JP2003051991A - Digital camera - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a digital camera that can obtain a photographed image in a short time, from which a fixed pattern noise is eliminated. SOLUTION: When a mechanical shutter 16 is opened, by activating a CCD 12, a main image signal resulting from photographing an object can be obtained. When the mechanical shutter 16 is closed, by activating the CCD 12, a black image signal corresponding to the fixed pattern noise of the CCD 12 itself can be obtained. Furthermore, by subtracting black image data Db based on a black image signal from main image data Da based on a main image signal the fixed pattern noise included in the main image data Da is eliminated from the data Da. An exposure period Tb to obtain the black image signal is 1/N of an exposure period Ta to obtain the main image signal. Then a gain Gb of an AGC circuit 26, to amplify the black image signal is a multiple of N of a gain Ga, to amplify the main image signal.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a digital camera, and more particularly to, for example, a digital camera provided with an image pickup device for converting a subject image incident on a light receiving surface into an electric signal.

[0002]

2. Description of the Related Art Generally, in this type of digital camera,
CCD (Charge Coupled Device) type or CMOS (Complementary Metal Oxide Semicond)
uctor) type is used. In any of the image pickup devices, fixed pattern noise occurs due to variations in output characteristics of each pixel, dark current unevenness, and the like.

As a technique for removing such fixed pattern noise, conventionally, for example, Japanese Patent Application Laid-Open No. 6-197285 [H04N5 / 3] filed and published on July 15, 1994.
35]. This technology detects fixed pattern noise by driving the image sensor without exposing it, stores the data in a memory, and actually captures an image of a subject, and then outputs the signal from the image sensor output signal at that time. The fixed pattern noise is removed by subtracting the data recorded in.

[0004]

However, in the above-mentioned prior art, it takes as long as the time when the subject is actually photographed to detect the fixed pattern noise.
Therefore, there is a problem that it takes simply twice as long time to obtain one captured image as compared with the case where the fixed pattern noise is not removed.

Therefore, a main object of the present invention is to provide a digital camera capable of removing fixed pattern noise in a shorter time than ever before.

[0006]

SUMMARY OF THE INVENTION According to the present invention, a first image pickup device is provided with a subject image incident on a light receiving surface of the image pickup device.
A first activating means for activating for a period, a first applying means for applying a first gain to a first image signal output from the image sensor by the activation for the first period, and an image sensor with the light receiving surface shielded. Second activating means for activating for a second period shorter than the first period, and second activating means for imparting a second gain larger than the first gain to the second image signal output from the image sensor by the activation for the second period. The digital camera includes: a second adding unit; and a subtracting unit that subtracts the second image signal having the second gain from the first image signal having the first gain to output a corrected image signal.

[0007]

According to the present invention, the first activation means activates the image pickup device in a state in which the object image is incident on the light receiving surface of the image pickup device for the first period. As a result, the image sensor outputs the first image signal corresponding to the subject image. Note that the first image signal also includes fixed pattern noise included in the image pickup device itself. Then, the first gain is imparted to the first image signal by the first imparting means.
On the other hand, the second activation means activates the image pickup device in a state where the light receiving surface of the image pickup device is shielded for the second period.
As a result, the image sensor outputs the so-called black level second image signal including the fixed pattern noise described above. That is, the fixed pattern noise can be detected by obtaining the second image signal. Since the second period is shorter than the first period, the magnitude of fixed pattern noise included in the second image signal is smaller than the magnitude of fixed pattern noise included in the first image signal. However, since the second gain that is larger than the first gain is added to the second image signal by the second adding unit, the magnitude of the fixed pattern noise included in the second image signal also increases accordingly. Then, the subtraction unit subtracts the second image signal to which the second gain has been added from the first image signal to which the first gain has been added. As a result, the corrected image signal obtained by removing the fixed pattern noise from the first image signal is output from the subtraction unit.

The second period is preferably 1 / N times the first period (N> 1). In this case, the second gain is N times the first gain.

In one embodiment of the present invention, the subtracting means is
A first storage unit for storing the first image data based on the first image signal; and a second storage unit for storing the second image data based on the second image signal.
It is desirable to subtract the image data.

[0010]

According to the present invention, the second period for detecting fixed pattern noise is shorter than the first period for actually photographing a subject. Therefore, it is possible to remove the fixed pattern noise in a short time as compared with the above-described conventional technique in which the fixed pattern noise takes the same length of time as when the subject is actually photographed. .

The above-mentioned objects, other objects, features and advantages of the present invention will become more apparent from the following detailed description of the embodiments with reference to the drawings.

[0012]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1, a digital camera 10 of this embodiment is a CCD type solid-state image pickup device (hereinafter, simply CC.
Say D. ) 12. A mechanical shutter (shutter plate) 16 is provided between the light receiving surface 12 a of the CCD 12 and the lens 14, and the mechanical shutter 16 is
It opens and closes in response to a shutter control signal supplied from a shutter control circuit 18 described later via a shutter driver circuit 20. By opening and closing the mechanical shutter 16 in this manner, the light receiving surface 12a of the CCD 12 is exposed to the outside through the lens 14 or is shielded from light.

The CCD 12 itself also has an electronic shutter function. That is, the CCD 12 has a C
The electronic shutter is opened / closed in response to the electronic shutter control signal supplied from the CD control circuit 22. When the electronic shutter is open, that is, during the exposure period, the CCD 12
The light incident on the light receiving surface 12a is converted into electric energy (charge) and stored. Then, when the exposure period ends, C
The CD 12 closes the electronic shutter, and sequentially outputs the accumulated charges in an interlaced scan format in response to the read control signal supplied from the CCD control circuit 22, that is, outputs an analog image signal.

The image signal output from the CCD 12 is C
The signal is input to a DS (Correlated Double Sampling) circuit 24, where it is subjected to correlated double sampling processing, and then AGC.
(Automatic Gain Control) circuit 26. A
The GC circuit 26 amplifies the input image signal with a gain (amplification factor) according to a gain control signal supplied from a gain control circuit 28 described later. The image signal after this amplification is
It is input to the A / D conversion circuit 30, converted into digital image data here, and then input to the memory buffer circuit 32. The memory buffer circuit 32 rearranges the input image data into progressive scan format data and stores the rearranged data in an SDRAM (Synchr
onous dynamic RAM) 34.

The SDRAM 34 has two storage areas, a main image storage area 36 and a black image storage area 38, as storage destinations of the input image data. Which of the storage areas 36 and 38 is to store the image data depends on a memory control signal supplied from a memory control circuit 40 described later.

By the way, the shutter control circuit 18,
The CCD control circuit 22, the gain control circuit 28, the memory buffer circuit 32, and the memory control circuit 40 are integrated into one AS.
It is configured by an IC (Application Specified IC) 42. And these circuits 18, 22, 28, 3
The operations of 2 and 40 are both controlled by the CPU 44.

That is, when the shutter button (not shown) forming the operation key 46 is pressed, the CPU 44 operates in the operation mode (program AE) set at that time.
The optimum exposure period Ta is determined in accordance with (Automatic Exposure) mode, shutter speed priority mode, etc. Then, after controlling the shutter control circuit 18 to open the mechanical shutter 16, the CCD control circuit 22 is controlled to open the electronic shutter of the CCD 12 for the determined exposure period Ta. As a result, the CCD 12 outputs an image signal, that is, a main image signal, which corresponds to an optical image of a subject (not shown) that is incident on the light receiving surface 12a via the lens 14. Note that this main image signal includes
As exaggeratedly shown in (a), the fixed pattern noise 48 included in the CCD 12 itself is included.

The main image signal output from the CCD 12 passes through the CDS circuit 24 and the AGC circuit 2 as described above.
6 is input, but at this time, the CPU 44 controls the gain control circuit 28 so that the gain of the AGC circuit 26 is set to a certain gain Ga. As a result, the AGC circuit 2
The main image signal to which the gain Ga is added in 6 is A /
It is digitized by the D conversion circuit 30. And
The digitized main image data Da is input to the memory buffer circuit 32, where the CPU 44 performs scan conversion processing and then input to the SDRAM 34. Then, the CPU 44 controls the memory control circuit 40 so that the main image data Da input to the SDRAM 34 is stored in the main image storage area 36.

After storing the main image data Da in the main image storage area 36 in this way, the CPU 44 controls the shutter control circuit 18 to close the mechanical shutter 16 next. Then, in this state, N of the above-described exposure period Ta
The CCD control circuit 22 is controlled to open the electronic shutter of the CCD 12 for one-half period Tb (= Ta / N). As a result, the CCD 12 outputs a so-called black image signal having a black level including the fixed pattern noise 50, as shown in FIG. However, the magnitude Vb ′ of the fixed pattern noise 50 included in this black image signal
Is proportional to the length of the exposure period Tb, which is 1 / N of the magnitude Vb of the fixed pattern noise 48 included in the main image signal (that is, Vb '= Vb / N).

This black image signal is also input to the AGC circuit 26 via the CDS circuit 24, but at this time, the CPU 44
Controls the gain control circuit 28 so that the gain of the AGC circuit 26 is set to a gain Gb (= N · Ga) that is N times as high as when the main image signal is amplified. As a result, FIG.
As shown in (c), the magnitude Vb ′ of the fixed pattern noise 50 included in the black image signal is multiplied by N to be approximately equal to the magnitude Vb of the fixed pattern noise 48 (N · Vb′≈V).
b). Then, the black image signal to which the gain Gb is added in this way is digitized by the A / D conversion circuit 30, and the digitized black image data Db is
It is input to the SDRAM 34 via the memory buffer circuit 32. Then, the CPU 44 controls the memory control circuit 40 so that the black image data Db input to the SDRAM 34 is stored in the black image storage area 38.

After storing the black image data Db in the black image storage area 38 in this way, the CPU 44 uses the main image data Da previously stored in the main image storage area 36 to store the black image stored in the black image storage area 38 this time. The data Db is subtracted. As a result, as shown in FIG. 2D, data [Da-Db] obtained by removing the fixed pattern noise 48 contained in the main image data Da is obtained. And CP
The U44 controls the memory control circuit 40 to store the subtracted data [Da-Db] in the main image storage area 36 as new main image data Da. Then, the new main image data Da stored in the main image storage area 36 is input to a signal processing circuit at a subsequent stage (not shown).

As described above, according to this embodiment, the exposure period Tb for detecting the fixed pattern noise 48 (50) is N of the exposure period Ta when the subject is actually photographed.
Since the length of the fixed pattern noise is one-hundredth, the fixed pattern noise 48 can be detected in a short time as compared with the above-described conventional technique in which it takes the same length of time to detect the fixed pattern noise as when actually capturing an object. Can be detected and removed.
Therefore, for example, assuming that the exposure period Ta required for actually photographing a subject is Ta = 4 [msec], in the above-mentioned conventional technique, it is 8 [, which is twice Ta, to obtain one photographed image. According to this embodiment, 6 [msec] (∵T = Ta + Ta / N (∵Tb = Ta /
N)) is enough.

Needless to say, the shooting time can be further shortened by increasing the value of N. However, if the value of N is too large, the fixed pattern noise 48 may not be accurately removed due to the influence of the offset characteristic and the input / output characteristic of the AGC circuit 26. Therefore, the value of N is set in consideration of this point. It is desirable to decide.

The effect of this embodiment is more effective as the exposure period Ta required for actually photographing a subject is longer. This is the case, for example, when shooting in a dark place by the program AE mode or aperture priority mode, or setting the shutter speed slower by the shutter speed priority mode or manual mode. It should be noted that these operation modes can be arbitrarily selected by a mode selection button (not shown) which constitutes an operation key.

Further, although not shown in the figure, the operation key is provided with a noise removal function ON / OFF button for enabling or disabling the noise removal function according to this embodiment. That is, this noise removal function is ON /
When the OFF button is ON, the CPU 44
As described above, each circuit 18, 22, 28 in the ASIC 42,
By controlling 32 and 40, the main image data D
The fixed pattern noise 48 is removed from a, and the data after this removal is input to the signal processing circuit in the subsequent stage. On the other hand, when the noise removal function ON / OFF button is OFF, the CPU 44 does not perform the control as described above, and inputs only the main image data Da based on the exposure period Ta as it is to the signal processing circuit in the subsequent stage.

Specifically, the CPU 44 executes each process shown in the flow chart of FIG. 3 in response to the depression of the shutter button described above. The program for controlling the operation of the CPU 44 according to the flow chart of FIG. 3 is stored in a program memory (not shown). Further, when the shutter button is pressed, the mechanical shutter 16
Is open.

That is, when the shutter button is pressed, the CPU 44 determines the optimum exposure period Ta for photographing the subject in step S1. This decision
Follow the operation mode at that time.

Then, in step S3, the CPU 44 controls the memory control circuit 40 to set the main image storage area 36 of the SDRAM 34 as a data storage destination, and in step S5, sets the gain of the AGC circuit 26 to Ga. The gain control circuit 28 is controlled so that Further, in step S7, the light receiving surface 1 of the CCD 12 is exposed for the exposure period Ta determined in step S1.
After setting the shutter speed of the electronic shutter so as to expose 2a, in step S9, the CCD control circuit 22 is controlled to open the electronic shutter and start exposure. As a result, the optical image of the subject is incident on the light receiving surface 12a of the CCD 12.

If it is determined in step S11 that the exposure period Ta has elapsed since the electronic shutter was opened in step S9, the CPU 44 closes the electronic shutter by C
It controls the CD control circuit 22. Then, step S13
In step S15, after controlling the shutter control circuit 18 to close the mechanical shutter 16, in step S15
The CCD control circuit 22 is controlled so as to read the image signal (ODD) of the odd line from 12 and read the image signal (EVEN) of the even line in step S17. As a result, the CCD 12 outputs the main image signal including the fixed pattern noise 48.

Then, in step S19, the CPU 44 controls the memory control circuit 40 so that the main image data Da based on the main image signal is stored in the main image storage area 36 (strictly speaking, at the previous stage). , Scan conversion processing of the main image data Da is performed using the memory buffer circuit 32). Then, in step S21, the CPU 44 turns on the noise removal function ON / OFF button described above.
It is determined whether or not N is set. If it is determined that the noise removal function ON / OFF button is ON,
The CPU 44 proceeds to step S23.

In step S23, the CPU 44
Calculates the exposure period Tb for obtaining the black image signal.
Then, in step S25, the memory control circuit 40 is controlled to set the black image storage area 38 of the SDRAM 34 as a data storage destination. Further, step S2
7, the gain Gb of the AGC circuit 26 is calculated so as to be added to the black image signal, and the gain Gb obtained by this calculation is calculated in step S29.
Set to. Then, in step S31, the shutter speed of the electronic shutter of the CCD 12 is set based on the exposure period Tb obtained in step S23, and then in step S31.
At 33, the CCD control circuit 22 is controlled to open the electronic shutter and start exposure.

In step S35, step S33
When it is determined that the exposure period Tb has elapsed since the electronic shutter was opened, the CPU 44 controls the CCD control circuit 22 to close the electronic shutter. And step S3
7, the image signal (OD) of the odd line from the CCD 12
CC) so that the image signal (EVEN) of the even line is read in step S39 while reading D).
The D control circuit 22 is controlled. This allows the CCD 12
Outputs a black image signal corresponding to the fixed pattern noise 50.

Then, in step S41, the CPU 44 controls the memory control circuit 40 to store the black image data Db based on the black image signal in the black image storage area 38. Then, in step S43, from the main image data Da stored in the main image storage area 36 in step S19 described above, the black image storage area 3 is selected in step S41 this time.
The black image data Db stored in 8 is subtracted, and the subtracted data [Da-Db] is used as new main image data D.
The memory control circuit 40 is controlled so as to be stored in the main image storage area 36 as a.

After rewriting the main image data Da in this way, the CPU 44 proceeds to step S45. Then, in step S45, the shutter control circuit 18 is controlled to open the mechanical shutter 16, and then, as shown in FIG.
The series of processes shown in the flowchart of FIG. In step S21 described above, the noise removal function is turned ON / O.
When it is determined that the FF button is off, the CPU
The process directly proceeds to step S45.

In this embodiment, the interlaced scan type CCD 12 is used, but the progressive scan type CCD 12 may be used. If the progressive scan type is used in this way, the memory buffer circuit 32 becomes unnecessary. Further, not only the CCD 12 but also another type of image pickup device such as a CMOS type may be used.

Then, by changing the gain of the AGC circuit 26 (Ga or Gb), the magnitude Vb 'of the fixed pattern noise 50 contained in the black image signal and the fixed pattern noise 48 contained in the main image signal are reduced. Size Vb
And the same, but not limited to this. That is, A / D
By performing a predetermined digital process in the portion subsequent to the conversion circuit 30, the same operation as changing the gain of the AGC circuit 26 may be obtained as a result.

Further, in this embodiment, after the main image signal (main image data Da) is obtained, the black image signal (black image data Db) is obtained. However, each image signal is reversed in the reverse order. (Each image data Da and Db) may be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing a schematic configuration of an embodiment of the present invention.

FIG. 2 is an illustrative view for explaining a process of noise removal in the embodiment of FIG. 1, and is a drawing in which a certain line is focused.

FIG. 3 is a flowchart showing the operation of the CPU in the embodiment of FIG.

[Explanation of symbols]

10 ... Digital camera 12 ... CCD type solid-state image sensor 12a ... light receiving surface 14 ... Lens 16 ... Mechanical shutter 26 ... AGC circuit 34 ... SDRAM 36 ... Main image storage area 38 ... Black image storage area 44 ... CPU

Claims (3)

[Claims] 1. A first activating means for activating an image pickup device for a first period while a subject image is incident on a light receiving surface of the image pickup device.
Activation means, first imparting means for imparting a first gain to a first image signal output from the image sensor by activation for the first period, the first image sensor with the light receiving surface shielded from light A second activation means for activating for a second period shorter than the period; and a second gain larger than the first gain for the second image signal output from the image sensor by the activation for the second period. Second adding means, and the second image from the first image signal to which the first gain is added
A digital camera comprising subtraction means for subtracting a second image signal to which a gain has been added to output a corrected image signal. 2. The digital camera according to claim 1, wherein the second period is 1 / N times (N> 1) times the first period, and the second gain is N times the first gain. 3. The subtraction means includes first storage means for storing first image data based on the first image signal, and second storage means for storing second image data based on the second image signal. 3. The digital camera according to claim 1, wherein the second image data is subtracted from the first image data.