Patent Publication Number: US-10791251-B2

Title: Imaging device and imaging system

Description:
This application is a continuation of U.S. application Ser. No. 15/709,816, filed Sep. 20, 2017, which claims priority to Japan 2016-198662, filed on Oct. 7, 2016, the contents of each of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to an imaging device and an imaging system. 
     Description of the Related Art 
     In the field of electronic cameras, there is a technology of adding capturing information such as the date of capturing, the location of capturing, or the like to image data (Japanese Patent Application Laid-Open No. 2009-225229). In the art of Japanese Patent Application Laid-Open No. 2009-225229, capturing information is added to image data based on the Exchangeable Image File (Exif) format in the Design rule of Camera File System (DCF) specification. 
     The prior art merely adds one piece of time information to one frame of a motion image. Therefore, in the prior art, time information corresponding to a processing timing of a pixel signal on one row of a plurality of rows and time information corresponding to a processing timing of a pixel signal on another row of the plurality of rows cannot be obtained. 
     SUMMARY OF THE INVENTION 
     An imaging device according to one aspect of the present invention includes: a plurality of pixels arranged in a plurality of rows, in which each of the plurality of pixels outputs a pixel signal; a row scanning unit that scans the plurality of pixels on a row basis; and an output unit that outputs first time information corresponding to a processing timing of the pixel signal on one of the plurality of rows and second time information corresponding to the pixel signal on another of the plurality of rows and having a different value from the first time information. 
     An imaging system according to another aspect of the present invention includes an imaging device and an information generation unit that generates time information. The imaging device includes a plurality of pixels arranged in a plurality of rows, a row scanning unit that scans the plurality of pixels on a row basis, a readout unit that reads out pixel signals respectively from the plurality of pixels on a selected row, and an output unit that outputs a plurality of the pixel signals from the readout unit as image data on a frame basis. The information generation unit generates, as the time information, first time information corresponding to a processing timing of the pixel signal on one of the plurality of rows and second time information corresponding to the pixel signal on another of the plurality of rows and having a different value from the first time information. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an imaging system of a first embodiment. 
         FIG. 2  is a block diagram of an imaging device of the first embodiment. 
         FIG. 3  is an equivalent circuit diagram of a pixel of the first embodiment. 
         FIG. 4  is a timing chart of control signals of the first embodiment. 
         FIG. 5  is a timing chart of the imaging system of the first embodiment. 
         FIG. 6A  and  FIG. 6B  are diagrams illustrating an adding method of time information to image data in the first embodiment. 
         FIG. 7A  and  FIG. 7B  are diagrams illustrating a capturing object and time information in the first embodiment. 
         FIG. 8  is a timing chart of an imaging system of a second embodiment. 
         FIG. 9  is a timing chart of an imaging system of a third embodiment. 
         FIG. 10  is a timing chart of an imaging system of a fourth embodiment. 
         FIG. 11  is an equivalent circuit diagram of a pixel of a fifth embodiment. 
         FIG. 12  is a timing chart of an imaging system of the fifth embodiment. 
         FIG. 13A  and  FIG. 13B  are diagrams illustrating a capturing object and time information of the fifth embodiment. 
         FIG. 14  is a block diagram of an imaging system of a sixth embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will now be described in detail in accordance with the accompanying drawings. 
     First Embodiment 
       FIG. 1  is a block diagram of an imaging system of the first embodiment of the present invention. As illustrated in  FIG. 1 , the imaging system includes an imaging device  10 , an optical system  11 , an Analog Front End (AFE)  12 , a Digital Signal Processor (DSP)  13 , an image memory  14 , a pulse generation unit  15 , a time acquisition unit  16 , a time information generation unit  17 , and an information addition unit  18 . 
     The imaging device  10  is a CMOS area sensor in the present embodiment, which has a plurality of pixels two-dimensionally aligned and converts an optical signal that has entered via the optical system  11  such as a lens into a pixel signal that is an electrical signal. The AFE  12  includes a differential amplifier, a clamping circuit, and an analog-to-digital conversion circuit to output digital pixel data. The DSP  13  is a circuit that processes pixel data and performs gradation correction such as a gamma process, white balance correction, digital signal processing such as de-pixelization, noise reduction, or the like. The image memory  14  is a frame memory that temporarily holds image data resulted from digital signal processing. The pulse generation unit  15  generates a vertical synchronization signal VD and a horizontal synchronization signal HD of the imaging device  10  and a control signal of the imaging device  10 . 
     The time acquisition unit  16  has an antenna that receives radio waves from a plurality of Global Positioning System (GPS) satellites, a receiving circuit that processes a baseband signal, and a demodulation circuit that demodulates information contained in the received signal. Accurate information of the longitude, latitude, and altitude of the imaging system can be acquired based on time differences among received radio waves from the plurality of GPS satellites. Further, a received radio wave from the GPS satellite includes GPS time information (GPS week information, GPS second information) as the reference time and leap-second information. The GPS week information is information circulating from 0 to 1023 started from Jan. 6, 1980. The GPS second information is elapsed second information within the GPS week and can take a value within 604800 seconds=7 days×24 hours×60 minutes×60 seconds. The leap-second information represents a difference between the GPS time and the Universal Time Coordinated (UTC) and is used in converting the GPS time into the UTC. Further, in demodulating the received radio wave, the time acquisition unit  16  outputs an accurate signal of one pulse per second called a Pulse Per Second (PPS) signal. 
     The time information generation unit  17  includes a clock circuit and uses the PPS signal and the horizontal synchronization signal HD to generate time information having a higher time-resolution than the GPS time information. The time information generation unit  17  counts a counter that is counted up at 10 MHz, that is, by 10 −7  seconds, for example, and resets the counter at a rising edge of the PPS signal. The use of the PPS signal for resetting a counter allows for generating accurate time information in which errors are not accumulated. 
     The information addition unit  18  adds, to image data, time information generated by the time information generation unit  17 . Furthermore, the information addition unit  18  can add, to image data, location information or capturing information in addition to the time information. An output device (an output unit)  19  can output image data on a frame basis and may include a digital interface, an interface for a storage medium such as a memory card, a display circuit for a display device such as a display, or the like. 
       FIG. 2  is a block diagram of the imaging device  10 . The imaging device  10  includes a pixel unit  100 , a row scanning circuit (a row scanning unit)  120 , column readout circuits (readout units)  130 A and  130 B, column scanning circuits  150 A and  150 B, a timing generator  160 , and output circuits  171 A,  172 A,  171 B, and  172 B. In the pixel unit  100 , a plurality of pixels  110  are arranged in a two-dimensional matrix in the row direction and the column direction. While the pixel unit  100  of  FIG. 2  includes the pixels  110  of n rows by m columns, the limited number of pixels  110  are depicted for simplified illustration. Note that, in the present specification, the row direction corresponds to the horizontal direction in the drawings, and the column direction corresponds to the vertical direction in the drawings. In the pixel unit  100 , ranging rows in which focus detection pixels that output signals used for focus detection are arranged and a plurality of capturing rows in which capturing pixels that output signals used for generating an image are arranged are provided. Further, some of the pixels  110  of the pixel unit  100  are light-shielded as an optical black (OB) pixels. 
     Each of the pixels  110  includes a color filter, a micro lens, a photoelectric conversion unit, a floating diffusion part, a reset transistor, a transfer transistor, an amplification transistor, a selection transistor, and the like. The color filter is primary colors of red, blue, and green, for example, and is provided in each pixel  110  according to the Bayer arrangement. 
     In response to receiving control signals from the timing generator  160 , the row scanning circuit  120  performs readout scan on the pixel unit  100  on a row basis. That is, control signals V 1  to Vn are provided from the row scanning circuit  120  to pixel rows on which a plurality of pixels  110  are arranged, and photoelectrically converted signals are read out from the pixels  110 . Each of the control signals V 1  to Vn includes control signals that turns on or off the reset transistor, the transfer transistor, and the selection transistor included in the pixel  110 . The plurality of pixels  110  in the column direction are connected to 4×m column signal lines L( 1 - 1 ) to L( 1 - 4 ), . . . , L(m- 1 ) to L(m- 4 ), respectively. For example, a plurality of pixels  110  on the first column are connected to the column signal lines L( 1 - 1 ), L( 1 - 2 ), L( 1 - 3 ), and L( 1 - 4 ), and a plurality of pixels  110  on the second column are connected to the column signal lines L( 2 - 1 ), L( 2 - 2 ), L( 2 - 3 ), and L( 2 - 4 ). The column signal lines L( 1 - 1 ), L( 1 - 2 ), . . . , L(m- 1 ), and L(m- 2 ) are connected to the column circuits  131 A and  132 A of the column readout circuit  130 A, respectively. 
     The column circuits  131 A and the column circuits  132 A are scanned by the column scanning circuit  150 A to sequentially output pixel signals to the output circuits  171 A and  172 A. Similarly, the column signal lines L( 1 - 3 ), L( 1 - 4 ), . . . , L(m- 3 ), and L(m- 4 ) are connected to the column circuits  131 B and  132 B of the column readout circuit  130 B, respectively. The column circuits  131 B and the column circuits  132 B are scanned by the column scanning circuit  150 B to sequentially output pixel signals to the output circuits  171 B and  172 B. The pixel signals from the output circuits  171 A,  172 A,  171 B, and  172 B are output to the AFE  12  of  FIG. 1 . 
     In the present embodiment, since four column circuits  131 A,  132 A,  131 B, and  132 B are provided to the pixels  110  on each column, pixel signals from the pixels  110  on four rows at most can be simultaneously read out. Note that the number of pixel rows on which simultaneous readout is possible is not limited to four and may be one, two, or more than four. Furthermore, pixel signals on a plurality of rows may be added and read out. 
     Each of the column circuits  131 A,  132 A,  131 B, and  132 B includes a constant current source that is a load of an amplification transistor, a differential amplification circuit that amplifies a pixel signal, a holding circuit that temporarily holds a pixel signal, and the like. The timing generator  160  functions as a control unit and outputs the vertical synchronization signal VD, the horizontal synchronization signal HD, and a control signal based on a reference clock (not illustrated) to the row scanning circuit  120  and the column scanning circuits  150 A and  150 B. 
       FIG. 3  illustrates an equivalent circuit of the pixel  110  of the imaging device  10  according to the present embodiment. The pixel  110  includes a photoelectric conversion unit PD, a floating diffusion part FD, a transfer transistor M 1 , an amplification transistor M 3 , a selection transistor M 4 , and a reset transistor M 5 . 
     The photoelectric conversion unit PD photoelectrically converts an incident light that has passed through a color filter and a micro lens and accumulates charges generated by the photoelectric conversion. When turned on, the transfer transistor M 1  transfers charges of the photoelectric conversion unit PD to the floating diffusion part FD of the amplification transistor M 3 . The drain of the amplification transistor M 3  is connected to the power source voltage line, and the source thereof outputs a signal based on the voltage of the floating diffusion part FD to a column signal line L via the selection transistor M 4 . A constant current source  115  is connected to the column signal line L. When turned on, the reset transistor M 5  resets the voltage of the floating diffusion part FD. 
     A common control signal is supplied to the pixels  110  on the same row from the row scanning circuit  120 . That is, the control signals TX, SEL, and RES are supplied to the transfer transistor M 1 , the selection transistor M 4 , and the reset transistor M 5 , respectively, on the same row. Each of these transistors is turned on when the corresponding control signal is a high level, and turned off when the corresponding control signal is a low level. Note that a plurality of pixels  110  may share a single amplification transistor M 3 . 
       FIG. 4  is a timing chart of control signals according to the present embodiment and represents the operation of readout of a pixel signal.  FIG. 4  illustrates the control signal SEL supplied to the selection transistor M 4 , the control signal RES supplied to the reset transistor M 5 , and the control signal TX supplied to the transfer transistor M 1 . Each of the transistors M 1 , M 4 , and M 5  is turned on when the corresponding control signal is a high level, and turned off when the corresponding control signal is a low level. 
     The operation of readout of a pixel signal will be described below with reference to  FIG. 2  and  FIG. 3 . First, the row scanning circuit  120  sets the control signal SEL to a high level to turn on the selection transistor M 4  and selects the pixel  110  from which a signal is read out. Next, the row scanning circuit  120  sets the control signal RES to a high level to turn on the reset transistor M 5 . Once the reset transistor M 5  is turned on, the voltage of the floating diffusion part FD is reset to the power source voltage. After the reset transistor M 5  is turned off, the column readout circuits  130 A and  130 B perform readout of pixel signals at the time of reset from the column signal line L (N-read). The row scanning circuit  120  sets the control signal TX to a high level to turn on the transfer transistor M 1  and transfers charges of the photoelectric conversion unit PD to the floating diffusion part FD. The column readout circuits  130 A and  130 B perform readout of pixel signals at the time of photoelectric conversion from the column signal line L (S-read). The pixel signals read out in such a way are subjected to a correlated double sampling process in the column readout circuits  130 A and  130 B and are output from the output circuits  171 A,  172 A,  171 B, and  172 B. Note that the correlated double sampling process may be performed after analog-to-digital (AD) conversion of pixel signals. 
       FIG. 5  is a timing chart of an imaging system of the present embodiment and illustrates the vertical synchronization signal VD, the horizontal synchronization signal HD, a counter, and the PPS signal. The counter is a digital value that is counted up every 10 −7  seconds in the time information generation unit  17  and reset at a rising edge of the PPS signal of the GPS. 
     Once the imaging system is powered on, the pulse generation unit  15  outputs the vertical synchronization signal VD, the horizontal synchronization signal HD, and the control signal, and the imaging device  10  starts a capturing operation. The time acquisition unit  16  receives radio waves from the GPS satellites and starts acquiring GPS location information and GPS time information. 
     At the time t 1 , in response to acquiring the PPS signal from the received radio wave, the time acquisition unit  16  outputs the PPS signal to the time information generation unit  17 . In response to detecting a rising edge of the PPS signal, the time information generation unit  17  resets the counter. The use of the counter that is reset synchronized with the PPS signal allows for generating time information having a time resolution of a unit of μ second that is higher than the time resolution of one second, which is the update period of the GPS time information. At the same time as the time t 1 , the time acquisition unit  16  acquires GPS time information including GPS week information and GPS second information from the received radio wave and converts the GPS time information into time information in a form of year/month/date/hour/minute/second. The GPS time information is acquired every one second of the PPS signal asynchronously with a drive timing of the imaging device  10 . 
     When the vertical synchronization signal VD becomes a high level at the time t 2  and then becomes a low level, a vertical scan period of one frame starts in the imaging device  10 . The vertical synchronization signal VD is a signal that is asynchronous with the PPS signal. 
     When the horizontal synchronization signal HD becomes a high level at the time t 3  and then becomes a low level, a first horizontal scan period starts in the imaging device  10 . Note that, in the following description, the time of a rising edge and the time of a falling edge of the horizontal synchronization signal HD may be collectively referred to as the start time of the horizontal scan period. In the imaging device  10 , the row scanning circuit  120  selects the first to fourth pixel rows, and the column readout circuits  130 A and  130 B read out pixel signals from four rows. The AFE  12  converts a pixel signal into digital image data, and the DSP  13  outputs the image data resulted after signal processing to the image memory  14  and the information addition unit  18 . The time information generation unit  17  outputs the counter value at the time t 3  to the information addition unit  18  as time information. The information addition unit  18  adds time information, location information, and the like to the image data on the first to fourth pixel rows. Thereby, the start time t 3  of the first horizontal scan period can be recorded in a time resolution of 10 −7  seconds. 
     When the horizontal synchronization signal HD becomes a high level at the time t 3 - 2  and then becomes a low level, the second horizontal scan period starts. The information addition unit  18  adds the counter value at the time t 3 - 2  to the fifth to eighth pixel rows as time information. Similarly, at the time t 3 - 3 , . . . , t 3 - 300  when the horizontal synchronization signal HD becomes a high level, the time information at the start time t 3 - 3 , . . . , t 3 - 300  of each horizontal scan period is added to the image data. In the present embodiment, since four pixel rows are simultaneously read out within one horizontal scan period, accurate time information can be added on a four-pixel-row basis. In such a way, by adding the start time of the horizontal scan period to a part of the rows of image data, the row of image data and time information can be recorded in association with each other. 
     Thereby, time information with a high time-resolution can be known for all the rows of saved image data. In particular, when cut out of a Region of Interest (ROI) is performed from image data or when reduced drive of the imaging device  10  or the like is performed, the acquisition time of each row can be accurately known. In such a case, the advantage of the present embodiment becomes more significant. 
       FIG. 6A  and  FIG. 6B  are diagrams illustrating a method of adding time information to image data in the present embodiment and illustrate image data output from the information addition unit  18 .  FIG. 6A  illustrates image data of one frame. Image data of one frame includes a data section A 1  in which time information, location information, or the like is recorded, a data section A 2  for horizontal OB pixels, a data section A 3  for vertical OB pixels, and a data section A 4  for effective pixels.  FIG. 6B  illustrates a data area surrounded with a dashed line of  FIG. 6A , in which one-pixel data is illustrated by one block. When one-pixel data is illustrated by 16-bit data, that is, by two bytes, information of ASCII code, two-byte character, or the like can be written in one pixel. 
     The data section A 1  for the time information or the like may be added to the left side of the horizontal OB data (that is, the preceding time) or may be added to the right side of the effective pixel data (that is, the subsequent time), for example. Furthermore, the data section A 2  for the horizontal OB may be replaced with the data section A 1  for time information or the like. In the present embodiment, the timing of determining time information is the time of starting outputting the horizontal synchronization signal HD, that is, image data of a scan row. Thus, data of the leading part of the horizontal OB closest to the horizontal synchronization signal HD is replaced with the data part A 1  of time information. 
     The imaging device  10  of the present embodiment can simultaneously output pixel signals of 16 channels of four rows by four columns by using progressive scan. For example, when the number of scan rows (the number of times of scan) in one frame is 300, the total number of pixel rows will be 1200 pixel rows=4 pixel rows×300. When the number of times of column scan per horizontal scan is 540, the total number of pixel columns will be 2160 pixel columns=4 pixel columns×540. Further, the imaging device  10  can perform a rolling shutter operation, and image data converted into a digital form is output in 16 bits. Note that the number of pixels and the number of bits of image data are not limited to the above-described example. 
     Time information or the like is stored in the leading four pixel columns of the four pixel rows associated with one scan row of the horizontal OB data, that is, 16 pixels in a form of binary data. Here, the leading 16 pixels of the OB data are denoted as px 00 , px 01 , px 02 , px 03 , px 10 , px 11 , px 12 , px 13 , px 20 , px 21 , px 22 , px 23 , px 30 , px 31 , px 32 , and px 33 , respectively. In such a way, 16 pixels of four by four can be handled as a data unit A 11 . Each of the pixels px 00  to px 33  has 16 bits (b 0  to b 15 ), and time information or the like is allocated to each bit of the pixels px 00  to px 33  as indicated below. In the present embodiment, location information and capturing information in addition to time information are recorded as described below. 
     Location information includes longitude information and latitude information. Data indicating whether the location information is enabled or disabled is stored in the bit b 15  of the pixel px 11 . That is, the bit b 15  is “0” when acquisition of location information is enabled and “1” when acquisition of location information is disabled. The longitude information is denoted by coordinates of a direction (south (S) or north (N)), degrees (dd), and minutes (mm. mmmm). The information of direction is stored in the bit b 15  of the pixel px 13 , where the data bit is “0” when the direction is S, and the data bit is “1” when the direction is N. The degree (dd) ranges from 0 to 90 and is stored in the bits b 8  to b 14  of the pixel px 11  as 7-bit data. The minute (mm. mmmm) ranges from 0 to 59.9999, and the integer part thereof is stored in the bits b 0  to b 5  of the pixel px 11  as 6-bit data. The fraction part is stored in the bits b 0  to b 13  of the pixel px 13  as 14-bit data. The latitude information is denoted by coordinates of a direction (east (E) or west (W)), degrees (ddd), and minutes (mm. mmmm). The information of direction is stored in the bit b 15  of the pixel px 33 , where the data bit is “0” when the direction is E, and the data bit is “1” when the direction is W. The degree (ddd) ranges from 0 to 180 and is stored in the bits b 8  to b 15  of the pixel px 31  as 8-bit data. The minute (mm. mmmm) ranges from 0 to 59.9999, the two-digit integer part thereof is stored in the bits b 0  to b 5  of the pixel px 31  as 6-bit data, and the four-digit fraction part is stored in the bits b 0  to b 13  of the pixel px 33  as 14-bit data. 
     The GPS time information includes the leap-second information, the GPS week information, and the GPS second information as described above. The data indicating whether the GPS leap-second information is enabled or disabled is stored in the bit b 15  of the pixel px 01 . The data bit is when acquisition of the leap-second information is enabled, and the data bit is “1” when acquisition of the leap-second information is disabled. The leap-second information is denoted in a numerical value of −128 to 127 and stored in the bit b 0  to b 7  of the pixel px 01  as 8-bit data. 
     The information indicating whether the GPS week information is enabled or disabled is stored in the bit b 15  of the pixel px 10 . The data bit is “0” when acquisition of the GPS week information is enabled, and the data bit is “1” when acquisition of the GPS week information is disabled. As described above, the week information obtained from the GPS is a week number starting from Jan. 6, 1980 and circulating from 0 to 1023, and the current GPS week information is denoted by (1024+week number). When recorded from Mar. 27, 2016 to Nov. 11, 2035, the GPS week information is 12-bit data from 1890 to 2914. The GPS week information converted in such a way is stored in the bits b 0  to b 11  of the pixels px 10 . 
     The data indicating whether the GPS second information is enabled or disabled is stored in the bit b 15  of the pixel px 12 . The data bit is “0” when acquisition of the GPS second information is enabled, and the data bit is “1” when acquisition of the GPS second information is disabled. As described above, the GPS second information is progress information within the GPS week and may take a value within 604800 seconds=7 days×24 hours×60 minutes×60 seconds. The GPS second information is stored in the bits b 0  to b 15  of the pixels px 03  and bits b 0  to b 3  of the pixels px 12  as 20-bit data from 0 to 604799. 
     The data indicating whether or not the PPS signal is received is stored in the bit b 15  of the pixel px 30 . The data bit is “0” when the PPS signal is received, and the data bit is when no PPS signal is received. Further, the data indicating whether or not the counter is reset is stored in the bit b 15  of the pixel px 20 . After the imaging system is started up, the data bit is “0” when the counter has been reset by the PPS signal at least once, and the data bit is “1” when the counter has not been reset. The time information in a unit of 10 −7  seconds reset by the PPS signal is 20-bit data from 0 to 999999 and stored in the bits b 0  to b 15  of the pixels px 21  and bits b 0  to b 3  of the pixels px 30 . 
     Further, in the present embodiment, in order to supplement time information and location information, capturing information indicating capturing conditions such as exposure time, a frame number, a scan row number, or the like may be added to the image data. The exposure time is set in the imaging device  10  and represented by 30-bit data of 0 to 600000000μ seconds. The data of the exposure time is stored in the bits b 0  to b 15  of the pixel px 23  and the bits b 0  to b 13  of the pixel px 32 . The frame numbers are continuous values counted up on a frame basis and are stored in the bits b 0  to b 15  of the pixel px 00  as 16-bit data from 0 to 0xFFFF. By confirming the frame number after capturing, missing of the continuously captured frames can be easily known. The scan row number is represented by 9-bit data from 0 to 299 and stored in the bits b 0  to b 8  of the pixel px 20 . The scan row number is added for the purpose of easier confirmation of the association of the time information and the location information with the captured row. In such a way, together with accurate time information at the starting time of the horizontal scan period, the location information and the capturing information are added to the OB data section on each scan row. 
       FIG. 7A  and  FIG. 7B  are diagrams illustrating a capturing object and time information in the present embodiment.  FIG. 7A  illustrates motion of a subject within a capturing area, and  FIG. 7B  illustrates capturing images of respective frames. It is here assumed that the number of frames per second is 60 fps (16.7 msec/frame) and the number of scan rows by simultaneous readout of four pixel rows is 300 (1200 pixel rows). In the first frame, a subject is captured in the first scan row (the first to fourth pixel rows). The time information of year, month, day, hour, minute, and second “2016/5/27 02:03:04” and time information in a unit of 10 −7  seconds “000000000” are recorded in the leading part of the OB data section of the first scan row. In the second frame, the subject is located on the second scan row (the fifth to eighth pixel rows), and the time information in a unit of 10 −7  seconds is “01672222”. The subject further moves and is captured on the 300th acquisition row of the 100th frame. At this time, the time information of year, month, day, hour, minute, and second “2016/5/27 02:03:05” and time information in a unit of 10 −7  seconds “68333333” are recorded in the OB data section of the scan row. 
     As discussed above, according to the present embodiment, when a subject moves within a capturing area, a capturing position within a frame where the subject is captured and absolute time information at capturing can be recorded in accurate association with each other. In the conventional imaging system, only the start time or the end time of capturing or the save time is added on a frame basis. Further, the time information to be added is information in a unit of one second that depends on the GPS time information, and thus the capturing position of a subject within a frame and the absolute time information cannot be accurately associated with each other. The imaging system of the present embodiment generates first time information corresponding to a process timing of a pixel signal of one of the plurality of rows and second time information that is time information corresponding to a pixel signal of another row of the plurality of rows and has a value different from the first time information. That is, time information having a higher resolution than one frame period is generated, and thereby information of capturing position of a subject within a frame (scan row number) and time information are recorded in association with each other. Thereby, the capturing time of the subject, that is, the moment when the subject is present at a certain point can be recorded as more accurate absolute time information. 
     In an astronomical observation, when capturing a motion image of a moving object such as a meteor, space debris, or the like, the location information of the moving object and the accurate absolute time information thereof will be important in order to calculate the accurate orbit of the moving object. Here, the absolute time refers to the International Atomic Time defined by the universal time coordinated (UTC) or the like, for example. Further, in the field of so-called remote sensing for observing a far object, capturing of a moving object or capturing from a moving object such as an airplane, an artificial satellite, or the like is performed. Also in this case, with the location of an object with respect to the earth and accurate time information, it is possible to obtain more pieces of information on the object. According to the present embodiment, when capturing a motion image of a moving object such as a meteor, space debris, or the like in an astronomical observation, for example, the position of the moving object within a frame and the accurate absolute time information can be recorded in association with each other. 
     Although time information (the time t 1  of  FIG. 5 ) that is the reference of a counter in a unit of 10 −7  seconds is defined by the PPS signal obtained from the GPS satellite in the present embodiment, the present invention is not limited thereto. For example, the standard radio wave may be received, or information of a time server may be acquired from a network. Further, although the synchronization signals VD and HD are generated by the pulse generation unit  15 , a synchronization signal generated by the imaging device  10  may be detected to acquire the acquisition time of the counter (the time t 3  of  FIG. 5 ). Furthermore, although the counter operates at 10 MHz, that is, a cycle of 10 −7  seconds, the counter is not limited to the above example as long as it has a higher time-resolution than the PPS signal or the frame period that is used as the reference of the counter. Further, although the time information generation unit  17  is provided outside the imaging device  10  in the present embodiment, the imaging device  10  may have the time information generation unit  17 . 
     Further, although the acquisition timing of time information is at the rising time of the horizontal synchronization signal HD (the time t 3  of  FIG. 5 ) in the present embodiment, various time may be the reference as long as it is associated with a scan row of the imaging device  10 . For example, as described later, any time associated with the processing timing of a pixel signal can be used, such as the falling time of the horizontal synchronization signal HD, the start time or the end time of exposure of each acquisition row, the start time or the end time of readout of the column readout circuits  130 A and  130 B, the end time of charge transfer from the photoelectric conversion unit to the floating diffusion part, or the like. 
     Although the information addition unit  18  stores time information for all the scan rows (1 to 300) of one frame, the present invention is not limited thereto. For example, the row for storing time information may be one of two rows such as even rows or odd rows, may be one of any multiple rows, or may be a row selected at random. Furthermore, time information may be stored only on the row on which a subject is recognized. Further, the storage location of time information is not limited to the above-described examples and may be output separately from image data as long as the association between the time information and the captured row is clear. In such a way, the row for storing time information may be properly changed in accordance with a condition such as an output format of image data, a memory capacity, a drive specification of the imaging device, acquired information, or the like. 
     Second Embodiment 
       FIG. 8  is a timing chart of an imaging system of the present embodiment. In the present embodiment, the time information generation unit  17  outputs a counter value as time information at the falling time of the horizontal synchronization signal HD. Since other configurations and the control method are similar to those of the first embodiment, features different from the first embodiment will be mainly described. 
     At the time t 1 , in response to detecting a rising edge of the PPS signal, the time information generation unit  17  resets a counter and counts up the counter every 10 −7  seconds. The vertical synchronization signal VD becomes a high level at the time t 2  and then becomes a low level, a vertical scan period of one frame starts in the imaging device  10 . The horizontal synchronization signal HD becomes a high level and then becomes a low level at the time t 4 , and thereby a horizontal scan period starts in the imaging device  10 . At the same time, the time information generation unit  17  can record the start time t 4  of the horizontal scan period at a high time-resolution by recording the counter value in the OB data section. In a similar manner, at the time t 4 - 2 , t 4 - 3 , . . . , t 4 - 300  when the horizontal synchronization signal HD becomes a low level, the time information generation unit  17  records the counter value and the scan row in association with each other. Also in the present embodiment, accurate time information can be recorded in a similar manner to the first embodiment. 
     Third Embodiment 
       FIG. 9  is a timing chart of an imaging system of the present embodiment. In the present embodiment, the time information generation unit  17  outputs the counter value as time information at the time t 5  when the column readout circuits  130 A and  130 B end readout of pixel signals. In the following description, the time when the column readout circuits  130 A and  130 B end readout may be called an end time of a horizontal scan period. Since other configurations and the control method are similar to those of the first embodiment, features different from those of the first embodiment will be mainly described. 
     At the time t 1 , in response to detecting a rising edge of the PPS signal, the time information generation unit  17  resets a counter and counts up the counter. The vertical synchronization signal VD becomes a high level at the time t 2  and then becomes a low level, a vertical scan period of one frame starts in the imaging device  10 . The horizontal synchronization signal HD becomes a high level and then becomes a low level, and thereby a horizontal scan period starts in the imaging device  10 . The column readout circuits  130 A and  130 B read out image signals of one scan row (four pixel rows), and the column scan circuits  150 A and  150 B scan the column readout circuits  130 A and  130 B. Thereby, pixel signals are sequentially output from the imaging device  10  on a four-pixel-column basis (on a 16-pixel basis). For one scan row, 540 times of output of the pixel signal is performed, and upon the completion of the last 540th output, the timing generator  160  generates a pulse signal HSR_END indicating the end of the output of the pixel signal. At the time t 5 , at the same time when the pulse signal HSR_END becomes a high level, the time information generation unit  17  records the counter value as time information. Thereby, the end time t 5  of the horizontal scan can be recorded with a high time-resolution. In a similar manner, once the pulse signal HSR_END of the end of the horizontal scan becomes a high level, the counter values at the time t 5 - 2 , t 5 - 3 , . . . , t 5 - 300  are added to a part of the scan row. This allows all the scan rows of image data and the absolute time information to be associated with each other with a high time-resolution. 
     Note that, in the present embodiment, the counter value may be recorded as time information at the time of the column readout circuits  130 A and  130 B starting readout of pixel signals. That is, the time when the pixel signal is output from the imaging device  10  may be recorded as time information on each scan row. 
     Fourth Embodiment 
       FIG. 10  is a timing chart of an imaging system of the present embodiment. In the present embodiment, at the time when the transfer transistor M 1  is transitioned from an on-state to an off-state and charge transfer from the photoelectric conversion unit PD ends, the counter value is recorded as time information. The control signal TX becomes a high level and thereby the transfer transistor M 1  is turned on, and charges generated at the photoelectric conversion unit PD are transferred to the floating diffusion part FD. When the pixel  110  performs a rolling shutter operation, the control signal TX determines the end time of charge accumulation at the photoelectric conversion unit PD. The imaging system of the present embodiment will be described below mainly for the features different from those of the first embodiment. 
     Once the horizontal synchronization signal HD becomes a high level and then becomes a low level at the time t 4 , the pixels  110  on the first scan row (the first to fourth pixel rows) are selected by the row scanning circuit  120 , and pixel signals are read out by the column readout circuits  130 A and  130 B. That is, as illustrated in  FIG. 4 , after the photoelectric conversion unit PD is reset, the control signal TX becomes a high level, and the transfer transistor M 1  is turned on. This causes charges to be transferred from the photoelectric conversion units PD to the floating diffusion parts FD, and voltages in accordance with charges are read out by the column readout circuits  130 A and  130 B from the amplification transistors M 3  via the column signal lines L. 
     At the time t 6 , the control signal TX becomes a low level, and the transfer transistor M 1  is turned off. Thereby, accumulation and readout of charges of the photoelectric conversion unit PD end. At the same time, the time information generation unit  17  outputs the counter value at the time t 6  as time information. 
     The time t 6  when the control signal TX becomes a low level matches the end time of charge accumulation in the pixel  110 . Therefore, the counter value at the time t 6  represents the capturing time of the imaging device  10  in a more accurate manner. 
     In a column scan period, the column readout circuits  130 A and  130 B are scanned by the column scanning circuits  150 A and  150 B, and pixel signals are sequentially output from the imaging device  10 . The information addition unit  18  adds the time information at the time t 6  to the scan row associated with image data. Similarly, at the time t 6 - 2 , t 6 - 3 , . . . , t 6 - 300 , the counter value is added as accurate time information. 
     By adding the end time of charge transfer as time information to the scan row as described above, a scan row and capturing time can be recorded in association with each other with a high time-resolution. Further, since the counter is synchronized with the absolute time obtained from a GPS satellite, the end time of charge accumulation can be recorded as the significantly accurate absolute time. 
     Fifth Embodiment 
     While the imaging device  10  can perform a normal capturing operation or a rolling shutter operation in the first to fifth embodiments, the imaging device  10  can perform a global electronic shutter operation in the present embodiment. In the global electronic shutter, exposure operations of all the pixels are performed at the same time. In the present embodiment, time information is determined with respect to the reference of the end time of exposure operations for all the pixels. 
       FIG. 11  illustrates an equivalent circuit of the pixel  110  according to the present embodiment. The pixel  110  includes a photoelectric conversion unit PD, a holding unit C 1 , a floating diffusion part FD, a first transfer transistor M 1 , a second transfer transistor M 2 , an amplification transistor M 3 , a selection transistor M 4 , a reset transistor M 5 , and an overflow transistor M 6 . When turned on, the first transfer transistor M 1  transfers charges of the photoelectric conversion unit PD to the holding unit C 1 , and the holding unit C 1  holds the charges transferred from the photoelectric conversion unit PD. In all the pixels  110 , the first transfer transistors M 1  are simultaneously driven, and thereby the timings of the end of exposure of the photoelectric conversion units PD can be controlled all at once. That is, by controlling the control signals TX 1  on respective rows to be simultaneously turned on or off, it is possible to simultaneously control exposure timings in all the pixels  110 . When turned on, the overflow transistor M 6  can drain charges of the photoelectric conversion unit PD to the overflow drain such as a power source node. According to the present embodiment, while the holding unit C 1  holds charges, the photoelectric conversion unit PD can accumulate newly generated charges, which can realize the global electronic shutter function that causes the timings of photoelectric conversion in all the pixels  110  to be matched each other. 
       FIG. 12  is a timing chart of the imaging system of the present embodiment. First, the control signals OFG are set to a high level and thereby the overflow transistors M 6  are turned on in all the pixels  110 , and charges of the photoelectric conversion units PD are drained. Once the overflow transistors M 6  are turned off, an exposure period in the photoelectric conversion units PD starts in all the pixels  110 . Then, in all the pixels  110 , the control signals TX 1  become a high level, and the first transfer transistors M 1  are turned on. This causes charges of the photoelectric conversion units PD to be transferred to the holding units C 1 . 
     At the time t 7 , the control signals TX 1  become a low level in all the pixels  110 , and the exposure period ends. At the same time, the time information generation unit  17  outputs the counter value at the time t 7  as time information. Then, pixel signals on respective rows are sequentially read out by the column readout circuits  130 A and  130 B and output from the imaging device  10 . The information addition unit  18  adds the same counter value to respective scan rows sequentially output from the imaging device  10 . Thereby, the time t 7  of the end of exposure of all the pixels  110  can be recorded with a high time-resolution. 
     Similarly, in each frame of a motion image, capturing operations having the matched exposure period are performed, and each end time of the exposure period is added to image data. That is, the time information generation unit  17  adds the counter values at the time t 7 - 2 , t 7 - 3 , . . . to image data of respective frames. Therefore, the capturing time of respective frames (the end time of exposure) can be associated with image data and recorded with a high time-resolution. 
       FIG. 13A  and  FIG. 13B  are diagrams illustrating a capturing object and time information in the present embodiment.  FIG. 13A  illustrates the motion of a subject within the capturing area, and  FIG. 13B  illustrates capturing images of respective frames. In the conventional imaging system, the time resolution is in a unit of one second that depends on the GPS time information for one capturing frame. Thus, if capturing is performed with exposure time less than one second in one frame, the same time information (for example, 02:03:04) is undesirably recorded over a plurality of frames. On the other hand, according to the present embodiment, the end time of exposure of each frame can be recorded as time information with a high resolution. This allows for accurately recording the capturing time of each frame. 
     Sixth Embodiment 
       FIG. 14  is a block diagram of an imaging system of the present embodiment. In the first to fifth embodiments, the imaging device  10  is driven based on a synchronization signal generated by the pulse generation unit  15 . In the present embodiment, instead of the pulse generation unit  15 , a synchronization signal detection unit  15   a  is provided, which can detect the synchronization signal generated by the imaging device  10 . 
     The imaging device  10  may take various configurations, and there is a configuration which generates the vertical synchronization signal VD and the horizontal synchronization signal HD inside the imaging device  10  or a configuration which generates the horizontal synchronization signal HD inside the imaging device  10  based on the vertical synchronization signal VD supplied from the outside. When such the imaging device  10  is used, it is preferable to detect a synchronization signal of the imaging device  10 . 
     According to the present embodiment, the synchronization signal detection unit  15   a  can detect the vertical synchronization signal VD and the horizontal synchronization signal HD in the imaging device  10  and output the detected signals to the time information generation unit  17 . Since other configurations are similar to those of the first to fifth embodiments, the description thereof will be omitted. Also in the present embodiment, addition of time information to the row of image data allows for accurately identifying the acquisition time of each row. 
     Other Embodiments 
     The embodiments described above are mere examples of the present invention, and modified implementations are possible within the scope not departing from the spirit of the present invention. For example, the imaging device is not limited to a CMOS image sensor and may be a CCD image sensor. Further, an internal clock which is periodically calibrated by a radio wave from the GPS satellite may be provided in the imaging system, and the absolute time information may be generated based on the internal clock when it is difficult to receive the radio wave from the GPS satellite. 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2016-198662, filed Oct. 7, 2016, which is hereby incorporated by reference herein in its entirety.