JP4327927B2 - Imaging device - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
  The present invention relates to an image pickup apparatus using an image pickup element such as a CCD, and more particularly to an image pickup apparatus capable of preventing image quality deterioration due to mixing of unnecessary charges during long exposure.
[0002]
[Prior art]
  Imaging devices (cameras) using a solid-state imaging device typified by a CCD are widely known, and so-called movie video cameras that obtain continuous image output for moving images, as well as still videos mainly for still images. Cameras have also formed a market centering on so-called digital cameras that employ a digital recording system.
[0003]
  A camera using a CCD drives vertical and horizontal charge transfer paths (VCCD and HCCD) when reading an image signal from a CCD image sensor. This drive entails considerable power consumption. With regard to a movie video camera, exposure and readout are performed continuously every field in order to obtain images continuously. In addition to the intended still image shooting, the still video camera has an imaging function because there is a need for movie-like image information for the AF, AE, AWB and other automated functions and electronic viewfinder functions. In general, exposure and readout are performed continuously every field.
[0004]
  However, since the driving of the transfer path involves considerable power consumption as described above, if it is not necessary for achieving the function, it is extremely effective for power saving if it is stopped. Conventionally, the following techniques have been proposed from such a viewpoint. First, Japanese Patent Laid-Open No. 9-168118 proposes a device that saves power by stopping CCD driving in a field period that is not an effective image readout field during two-field exposure. Japanese Patent Publication No. 7-44663 proposes a CCD with a horizontal OFD (overflow drain) structure that saves power by temporarily stopping the CCD drive during a long shutter operation (high-speed drive for charge sweeping). ing.
[0005]
[Problems to be solved by the invention]
  By the way, the technique disclosed in the above Japanese Patent Laid-Open No. 9-168118 is basically based on a movie camera, and although it is a shutter for a long time, the content is limited to two-field exposure. It does not relate to general long-time exposure as intended by the present invention. As clearly shown in FIG. 4 which is a main diagram of the embodiment of the publication, the stop period of the vertical drive pulse φV and the horizontal drive pulse φH which are charge transfer pulses is a signal charge read pulse. Until the time of 15 outputs, all “non-signal readout period” is covered.
[0006]
  In such a drive system, image quality degradation due to smear or dark charge actually occurs. That is, the subject light is always incident on the entire screen area of the image pickup device during the exposure period (actually, not always during the exposure time unless a separate light shielding means such as an optical shutter acts). The charges that could not be trapped in the accumulation region are mixed into the transfer path as smear, causing image quality degradation. Also, dark charges generated by thermal noise and insulation failure are mixed into the transfer path and cause image quality degradation. In normal movie operation, these unwanted charges are read and discharged together with the image signal (usually included as negligible noise) because the transfer path is always driven. However, when the CCD drive is stopped as in the above-mentioned publication, the stop period is not discharged, and accordingly, the accumulation effect is generated and the image quality deterioration is increased. This becomes a more prominent problem because the longer the exposure time, the longer the drive stop period, that is, the accumulation time of unwanted charge.
[0007]
  Further, the one disclosed in the above Japanese Patent Publication No. 7-44663 discloses an electronic shutter using a CCD having a so-called lateral OFD (overflow drain) structure which does not have means for directly discharging charges accumulated in the pixel region to the semiconductor substrate. To achieve this, it is a technology that uses “high-speed sweep” (high-speed sweep of the transfer path) that discharges unnecessary charges corresponding to the start of exposure via the transfer path. Since the power consumption is large, the CCD drive is stopped for the unnecessary period to save power. A technique for performing “high-speed sweeping” again before signal reading is also described in order to cope with the problem of mixing unnecessary charges into the transfer path related to the CCD drive stop.
[0008]
  However, there are the following two problems in the publication of this publication.
(1) In the CCD with the horizontal OFD structure as described above, “high-speed sweep” is essential for the original shutter operation. However, the electric charge accumulated in the pixel region by the vertical OFD is directly discharged to the semiconductor substrate. From the viewpoint of the current technical level capable of realizing the shutter, if “high-speed sweep” is performed prior to signal readout, a peak of driving power occurs only during this period. The occurrence of a power peak is well known to reduce the battery life of battery-operated devices such as cameras in particular, and this is a very big problem regarding the design of the entire imaging apparatus.
(2) In the “high-speed sweep” described in the above-mentioned publication, the readout of the video signal is unnecessary for the purpose of discharging the electric charge for the shutter operation, so the drive frequency of the vertical transfer path (VCCD) is simply set higher than usual. This is a high setting. Therefore, charge transfer from the vertical transfer path to the horizontal transfer path is not limited to within the H blanking period, but is also performed during the H video period. (See FIG. 5 of the above publication: vertical drive pulse HD effectively corresponds to the H blanking period, and φV1 and φV2 are drive pulses for the vertical transfer path.) Therefore, normal during the “high-speed sweep” period A video signal output cannot be obtained, and therefore OB (optical black) is not normally output. (As a matter of course, since OB is used for the reference level as a video signal of light input 0 by shading a part of the pixels of the image sensor, its function is not obtained until a normal video signal output as an electric signal is obtained. In addition, it is clear that a normal video signal cannot be obtained during the drive stop period, and therefore OB is not output normally.
[0009]
  In such an abnormal OB output period during the drive stop period or the high-speed sweep period, if the read control is performed using the same signal processing as the conventional one, the OB clamp operation performs clamping based on an inappropriate signal level. Will end up. Since such a period itself is a period in which video output is not required, there is no problem as long as that is the case. However, as a result, the charge amount (potential difference) of the coupling capacitor may deviate greatly from the original normal clamping operation value. Then, when returning to normal signal readout control, the return time to normal clamp operation (which is governed by the time constant of the clamp circuit determined by the impedance of the clamp switch and the capacitance of the coupling capacitor) is an actual circuit. However, since there are cases where several milliseconds are required due to various design constraints, in such a case, the image quality of a portion corresponding to several milliseconds for the upper readout period of the target still image is deteriorated.
[0010]
  The present invention has been made to solve the above-described problems in the conventional imaging apparatus., DrivingAn object of the present invention is to provide an imaging apparatus capable of avoiding a clamp abnormality due to an influence of a period during which a normal output cannot be obtained, such as a motion stop period.
[0011]
[Means for Solving the Problems]
  In order to solve the above problem, the invention according to claim 1 is configured to transfer the charge accumulated in the photoelectric conversion unit over the exposure period to the vertical charge transfer means according to a predetermined charge transfer command. A charge transfer type imaging device configured to be transferred to a horizontal charge transfer unit by a transfer unit and read out as an image signal by being further transferred by the horizontal charge transfer unit; and the vertical and horizontal charge transfer units of the image sensor In driving, driving of the vertical charge transfer means is prohibited at least during a period other than the horizontal blanking period provided as a non-video signal output period for horizontal charge transfer, and only during the horizontal blanking period of the vertical charge transfer means. An image output enabling drive mode for driving, and a drive stop mode for substantially stopping the driving of the vertical and horizontal charge transfer means A drive means having,Optical black clamp circuit for optical black clamp,Output of at least the charge transfer commandAnd the above optical black clamp circuitAnd a reading control means for reading out the image signal of the imaging element in a predetermined reading mode by driving the driving means, the reading control means at the end of the exposure period The driving means is operated for at least a predetermined period in a period prior to the first time point preceding the second time point for outputting the charge transfer command.the aboveIn the drive stop mode, the drive means is turned on for the period from the first time point to the second time point.the aboveThe vertical and horizontal charge transfer means is driven as an image output enabling drive mode.In addition, the optical black clamping operation is executed for at least a predetermined period in the period from the first time point to the second time point.It is comprised so that.
[0012]
  In the imaging apparatus configured as described above, during long-time exposure, driving is temporarily stopped with a predetermined period as a driving stop mode. However, prior to reading of an image signal after the end of the exposure period, one frame period, etc. The drive for discharging unnecessary charges in the transfer means for a predetermined period is performed in the image output enable mode, so that it does not generate a wasteful power peak such as high-speed sweeping drive. In addition, it is possible to suppress the occurrence of image deterioration due to the mixing of unnecessary charges such as smear due to special driving.Also, by performing OB clamping during the drive period in the image output enable mode for discharging electric charges, even if an OB clamp abnormality occurs during an abnormal output period during which a normal output cannot be obtained, such as a drive stop period, etc. Therefore, it is possible to recover the OB clamp, and it is possible to prevent the image quality deterioration due to the OB clamp abnormality.
[0013]
  And claims2The invention according to claim1In the imaging apparatus, the readout control unit may further stop the optical black clamp operation for a period before the first time point and a period other than the period in which the driving is performed in the image output enable driving mode. It is.
[0014]
  As described above, since the OB clamp is stopped in the abnormal output period preceding the drive period in the image output enabling mode for discharging the electric charge, the OB clamp is not abnormal and the normal output such as the drive stop period is performed. Therefore, it is possible to avoid an OB clamp abnormality due to the influence of a period during which no image quality is obtained, and to prevent image quality deterioration.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
  Next, embodiments of the invention will be described. FIG. 1 is a schematic block diagram illustrating an embodiment of an imaging apparatus according to the present invention. In FIG. 1, 101 is a lens system, 102 is a lens driving mechanism, 103 is an exposure control mechanism, 104 is a filter system, 105 is a CCD imaging device, 106 is a CCD driver, 107 is a preprocess circuit (including an AD converter), 108 is a digital process circuit (memory as hardware, all digital processes are included as processing), 109 is a card interface, 110 is a memory card, 111 is an LCD image display system, 112 is a system controller (mainly includes a microcomputer) , 113 is an operation switch system, 114 is an operation display system, 115 is a strobe, 116 is a lens driver, and 117 is an exposure control driver.
[0016]
  FIG. 2 is a circuit configuration diagram of the periphery of the CCD drive circuit and the OB clamp circuit. The peripheral portion includes a CCD image pickup device 105, a CCD driver 106, a preprocess circuit 107, and a system controller 112. The preprocess circuit 107 includes a sample hold (SH) circuit 201, a clamp circuit 202, a GCA 203, and an AD converter 204. It consists of Reference numeral 205 denotes a coupling capacitor. Of the commands from the system controller 112 to the CCD driver 106, only the charge discharge pulse VSUB, the charge transfer pulse TG, and the drive mode signals DM1 and DM2 are shown, and the others are omitted. Of the commands from the system controller 112 to the preprocess circuit 107, only the clamp operation ON signal CLON of the clamp circuit 202 is shown, and the others are omitted.
[0017]
  3A and 3B are timing charts showing CCD drive modes corresponding to the drive mode signals DM1 and DM2, respectively. φV and φH are a VCCD driving pulse and an HCCD driving pulse, respectively, and HBLK is a horizontal blanking signal. The VCCD and HCCD are driven by known 4-phase / 2-phase pulses and sequentially transfer charges. However, for convenience of explanation, one pulse is output for each phase by driving once (ie, the minimum unit of charge transfer). (A) and (B) in FIG. 3 assume that only one phase representing a 4-phase / 2-phase pulse is drawn. Note that (A) and (B) in FIG.
[0018]
  The drive corresponding to the drive mode signal DM1 shown in FIG. 3A is a drive that reads out all the pixel charges that are generally used conventionally, and the VCCD is driven within the horizontal blanking period. Only once. In the present specification, this drive is referred to as a 1 × speed drive in contrast to a drive corresponding to a drive mode signal DM2 described later.
[0019]
  FIG. 3B shows the driving corresponding to the driving mode signal DM2, and the driving of the VCCD is performed twice only within the horizontal blanking period. Accordingly, pixel charges are output by adding two vertical pixels at the time of transfer from the vertical transfer path (VCCD) to the horizontal transfer path (HCCD), and the readout time for one frame is (A in FIG. ) Is reduced to one half of the 1 × speed driving shown in FIG. This driving is referred to as double speed driving in this specification. In addition, in addition, in the known double speed drive in which the double speed drive or the VCCD which is an extension concept thereof is driven n times only within the horizontal blanking period, the single charge speed drive is performed by adding pixel charges. In contrast to the “high-speed sweep” drive in the above-described prior art, an output signal that can be used as a video signal can be obtained although there is information lost. Therefore, all of these drive modes are drive modes capable of outputting an image in the same manner as the 1 × speed drive.
[0020]
  FIG. 4 is an imaging timing chart for explaining the first imaging mode in the present embodiment. This imaging mode corresponds to the exposure operation in the “high quality mode” of the imaging apparatus according to the present embodiment, and signal readout for image recording is performed in the 1 × speed mode. Prior to the start of exposure, the CCD drive is repeated in the 1 × speed mode, and the sign of TA = T−F1 is determined to determine whether the exposure time T is longer than 1 frame period F1 in the 1 × speed mode. ing. FIG. 4 shows a case where TA> 0. (Note that when TA ≦ 0, conventionally well-known control is performed in which the CCD driving is always executed in the 1 × speed mode without interruption, and the description thereof is omitted.)
[0021]
  Upon receiving an exposure command (not shown), the system controller 112 has a predetermined timing (time t₁) To output the charge discharge pulse VSUB, and at the same time as the exposure is started, the CCD drive mode signal DM1 and the clamp operation on signal CLON are set to L. After that time TA has elapsed (time t₂), The CCD drive mode signal DM1 and the clamp operation on signal CLON are set to H. Furthermore, when time F1 has elapsed, that is, time t₁Time after the exposure time T has elapsed (time t_Three) To output a charge transfer pulse TG to transfer the signal charges in the pixel region to the vertical transfer path (VCCD). The transferred charge is then time t_FourAre sequentially read over one frame period of 1 × speed, and are appropriately processed and recorded in a subsequent circuit.
[0022]
  According to such imaging control, t₁~ T₂During this period, the CCD driving power is reduced to 0 to save power, and the clamp operation is stopped and the clamp switch remains open, so that the coupling capacitor is not charged / discharged by an abnormal video signal output. . And t₂~ T_ThreeIn this period, transfer for one frame is performed and unnecessary charges due to smear and dark charges in the transfer path are discharged, and OB clamping is performed during this period. Even when the ring capacitor is abnormally charged / discharged, the normal OB clamp can be recovered, so that the image quality of the recorded image is not deteriorated. Further, the peak power generated in this period is the same as that in the reading period for subsequent recording, and no new peak is generated.
[0023]
  FIG. 5 is an imaging timing chart for explaining the second imaging mode in the present embodiment. This imaging mode corresponds to the exposure operation in the “low image quality mode” of the imaging apparatus according to the present embodiment, and signal readout for image recording is performed in the double speed mode. Prior to the start of exposure, the CCD drive is repeated in the double speed mode, and the sign of TA = T−F2 is determined to determine whether the exposure time T is longer than one frame period F2 in the double speed mode. ing. FIG. 5 shows a case where TA> 0. (Note that, when TA ≦ 0, conventionally well-known control is performed in which the CCD driving is always executed in the double speed mode without interruption, and the description thereof is omitted.)
[0024]
  Upon receiving an exposure command (not shown), the system controller 112 has a predetermined timing (time t_Five) To output the charge discharge pulse VSUB, and at the same time as the exposure is started, the CCD drive mode signal DM2 and the clamp operation on signal CLON are set to L. After that time TA has elapsed (time t₆), The CCD drive mode signal DM2 and the clamp operation on signal CLON are set to H. Further, when time F2 has elapsed, that is, t_FiveTime after the exposure time T has elapsed (time t₇) To output a charge transfer pulse TG to transfer the signal charges in the pixel region to the vertical transfer path (VCCD). The transferred charge is then time t₈Are sequentially read over one frame period of double speed, and are appropriately processed and recorded in a subsequent circuit.
[0025]
  According to such imaging control, t_Five~ T₆During the period, the CCD driving power is reduced to 0 to save power, and the clamp operation is stopped and the clamp switch is kept open, so that the coupling capacitor is not charged / discharged by an abnormal video signal output. And t₆~ T₇During the period, transfer for one frame is performed and unnecessary charges due to smear and dark charges in the transfer path are discharged, and OB clamping is performed over this period. Even when the capacitor is abnormally charged / discharged, the normal OB clamp can be recovered, so that the image quality of the recorded image does not deteriorate. Further, the peak power generated in this period is the same as that in the reading period for subsequent recording, and no new peak is generated.
[0026]
  An imaging timing chart for explaining the third imaging mode in the present embodiment is shown by rewriting a part of FIG. 5 with a dotted line. This imaging mode corresponds to the exposure operation in the “high image quality mode” of the imaging apparatus according to the present embodiment, and signal readout for image recording is performed in the 1 × speed mode. However, prior to the start of exposure, the CCD drive is repeated in the double speed mode in order to cope with moving image output, and it is determined whether the exposure time T is longer than one frame period F2 in the double speed mode. Therefore, the sign of TA = T−F2 is determined. FIG. 5 shows a case where TA> 0. (It should be noted that when TA ≦ 0, the CCD driving is always performed without interruption in the double speed mode, and then the single speed driving is performed only at the time of reading for recording.)
[0027]
  Upon receiving an exposure command (not shown), the system controller 112 has a predetermined timing (time t_Five), The charge discharge pulse VSUB is output to start exposure, and at the same time, the CCD drive mode signal DM2 and the clamp operation on signal CLON are set to L. After that time TA has elapsed (time t₆), The CCD drive mode signal DM2 and the clamp operation on signal CLON are set to H. Further, when time F2 has elapsed, that is, t_FiveTime after the exposure time T has elapsed (time t₇) To output a charge transfer pulse TG to transfer the signal charges in the pixel region to the vertical transfer path (VCCD). At the same time, the CCD drive mode signal DM2 is set to L and the CCD drive mode signal DM1 is set to H. The transferred charge is then time t₉Are sequentially read over one frame period at 1 × speed, and are appropriately processed and recorded in a subsequent circuit.
[0028]
  According to such imaging control, t_Five~ T₆During the period, the CCD driving power is reduced to 0 to save power, and the clamp operation is stopped and the clamp switch is kept open, so that the coupling capacitor is not charged / discharged by an abnormal video signal output. And t₆~ T₇During this period, transfer for one frame is performed at double speed, unnecessary charges due to smear and dark charges in the transfer path are discharged, and OB clamping is performed over this period. Even when abnormal charging / discharging of the coupling capacitor has been performed before, normal OB clamping can be recovered, so that the image quality of the recorded image does not deteriorate. The peak power generated during this period is approximately twice that of the readout period for subsequent recording. However, if it is considered including the video image output period after that, a new peak is generated. No. Since the unnecessary charge discharging period can be shortened to ½ compared to the case of the first imaging mode, it can be applied to a smaller exposure time T.
[0029]
  In addition to the above, various modifications are possible for the imaging mode. First, the time t until the exposure period is reached₁Before or after reading for recording (time t_FourOr t₈, Or t in the third imaging mode₉Thereafter, if not necessary, the CCD drive is not performed. In this case, it goes without saying that the power saving effect as a whole is further increased. Alternatively, the CCD driving period for discharging unnecessary charges may be set longer than the above imaging modes (when the exposure time T permits), if necessary, at the expense of a little power saving effect.
[0030]
  In addition to this aspect, it is of course possible to control on / off of the OB clamp at a timing different from the CCD driving period for discharging unnecessary charges. Actually, if the OB clamp is turned off during the abnormal output period of the video signal (this is a configuration corresponding to claim 4 alone), the coupling capacitor is not charged or discharged abnormally, so that reading for recording is performed. Even if the pre-clamping prior to is omitted, there may be no significant problem.
[0031]
  On the other hand, if pre-clamping is performed for a sufficient time with respect to the time constant of the OB clamp (this is a configuration corresponding to claim 3 alone), the clamp is turned on during the abnormal output period and the coupling capacitor is abnormally charged. Even if a discharge occurs, it usually does not matter.
[0032]
  In the above imaging mode, only the double speed drive is used, but it goes without saying that the present invention can be applied to any n-times speed drive in the same manner. Furthermore, although the CCD scanning method is not referred to in the above description, any type of progressive (sequential) scanning and interlaced scanning may be used. In this case, one frame period of the present embodiment is also in the claims. As defined, it is “a period required to transfer all the signal charges transferred by one charge transfer command in the image output enabling drive mode from the vertical charge transfer means to the horizontal charge transfer means”. Needless to say, in an interlaced scanning type image sensor, a so-called one field period corresponds to this. Of course, the imaging element may be either monochrome or color.
[0033]
【The invention's effect】
  As described above based on the embodiments, the claims1According to the invention, during long exposure, driving for discharging unnecessary charges in the transfer means is performed for a predetermined period such as one frame period prior to reading of the image signal after the end of the exposure period. Since it is performed in the image output enable mode, it suppresses the occurrence of image deterioration due to the mixing of unnecessary charges such as smear due to special drive under conditions that do not cause useless power peaks such as high-speed sweeping drive. AndAlso drivingEven if an OB clamp abnormality occurs during an abnormal output period in which a normal output cannot be obtained, such as a motion stop period, it can be restored to a normal OB clamp, and image quality deterioration due to an OB clamp abnormality can be prevented. Claims2According to the invention, since the OB clamp is stopped in the abnormal output period preceding the drive period in the image output enabling mode for discharging electric charges, no abnormality of the OB clamp occurs and the image quality deteriorates. Can be prevented.
[Brief description of the drawings]
FIG. 1 is a block configuration diagram showing an embodiment of an imaging apparatus according to the present invention.
FIG. 2 is a block configuration diagram showing a detailed configuration of a CCD driver and a preprocess circuit portion in the embodiment shown in FIG. 1;
FIG. 3 is a timing chart for explaining CCD driving modes of 1 × speed driving and 2 × speed driving in the embodiment;
FIG. 4 is an imaging timing chart for explaining a first imaging mode in the embodiment.
FIG. 5 is an imaging timing chart for explaining second and third imaging modes in the embodiment.
[Explanation of symbols]
  101 Lens system
  102 Lens drive system
  103 Exposure control mechanism
  104 Filter system
  105 CCD image sensor
  106 CCD driver
  107 Pre-processing circuit
  108 Digital process circuit
  109 card interface
  110 Memory card
  111 LCD image display system
  112 System controller
  113 Operation switch system
  114 Operation display system
  115 Strobe
  116 Lens driver
  117 Exposure control driver

Claims (2)

The charge accumulated in the photoelectric conversion unit over the exposure period is transferred to the vertical charge transfer means according to a predetermined charge transfer command, transferred to the horizontal charge transfer means by the vertical charge transfer means, and further the horizontal charge transfer A charge transfer type imaging device configured to read out as an image signal by transferring by means;
In driving the vertical and horizontal charge transfer means of the image pickup device, driving of the vertical charge transfer means in a period other than a horizontal blanking period provided as a non-video signal output period for at least horizontal charge transfer is prohibited. A drive means having an image output enabling drive mode for driving the vertical charge transfer means only in a horizontal blanking period and a drive stop mode for substantially stopping the drive of the vertical and horizontal charge transfer means;
An optical black clamp circuit for performing optical black clamp, and at least the output of the charge transfer command and the optical black clamp circuit are controlled, and the driving means is driven to read the image signal of the image sensor in a predetermined readout mode. An image pickup apparatus comprising a read control means for reading out at
It said read control means, the drive stops at least a predetermined period the drive means also in the previous period than the first time point preceding a predetermined time relative to the second time point to output an end serving as the charge transfer command exposure period and mode, the above over the first time after the period leading up to the second time point, the drive means as the image output can drive mode to drive the said vertical and horizontal charge transfer means, further said first imaging apparatus characterized by being configured to so that allowed perform at least a predetermined period optical black clamp operation at the time after the period leading to the second time point. 2. The imaging according to claim 1, wherein the readout control unit further stops the optical black clamp operation for a period before the first time point and a period other than the period in which the driving is performed in the image output enabling drive mode. apparatus.

Images