Electronic still camera

Abstract: An electonic still camera comprising: a charge accumulation type of solid image pickup device (104) for forming a video signal, said image pickup device having light-receiving portions corresponding to odd field and light-receiving portions corresponding to even field; an optical system (101, 102) for focusing an object light on said image pickup device; shutter device (103) disposed between said image pickup device and said optical system, said shutter device is opened a predetermined preparatory time after a shutter releasing operation to an exposure of said image pickup device; a generator (108) for generating a first timing signal by which said image pickup device outputs the electric charge of the light-receiving portions corresponding to odd field and a second timing signal by which said image pickup device outputs the electric charge of the light-receiving portions corresponding to even field, said first and second timing signals being in a synchronized relation; and a controller (109) for making the generator output said timing signals at a first period for a predetermined time long from the shutter releasing and, after the lapse of the predetermined time, making it output said timing signals at a second period longer than the first period.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an electronic still camera and particularly to the picture-taking operation of the camera. Related Background Art A typical construciton of the circuit of the prior art electronic still camera is shown in Fig. 5. In the arrangement shown in Fig. 5, an object image is focused on a light receiving surface of a solid image pickup element 104 through a lens 101 under an exposure condition selected by a diaphragm 102 and a mechanical shutter 103. An output produced from the solid image pickup element is converted into a video signal based on a format by a signal processing circuit 105 and then FM-modulated by a recording circuit 106. The modulated signal is recorded in a magnetic recording disk at a recording portion 107. A timing pulse generator 108 generates a driving pulse for the image pickup element 105 and a timing pulse required for the signal processing circuit 105. An operation control unit (central processing unit, CPU) 109 controls all of the timings for a series of take-up operations. In the above-shown conventional apparatus, a picture is taken through the following operational steps whose time chart is shown in Fig. 6. A take-up operation starts with falling of a release signal and a power supply for every circuit shown above is switched ON. At the falling of 1CMg signal, the preceding shutter blade of the mechanical shutter starts running and at the falling of 2CMg signal, the trailing shutter blade starts running. In other words, an exposure is started at the start of the preceding shutter blade and is ended at the completion of running of the trailing shutter blade. In Fig. 6, Vp signal is a signal synchronized with a vertical synchronizing signal with a delay of about 70 usec. A TG pulse signal later described is within the Vp pulse. The start of the preceding shutter blade mentioned above is in synchronism with a time immediately after the Vp signal. PG signal is a signal for the start position of the magnetic disk of the recording portion. The signal is so controlled that one rotation time of the disk is coincident with one period of the vertical synchronization time. RESET signal is a signal used for the phase shift of PG signal and Vp signal during recording and for another similar purpose. At the falling of this RESET signal, a synchronizing signal from the odd field side of the vertical synchronizing signal as well as every timing pulse start. Odd TG and even TG signals are signals for readout of charge from the solid image pickup element. The odd TG signal is used for readout of odd field pixel and even TG signal for readout of even field pixel. CCD control signal is a signal for masking the charge readout signal. The latter signal is masked for the time of from exposure start to a time point immediately before recording start. The time during which the exposure charge is read out from the image pickup element and recorded in the magnetic disk is determined by a recording time signal. In the prior art apparatus, as seen from the timing chart, a discharging of unnecessary charge on the solid image pickup element (especially discharging from its light-receiving part) is carried out during the time of from release to exposure start for the purpose of removing any noise as caused by dark current. Before releasing, since the power supply to the image pickup element is not ON, the element is in a thermally balanced state. In other words, the wells of light-receiving part and transfer part are full of charge. In order to completely discharge it so as to empty the wells after releasing, readout of charge must be carried out at least more than three times continuously after releasing. To this end, usualy, the readout operation is carried out at the rate of one time per about 33ms. (two vertical synchronization periods) in the frame accumulation mode, with odd field pixel and even field pixel being read out alternately each for one vertical synchronization period. This means that the drainage of unnecessary charge on the image pickup element must be carried for a time long more than 100ms from releasing. The readout of charge of odd field pixel is effected in response to odd TG pulse and the readout of charge of even field pixel is effected in response to even TG pulse. However, this drainage of unnecessary charge in the operation mode of frame accumulation involves the following problem. As previously noted, in order to fully discharge the unnecessary charge, at least three times of readout are required for each field pixel. Therefore, in this case, an exposure can be started only after each field pixel having been read out at least three times or more. This means that the time passed from releasing to exposure start (time lag) is long. In order to shorten the time lag, the number of times of readout must be decreased. But, in this case, the remaining unnecessary charge is increased and, therefore, the noise increases up. US-A-4,760,452 discloses an electronic store camera in which charges due to dark current are rapidly discharged from an imaging device before an exposure operation and after the exposure operation an image is read from the imaging device. The charges due to dark current may not however be fully discharged during the rapid discharge and may affect the image recorded. SUMMARY OF THE INVENTION Accordingly, it would be desirable to complete the drainage of the unnecessary charge from the solid image pickup element (especially drainage of charge on the light receiving portion) in a shorter time from the time of releasing and to reduce the period from releasing to exposure start. There is thus provided an electronic still camera comprising: a charge accumulation type of solid state image pickup means for forming a video signal; an optical system for focusing light from an object on said image pickup means; signal generation means for generating timing signals to control reading out of accumulated electric charges from said image pickup; and control means for controlling said signal generation means to output timing signals at a first rate during a predetermined period and at a second rate, slower than said first rate, characterised in that said image pickup means has light receiving portions corresponding to odd and even light receiving portions, and in that said signal generation means generates a first timing signal for reading electric charges out of odd field light receiving portions and a second timing signal for reading electric charges out of said even field light receiving portions, said first and second timing signals being in a synchronised relation, and further characterised by outputting said timing signals at said second rate from the end of said predetermined period. There is further provided an electronic still camera comprising an image pickup device having charge storing elements, and control means for supplying timing signals to the device at a first rate during a predetermined period to discharge the elements and at a second rate, slower than said first rate, to cause charges stored by the elements to be read out in order to form a signal representing an image received by the device, characterised in that said timing signals are supplied to said elements at said second rate immediately following said predetermined period. There is further provided an electronic still camera comprising: a charge accumulation type of image pickup means for a video signal; an optical system for focusing light from an object on said image pickup means; and generating means for generating a timing signal by which said image pickup means outputs electrical charges accumulated in said image pickup means; characterised by control means for resetting the timing signal after the lapse of a predetermined time from a shutter releasing operation. The invention may be achieved by: 1. carrying out the operation in the field accumulation mode (TG pulses are generated almost at the same time on the light receiving parts of the odd field and the even field) for an arbitrary period from releasing and thereafter returning the mode back to the ordinary frame accumulation mode; 2. carrying out the operation of the field accumulation in an electronic shutter mode (two TG pulses are generated almost at the same time on the light receiving parts of the odd and even fields for one vertical synchronization period) for an arbitrary period from releasing and thereafter returning the mode back to the ordinary accumulation mode. The features described above bring about the following advantages over the prior art. 1. Since the operation is in the field accumulation mode for an arbitrary period from releasing, the readout of charge from the solid image pickup element is doubled, a complete drainage of the unnecessary charge is attainable and the time lag is reduced markedly. 2. Since the field accumulation in an electronic shutter mode is carried for an arbitrary period from releasing, the readout of charge from the solid image pickup element is increased by four times, a complete drainage of the unnecessary charge is assured and the time lag is reduced to a further extent. BRIEF DESCRIPTION OF THE DRAWING Fig. 1A is a circuit diagram showing a first embodiment of the timing pulse generator according to the present invention; Fig. 1B is a schematic view of a solid image pickup element; Fig. 2 is a flow chart of operation of the embodiment; Fig. 3 is a time chart of the embodiment; Fig. 4 is a circuit diagram showing a second embodiment of the timing pulse generator according to the invention; Fig. 5 is a block diagram showing the construction of a circuit in an electronic still camera; and Fig. 6 is a time chart of the prior art apparatus. DESCRIPTION OF THE PREFERRED EMBODIMENTS The manner of operation of an embodiment of the present invention will be described below with reference to Figs. 1A, 2 and 3. Referring to Fig. 1A, a timing pulse generator 80 according to the invention comprises a synchronizing signal generator 81, a frame accumulation pulse generator 82, a field accumulation pulse generator 83 and a selector 84. The synchronizing signal generator 81 generates a horizontal synchronizing signal HD, a vertical synchronizing signal VD, a synchronizing signal VP whose period is equal to that of the vertical synchronizing signal, and a synchronizing signal for driving the pulse generators 82 and 83. The selector 84 selects one of the pulse generator 82 and 83 in response to a signal from the operation control unit 109. The frame accumulation pulse generator 82 and the field accumulation pulse generator 83 generate a pulse for driving a vertical transfer shift register 104 and a horizontal transfer shift register 104H of the solid image pickup element shown in Fig. 1B and a pulse for driving a transfer gate. The gate 85 controls the input of a transfer gate (TG) pulse to the solid image pickup element in accordance with a masking signal from CPU. In the frame accumulation mode in which the operation of the solid image pickup element is carried out in accordance with the frame accumulation pulse generator 82, the light-receiving parts of the image pickup element are divided into odd field pixels O and even field pixels E as shown in Fig. 1B, and a time difference of 1/60 sec. is set between the timing of readout charges on the light-receiving part corresponding to the odd field pixels and the timing of readout of charges on the light-receiving part corresponding to the even field pixels. For each field, the charge accumulation time is set to 1/60 sec. In the field accumulation mode in which the operation of the solid image pickup element is carried out in accordance with the field accumulation pulse generator 83, the charge accumulation time is 1/60 sec., and odd TG pulse and even TG pulse are generated every 1/60 sec. with a phase shift of 60µ sec. between the two pulses. The transfer gate pulse has a width narrower than that of the synchronizing signal Vp and is generated within the signal Vp. With an operation of releasing, a series of operations shown in the flow chart starts. At the releasing, the reset signal is Low Level and the timing pulse generator is not operating. The accumulation mode selection signal (Frame/Field in Fig. 3) to the solid image pickup element is also Low Level and the selector 84 selects the field accumulation pulse generator. An internal timer within CPU starts counting from the time point of releasing. At the step S2, a discrimination is made as to whether the internal timer has counted 3msec. or not. When the timer has counted 3msec., the reset signal is released at the step S3. Thus, the reset signal is turned to High Level by which the timing pulse generator is brought into operation to generate every pulse. At the step S4, a discrimination is made as to whether the internal timer has counted 53msec. from the releasing the reset at S3. During the time, on the solid image pickup element, a readout of charge is being carried out in the field accumulation mode. When the timer has counted 53msec., the reset signal is momentarily turned to Low Level for 10µ sec. at the step S5 to initialize the timing pulse generator. At the same time, the accumulation mode is changed over to the frame accumulation mode at the step S6 and the frame accumulation pulse generator 82 is selected. The technique for resetting the synchronizing signal is disclosed in US-A-4,710,825 assigned to the same assignee as that of the present application. At the step S7, a discrimination is made as to whether the internal timer has counted 30msec. from the resetting at S5. After verifying the generation of Vp signal at the step S8, a mask signal (CCD control in Fig. 3) is turned to High Level at the step S9. During the time when the mask signal is High Level, no TG pulse is supplied to the image puckup element. That is to say, this time is a charge readout inhibition time. At the step S10, the motion of the preceding and trailing shutter blades is controlled. After verifying the generation of PG signal at the step S11, the synchronizing signal generator 81 is reset at the step S12 in the same manner as at the step S5. At the step S13, like the step S11, it is checked whether PG signal is generated. When the generation of the signal is confirmed, the mask signal is turned to Low Level at the step S14 to allow recording. At the step S15, an operation of recording is started and the video signal is recorded in the magnetic disk. Fig. 4 shows a second embodiment of the invention. In the timing pulse generator 180 according to the second embodiment, an electronic shutter pulse generator 86 is used in place of the field accumulation pulse generator 83 previously shown. The electronic shutter pulse generator functions as an electronic shutter. The manner of operation of the electronic shutter is disclosed in US-A-4,743,778 assigned to the same assignee as that of the present patent application. In brief, the electronic shutter controls the charge accumulation time by the time interval at which a transfer gate pulse is generated, and drives the vertical transfer shift register at a higher speed than usual for at least a portion of the charge accumulaiton time. Therefore, the number of transfer gate pulses odd TG and even TG generated for a unit time is doubled and the time passed up to the change-over of mode to the frame accumulation mode can be reduced as compared with the previous embodiment in which the field accumulation pulse generator 83 been used. Fig. 3 is a time chart showing the manner of operation of the embodiment of the invention. The construction of the circuit of the embodiment corresponds to that of the prior art shown in Fig. 5 with the exception of the timing pulse generator circuit. In the operational time chart shown in Fig. 3 a take-up operation starts with falling of a release signal and a power supply to every circuit shown above is switched ON. At the falling of 1CMg signal, the preceding shutter blade of the mechanical shutter starts running and at the falling of 2CMg signal, the trailing shutter blade starts running. In other words, an exposure is started at the start of the preceding shutter blade and the exposure is ended at the completion of running of the trailing shutter blade. Vp is a signal synchronized with the vertical synchronizing signal with a delay of about 70 usec. As previously described, TG pulse signal is within the Vp pulse. The start of the preceding shutter blade mentioned above is in synchronism with a time immediately after the Vp signal. PG is a signal for the recording start position of the magnetic disk at the recording portion. The signal is so controlled that one rotation time of the disk is conincident with one period of the vertical synchronization time. The reset signal RESET is used for the phase shift of PG signal and Vp signal during recording. The frame/field signal is a accumulation mode selection signal. When the signal is High Level, the frame accumulation mode is selected, and when Low Level, the mode is the field accumulation mode. The odd TG and even TG are signals for readout of charge from the solid image pickup element. The odd TG signal is used for readout of odd field pixel and even TG signal for readout of even field pixel. In this embodiment, when the mode is the electronic shutter mode, two TG pulses are generated for each field during one vertical synchronization period of time. CCD control signal is a singal for making (inhibiting) the charge readout signal. The latter signal is masked for the time of from exposure start to a time point directly before the start of recording. The time during which the exposed charge is readout from the image pickup element and recorded in the magnetic disk is determined by the recording time signal. In this embodiment, the mode is in the field accumulation mode during the time of 56msec. from releasing and during the time the charge of the solid image pickup element is read out four times per field so that unnecessary charge are completely discharged. After discharged, with a resetting operation, the mode is changed over to the frame accumulation mode. After the frame accumulation has got in a stable state, that is, at the time point of 90msec. after releasing, an exposure is started. If the field accumulation is carried out in the electronic shutter mode from the time of releasing, the time lag can be reduced further by two vertical synchronization periods of time. The time point at which the resetting is carried out can be selected at will. As will be understood from the foregoings, the present invention has the following advantages over the prior art: The drainage of unnecessary charge from the solid image pickup element can be carried out completely for a shorter time. The time lag from releasing to exposure start can be reduced. The time point at which a resetting is carried out to change over the mode from field accumulation mode to frame accumulation mode, can be selected at will. Therefore, man can adjust the time lag from releasing to exposure start to any desired time length.

Claims: An electronic still camera comprising: a charge accumulation type of solid state image pickup means (104) for forming a video signal; an optical system (101) for focusing light from an object on said image pickup means (104); signal generation means (80,85;180,85;108,109) for generating timing signals (ODD TG, EVEN TG) to control reading out of accumulated electric charges from said image pickup (104); and control means (84;109) for controlling said signal generation means (80,85;180,85;108,109) to output timing signals (ODD TG, EVEN TG) at a first rate during a predetermined period and at a second rate, slower than said first rate, characterised in that said image pickup means (104) has light receiving portions corresponding to odd (O) and even (E) light receiving portions, and in that said signal generation means (80,85;180,85;108,109) generates a first timing signal (ODD TG) for reading electric charges out of odd field light receiving portions (O) and a second timing signal (EVEN TG) for reading electric charges out of said even field light receiving portions (E), said first and second timing signals (EVEN TG, ODD TG) being in a synchronised relation, and further characterised in that said signal generation means is adapted for outputting said timing signals (EVEN TG, ODD TG) at said second rate from the end of said predetermined period. An electronic still camera as claimed in claim 1 including shutter means (102) disposed between said image pickup means (104) and said optical system (101), said shutter means (102) being opened after the lapse of a predetermined preparatory period from a shutter releasing operation to start an exposure of said image pickup means (104). An electronic still camera according to claim 1 or claim 2, which further comprises recording means (105,106,107,109) for recording image information formed on the basis of the electric charges outputted from said image pickup means (104) in response to said first and second timing signals (EVEN TG, ODD TG) when said first and second timing signals (EVEN TG, ODD TG) are outputted at said second rate. An electronic still camera according to claim 2 or claim 3 when dependent upon claim 2 wherein said predetermined preparatory period is a period from a shutter releasing operation to a time point before the exposure start. An electronic still camera according to any one of the preceding claims wherein said first and second timing signals (EVEN TG, ODD TG) are generated substantially at the same time during said predetermined period, and, after said predetermined time, said first and second timing signals are generated alternately at regular intervals. An electronic still camera according to any one of the preceding claims wherein said second rate is half said first rate. An electronic still camera according to claim 1, wherein said signal generation means (80,85;180,85;108,109) output said first and second timing signals to each of the light receiving portions (O,E) in the form of two continuous pulses during said predetermined period and, after said predetermined period, said signal generation means (80,85;180,85;108,109) outputs said first and second timing signals to each of the light receiving portions (O,E) in the form of pulses. An electronic still camera comprising an image pickup device (104) having charge storing elements (O,E), and control means (108,109;80,85;180,85) for supplying timing signals to the device at a first rate during a predetermined period to discharge the elements (O,E) and at a second rate, slower than said first rate, to cause charges stored by the elements (O,E) to be read out in order to form a signal representing an image received by the device, characterised in that said control means is adapted for supplying said timing signals to said elements (O,E) at said second rate immediately following said predetermined period. An electronic still camera comprising: a charge accumulation type of image pickup means (104) for a video signal; an optical system (101) for focusing light from an object on said image pickup means (104); and generating means (108,109) for generating a timing signal by which said image pickup means (104) outputs electrical charges accumulated in said image pickup means (104); characterised by control means (109) for resetting the timing signal after the lapse of a predetermined time from a shutter releasing operation.

Inventors: SUZUKI MASAHIRO, SUZUKI, MASAHIRO
Applicants: NIPPON KOGAKU KK, NIKON CORPORATION
Application year: 1989
Main classification: H04N3
IPC codes: H04N3, H04N5