriers of the floating diffusion regions of all the pixel units are the same in height. No data read is performed at this time.
As shown in fig. 5 and 10, in the differential pixel transfer signal reading phase, the reset MOS transistor RST is turned off, the transfer MOS transistor TX is turned on, and the charges at the photodiode PD are all transferred to the floating diffusion FD. The potential barriers at the point of the floating diffusion FD are different due to the different amount of charge generated by the exposure (as shown in fig. 10, the differential pixel transfer signal Vsig for different pixel cells is read at this time).
The differential reset signal Vrst and the differential pixel transfer signal Vsig are both related to the threshold voltage Vth of the reset MOS transistor RST, and the voltage V1 at the time of hard reset is not related to the threshold voltage of the reset MOS transistor RST, and V1 is VDD. Regardless of gain amplification, Vrst is VDD-Vth and Vsig is V1-Q/C. Q is the total charge amount at the exposure phase PD and C is the capacitance of the floating diffusion FD. Q is related to the threshold voltage Vth of the reset MOS transistor RST, and the larger Vth is, the larger Q is, and the smaller Vsig is, namely Vsig is in negative correlation with Vth. Meanwhile, the larger Vth, the smaller Vrst, i.e., Vrst is negatively correlated with Vth. The final pixel exposure signal Vpd-Vrst-Vsig is independent of Vth.
In the complete signal reading period after one exposure, the invention enables the suspended diffusion region to carry out two times of resetting and two times of signal reading. The two times of resetting are respectively soft resetting and hard resetting, the difference reset signal generated by the soft resetting is an effective reset signal read by the image sensor, the hard resetting is only to ensure that the electric potentials of the suspended diffusion regions before the pixel signal transmission are equal, the image sensor does not read the reset signal at the moment, and finally the difference pixel transmission signal read after the pixel signal transmission contains the difference of the electric charge quantity generated in the exposure stage. Because the difference reset signal and the difference pixel transmission signal which are read twice are in negative correlation with the threshold voltage of the reset MOS tube, the exposure signal from which the fixed mode noise is removed is obtained by subtracting the difference reset signal and the difference pixel transmission signal. The invention introduces the same difference with the exposure signal in the signal reading process, and simply and effectively eliminates the fixed mode noise in the image through subtraction processing, and the frame rate of the image sensor can not be reduced.
The above description is only a preferred embodiment of the present invention, and the embodiment is not intended to limit the scope of the present invention, so that all equivalent structural changes made by using the contents of the specification and the drawings of the present invention should be included in the scope of the appended claims.
Claims (5)
1. A method for removing fixed pattern noise based on difference reset is characterized by comprising the following steps:
s01: carrying out single-frame segmented exposure on the pixel array, wherein the single-frame segmented exposure comprises low-full-well exposure and high-full-well exposure which are respectively carried out in the same frame; the pixel array comprises a pixel unit, the pixel unit comprises a transmission MOS tube, a photodiode, a reset MOS tube and a suspension diffusion region, the photodiode is connected with the suspension diffusion region through the transmission MOS tube, and the suspension diffusion region is also connected with a reset signal through the reset MOS tube; the threshold voltages of the reset MOS tubes of all the pixel units in the pixel array are different, so that the total charges accumulated in the same frame of exposure of different pixel units are different, and the total charges accumulated in the exposure of the pixel units are positively correlated with the threshold voltage of the reset MOS tubes in the pixel units;
s02: the method for reading signals of the pixel array specifically comprises the following steps:
s021: carrying out soft reset: setting the reset signal in the pixel unit to be an intermediate voltage which is less than the power supply voltage and greater than the grounding voltage, and reading a differential reset signal; the reset MOS tube source-drain terminal voltages in each pixel unit are not equal, the reset voltages of the suspended diffusion regions are not equal, and the reset voltages of the suspended diffusion regions in the pixel units are inversely related to the reset MOS tube threshold voltage in the pixel units;
s022: and (3) carrying out hard reset: setting a reset signal in the pixel unit to be higher than or equal to the sum of a power supply voltage and a threshold voltage of a reset MOS tube; the reset MOS tube source and drain voltages in the pixel units are equal, the reset voltages of the suspended diffusion regions are also equal, and the reset voltage of the suspended diffusion region of each pixel unit is equal to the power supply voltage;
s023: opening the transmission MOS tube to enable an exposure signal in the photodiode to be transmitted to the suspension diffusion region, and reading a differential pixel transmission signal; the difference pixel transmission signal of each pixel unit is inversely related to the threshold voltage of the reset MOS tube in the pixel unit;
s03: and subtracting the difference pixel transmission signal and the difference reset signal to obtain an exposure signal with the fixed pattern noise removed.
2. The method for removing fixed pattern noise based on differential reset of claim 1, wherein the step S01 is implemented by controlling pixel units to be exposed in a single frame segment by resetting MOS transistors.
3. The method according to claim 2, wherein the reset MOS transistor controls the pixel unit to be exposed with a low full well volume first and then with a high full well volume.
4. The method according to claim 2, wherein in step S01, a single frame of a low full-well exposure and B high full-well exposure is performed on the pixel array, where a and B are positive integers greater than 0, and the full-well charge amounts corresponding to the a low full-well exposure are different and the full-well charge amounts corresponding to the B high full-well exposure are different.
5. The method for removing fixed pattern noise based on differential reset as claimed in claim 1, wherein when performing the hard reset in step S022, no reset signal reading is performed.
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