1.  Field of the Invention
The present invention relates to solid state image pickup apparatus which electrically processes the signal photoelectrically converted by a solid state image pickup device, and which is suitable for an electronic still camera which records the signal as a still picture on a medium such as a magnetic sheet or disc. 
2.  Related Background Art
FIG.  1 illustrates an interline type CCD as one example of a solid state image pickup element used very often in an electronic still camera.  In FIG.  1, reference numeral 101 denotes the interline type CCD as a solid state image pickup device; 102, photodiodes which convert light to electric charges and store them; 103, a vertical CCD which vertically transfers the electric charges received from the photodiodes, stage by stage, for each horizontal scanning interval; V1-V4, a transfer terminal for the vertical CCD in which V1 also functions as a transfer gating terminal to transfer electric charges in an odd number of lines of photodiodes to the vertical CCD and V3 also functions as a transfer gating terminal corresponding to an even number of lines of photodiodes.  The vertical CCD is driven by four-phase transfer pulses applied to terminals V1-V4.  Reference numeral 104 denotes a horizontal CCD which horizontally transfers the electric charges transferred from the vertical CCD 103 by one stage for each horizontal scanning period.  H1 and H2 each denote a transfer terminal for the horizontal CCD and are driven by two-phase pulses.  Reference numeral 105 denotes an output amplifier for converting electric charges to a voltage and outputting the voltage.  Reference VOUT denotes an output terminal. 
FIG.  2 shows the arrangement of color filters suitable for an electronic still camera.  The n-th line includes alternating Mg (magenta) and G (green) filters; the (n+1)-th line includes alternating Ye (yellow) and Cy (cyan) filters.  Mg, G and Ye, Cy are inverted in order of arrangement in each field as shown. 
FIG.  3 shows the drive timing used for performing the frame reading of pixel information in the solid state image pickup device 101.  The electric charges in the n-th line photodiode are transferred to the vertical CCD 103 by rendering V1 high at a time T1, and read between times T2 and T3.  The electric charges in the n'-th line are transferred to the vertical CCD at a time T3 and read at a time T4.  Thus, the electric charges corresponding to Ye, Cy, Mg and G are read separately pixel by pixel. 
FIG.  4 illustrates a process for producing a luminance signal and a color signal from the read signals.  As shown collectively in FIG.  4, the luminance signal (hereinafter referred to also as a Y-signal) is obtained from each line and color difference signals Mg-G or R-B are obtained line-sequentially from every other line. 
FIG.  5 is a block diagram of an electronic still camera.  Reference numeral 201 denotes a lens; 202, a diaphram; 203, a shutter; and 204, a driving circuit for the shutter and diaphram.  The solid state image pickup device 101 includes an interline type CCD, as mentioned above.  Reference numeral 205 denotes an image pickup device driving circuit; 206, a clock generating circuit; 207, an image pickup signal processing circuit which processes the output of the image pickup device to obtain the luminance signal and the line-sequential color difference signals sequentially; 208, a frequency modulation circuit which frequency-modulates the luminance signal and the line-sequential color difference signals; 209, a REC amplifier which amplifies the frequency-modulated signal so as to enable magnetic recording; 210, a magnetic head; 211, a magnetic sheet as a recording medium; 212, a motor to drive the magnetic sheet; 213, a motor servo circuit; 214, a system control circuit which controls the operation of the whole system; 215, a shutter release switch which is turned on to start a still picture pickup sequence. 
FIG.  6 is a block diagram of the image pickup signal processing circuit 207.  Reference numeral 301 denotes a CDS circuit to eliminate noise from the solid state image pickup device 101; 302, a clamping circuit to clamp the output of the CDS circuit 301 to a constant DC level; 303, a gamma correction circuit to gamma correct the output of the clamping circuit 302; 304, an AGC circuit to adjust the gain of the output from the gamma correction circuit 303; 305, a luminance low pass filter (hereinafter referred as Y.multidot. LPF) which allows the passage of the luminance signal band alone; 306, an adder to add the synchronizing signal and the luminance signal to produce a luminance+synchronization (hereinafter referred to as Y+S) signal; 307, a sample and hold circuit to sample and hold the Mg or Ye signal; 308, a sample and hold circuit to sample and hold the G or Cy signal; 309, a white balancing circuit (hereinafter referred to as a WB circuit A) to correct the gain of the output from the sample and hold circuit 308 such that the color difference signal Mg-G corresponding to a white subject is nullified; 310, a white balancing circuit (hereinafter referred to a WB circuit B) to correct the gain of the output from the sample and hold circuit 308 such that the color difference signal R-B corresponding to a white subject is nullified; 319, a color temperature sensor to sense the color temperature of a light source; usually, two color (R and B) sensors or three color (R, G and B) sensors are used; 320, a color temperature detecting circuit which detects the color temperature from the output of the color temperature sensor 319 and determines the gain of the white balancing circuit; 312, a switch for switching between the output signals from the WB circuits A 309 and B 310 for each horizontal scanning interval; 311, a subtracting circuit to produce a color difference signal; 313, a low pass color filter (hereinafter referred to as a C.multidot. LPF) which allows the passage of a color signal band alone; PI, a phase inversion circuit; 314, a gamma correction circuit for the color signal; 315, an AGC circuit for the color signal; 316, a gain correction circuit for Mg-G; 317, a gain correction circuit for R-B; 318, an axis conversion circuit to convert color difference signals Mg-G and G-B to color difference signals R-Y and B-Y.
FIG.  7 illustrates a sequence for driving the electronic still camera.  When the shutter release switch 215 is turned on at time T1, unnecessary electric charges in the A-field are cleared.  Unnecessary electric charges in the B-field are then cleared at time T2 delayed by a time of one field interval.  The shutter 203 is opened at time T3 to start exposure.  The reading of the photodiode in the A-field is started at time T4, and the resulting signal is processed simultaneously and recorded as a still picture on the magnetic sheet 209.  The reading of the photodiodes in the A-field is started at time T5 one field later than T4, and the resulting signal is processed simultaneously and recorded on the magnetic sheet 209 as a still picture.  The next B-field signal D"' is recorded during a time of one field interval between T5 and T6. 
When the luminance signal in the n-th line is made of Mg and G, and the luminance signal in the (n+1)-th line is made of Ye and Cy, it is very difficult to control the spectral characteristic of the color filters such that the values (Mg+G) and (Ye+Cy) coincide completely.  Especially, such control is impossible with a colored object.  Therefore, a phenomenon would occur in which luminance signals differ from line to line as shown in FIG.  8.  This phenomenon is referred to as a luminance step and appears as a horizontal stripe on the screen to thereby impair the picture quality. 
There is a method of averaging the vertical luminance signals by causing their vertical signal components to pass through a low pass filter in order to solve the problem.  This method can solve the problem of the luminance step, but an image would be blurred in its portion containing many high frequency components.