JP2007036332A - Imaging apparatus and drive method thereof - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an imaging apparatus capable of forming a high-grade image by performing dark noise correction stably regardless of temperature, photographing sensitivity, or the like; and to provide a drive method of the imaging apparatus in the imaging apparatus having a CCD capable of reading multiple fields. <P>SOLUTION: The imaging apparatus comprises a CCD area sensor capable of reading multiple fields, a clamping means for clamping an image signal generated by the CCD area sensor, a clamp control means for controlling the operation of the clamping means, a detection means for detecting the temperature of the CCD area sensor, and a photographing sensitivity setting means for setting the photographing sensitivity of the CCD area sensor. In the imaging apparatus, the clamp control means controls the operation of the clamping means at least in a second field, based on the temperature or photographing sensitivity of the CCD area sensor in a still image mode. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to an imaging apparatus and a driving method thereof, and more particularly, to an imaging apparatus that performs dark noise correction of an imaging element.

  In an imaging apparatus such as an electronic camera, a scanner, and a video camera, an image is formed by converting a subject optical image into an image signal by an imaging device typified by a CCD (Charge Coupled Device). In these imaging apparatuses, there is a case where image noise caused by dark current generated in an imaging element called dark noise (thermal noise) is generated. Since dark noise has a large effect on image reproducibility and image quality, it is necessary to remove the influence by operations such as correction in order to realize high-quality image formation. Various techniques for efficiently correcting dark noise have been studied in the field of imaging devices.

  Here, a dark noise correction method generally performed will be described.

  First, the configuration of the CCD and its driving method will be described.

  Various types of imaging devices are known. For example, the configuration of a vertical overflow drain CCD will be described with reference to FIG. FIG. 7A is a schematic diagram showing a pixel configuration of the CCD 50, and FIG. 7B is a schematic diagram showing a CCD output signal of an arbitrary horizontal line that scans the light receiving region 52.

  As shown in FIG. 7A, the CCD 50 includes a light receiving area 52 that converts an optical image of a subject into an image signal, and a light shielding area 53 that forms an image signal for detecting dark noise (hereinafter, the light shielding area 53 is referred to as an OB area). ; Also referred to as an optical black region), and the light shielding region 53 has a structure arranged around the light receiving region 52.

  In the CCD 50, photodiodes 51 corresponding to photoelectric conversion units are arranged in units of pixels in the light receiving region 52 and the OB region 53. The photodiode 51 is connected to a vertical shift register (vertical CCD) 54 as shown in FIG. 7A, and the vertical shift register 54 is further provided with a horizontal shift register (horizontal CCD) 55 provided below the CCD 50. Connect with.

  The photodiode 51 provided in the OB region 53 is optically completely shielded from light, and only a charge signal resulting from dark noise caused by heat is formed. In the present invention, a charge signal caused by dark noise is also called an image signal.

  Next, a driving method of the CCD 50 having the above configuration will be described. The driving method of the CCD 50 includes two driving modes generally called “live view mode” and “still image mode”. The live view mode is a mode in which an image signal is output from the CCD 50 while pixel thinning is performed at regular intervals to determine an angle of view or the like in photographing, and is finally displayed on the image display unit. The still image mode is a mode in which there is a charge sweeping period by high-speed transfer of the vertical shift register 54 when driving the CCD 50, and is a mode normally used when shooting a still image, and is referred to as a “still image shooting mode” Used.

  In the live view mode, the charge signal generated by the photodiode 51 is read to the vertical shift register 54 by the charge read pulse φTG1 or φTG3 output from the timing generator that drives the CCD 50. The read charge signal is transferred in the vertical direction by vertical transfer pulses φV1 to φV4 output from the timing generator. The timing generator outputs horizontal transfer pulses φH 1 and φH 2 to the horizontal shift register 55 each time a charge signal for one horizontal line is supplied from each vertical shift register 54 to the horizontal shift register 55. In response to the horizontal transfer pulses φH1 and φH2, charge signals corresponding to one horizontal line are transferred in the horizontal direction. The horizontally transferred charge signal is output to the outside through the output unit 57. In this manner, a charge signal corresponding to one screen generated by each photodiode 51 is output from the CCD 50. As shown in FIG. 7B, the image signal output from the CCD 50 becomes a CCD output signal composed of a dummy pixel signal, a front stage OB signal, an image signal, and a rear stage OB signal from the front edge.

  On the other hand, in the still image mode, before the charges accumulated in the photodiode 51 are read out to the vertical shift register 54, an unnecessary charge generated in the vertical shift register 54 is quickly swept out. In the vertical shift register 54, unnecessary charges may remain due to the influence of dark noise, smear, blooming, and the like. If unnecessary charges remaining in the vertical shift register 54 are not swept out before the charge signal accumulated in the photodiode 51 is read out to the vertical shift register 54, unnecessary charges are generated as image noise. Therefore, a high-speed sweeping period is provided for sweeping out unnecessary charges remaining in the vertical shift register 54 at once. When the high-speed sweep is completed, the charge signal accumulated in the photodiode 51 is read out to the vertical shift register 54, and then the image signal is output from the CCD 50 through the same operation as that performed in the above-described live view mode. Is done.

  Here, a dark noise correction method that is normally performed on an image signal read from the CCD 50 will be described.

  In the live view mode, the dark noise correction is normally performed on pixels constituting the OB area at the right end of the imaging surface of the CCD 50 shown in FIG. 7A (only two pixels are shown in the figure, but several tens of pixels are shown). The dark noise generated in step 1 is extracted, and the image signal is corrected in accordance with the extracted dark noise level. Specifically, dark noise is extracted from the subsequent stage OB signal of the CCD output signal shown in FIG. 7B based on a horizontal clamp signal (not shown), and the black level of the image signal is based on the extracted dark noise. A black level reference voltage is set with reference potential as. Based on the black level reference voltage set in this way, a clamping operation is performed in a CDS (correlated double sampling) circuit, whereby dark noise correction is performed.

  However, in the still image mode, due to the existence of the above-described high-speed sweep-out period, high-precision dark noise correction cannot be performed, and a step called V sag may occur in the image signal.

  The dark noise correction operation in the still image mode will be described with reference to FIG. 3A shows the vertical synchronization signal (VD), FIG. 3B shows the read operation mode, FIG. 3C shows the dark noise level, FIG. 3D shows the clamp signal (CLP), FIG. 3 (e) shows the clamp potential.

  When the CCD 50 is driven in the still image mode, there is a high-speed sweep-out period tc of the vertical shift register 54 as shown in FIG. In the high-speed sweep-out period tc, noise components such as dark noise and smear are discharged via the output unit 57 by transferring the vertical shift register 54 at high speed. Therefore, as shown in FIG. Stopped. Further, as shown in FIG. 3B, after the high-speed sweep period tc, reading of the image signal is started, and reading of pixels (corresponding to ten or more pixels) constituting the OB area below the imaging surface of the CCD 50 is first performed. Is done. Even in the lower OB signal readout period tb1, the normal clamping operation is stopped as shown in FIG. Therefore, the clamp potential in the high-speed sweep period tc during which the clamp operation is stopped and the lower OB signal readout period tb1 is very low as shown in FIG.

  On the other hand, the amount of dark noise generated in the vertical shift register 54 greatly affects the temperature. That is, the amount of dark noise generated in the vertical shift register 54 increases because the use time of the image pickup device becomes longer and the power consumption increases to generate heat or the environmental temperature in which the image pickup device is used increases. For this reason, the high-speed sweep operation performed in the high-speed sweep-out period tc cannot follow the generation of dark noise, and the dark noise is accumulated even in the high-speed sweep-out period tc. As shown in FIG. 3C, the dark noise level reaches Vob at the end of the lower OB signal read period tb1.

  In this state, when the lower OB signal readout period tb1 ends and the image signal readout period ts in which the clamping operation is performed is started, as shown in FIG. 3D, the time constant of the clamping circuit is started although the clamping operation is started. Depending on the magnitude, it is difficult to instantaneously obtain a clamp potential corresponding to the dark noise level Vob. In addition, as indicated by a solid line in FIG. 3E, the clamp potential must reach the clamp potential Vcp corresponding to the dark noise level Vob instantaneously after the start of the clamp operation. Depending on the size, tracking becomes difficult, and as shown by the broken line, it gradually rises to reach the required clamp potential. The potential difference between them is generated as a V sag (step) in the image signal, which is a factor that greatly impairs the image quality.

  In this way, in the still image mode, V sag is generated in the image signal and affects the image quality. Therefore, the influence must be reduced by an appropriate dark noise correction operation or the like. In the field of imaging devices, various techniques have been studied so far that enable dark noise correction with high accuracy.

For example, an analog clamp unit and a digital clamp unit are provided as dark noise correction means, and the operation of the digital clamp unit is selected from two operation modes of a field clamp and a line clamp according to the driving mode of the CCD. There is a technique for suppressing the occurrence of V sag (step) in the still image mode by selecting a line clamp operation during driving (see, for example, Patent Document 1).
JP 2001-313864 A (see paragraph 0033 and the like)

  By the way, in recent years, with the increase in performance of image pickup apparatuses, the increase in the number of pixels and the increase in density of the image pickup element have been accelerated. It has a great influence on. In order to compensate for this decrease in sensitivity, a CCD capable of multi-field readout has been used. Multi-field readout is based on the normal frame readout (frame readout method called two-field readout, which reads out the charges generated by the photodiode in two steps) when reading out the charges generated by the photodiode. In this method, the number of divisions is further increased, and reading is performed by dividing into three or more fields.

  In such multi-field reading, the image signals read out divided into a plurality of times are combined to form a single image, so it is necessary to match the signal levels of the image signals read out in each field. is there. In particular, the black level that serves as the reference for the image signal must be matched with high accuracy, and dark noise correction that affects the black level of the image signal must be reliably performed.

  Further, the V sag (step) caused by dark noise or a clamping error during dark noise correction is affected by the photographing sensitivity. That is, when the photographing sensitivity is high, the gain of the signal processing circuit is set high, so that dark noise and V sag (step) are amplified and appear as large noise on the image signal.

  Further, since the amount of dark noise is affected by temperature as described above, the amount of dark noise increases as the temperature of the image sensor increases.

  Therefore, it is necessary to correct dark noise under conditions that are not affected by these factors, depending on temperature and photographing sensitivity. In particular, in a CCD capable of multi-field readout, dark noise must be corrected reliably to match the signal level between fields with high accuracy.

  In the above-mentioned Patent Document 1, when the CCD is driven in the still image mode, the line clamp operation is performed to suppress the occurrence of V sag (step) in the still image mode. However, in a CCD capable of multi-field readout, the amount of dark noise increases because the charge accumulation time is longer in the field read out later than in the field read out first. That is, the dark noise level at the start of the clamp operation becomes higher as the field is read later.

  Here, how the dark noise changes during multi-field readout will be described with reference to FIG. 4A to 4E show the same signals, dark noise levels, and the like as in FIGS. 3A to 3E.

  Since the amount of the dark noise increases as the charge accumulation time becomes longer, the dark noise level is higher than the dark noise level Vob in the first field, as shown in FIG. 4C. Vob2 is higher, and the dark noise level Vob3 in the third field is higher than the dark noise level Vob2 in the second field. As described above, when the dark noise level becomes high, it is considered that it is difficult to instantaneously obtain an appropriate clamp potential even if the line clamp operation is performed.

  Further, the V sag (step) caused by dark noise or a clamping error at the time of dark noise correction is greatly influenced by temperature and photographing sensitivity, but Patent Document 1 relates to the influence of temperature and photographing sensitivity. However, it did not suggest that dark noise correction could be performed stably.

  The present invention has been made in view of the above problems, and enables an image pickup apparatus including a CCD capable of multi-field readout to stably perform dark noise correction without being affected by temperature, shooting sensitivity, and the like. Accordingly, an object of the present invention is to provide an imaging apparatus capable of forming a high-quality image and a driving method thereof.

  The object is achieved by the invention described in any one of claims 1 to 6 below.

(Claim 1)
CCD area sensor capable of multi-field readout;
Clamping means for clamping an image signal generated by the CCD area sensor;
Clamp control means for controlling the operation of the clamp means;
Detection means for detecting the temperature of the CCD area sensor, or
A photographing sensitivity setting means for setting a photographing sensitivity of the CCD area sensor;
When the CCD area sensor is driven in a still image mode, the image signal generated by the CCD area sensor is read out after sweeping out unnecessary charges filling the vertical shift register of the CCD area sensor.
The clamp control means controls the operation of the clamp means in at least the second field based on the temperature of the CCD area sensor detected by the detection means or the photographing sensitivity set by the photographing sensitivity setting means. An imaging apparatus characterized by that.

(Claim 2)
The clamp control means controls the operation of the clamp means so as to change a clamp area or a clamp cycle performed by the clamp means on the image signal. The imaging apparatus according to 1.

(Claim 3)
The clamp control means includes
The higher the temperature of the CCD area sensor or the set shooting sensitivity,
The imaging apparatus according to claim 2, wherein the operation of the clamping unit is controlled so as to widen a clamping region performed on the image signal by the clamping unit or to delay a clamping cycle. .

(Claim 4)
A driving method of an imaging apparatus having a CCD area sensor capable of multi-field readout,
When driving the CCD area sensor in the still image mode, the image signal generated by the CCD area sensor is read after unnecessary charges filling the vertical shift register of the CCD area sensor are swept out.
At least in the second field, the image signal generated by the CCD area sensor is clamped on the basis of the temperature of the CCD area sensor or the photographing sensitivity.

(Claim 5)
5. The method of driving an imaging apparatus according to claim 4, wherein clamping processing is performed by changing a clamp area or a clamp cycle performed on the image signal.

(Claim 6)
The higher the temperature of the CCD area sensor or the shooting sensitivity,
6. The method of driving an imaging apparatus according to claim 5, wherein the clamping process is performed by expanding a clamp area to be performed on the image signal or delaying a clamp cycle.

  According to the first and fourth aspects of the present invention, when the CCD area sensor capable of multi-field reading is driven in the still image mode, the operation of the clamping means can be controlled based on the temperature of the CCD area sensor or the photographing sensitivity. I did it. Therefore, in still image mode, dark noise can be corrected with an optimum clamping method according to temperature and shooting sensitivity so that generation of V sag (step), which has been difficult to remove in the past, can be suppressed. became. As a result, in multi-field readout, it is possible to match the signal level of the image signal between fields with high accuracy without being affected by temperature, imaging sensitivity, etc., and an imaging device capable of forming a high-quality image And a driving method thereof can be provided.

  According to the second and fifth aspects of the present invention, the clamp area and the clamp cycle performed by the clamp unit on the image signal are changed based on the temperature of the CCD area sensor or the photographing sensitivity. Therefore, it is possible to reliably perform dark noise correction without being affected by temperature, photographing sensitivity, etc., by performing clamping with an optimum clamping region or clamping cycle according to temperature and photographing sensitivity.

  According to the third and sixth aspects of the present invention, as the temperature of the CCD area sensor or the photographing sensitivity increases, the clamping unit performs a wider clamping area on the image signal, or the clamping cycle is delayed. I did it. Therefore, it is possible to reliably correct dark noise by clamping a wide area of the image signal or slowly clamping the image signal at high temperature and high sensitivity, where dark noise and V sag (step) are likely to occur. became. As a result, in multi-field readout, the signal level of the image signal between fields can be matched with high accuracy without being affected by temperature, photographing sensitivity, and the like.

  As a specific embodiment of the imaging apparatus according to the present invention, a digital camera is typical, but a mobile phone with a camera, a digital video camera, and the like are also included.

  The appearance of a digital camera, which is one of the representative embodiments of the imaging apparatus according to the present invention, will be described with reference to FIG. FIG. 1A is a side view of a digital camera 1 according to the present invention, and FIG. As shown in FIG. 1A, the digital camera 1 includes a camera body 2 and a lens unit 3.

  The lens unit 3 includes a photographing lens having a macro zoom (not shown), a diaphragm, a shutter, and the like.

  The camera body 2 includes an LCD monitor 211 formed of an LCD (Liquid Crystal Display), an EVF (Electronic View Finder) 210, and external connection terminals for connecting the digital camera 1 to a personal computer (not shown). The image signal captured by the CCD area sensor 241 described later is subjected to predetermined signal processing, image display on the LCD monitor 211 and EVF 210, and image recording on a recording medium such as a memory card 277 described later, Alternatively, processing such as image transfer to a personal computer is performed.

  As shown in FIG. 1A, a built-in flash 212 that is hopped up when necessary is provided on the upper surface of the camera body 2.

  Further, as shown in FIG. 1B, an LCD for displaying a photographed image, reproducing a recorded image, or displaying a GUI such as a menu screen or various statuses is displayed at a substantially central portion on the back of the camera body 2. A monitor 211 and an EVF 210 are provided.

  On the upper surface of the camera body 2, as shown in FIG. 1B, there are provided a shutter button 201 and a mode setting dial 203 for setting the operation mode of the digital camera 1 near the shutter button 201. The operation mode of the digital camera 1 set by the mode setting dial 203 includes “still image mode”, “moving image mode”, “reproduction mode”, and the like.

  For example, the still image mode is a mode for taking a picture from the standby state to the shooting through the exposure control process, and the reproduction mode is a reproduction display of the photographed image recorded on the memory card 277 on the LCD monitor 211 or the EVF 210. It is a mode to do.

  Further, as shown in FIG. 1B, a digital switch button 209 for changing the zoom magnification of the digital zoom is provided near the main switch 202, as shown in FIG. ing.

  Further, as shown in FIG. 1B, a menu button 207 for displaying a menu on the LCD monitor 211 and an image on the LCD monitor 211 near the menu button 207 are displayed at the lower back of the camera body 2. Is provided with a quick view button 208.

  Furthermore, a selection / determination button 206 for performing various settings is provided at a substantially central portion on the back of the camera body 2. The selection / determination button 206 includes a jog dial (cross key) 204 for selecting various settings and a determination button 205 for confirming the setting selected with the jog dial (cross key) 204.

  The selection / determination button 206 is used to set the shooting sensitivity of the digital camera 1. Specifically, for example, a shooting sensitivity setting mode is entered on a menu screen displayed on the LCD monitor 211. There are ISO 50 to ISO 400, for example, and the jog dial 204 is operated to select a desired imaging sensitivity. Next, the photographing sensitivity selected by the jog dial 204 is confirmed by operating the decision button 205. A camera control CPU 291 (to be described later) adjusts the gain of the AGC circuit 243 based on the photographing sensitivity selected by the selection / determination button 206. As described above, the selection / decision button 206 functions as a photographing sensitivity setting unit in the imaging apparatus according to the present invention.

  Next, the control system of the digital camera 1 will be described with reference to FIG. FIG. 2 is a block diagram of a control system of the digital camera 1 according to the present invention. In FIG. 2, the same members as those shown in FIG.

  The CCD area sensor 241 (hereinafter abbreviated as CCD 241) arranges each color transmission filter of R (red) light, G (green) light, and B (blue) light in a checkered pattern in pixel units (pixel units). The subject light image formed by the lens unit 3 is photoelectrically converted by the color area imaging sensor, and image signals (each pixel) of each color component of R (red) light, G (green) light, and B (blue) light A signal composed of a signal sequence of pixel signals received in units), and has an interline type element structure capable of multi-field readout.

  In the CCD 241, as shown in FIG. 7A, photodiodes 51 are arranged in units of pixels in the light receiving region 52 and the OB region 53. As shown in FIG. 7A, the photodiode 51 is connected to a vertical shift register 54, and the vertical shift register 54 is connected to a horizontal shift register 55 provided below the CCD 241, and is stored in the photodiode 51. The charged signal is amplified by the output unit 57 and output to the outside through these parts.

  The timing generator 248 generates a drive control signal for the CCD 241 based on a reference clock transmitted from a reference clock generator 271 described later. The drive control signal generated by the timing generator 246 includes, for example, an integration start / end timing signal for controlling the exposure start and end timings in the CCD 241, and a readout control signal (horizontal synchronization signal, vertical synchronization) of the light reception signal of each pixel. Clock signals such as signals, transfer signals, and the like. When these clock signals are supplied to the CCD 241, the CCD 241 performs drive control corresponding to each clock signal.

  The timing generator 248 generates a clamp signal used in a CDS circuit 243 and a switch circuit 246 in a signal processing circuit 242 described later based on a reference clock transmitted from a reference clock generation unit 271 described later. The form and cycle of the clamp signal generated by the timing generator 248 are controlled by a camera control CPU 291 described later.

  Next, the signal processing circuit 242 includes a CDS circuit 243, an AGC circuit 244, an A / D converter 245, a switch circuit 246, and a black level reference voltage setting unit 247. Is performed. Hereinafter, a predetermined process for the image signal performed by the signal processing circuit 242 will be described.

  A CDS (correlated double sampling) circuit 243 reduces noise generated at the time of reading from an image signal read from the CCD 241, or performs OB clamping based on the black level reference voltage generated by the black level reference voltage setting unit 247. The operation is performed to correct dark noise.

  An AGC (automatic gain control) circuit 244 controls the photographing sensitivity by adjusting the gain of the image signal processed by the CDS circuit 243 based on the photographing sensitivity set by the selection / determination button 206.

  The A / D converter 245 converts each pixel signal constituting the image signal input from the AGC circuit 244 into a digital signal. The A / D converter 245 converts each pixel signal of an analog signal into, for example, a 14-bit digital signal based on the A / D conversion clock input from the reference clock generation unit 271.

  The switch circuit 246, based on the clamp signal sent from the timing generator 248, from the image signal output from the AGC circuit 244, an image signal in the OB area composed of dark noise components included in the image signal, or a dummy A pixel signal is extracted.

  The black level reference voltage setting unit 247 generates a black level reference voltage used when performing dark noise correction in the CDS circuit 243 based on the image signal extracted by the switch circuit 246 or the dummy pixel signal. It is.

  That is, the timing generator 248, the CDS circuit 243, the switch circuit 246, and the black level reference voltage setting unit 247 function as clamping means in the imaging apparatus according to the present invention.

  With such a configuration, in the present invention, when the CCD 241 capable of multi-field readout is driven in the still image mode, an optimum clamping method is selected according to the temperature and photographing sensitivity, and dark noise is selected by the selected clamping method. By correcting the above, the clamping operation performed in the digital camera 1 is controlled so as to suppress the occurrence of V sag (step) that has been difficult to remove in the past. A detailed description of clamp operation control based on temperature and imaging sensitivity will be given later.

  In this manner, the image signal read out by the CCD 241 is subjected to predetermined processing by the signal processing circuit 242 and converted into a digital image signal. The digitized image signal is captured by the image processing CPU 261 and subjected to predetermined processing.

  The image processing CPU 261 is composed of a microcomputer, and comprehensively controls image signal processing operations performed by the digital camera 1. In the following, processing for a digital image signal performed by the image processing CPU 261 will be described.

  First, the image signal captured by the image processing CPU 261 is written into the image memory 275 in synchronization with the reading of the image signal output from the CCD 241. That is, the digital image signal used for the processing performed by the image processing CPU 261 is once recorded in the image memory 275, taken out from the image memory 275, and used for processing in each block.

  As shown in FIG. 2, the image processing CPU 261 includes, for example, a black level correction unit 263, a pixel interpolation unit 264, a resolution conversion unit 265, a white balance control unit 266, a gamma correction unit 267, a matrix calculation unit 268, a shading correction unit 269, The image processing unit 262 includes an image compression unit 270 and the like, and a reference clock generation unit 271, and the image processing unit 262 performs known image signal processing on the digital image signal extracted from the image memory 275. Then, the digital image signal that has been subjected to the predetermined processing in these parts is stored in the image memory 275 again.

  The reference clock generation unit 271 is a circuit that generates a reference clock used for driving control of the digital camera 1 and supplies the reference clock to each circuit. Specific examples of the reference clock in the present invention include a reference clock used for the timing generator 246, an A / D conversion clock used for the A / D converter 245, and the like. Generate a clock.

  Next, the LCD monitor 211 performs display of an image signal captured by the CCD 241 and GUI display.

  The LCD drive circuit 279 includes a buffer memory (VRAM) that temporarily stores an image signal read from the image memory 275 by the image processing CPU 261, and displays the image signal on the LCD monitor 211 as a field image.

  The temperature sensor 293 is provided in the vicinity of the CCD 241, detects the temperature of the CCD 241, and sends the detected temperature information to a camera control CPU 291 described later. That is, the temperature sensor 293 functions as a detection unit in the imaging apparatus according to the present invention.

  The camera control CPU 291 is composed of a microcomputer, and based on switch signals generated by switch operations of a switch group 295 described later, the camera body CPU 2 and the lens unit 3 are sequentially controlled to drive the digital camera 1. Centrally control the operation.

  In addition, the camera control CPU 291 performs a timing generator 248, a CDS circuit 243, a switch circuit 246, based on the temperature of the CCD 241 detected by the temperature sensor 293 or the photographing sensitivity selected by the selection / decision button 206 in the still image mode. The clamp operation performed by the black level reference voltage setting unit 247 and the like is controlled. That is, the camera control CPU 291 functions as a clamp control unit in the imaging apparatus according to the present invention. A detailed description of clamp operation control based on temperature and imaging sensitivity will be given later.

  As described above, in the digital camera 1, the image processing CPU 261 performs the signal processing as described above on the image signal captured from the CCD 241, and records the image signal subjected to these processing in the image memory 275. .

  The digital camera 1 according to the present invention records the image signal captured from the CCD 241 under the mode set by the mode setting dial 203 in the image memory 275 or displays it on the LCD monitor 211 or EVF 210.

  In a shooting standby state (for example, a state in which the mode setting dial 203 is set to “still image mode”), image signals are taken in from the CCD 241 through the signal processing circuit 242 and image processing CPU 261 at predetermined intervals such as every 1/30 seconds. ing. As described above, the captured image signal is subjected to predetermined signal processing at a portion from the black level correction unit 263 to the shading correction unit 269 of the image processing unit 262 in the image processing CPU 261, and then as a digital image signal. It is recorded in the image memory 275. Then, the image signal recorded in the image memory 275 is read out, and the read image signal is displayed on the LCD monitor 211 or EVF 210 as a field image via the LCD drive circuit 279 or EVF drive circuit 280.

  Further, at the time of image recording, the image signal is compressed by the image compression unit 270 of the image processing unit 262 in the image processing CPU 261 to obtain an image of the set recording resolution, and the obtained compressed image is a memory card driver 276. To the memory card 277.

In the reproduction mode (the state in which “reproduction mode” is set by the mode setting dial 203), the image signal read from the memory card 277 is subjected to predetermined signal processing by the image processing CPU 261, and is driven by the LCD drive circuit 279 or EVF drive. 2 is displayed on the LCD monitor 211 and the EVF 210 via the circuit 286. The switch group 295 in FIG. 2 includes the shutter button 201, the main switch 202, the mode setting dial 203, the selection / decision button 206, the menu button 207 in FIG. Switches corresponding to the quick view button 208, the digital zoom button 209, and the like.

  Here, the clamp operation control in the still image mode will be described.

  In the conventional clamping operation in the still image mode, as described above, the clamping operation is stopped during the high-speed sweep-out period tc and the lower OB signal readout period tb1. Then, after the end of the lower OB signal readout period tb1, when the image signal readout period ts starts, the clamp operation starts as shown in FIG. 3D, but during the period when the clamp operation is stopped, as shown in FIG. As shown in (c), it is difficult to instantaneously obtain a clamp potential Vcp corresponding to the dark noise level that is increased by the influence of temperature and accumulated up to Vob due to the time constant of the clamp circuit. Originally, as indicated by the solid line in FIG. 3E, the clamp potential must instantaneously reach the clamp potential Vcp corresponding to the dark noise level Vob when the clamp operation is started, but the magnitude of the time constant of the clamp circuit Cannot follow, and gradually rises as shown by the broken line to reach the originally required clamp potential. The potential difference between them occurs as a V sag (step) in the image signal, and the image quality is greatly impaired.

  Further, the V sag (step) due to dark noise or a clamping error at the time of dark noise correction is greatly influenced by the photographing sensitivity. That is, when the imaging sensitivity is high, the gain of the signal processing circuit is set high, so that dark noise and V sag (step) are amplified and appear as large noise in the image signal.

  Further, in a CCD capable of multi-field readout, the amount of dark noise increases due to the longer charge accumulation time in the field read out later than the field read out first. That is, the dark noise level at the start of the clamp operation becomes higher as the field is read later. When the dark noise level becomes high in this way, it is increasingly difficult to obtain an appropriate clamp potential instantaneously, and it is very difficult to match the signal level of the image signal between fields with high accuracy.

  According to the present invention, when a CCD capable of multi-field readout is driven in a still image mode, an optimum clamping method is selected according to temperature and photographing sensitivity, and dark noise is corrected by the selected clamping method. The clamping operation performed by the digital camera 1 is controlled so as to suppress the occurrence of V sag (step) that has been difficult to remove in the past.

  Here, the clamp operation control in the still image mode in the digital camera 1 according to the present invention will be described in detail with reference to FIG.

  FIG. 5A shows a vertical synchronization signal (VD). FIG. 5B shows a read operation mode. FIG. 5C shows the dark noise level. FIG. 5D shows a clamp signal (CLP). FIG. 5E shows the clamp potential.

  Conventionally, after the lower OB signal readout period tb1, the clamping operation is started from the time when the image signal readout period ts is started. However, in the digital camera 1 according to the present invention, dark noise and V sag (step) are present. When a large temperature and high sensitivity are generated, as shown in FIG. 5D, after the high-speed sweep period tc ends, the clamp operation starts from the time when the lower OB signal read period tb1 starts.

  Immediately after the start of the clamp operation, it is difficult to instantaneously obtain an appropriate clamp potential depending on the magnitude of the time constant of the clamp circuit as described above. Therefore, in order to stabilize the clamp potential before shifting to the image signal readout period ts, the clamp operation is started immediately after the end of the high-speed sweep-out period tc, and the image signal is affected as shown in FIG. The clamp potential is made to follow and stabilize the dark noise level during the lower OB signal readout period tb1, and the clamp potential Vcp corresponding to the dark noise level Vob is obtained at the time when the image signal readout period ts is reached. Yes.

  Specifically, as shown in Table 1, the camera control CPU 291 previously sets a plurality of optimum clamping operation modes according to temperature or photographing sensitivity, and detects the temperature or selection of the CCD 241 detected by the temperature sensor 293. Based on the photographing sensitivity selected by the / decision button 206, an optimum clamping operation mode is selected from a plurality of types of clamping operation modes. Based on the selected clamp operation mode, the clamp operation performed by the timing generator 248, the CDS circuit 243, the switch circuit 246, the black level reference voltage setting unit 247, and the like is controlled.

  Here, clamp operation modes A1 to A8 in Table 1 will be described with reference to FIG. FIG. 6A shows a clamp area in the CCD 50. FIG. 6B shows clamp signals CLP1 to CLP6 corresponding to the clamp operation modes A1 to A6, respectively.

  In the clamp operation mode A1, the post-stage OB signal generated in the post-stage OB area OB1 shown in FIG. 6A is extracted by the clamp signal CLP1 shown in FIG. 6B, and the image signal is clamped.

  In the clamp operation mode A2, in addition to the signal extracted in the clamp operation mode A1, the previous stage OB generated in the previous stage OB area OB2 shown in FIG. The signal is also extracted and the image signal is clamped.

  In the clamp operation mode A3, in addition to the signal extracted in the clamp operation mode A2, a clamp signal CLP3 shown in FIG. 6B is used to display a part OB3 (for example, a rear stage) of the lower OB region shown in FIG. The lower OB signal generated by the OB unit) is also extracted and the image signal is clamped.

  In the clamp operation mode A4, in addition to the signal extracted in the clamp operation mode A3, the clamp signal CLP4 shown in FIG. 6B is used to generate the lower OB signal generated in the lower OB area OB4 shown in FIG. Are also extracted and the image signal is clamped.

  In the clamp operation mode A5, in addition to the signal extracted in the clamp operation mode A4, a dummy pixel signal generated by the dummy pixel DM shown in FIG. 6A is obtained by a clamp signal CLP5 shown in FIG. Are also extracted and the image signal is clamped.

  In the clamp operation mode A6, as shown in FIG. 6B, the clamp operation mode A6 includes a clamp signal CLP6 having a cycle tw2 that is later than the cycle tw1 of the clamp signal in the lower OB signal readout period tb1 in the clamp operation mode A5. The same signal as the signal extracted in A5 is extracted, and the image signal is clamped.

  In the clamp operation mode A7, the clamp of the image signal is performed at a slower cycle than the clamp operation performed in the lower OB signal readout period tb1 in the clamp operation mode A6.

  In the clamp operation mode A8, the image signal is clamped at a slower cycle than the clamp operation performed in the lower OB signal readout period tb1 in the clamp operation mode A7.

  Note that the period of the clamp signal during the image signal readout period ts in the clamp operation modes A6 to A8 is the same period as the period of the clamp signal during the image signal readout period ts in the clamp operation modes A1 to A5.

  As described above, the dark noise can be reliably corrected by performing the clamping operation in the optimum clamping operation mode according to the temperature or the photographing sensitivity.

  For example, in a case where the influence of dark noise is relatively low or at a low sensitivity, for example, when the temperature is set to 20 ° C. to 30 ° C. and the photographing sensitivity is set to ISO 50, a normal post-stage OB signal is performed. Alternatively, since the dark noise can be sufficiently corrected by the clamping operation using the preceding stage OB signal in addition to the subsequent stage OB signal, the camera control CPU 291 selects the clamping operation mode A1 or A2, and enters the selected clamping operation mode. Based on this, the clamping operation performed by the timing generator 248, the CDS circuit 243, the switch circuit 246, the black level reference voltage setting unit 247, etc. is controlled to correct the dark noise.

  Also, when the temperature or shooting sensitivity increases and dark noise increases, in the normal clamping operation, immediately after the start of the clamping operation, an appropriate value is instantaneously determined depending on the size of the time constant of the clamping circuit as described above. It is difficult to obtain a clamp potential. In this case, in order to stabilize the clamp potential before shifting to the image signal readout period ts, the camera control CPU 291 selects the clamp operation mode A3, A4, or A5, and the high-speed sweep period tc ends based on the selected clamp operation mode. The clamping operation is started immediately after. In this way, the clamp potential can be stabilized by following the dark noise level during the lower OB signal readout period tb1 that does not affect the image signal, so at the time when the image signal readout period ts starts. A clamp potential Vcp corresponding to the dark noise level Vob is obtained.

  Further, as the temperature or photographing sensitivity increases, the dark noise increases rapidly. However, in the clamp operation mode A6 or A7, the clamp cycle performed on the image signal is slowed down, and increases rapidly by slowly clamping. The clamp potential can be made to follow the dark noise, and the dark noise can be reliably corrected.

  As described above, in the imaging apparatus according to the present invention, when a CCD area sensor capable of multi-field readout is driven in the still image mode, an optimum clamping method is selected according to the temperature and the imaging sensitivity, and the selected clamping method is selected. By correcting dark noise by the method, it is possible to suppress the occurrence of V sag (step), which has been difficult to remove in the past. As a result, in multi-field readout, the signal level of the image signal between fields can be matched with high accuracy without being affected by temperature, photographing sensitivity, etc., and a high-quality image can be formed. .

1 is a schematic external view of a digital camera according to the present invention. It is a circuit block block diagram in the digital camera which concerns on this invention. It is a timing chart which shows the clamp operation | movement at the time of the still image mode in the conventional imaging device. It is a timing chart which shows the clamp operation at the time of multi-field reading in the conventional imaging device. 4 is a timing chart showing a clamping operation in a still image mode in the digital camera according to the present invention. 6 is a timing chart showing various clamp operation modes in a still image mode in the digital camera according to the present invention. It is a schematic diagram showing a CCD configuration and a CCD output signal in the digital camera according to the present invention.

Explanation of symbols

1 Digital Camera 2 Camera Body 201 Shutter Button 202 Main Switch 203 Mode Setting Dial 204 Jog Dial (Cross Key)
205 OK button 206 Select / OK button 207 Menu button 208 Quick view button 209 Digital zoom button 210 Electronic viewfinder (EVF)
211 LCD monitor 212 Built-in flash 241 CCD area sensor 242 Signal processing circuit 243 CDS circuit 244 AGC circuit 245 A / D converter 246 Switch circuit 247 Black level reference voltage setting unit 248 Timing generator 251 Aperture / shutter drive circuit 252 Focus / zoom motor Drive circuit 261 Image processing CPU
262 Image processing unit 263 Black level correction unit 264 Pixel complementation unit 265 Resolution conversion unit 266 White balance control unit 267 Gamma correction unit 268 Matrix calculation unit 269 Shading correction unit 270 Image compression unit 271 Reference clock generation unit 275 Image memory 276 Memory card driver 277 Memory card 278 External communication I / F
279 LCD drive circuit 280 EVF drive circuit 291 Camera control CPU
292 Flash control circuit 293 Temperature sensor 295 Switch group (shutter button, etc.)
3 Lens unit 351 Focus / zoom motor 352 Aperture 50 CCD
51 Photodiode 52 Light-receiving area 53 Light-shielding area 54 Vertical shift register (vertical CCD)
55 Horizontal shift register (horizontal CCD)
56 Dummy pixel 57 Output section

Claims (6)

CCD area sensor capable of multi-field readout;
Clamping means for clamping an image signal generated by the CCD area sensor;
Clamp control means for controlling the operation of the clamp means;
Detection means for detecting the temperature of the CCD area sensor, or
A photographing sensitivity setting means for setting a photographing sensitivity of the CCD area sensor;
When the CCD area sensor is driven in a still image mode, the image signal generated by the CCD area sensor is read out after sweeping out unnecessary charges filling the vertical shift register of the CCD area sensor.
The clamp control means controls the operation of the clamp means in at least the second field based on the temperature of the CCD area sensor detected by the detection means or the photographing sensitivity set by the photographing sensitivity setting means. An imaging apparatus characterized by that. The clamp control means controls the operation of the clamp means so as to change a clamp area or a clamp cycle performed by the clamp means on the image signal. The imaging apparatus according to 1. The clamp control means includes
The higher the temperature of the CCD area sensor or the set shooting sensitivity,
The imaging apparatus according to claim 2, wherein the operation of the clamping unit is controlled so as to widen a clamping region performed on the image signal by the clamping unit or to delay a clamping cycle. . A driving method of an imaging apparatus having a CCD area sensor capable of multi-field readout,
When driving the CCD area sensor in the still image mode, the image signal generated by the CCD area sensor is read after unnecessary charges filling the vertical shift register of the CCD area sensor are swept out.
At least in the second field, the image signal generated by the CCD area sensor is clamped on the basis of the temperature of the CCD area sensor or the photographing sensitivity. 5. The method of driving an imaging apparatus according to claim 4, wherein clamping processing is performed by changing a clamp area or a clamp cycle performed on the image signal. The higher the temperature of the CCD area sensor or the shooting sensitivity,
6. The method of driving an imaging apparatus according to claim 5, wherein the clamping process is performed by expanding a clamp area to be performed on the image signal or delaying a clamp cycle.

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