Abstract:
In the electronic still camera capable of field reading or frame reading and having a solid state image pickup element composed of a plurality of photosensitive elements arranged adjacent to each other, in reading, the electric charges of the odd numbered rows in the vertical direction of photosensitive elements and the electric charges of the even numbered rows of photosensitive elements are simultaneously cleared, and means are provided for shifting the clearings of the electric charges of the odd numbered rows of photosensitive elements and the electric charges of the even numbered rows of photosensitive elements from each other in time by one vertical scanning period in reading.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention: 
     This invention relates to electronic still cameras having a solid state image pickup element comprised of a plurality of photosensitive elements capable of field readout or frame readout and arranged adjacent to each other. 
     2. Description of the Related Art: 
     FIG. 1 is a diagram illustrating the circuit structure of an inter-line transfer CCD 1 which is used in the conventional electronic still camera. In the FIG., 13, 14 are respective photo-diodes for converting photo-signals to electric charges. 15 is a vertical transfer CCD (hereinafter referred to as V-CCD) for transferring the electric charges vertically. 16 is a horizontal transfer CCD (hereinafter referred to as H-CCD) for reading out the electric charges horizontally. The electrical charges are converted to voltages by an output amplifier 17. Also, 18 is terminals at which vertical transfer pulses of four phases for driving the V-CCD 15 enter. 19 are a terminal at which a horizontal transfer pulse for driving the H-CCD 16 enters. Upper and lower drains 20 and 21 for clearing the electric charges include gates for selectively clearing the electric charges. 
     In FIG. 1, of the input terminals 18 for the vertical transfer pulses, the terminals φ V1  and φ V3  also serve as transfer gate electrodes from the photo-diodes 13 and 14 to the V-CCD 15, and are driven by pulses of such three values as shown in FIG. 2. That is, when the pulses φ V1  and φ V3  become +V, the gates open and electric charges transfer from the photo-diodes 13 and 14 to those electrodes in the V-CCD 15 to which the pulses φ V1  and φ V3  are applied. After that, the usual 4-phase drive pulses φ V1  -φ V4  each are applied between 0 and -V shown in FIG. 2 to perform vertical transfer of the electric charges. 
     In such a conventional device as has been described above, when reading a frame, the time from the clearing to the reading of each photosensitive element differs between the adjacent upper and lower photosensitive elements each, by which difference is produced in the amount of dark current stored in each and the degree of unevenness. Therefore, there was a problem in that a flicker phenomenon and an unnatural image were produced. 
     SUMMARY OF THE INVENTION 
     This invention has been made to solve such a conventional problem, and its object is to provide an electronic still camera in which there is no difference in brightness between the upper and lower adjacent picture elements of an image-picked-up image, and an image signal having no flicker effect produced and freed from an unevenness can be obtained. 
     To achieve the above-described object, an embodiment of this invention has an arrangement that when reading a field, the electric charges of the above-described photosensitive elements arrayed in the odd numbered rows to the above-described vertical direction and of the above-described photosensitive elements arrayed in the even numbered rows are simultaneously cleared, and means is provided that when reading a frame, the electric charge clearings of the photosensitive elements arrayed in the odd numbered rows to the vertical direction and of the photosensitive elements arrayed in the even numbered rows are shifted from each other in time by one vertical scanning period. 
     By having the above-described arrangement, in the drive timing of the frame reading, the interval between the clearing of the photosensitive elements arrayed in the adjacent upper part and the clearing of the photosensitive elements arrayed in the adjacent lower part is made to be one vertical scanning period, thereby it being made possible that the times from the clear to the readout of the respective photosensitive elements are equal to each other. 
     Other objects and features of the present invention will become apparent from the following described specification and drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram illustrating the circuit structure of the inter-line transfer CCD used in the electronic still camera. 
     FIG. 2 is a diagram illustrating the transfer pulse for the electric charge from the photo-diode to the V-CCD. 
     FIG. 3 is a block diagram of an electronic still camera in this invention. 
     FIG. 4 is a timing chart illustrating the timing of field reading of a first embodiment of the invention. 
     FIG. 5 is a diagram in an enlarged scale illustrating a part A enclosed by dashed lines of FIG. 4. 
     FIG. 6 is a diagram illustrating the vertical transfer pulse for reverse transfer of field reading. 
     FIG. 7 is a drive timing chart of frame reading of the above-described first embodiment. 
     FIG. 8 is a diagram in enlarged scale of a part B enclosed by dashed lines of FIG. 7. 
     FIG. 9 is a diagram illustrating the vertical transfer for reverse transfer of frame reading. 
     FIG. 10 is a drive timing chart in frame reading of the inter-line transfer CCD for an electronic still camera in a second embodiment of this invention. 
     FIG. 11 is a diagram illustrating a third embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 shows a block diagram of the electronic still camera in this invention. 
     In the figure, ST is a shutter; 1 is an interline transfer CCD as shown in FIG. 1; 2 is a capacitor. A photo-signal is converted to a voltage by the CCD 1. By a sample-hold circuit 3, clock leak is removed. The potential of the signal is fixed to a predetermined potential by a clamp circuit 7 through an amplifier 4, a buffer 5 and a capacitor 6. It is supplied as the output signal to a processing circuit 9 where it undergoes gamma correction or like correction, being converted to a video signal, and further to a recording and/or reproducing device RP where it is recorded or reproduced. 
     Also, the CCD 1 is driven by the vertical transfer pulse outputted from the vertical driver circuit 10 and the horizontal transfer pulse outputted from the horizontal driver circuit 11 formed on the basis of the timing pulse made by the timing generating circuit 12. TR is a trigger button. 
     In the following, the first embodiment of the timings of field reading and frame reading by the driver circuit 10 is explained. 
     FIG. 4 is a timing chart illustrating the timing of field reading, FIG. 5 is a diagram showing in enlarged scale the part A enclosed by the dashed lines in FIG. 4. When the pulse is applied in such a timing as shown in FIG. 5, a potential gradient necessary to transfer electric charges is realized, and the electric charges of the V-CCD 15 are transferred vertically downward. 
     Also, the timing of the transfer pulse is changed as shown in FIG. 6, the electric charges of the interior of the V-CCD 15 can be reversely transferred. 
     In the field reading of FIG. 4, in accompaniment with the actuation of the trigger button TR, at a time t 0  before an exposure, the photo-diodes 13 and 14 are simultaneously cleared. In an interval until a time t 1 , the V-CCD 15 is cleared. In FIG. 4, by reversely transferring the electric charges of the photo-diode 13 to the upper drain 20, the clearing is performed. But, of course, an electric charge transfer to the lower drain 21 may be performed by the normal transfer. Further, after the exposure, at a time t 4 , the electric charges of both of the photo-diode 13 and the photo-diode 14 are applied and read out in a read period. 
     Further, timing of the frame reading may be as in FIG. 7. An enlarged diagram of the portion B enclosed by the dashed lines in FIG. 7 is shown in FIG. 8. In the case of FIG. 8 also, the electric charges are transferred vertically downward likewise as in the field reading. With timing as FIG. 9, reverse transfer can be performed. 
     In FIG. 7, in accompaniment with the actuation of the trigger button TR, at the time t 0  before an exposure, the electric charge of the photo-diode 13, and at a time t 2  the electric charge of the photo-diode 14 are respectively cleared by the reverse transfer. In this case also, clearing of both may be performed by the normal transfer. Likewise as the field reading of FIG. 4, the signals of the photo-diodes 13 and 14 may also be simultaneously cleared. And, in the read period, from a time t 6 , the signal of the photo-diode 13 is read. Subsequently, from a time t 7 , the signal of the photo-diode 14 is read. With timing, the time from the clearing (t 0 ) of the electric charge in the photo-diode 13 to the read start (t 6 ) and the time from the clearing (t 2 ) of the electric charge in the photo-diode 14 to the read start (t 7 ) differ by nearly one vertical period (1V). Therefore, as to the amounts of dark current collected respectively in the photo-diodes 13 and 14 during that time, the photo-diode 14 amount is more than the photo-diode 13 amount. 
     FIG. 10 follows a second embodiment of this invention which can solve the identified problem and shows a timing chart of the drive pulses of the drive circuit 10 in the frame read mode of the inter-line transfer CCD for the electronic still camera. In accompaniment with the actuation of the trigger button TR, at the time t 0 , the pulse φ V3  becomes +V, by which the electric charge of the photo-diode 13 is transferred to the V-CCD 15, and reversely transferred in the timing of FIG. 6 at a high speed and cleared. Also, the clearing may be performed by the normal transfer in the timing of FIG. 9 at the high speed. At the time t 2 , the electric charge of the photo-diode 14 is cleared in a similar way. During a period from a time t 4  to t 5 , the shutter ST opens, and electric charges are accumulated on the photo-diodes 13 and 14. And, during one vertical period (1V) from a time t 6  to t 7 , the electric charge of the photo-diode 13 is transferred in the vertical direction in the timing of FIG. 5, read out to the H-CCD 16, then during 1V from a time t 7  to t 8 , the electric charge of the photo-diode 14 is read out likewise. 
     In the embodiment of FIG. 10, by making the interval from the clearing (t 0 ) of the electric charge of the photo-diode 13 to the clearing (t 2 ) of the electric charge of the photo-diode 14 to be 1V, the time from the clearing of the electric charge of the photo-diode 13 to the readout and the time from the clearing of the electric charge of the photo-diode 14 to the readout can be equalized to each other. By this fact, the drawback of the first embodiment, that is, that in the photo-diode 13 and the photo-diode 14, the time from the clearing of the electric charge to the readout differs so that a difference arises in the amount of dark current collected in the photo-diode 13 and 14, can be solved. 
     FIG. 11 is a diagram illustrating practice in accordance with a third embodiment of this invention. Whilst in the example of FIG. 10, the clearing of the electric charges in the photo-diodes 13 and 14 is performed by high speed transfer in the vertical direction, in the example of FIG. 11, practice is that in accompaniment with the actuation of the trigger button TR, the clearing of the electric charges of the photo-diodes 13 and 14 is not performed at the high speed but through one vertical period likewise as in the reading. Even in this embodiment, the time from the clearing of the electric charge of the photo-diode 13 at the time t 0  to the readout of the photo-diode 14 at the time t 4  becomes equal to the time from the clearing of the electric charge of the photo-diode 14 at the time t 1  to the readout at the time t 5 . Therefore, a similar effect to the case of using the example of drive timing of FIG. 10 is obtained. 
     As has been described above, in the second and third embodiments, when reading a field, the electric charge clearing of the above-described photosensitive elements arrayed in the odd numbered rows in the above-described vertical direction and the above-described photosensitive elements arrayed in the even numbered rows are simultaneously performed, and means are provided such that when the above-described frame reading is performed, the electrical charge clearing of the above-described photosensitive elements arrayed in the odd numbered rows in the vertical direction and the above-described pohtosensitive elements arrayed in the even numbered rows are staggered by one vertical scanning period, so that the amount of dark current corrected in the photosensitive element can be equalized. Therefore, an advantage is produced that the flicker phenomenon is removed, and the dark current unevenness is made nearly of the same order.