JPH08289206A - Video camera and signal processing circuit of solid-state image sensor used for the same - Google Patents

Abstract

PURPOSE: To provide the image of a wide dynamic range by providing a noise removing circuit, with which the noise component of a saturated area in the output signal of a solid-state imaging device is removed at least, and using the output signal of this noise removing circuit as a video signal. CONSTITUTION: The CCD output signal of an imager 11 is sampled/held by a sample/hold(S/H) circuit 15, digitized by an A/D converter 16 and supplied to a signal processing circuit 20 later. Thus, the output signal of the A/D converter 16 is selected by a changeover switch 22 in the non-saturated area of the CCD output signal under the switching control of a switch control circuit 23 and the output signal of a median filter 21 is selected by the changeover switch 22 in the saturated area of the CCD output signal under the switching control of the switch control circuit 23. Namely, not only the non-saturated area of the CCD output signal but also its saturated area are used as the video signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a video camera,
In particular, the present invention relates to a video camera using a solid-state image sensor having a vertical overflow drain structure.

[0002]

2. Description of the Related Art FIG. 7 shows a sectional structure of a unit cell of a solid-state image pickup device having a vertical overflow drain structure.
In FIG. 7, a P well layer 72 on an N type semiconductor substrate 71
A photosensor section 73 made of an N-type impurity that photoelectrically converts incident light into a signal charge having a charge amount corresponding to the light amount and accumulates the signal charge is formed on the surface side of the. Also,
In the P well layer 72, a signal charge transfer region 75 made of N ⁺ type impurities is formed with the channel region 74 interposed therebetween. A polysilicon electrode 77 is arranged above the channel region 74 and the signal charge transfer region 75 with an oxide film 76 interposed therebetween.

The photosensor portion 7 is formed by the channel region 74 and a part of the polysilicon electrode 77 thereon.
Read-out gate section 7 for reading out the signal charges accumulated in 3
8 is configured, and a vertical charge transfer portion 79 that vertically transfers the signal charge read via the read gate portion 78 is configured by the signal charge transfer region 75 and the polysilicon electrode 77 thereon. A channel stop portion 80 is formed between the photo sensor portion 73 and the signal charge transfer region 75 of the adjacent pixel. Moreover, an insulating film 81 is formed on the polysilicon electrode 77.
The top of 1 is covered with a light shielding film 83 of aluminum (Al) or the like except for the opening 82 of the photo sensor portion 73.

FIG. 8 shows the potential distribution from A to B in the direction parallel to the substrate surface in FIG. 1 and the potential distribution from B to C in the substrate depth direction. In the figure, the potential distribution of the solid line a shows the state in which the signal charges are accumulated in the photo sensor unit 73, and the chain double-dashed line b from the photo sensor unit 73 to the vertical charge transfer unit 79 via the read gate unit 74. The state of reading out electric charges is shown. When the amount of light incident on the photo sensor unit 73 increases and exceeds a certain amount of light, the signal charges exceed a barrier (overflow barrier) in the depth direction of the substrate, and overflow to the substrate 71 starts. It should be noted that the substrate voltage Vsub. Applied to the substrate 71 is obtained so that the saturation charge amount set for each device can be obtained under the determined light amount. The voltage value of is set.

Here, in order to secure the blooming resistance, the potential (height) of the overflow barrier h1.
Is limited to be lower than the barrier potential h2 of the read gate portion 78 on the substrate surface side. Here, blooming is a phenomenon in which the photo sensor unit 73 is saturated when strong light is incident, signal charges overflow and enter the vertical charge transfer unit 79, and a white portion spreads around the photo sensor. It becomes more likely to occur as the potential of the portion 73 becomes shallower.

By the way, the potential h1 of the overflow barrier that determines the saturated charge amount is determined by the photosensor section 73.
Changes according to the amount of electric charge stored in, and as shown in FIG. 9, changes from P1 to P2 and P3 as the amount of electric charge increases. At this time, even in the overflow state in which the signal charge exceeds the saturated charge amount, the photosensor unit 73 has the sensitivity characteristic with respect to the incident light, and the characteristic of the output signal with respect to the incident light amount is as shown in FIG. This is called a Knee characteristic and is unique to the vertical overflow drain structure. The slope of this knee characteristic can be changed by the impurity concentration and depth of the P well layer 72 and the photo sensor portion 73 in FIG. 7.

[0007]

In the solid-state image pickup device having the vertical overflow drain structure described above, the screen is grainy when viewed on the reproduced screen due to the pixel-to-pixel variation of the potential h1 of the overflow barrier in the saturation region (knee portion). It is known that so-called overflow unevenness occurs. Therefore, when applied to a video camera, only the non-saturated region in the output signal of the solid-state image sensor is treated as a video signal, and the saturated region beyond that is electrically processed by the signal processing circuit in the subsequent stage of the solid-state image sensor. It was cut and not treated as a video signal.

Therefore, the conventional video camera using the solid-state image pickup device having the vertical overflow drain structure has a problem that the dynamic range of the image is small and the bright portion of the image information is crushed in white. In particular, under daylight under sunlight, when the aperture is adjusted to a dark part or to a bright part, white and black parts of the image will be generated respectively. The image could not be reproduced correctly.

A solid-state image pickup device in which the output signal of the saturation region of the solid-state image pickup device is also used as a video signal is disclosed in Japanese Patent Laid-Open No. 4-364679. The output signal is simply linearized and used as a video signal, and does not solve the above-mentioned problem regarding overflow unevenness.

The present invention has been made in view of the above-mentioned problems, and an object thereof is to solve the problem of overflow unevenness in the saturation region in the output signal of the solid-state image pickup device, and to solve the nonlinear region of the solid-state image pickup device. Another object of the present invention is to provide a video camera capable of expanding the dynamic range of an image by using the output signal of the above as an image signal.

[0011]

According to the present invention, in a video camera using a solid-state image sensor having a vertical overflow drain structure, a noise removing circuit for removing a noise component of at least a saturation region of an output signal of the solid-state image sensor is provided. A configuration is provided in which the output signal of the noise removal circuit is used as a video signal.

[0012]

In the video camera having the above-described structure, the noise removal circuit removes at least the noise component in the saturation region of the output signal of the solid-state image pickup device, so that the output signal in the saturation region, which has hitherto been unusable, is also converted into a video signal. Can be used. Then, by using the output signal in the saturation region as the video signal, the dynamic range of the video can be expanded.

[0013]

Embodiments of the present invention will now be described in detail with reference to the drawings. FIG. 1 is a block diagram showing an embodiment of a video camera according to the present invention. In FIG.
As the imager 11, a CCD solid-state image pickup device having a vertical overflow drain structure is used. The structure of the unit cell of the imager 11 is, for example,
The structure is as shown in FIG. This imager 1
1, the substrate voltage Vsub. Is applied, but this substrate voltage Vsu
b. Is Vsub. The voltage value can be switched by the control circuit 13.

Vsub. The control circuit 13 determines the substrate voltage Vsub. Is switched in two steps, for example. The detection sensor 14 has a configuration that optically detects the brightness of the subject, but is not limited to this. For example, the brightness of the subject is detected based on the signal level of the CCD output signal. It may have a configuration.

The CCD output signal of the imager 11 is sampled and held by the sample and hold (S / H) circuit 15, digitized by the A / D converter 16, and then supplied to the signal processing circuit 20 according to the present invention. The signal processing circuit 20 receives a median (median value) filter 21 as a noise removing circuit, which receives the digitized CCD output signal, and outputs the output signal of the median filter 21 and the output signal of the A / D converter 16 from each other. A changeover switch 22 for inputting and selectively outputting one of them, and a CCD
Based on the signal level of the output signal, the switch control circuit 23 for controlling the changeover switch 22 with the boundary (point a in FIG. 10) between the non-saturated region and the saturated region of the CCD output signal as a switching point, and the changeover switch 22. It has a D / A converter 27 for converting the output signal into an analog signal and outputting it as a video signal.

In this structure, in the non-saturated area of the CCD output signal, the output signal of the A / D converter 16 is selected by the changeover switch 22 by the switching control by the switch control circuit 23, and in the saturated area of the CCD output signal, the switch control circuit 23. The output signal of the median filter 21 is selected by the changeover switch 22 by the changeover control by. That is, not only the non-saturated area of the CCD output signal but also the saturated area is used as the video signal.

By the way, in the imager having the vertical overflow drain structure, in the saturation region where the signal charges photoelectrically converted and accumulated in the photosensor section exceed the saturation charge amount determined by the potential of the overflow barrier, as described above, The unevenness of the overflow occurs due to the variation of the potential of the overflow barrier for each pixel. This overflow unevenness is superimposed on the CCD output signal as fixed pattern noise. The median filter 21 is provided as a noise removal circuit that removes this fixed pattern noise.

Here, the median (median) is a value that comes to the center when one set of data strings is arranged in order of size. In the median filter 21, for example, as shown in FIG. 2, data of several points (n points) before and after the point a is taken in order to determine the value of the point a, and a value satisfying a certain condition (for example, a point A value closest to the average value of the n pieces of data as the center) is selected, and the median process is performed using the value as the data of the point a.

FIG. 3 shows an example of signal output when median processing of 5 bits is performed as an example. In the figure, the vertical axis represents the signal level and the horizontal axis represents the time, (a) is an example of an input image gray scale chart, and (b) is an input image gray scale chart (a).
The output signal with fixed pattern noise superimposed on
(C) represents an output signal (indicated by a black circle in the figure) after performing 5-bit median processing on the output signal (b) on which the fixed pattern noise is superimposed.

When the median filter 21 is used for the CCD area sensor (CCD imager 11) as in the present invention, the median processing is performed for each horizontal line. A flowchart of the processing procedure is shown in FIG. In the figure, first, the output of the pixel p after the median processing is set to F (p) (step S11), and then the arithmetic processing of the arithmetic expression shown in Formula 1 is executed (step S1).
2).

[Equation 1] Then, k (k = p-) at which | F (p) -f (k) | is minimized.
Let (n-1) / 2 to p + (n-1) / 2) be K (step S13), and let f (K) = F (p) (step S1).
4).

Then, it is judged whether n> p (step S
15), if n> p, p = p + 1 and the next pixel p
(Step S16), and then the process returns to step S11 to repeat the above-described processing for the next pixel p. On the other hand, if n ≦ p, it is determined whether or not the median processing is completed for all horizontal lines (step S
17) If there is a line for which the median process has not been completed, the process returns to step S11 and the above-described series of median processes is repeatedly executed until the median process is completed for all horizontal lines.

As described above, in the video camera using the CCD imager 11 having the vertical overflow drain structure, the signal processing is performed by using the noise removing circuit such as the median filter 21 which removes the fixed pattern noise included in the CCD output signal. By configuring the circuit 20, fixed pattern noise caused by unevenness of overflow is superposed, so that it is possible to use a CCD output signal in a saturation region, which has been conventionally unusable, as a video signal. Therefore, an image with a wide dynamic range can be captured.

In particular, since the signal processing circuit 20 is configured by using the median filter 21, the median filter 21 does not blur lines or contours in the image, and the fixed pattern noise, particularly the level in the pixel of interest and the vicinity thereof, is reduced. Since it is a non-linear smoothing filter having a function of removing largely different isolated points, a good imaging output can be obtained. In this embodiment, the changeover switch 22 is provided so that the CC at the boundary between the unsaturated region and the saturated region of the CCD output signal is CC.
Although the D output signal is switched, it is not always necessary to switch the CCD output signal, and the CCD output signal that has passed through the median filter 21 in both the non-saturated region and the saturated region can be used as the video signal. According to this, there is an effect that fixed pattern noise included in the CCD output signal can be removed even in the non-saturated region.

In the above embodiment, Vsub.
The control circuit 13 outputs C based on the detection output of the detection sensor 14.
The substrate voltage Vsub. Voltage value of 2
It is designed to switch to stages. Specifically, when the brightness of the subject is dark, the substrate voltage Vsub. Is set to the same voltage value as the conventional one, and when the brightness of the subject is bright, the substrate voltage Vsub. To a higher voltage value than when it was dark. Substrate voltage Vsub. 9 is set high, the potential h1 of the overflow barrier becomes low in the potential diagram of FIG. 9, so that as shown in FIG. 5, the substrate voltage Vsub. Sensitivity characteristic d2 when it is set higher than the sensitivity characteristic d2 when the voltage value of is set low
When 1 is smaller, the amount of incident light becomes saturated immediately.

As a result, the substrate voltage Vsub. If the voltage value of the
By configuring the signal processing circuit 20 of the imager 11 using a noise removing circuit such as the median filter 21,
The CCD output signal in the saturated region can be used as a video signal, and the potential h of the overflow barrier can be used.
Since 1 is low, the range up to the point where blooming occurs is extremely wide, and the dynamic range is greatly expanded when the brightness of the subject is bright.

As a result, when an image of strong light is picked up from a bright portion to some extent, the substrate voltage Vsub. By setting the voltage value of 2 to be high and setting it to the sensitivity characteristic d1 in FIG. 5, even a light amount of a level that causes whitening in an ordinary imaging method can be dealt with without whitening. Further, in the case of capturing an image under low illuminance such as at night, the point where the knee characteristic starts like the sensitivity characteristic d2 is shifted in order to secure the sensitivity of the low illuminance, and the CCD output signal of the non-saturated area is changed to the video signal. It can be dealt with by using as. in this case,
The CCD output signal in the non-saturated region is selected by the switching control of the changeover switch 22.

FIG. 6 is a block diagram showing another embodiment of the video camera according to the present invention. In this embodiment, the CCD
A noise removal circuit 20 for removing fixed pattern noise in the output signal has a circuit configuration including a continuous point processing circuit 24, a low-pass filter (LPF) 25, and an adder 26. Other than that, the configuration of FIG. Is the same as. The description of the same part will be omitted here because it is redundant. The continuous point processing circuit 24 receives the digitized CCD output signal as input, and performs processing for extracting only points where the signal level is the same and continuous, thereby obtaining a CCD signal.
The fixed pattern noise included in the output signal is removed. However, by extracting only the continuous points, only the line and contour information can be obtained as the output signal of the continuous point processing circuit 24.

On the other hand, the low-pass filter 25 receives the digitized CCD output signal as an input and passes only the low-frequency components thereof to remove fixed pattern noise having a high frequency contained in the CCD output signal. However, by cutting the frequency components in the high frequency band, the output signal of the low-pass filter 25 can be image information with a blurred line or outline. The adder 26 adds the output signals of the continuous point processing circuit 24 and the low pass filter 25. That is, in the adder 26, the information of only the line or the contour from the continuous point processing circuit 24 and the image information of the blurred line or the contour from the low-pass filter 25 are added to each other, thereby the video signal from which the fixed pattern noise is removed. Will be obtained.

As described above, the signal processing circuit 20 for processing the CCD output signal of the CCD imager 11 includes the continuous point processing circuit 24, the low pass filter 25 and the adder 26.
Since it is possible to satisfactorily remove or reduce the fixed pattern noise generated particularly in the saturated region of the CCD output signal without blurring the lines and contours in the image, the configuration similar to that in the previous embodiment is adopted. The CCD output signal in the saturation region can also be used as a video signal. Therefore, the dynamic range of the image can be expanded.

Also in this embodiment, the changeover switch 22 is omitted, and the CCD output signal that has passed through the continuous point processing circuit 24, the low pass filter 25 and the adder 26 is used as a video signal in both the non-saturation region and the saturation region. What is acceptable is the same as in the case of the previous embodiment.

[0031]

As described above, according to the present invention,
In a video camera using a solid-state image sensor with a vertical overflow drain structure, a noise removal circuit that removes noise components in at least the saturation region of the output signal of the solid-state image sensor is provided, and the output signal of this noise removal circuit is used as a video signal. Since the configuration is used, an image with a wide dynamic range can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of a video camera according to the present invention.

FIG. 2 is a conceptual diagram of median processing.

FIG. 3 is a diagram showing an example of signal output when 5-bit median processing is performed.

FIG. 4 is a flowchart showing a procedure of n-bit median processing.

FIG. 5 shows a substrate voltage Vsub. FIG. 6 is a diagram showing a change in sensitivity characteristic when is switched.

FIG. 6 is a block diagram showing another embodiment of the video camera according to the present invention.

FIG. 7 is a sectional structural view showing an example of a unit cell.

8 is a diagram showing a potential distribution from A to B in FIG. 7 in a direction parallel to the substrate surface and a potential distribution from B to C in a substrate depth direction.

FIG. 9 is a potential diagram of the photo sensor unit in the substrate depth direction.

FIG. 10 is a diagram showing a relationship between an incident light amount and a CCD output signal.

[Explanation of symbols]

 11 CCD imager 13 Vsub. Control circuit 14 Detection sensor 20 Signal processing circuit 21 Median filter 22 Changeover switch 23 Switch control circuit 24 Continuous point processing circuit 25 Low pass filter (LPF)

Claims (6)

[Claims] 1. A video camera using a solid-state imaging device having a vertical overflow drain structure, comprising a noise removal circuit for removing a noise component of at least a saturation region of an output signal of the solid-state imaging device. And a video camera. 2. The noise removing circuit is configured by a median filter.
The described video camera. 3. A switching switch for switching between the output signal of the solid-state image sensor and the output signal of the noise removal circuit at the boundary between the non-saturated region and the saturated region of the output signal of the solid-state image sensor. The video camera according to claim 1, wherein: 4. A detection means for detecting the brightness of an object,
2. The video camera according to claim 1, further comprising a substrate voltage control circuit that controls the substrate voltage of the solid-state image sensor according to the detection output of the detection means. 5. A detection means for detecting the brightness of an object,
3. The video camera according to claim 2, further comprising a substrate voltage control circuit that controls a substrate voltage of the solid-state image sensor according to a detection output of the detection means. 6. A signal processing circuit for processing an output signal of a solid-state imaging device having a vertical overflow drain structure, comprising a median filter for removing a noise component of at least a saturation region of the output signal of the solid-state imaging device. A signal processing circuit for a solid-state imaging device, which is characterized by being configured.