JP4719566B2 - Imaging apparatus and processing method thereof - Google Patents

Description

  The present invention relates to an imaging apparatus and a processing method thereof.

  In an image pickup apparatus using a solid-state image pickup device, the dark level changes according to shooting conditions such as storage time, ISO sensitivity setting, and temperature. Particularly noticeable is high-temperature and long-time exposure, which generates a lot of dark current, and dark current greatly changes the dark level from normal shooting (room temperature and short-time exposure), and it compresses the dynamic range. . For this reason, in the imaging apparatus as described above, it is necessary to always make the dark level constant by correcting the dark level and to secure a dynamic range.

  As a method for correcting the dark level, a clamp circuit using an integrating circuit using a capacitor as described in Patent Document 1 below, and an output of a D / A converter as described in Patent Document 2 below. A clamp circuit that provides direct feedback is proposed. In recent years, a configuration having an analog front end (AFE) having both an OB clamp function and an A / D conversion function for clamping an OB (optical black) level by such a clamp circuit is generally used. It has become.

  An image pickup apparatus having such a clamp circuit increases the time constant of the OB clamp and avoids darkness before and after one OB clamp operation in order to avoid degradation of image quality due to the clamp noise due to the noise of the pixel signal. The change of the level is made not to become large.

  However, when the image sensor is intermittently driven as in still image shooting, there is a large difference between the dark level immediately after the start of signal readout of the image sensor and the level at which the dark signal should be clamped (clamp target value). Therefore, it is necessary to perform a rapid clamping process. FIG. 7 shows the pixel region of the image sensor. The clamp processing must be converged to the approximate clamp target value within the period of VOB (light shielding region above the opening) where the signal is first read out.

  Patent Document 3 below proposes a method of performing a rapid OB clamp operation by continuously generating a clamp pulse (a signal for instructing clamp processing) within this VOB period.

JP-A-5-153428 JP 2000-224440 A JP 2001-145029 A

  In the OB clamping process as described above, after the OB pixel signal is sampled and before the calculated correction value is actually reflected in the input signal, the calculation time, the output change time of the D / A converter, the capacitor Time such as a charge / discharge period is required. Hereinafter, this period is referred to as a feedback period. Until the feedback period ends, the dark level of the input signal fluctuates due to the clamping process, and signal sampling for the next clamping process cannot be performed. Therefore, when the clamp pulse is continuously generated as described in Patent Document 3, the shortest cycle is determined by the sum of the OB sampling period and the feedback period.

  However, on the other hand, the drive pulse of the image sensor is output even during the feedback period, and the reading of the pixel signal has advanced. For this reason, pixel signals that can be sampled for OB clamping become a part of all OB pixels, and the pixel signals read from the image sensor during the feedback period are not used for the clamping process and are sent to the processing circuit in the subsequent stage. It flows as it is. For this reason, the pixels are completely useless for the OB clamping process.

  When the OB clamping process is continuously repeated, the image sensor signal is read out even during the feedback period after pixel signal sampling, so an image sensor with a large number of OB pixels is required to perform the desired number of times. Met.

  An object of the present invention is to effectively use OB pixels in the OB clamping process and obtain a sufficient OB clamping effect with an image sensor having a small number of OB pixels.

An imaging device of the present invention has an aperture pixel in an aperture area and an optical black pixel in a light-shielding area, converts an optical signal into an electrical signal, a pixel signal output from the optical black pixel, and a predetermined dark level. Clamping means for adjusting the offset of the pixel signal by feeding back the difference between the two, and drive signal generating means for supplying a horizontal shift register drive signal for driving a horizontal shift register of the image sensor to the horizontal shift register The drive signal generating means supplies the horizontal shift register drive signal during a period in which the pixel signal output from the optical black pixel is sampled for the offset adjustment, and feeds back a difference from the predetermined dark level. stopping the supply of the horizontal shift register drive signal to the period And wherein the door.
Further, the processing method of the imaging apparatus of the present invention includes an imaging element that has an aperture pixel in an aperture area and an optical black pixel in a light shielding area, converts an optical signal into an electrical signal, and a pixel signal output from the optical black pixel And a clamp means for adjusting the offset of the pixel signal by feeding back a difference between a predetermined dark level and a drive signal generation for supplying a horizontal shift register drive signal for driving a horizontal shift register of the image sensor to the horizontal shift register And a horizontal shift register drive signal is supplied during a period in which the pixel signal output from the optical black pixel is sampled for the offset adjustment, and the predetermined dark level is obtained. The horizontal shift register drive during the period of feedback of the difference Characterized by stopping the supply of the items.

  The pixel signal of the optical black pixel can be effectively used for the optical black clamp process, and the optical black clamp process is performed a sufficient number of times for convergence from the start of reading of the image sensor to the reading of the signal of the aperture pixel. be able to. Conversely, the area corresponding to the pixels that have been wasted with respect to the optical black clamp process is not necessary, and the chip area of the image sensor can be reduced.

  In an imaging apparatus such as a digital camera or a video camera, a CCD or a CMOS image sensor is generally used as an imaging element. Here, the camera operation of the image pickup apparatus having the above configuration will be described with reference to FIGS.

  FIG. 1 is a diagram illustrating a configuration example of a digital camera or a digital video camera according to an embodiment of the present invention. Reference numeral 101 denotes an image sensor, which uses a CCD or CMOS sensor and converts an optical signal into an electrical signal. Reference numeral 102 denotes an analog front end (AFE), which has a function of performing processing such as analog-digital conversion and OB clamping on a signal from the image sensor. The OB clamping process is a process of performing offset adjustment on the signal so that the output of the light shielding portion (optical black: OB) of the image sensor becomes a predetermined value. Reference numeral 103 denotes a DSP (Digital Signal Processor) which performs various correction processes and development processes on the data from the AFE 102. The DSP 103 controls various memories such as the ROM 106 and the RAM 107 and writes image data to the recording medium 108.

  A timing generation circuit 104 supplies a clock signal and a control signal to the image sensor 101, AFE 102, and DSP 103, and is controlled by the CPU 105.

  Reference numeral 105 denotes a CPU that controls the DSP 103, timing generation circuit 104, and camera functions using various parts (not shown) such as photometry and distance measurement. Each switch 109 to 111 and a mode dial 112 are connected to each other. Execute processing according to the state.

  A ROM 106 stores a camera control program and various correction data, and a RAM 107 temporarily stores image data and correction data processed by the DSP 103. The RAM 107 can be accessed at a higher speed than the ROM 106.

  Reference numeral 108 denotes a recording medium such as a compact flash (registered trademark) card for storing a photographed image, which is connected to a camera via a connector (not shown).

  Reference numeral 109 denotes a power switch for starting the camera, and reference numeral 110 denotes a shutter switch SW1 for instructing start of operations such as photometry processing and distance measurement processing. Reference numeral 111 denotes a shutter switch SW2 that drives a mirror and a shutter (not shown) and instructs the start of a series of imaging operations to write a signal read from the imaging element 101 to the recording medium 108 via the AFE 102 and the DSP 103.

  FIG. 2 is a flowchart showing control of the camera shown in FIG. First, in step S201, it is determined whether or not the power SW (switch) 109 for starting the camera is turned on. If it is off, step S201 is repeated. If the power SW 109 is on, it is determined in step S202 whether or not the mode dial 112 is set to the shooting mode. If the shooting mode is set, the process proceeds to step S204. If the other mode is set, processing corresponding to the mode selected in step S203 is performed, and the process returns to S201.

  In step S204, it is determined whether the shutter switch (SW1) 110 is on. If SW1 is off, the process of S204 is repeated. If SW1 is on, the process proceeds to step S205.

  In step S205, photometry processing for determining the aperture value and shutter speed and distance measurement processing for adjusting the focus of the photographing lens to the subject are performed using a photometry control unit and a distance measurement control unit (not shown).

  When the photometry / ranging process is completed, the state of the shutter switch (SW2) 111 is determined in a subsequent step S206. If SW2 is off, step S206 is repeated, and if it is on, the shooting process of step S207 is executed. Details of this photographing process will be described later.

  When the photographing process ends, the process proceeds to step S208, and the DSP 103 performs a developing process on the photographed image data. In step S209, the image data that has undergone the development process is compressed and stored in an empty area of the RAM 107.

  In step S210, the image data stored in the RAM 107 is read and recording processing on the recording medium 108 is executed. After the recording process is completed, the process returns to step S201 to prepare for the next shooting.

Next, details of the photographing process in step S207 will be described with reference to FIG.
First, in step S301, the mirror is moved to the mirror up position. In step S302, the aperture is driven to a predetermined aperture value based on the photometric data obtained in the above-described photometric processing (step S205).

  In step S303, the charge clear operation of the image sensor 101 is performed, and charge accumulation in step S304 is started. After the start of charge accumulation, the shutter is opened in step S305, and exposure of the image sensor 101 is started (step S306).

  Thereafter, in step S307, the process waits for the end of exposure according to the photometric data, and the shutter is closed in step S308.

  In step S309, the aperture is driven to the open aperture value, and in step S310, the mirror is driven to the mirror down position.

  In step S311, it waits until the set charge accumulation time elapses, and the charge accumulation of the image sensor 101 is terminated (step 312).

  Finally, in step S313, the signal from the image sensor 101 is read out, and a series of processing is terminated and the processing returns to the main processing.

  In reading the image signal in step S313, the image signal is read from the image sensor 101, and at the same time, the AFE 102 performs the above-described OB clamping process. Since the dark output level is adjusted to a desired value by this processing, the dynamic range of the image signal can be ensured even in shooting such as long-time exposure at a high temperature where a large amount of dark current is generated.

  Here, FIG. 4A shows drive signals for the image sensor 101 and the AFE 102 when the image signal is read in step S313. The start of readout of one screen is instructed by the vertical synchronization signal, and the imaging signal readout operation for each horizontal line is performed by the horizontal synchronization signal. FIG. 7 is a diagram illustrating a pixel region of the image sensor 101. The pixel area has an opening pixel in the opening and OB pixels in the light shielding portions VOB and HOB. The regions VOB and HOB are light shielding regions (OB). Here, the light shielding area on the upper side of the opening is referred to as VOB, and the light shielding area on the left side is referred to as HOB. In FIG. 4A, when a reading operation is started by a vertical synchronization signal, an imaging signal is read for each horizontal line from the VOB side of the pixel area. In the figure, a horizontal shift register drive pulse is a signal for driving the horizontal shift register of the image sensor 101, an OB clamp pulse is a signal for instructing the AFE 102 to perform OB clamp processing, and an AFE drive clock is an operation clock of the AFE 102. These are all supplied from the timing generation circuit 104. Lines 1 to 8 are regions where OB clamping processing is performed in the VOB, and Lines 9 and after are open regions (or regions where OB clamping processing is not performed even with VOB).

  FIG. 5 shows a signal for each horizontal line in FIG. FIG. 5A corresponds to each line of Lines 1 to 8 in FIG. 4A, that is, one line drive signal in the portion A in FIG. FIG. 5B corresponds to a one-line drive signal after Line 9, that is, a portion B in FIG. 7. In reading each line, first, an image pickup signal is transferred to a horizontal read line in the image pickup device 101 by various drive pulses (not shown) supplied to the image pickup device 101 by the timing generation circuit 104 in the horizontal transfer period. Next, pixel signals for one line are sequentially read by a horizontal shift register drive pulse in the horizontal scanning period. In this horizontal scanning period, the timing generation circuit 104 generates an OB clamp pulse for the AFE 102 to prompt an OB clamp process. When the OB clamp pulse is input, the AFE 102 samples pixel signals for a predetermined number of pixels from that time, and feeds back the difference between the average value and a desired dark output value to the offset adjustment circuit. As an example of the offset adjustment circuit, a type using an integrating circuit using a capacitor, a type feeding back the output of the D / A converter as it is, and the like are conceivable.

  A configuration example of the AFE 102 using the latter type is shown in FIG. Reference numeral 801 denotes an image signal input terminal to which an analog signal from the image sensor 101 is input. This analog signal is supplied to the non-inverting input terminal (+) of the amplifier circuit 802, multiplied by a predetermined gain, converted to a digital signal by the A / D conversion circuit 803, and then subjected to subsequent signal processing from the output terminal 804. Output to the system. Here, when an OB clamp pulse is input to the AFE 102 in accordance with the input of the OB pixel signal, an average value of pixel signals for a predetermined number of pixels is calculated by the averaging circuit 805 from that point. Next, the subtracter 806 subtracts the clamp target value from the average value, and the calculation unit 807 calculates the correction value from the result and sets it in the D / A converter 808. Offset adjustment is performed by feeding back the output of the D / A converter 808 to the inverting input terminal (−) of the amplifier circuit 802, and the dark level is adjusted to the clamp target value.

  The AFE 102 performs an optical black clamp process for adjusting the offset of the pixel signal of the image sensor 101 based on the signal of the OB pixel and a process for converting the pixel signal of the image sensor 101 from analog to digital.

  In such an OB clamp circuit, in order to avoid the influence of defective pixels and random noise, the feedback coefficient is generally reduced. For this reason, in order to converge the dark level to the clamp target value, it is necessary to repeatedly perform the OB clamp process. On the other hand, the dark level must be converged during the VOB period before the opening is read. Therefore, in the VOB period, the OB clamping process is performed a plurality of times in one line as shown in FIG. However, as shown in the figure, a single process includes a period required for sampling a predetermined number of OB pixels and a feedback period such as averaging of the sampled pixels, correction value calculation, and output change of the D / A converter. is necessary. Accordingly, the number of OB clamping processes that can be performed per line is limited. For this reason, it is necessary to increase the number of VOB lines in order to increase the number of repetitions. Increasing the number of VOB lines leads to an increase in the chip area of the image sensor, which directly increases the cost.

  Therefore, in the present embodiment, as shown in FIG. 6A, the supply of the horizontal shift register drive pulse to the image sensor 101 is stopped during the feedback period described above. On the other hand, the AFE drive clock continues to be supplied without stopping during this period. Conventionally, a signal read from the image sensor 101 during the feedback period is transmitted to the DSP 103 without being used for the OB clamp processing, but all pixel signals are OB clamped by stopping the horizontal shift register drive pulse. It can be used effectively for processing. At the time of reading the opening area, it is not necessary to stop the horizontal shift register drive pulse, and it is exactly the same as the conventional one (FIG. 5B) as shown in FIG. 6B. At this time, the read time per VOB line is naturally longer than that of the open area, but if compared with the same number of repetitions, the number of horizontal transfers can be reduced by the smaller number of VOB lines. As shown in (2), the overall readout time is rather short.

  FIG. 4B is a diagram showing driving signals for a plurality of lines according to this embodiment. Compared with the conventional example of FIG. 4A, the number of lines necessary to perform the same number of OB clamping processes is reduced to ¼ (2 lines compared to 8 lines in the conventional example). In addition, it can be seen that the line 3 and subsequent regions are open regions (or regions where OB clamping processing is not performed even with VOB), and the overall readout time is also shortened.

  As described above, according to the present embodiment, a sufficient OB clamp processing effect can be obtained with a smaller number of pixels than the conventional one, and at the same time, an effect of shortening the readout time can be obtained.

  In this embodiment, the OB clamping method has been described by taking as an example a type that directly feeds back the output of the D / A converter. However, the present embodiment is applicable to any type that requires time for feedback after sampling of the OB pixel signal, such as a type in which a clamp voltage is held by a charged / discharged capacitor and offset adjustment is performed. It is valid.

  As described above, in this embodiment, the horizontal shift register drive pulse of the image sensor is stopped during the feedback period of the OB clamp process, and the pixel signal is not read from the image sensor during the feedback operation period.

  The timing generation circuit 104 generates a horizontal shift register drive signal that reads the signal by driving the horizontal shift register of the image sensor 101, and generates a signal that instructs the AFE 102 to start the OB clamping process. At that time, the timing generation circuit 104 has a first time for generating the horizontal shift register drive signal and a second time for stopping the horizontal shift register drive signal in one line. The first time is a time after an OB clamping process start instruction, and the second time is a time after the first time has elapsed.

  The first time is a time required for pixel signal sampling for the OB clamping process. The second time is a time required to feed back the difference between the pixel signal sampled in the OB clamping process and the dark level to the offset adjustment circuit.

  As shown in FIGS. 4B and 6A, the timing generation circuit 104 has a plurality of times for generating the horizontal shift register drive signal and for stopping the horizontal shift register drive signal in one line. Repeat once.

  All the VOB pixel signals are effectively used for the OB clamping process, and the OB clamping process is performed a sufficient number of times for dark level convergence between the start of reading of the image sensor and the reading of the opening signal. be able to. Conversely, since the area corresponding to the number of pixels read out during the conventional feedback period and wasted for the clamping process is not necessary, the chip area of the image sensor can be reduced.

  The above-described embodiments are merely examples of implementation in carrying out the present invention, and the technical scope of the present invention should not be construed in a limited manner. That is, the present invention can be implemented in various forms without departing from the technical idea or the main features thereof.

It is a figure which shows the example of whole structure of a digital camera. It is a flowchart which shows the camera operation | movement of a digital camera. It is a flowchart which shows the imaging process of a digital camera. 3 is a timing chart showing drive signals for an image sensor and AFE. It is the timing chart which showed the drive signal in each conventional horizontal line. It is the timing chart which showed the drive signal in each horizontal line of the execution form of this invention. It is the figure which represented typically the image sensor pixel area in the embodiment of the present invention. It is the figure which showed the structural example of the clamp circuit.

Explanation of symbols

101 Image sensor 102 AFE
103 DSP
104 Timing generation circuit 105 CPU
106 ROM
107 RAM
108 Recording medium 109, 110, 111 Switch 112 Mode dial

Claims (4)

An imaging device having an aperture pixel in the aperture area and an optical black pixel in the light-shielding area, and converting an optical signal into an electrical signal;
Clamping means for performing offset adjustment of the pixel signal by feeding back a difference between a pixel signal output from the optical black pixel and a predetermined dark level;
Drive signal generating means for supplying a horizontal shift register drive signal for driving a horizontal shift register of the image sensor to the horizontal shift register;
The drive signal generation means supplies the horizontal shift register drive signal during a period in which the pixel signal output from the optical black pixel is sampled for the offset adjustment, and feeds back a difference from the predetermined dark level. In addition, the supply of the horizontal shift register drive signal is stopped . The imaging apparatus according to claim 1 , wherein the drive signal generation unit repeats the offset adjustment a plurality of times . An imaging device having an aperture pixel in the aperture area and an optical black pixel in the light-shielding area, and converting an optical signal into an electrical signal;
Clamping means for performing offset adjustment of the pixel signal by feeding back a difference between a pixel signal output from the optical black pixel and a predetermined dark level;
A processing method of an imaging apparatus, comprising: a drive signal generation unit that supplies a horizontal shift register drive signal for driving a horizontal shift register of the image sensor to the horizontal shift register,
The horizontal shift register drive signal is supplied during a period in which the pixel signal output from the optical black pixel is sampled for the offset adjustment, and the horizontal shift register drive signal is fed back during a period in which a difference from the predetermined dark level is fed back. The processing method of the imaging device, characterized in that the supply of is stopped . The processing method of the imaging apparatus according to claim 3, wherein the offset adjustment is repeated a plurality of times.

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