A) Field of the Invention
The present invention relates to a solid-state image pickup device to be used for a digital still camera having a movie mode, and in particular, to a moving picture imaging technique of a solid-state image pickup device.
B) Description of the Related Art
In the digital still camera using a solid-state image pickup device, it is essential to have an image confirming function (movie mode) to confirm an image according to a movie picture produced by the camera. The number of pixels of the solid-state image pickup device is increasing every year. Solid-state image pickup devices available in the market have about 330 thousand pixels in 1996, about 800 thousand pixels in 1997, 1.5 million pixels in 1998, and more than 3 million pixels in 2000.
As a result of increase in the number of pixels as above, it became difficult to retain the mobile picture output function with picture quality substantially equal to that of the digital still camera, as in the prior art. To keep the picture quality of the prior art also in a digital still camera with an increased number of pixels, the drive frequency must be increased in proportion to the increase in the number of pixels. However, the operation frequency is not easily increased. When the circuit operation frequency is increased, power consumption of the camera is increased and the life of the camera driving battery becomes shorter. Additionally, noise appears easily in the image.
When it is attempted to increase the number of pixels of the solid-state image pickup device with the operation frequency kept unchanged, the frame rate must be lowered in the mobile picture shooting operation. When the culling rate of pixels in the readout operation is increased to keep the frame rate, false colors easily appear.
FIG. 8 shows a principle of a readout technique for mobile pictures as an example of a thin-out readout operation in a plan view.
As can be seen from FIG. 8, a charge-coupled device (CCD) solid-state image pickup device A includes a semiconductor substrate and a large number of photoelectric converter elements 103 formed in a contour of a matrix on a two-dimensional surface 101 of the semiconductor substrate. For each column of the photoelectric converter elements 103, one vertical charge transfer channel region (vertical charge-coupled device, VCCD) 105 is disposed to transfer electric charge stored in the photoelectric converter elements 103 in a vertical direction.
In one end portions of the vertical charge transfer channel regions 105, a horizontal charge transfer channel region (horizontal charge-coupled device, HCCD) 107 is disposed. The region 107 receives the electric charge transferred from the vertical charge transfer channel regions 105 and then transfers the electric charges in a horizontal direction. In one end portion of the horizontal charge transfer channel region 107, an output amplifier 111 is disposed. The amplifier 111 amplifies the electric charge from the horizontal charge transfer channel region 107 and then outputs the amplified electric signals to an external device.
In the CCD solid-state image pickup device A, colors are arranged in a so-called Bayer layout. In this layout, a column including green (G), red (R), G, R, . . . in this order in a column direction and a column adjacent thereto including blue (B), G, B, G, . . . in this order in the column direction are alternately disposed in a row direction. A row including R, G, R, G, . . . in this order in the row direction and a row adjacent thereto including G, B, G, B, . . . in this order in the row direction are alternately disposed in the column direction.
The rows shown in FIG. 8 are classified into groups each of which includes eight rows cyclically assigned with reference numerals L1 to L8, respectively. On the vertical charge transfer channel region, two vertical charge transfer electrodes are provided for each photoelectric converter element row, to which four kinds of voltages φV1 to φV4 can be independently applied.
Description will now be given of a quarter thin-out readout operation conducted in the movie mode by the CCD solid-state image pickup device A configured as above.
Signal charge of G, B, G, B, . . . and signal charge of R, G, R, G, . . . are read respectively from the photoelectric converter element rows L5 and L2 to be transferred to the vertical charge transfer channel region 105. By sequentially applying drive voltages to the vertical charge transfer electrodes, the signal charge is then transferred in a direction to the horizontal charge transfer channel region 107 in a 4-phase drive operation.
From the unit of eight photoelectric converter element rows, pixel signals of pixels of two rows are read out. That is, signals are read from a quarter of the overall pixels. This is called “quarter thin-out readout operation”. In this method, the imaging or image pickup operation can be conducted at a speed four times higher than that of the overall pixel readout operation employed to read out a still picture.
However, this method is attended with a problem. That is, when this method is used, the amount of signals is also lowered to a quarter of the original amount, and hence sensitivity is deteriorated. In the movie mode, the frame rate is 1/30 second (s). When this frame rate is kept unchanged, the sensitivity is insufficient in a dark place in most cases. Being different from the operation in the still picture mode, it is difficult to use a flash in the movie mode. Also, it is difficult in the movie mode to shoot a mobile picture using a long period of exposure time.
Additionally, since the signals are read out only from the photoelectric converter element rows L2 and L5, the intervals of rows between the photoelectric converter element rows for the signal readout operation are two and four. The intervals of two rows and four rows appear repeatedly. In other words, the photoelectric converter element rows for the signal readout operation are arranged with unequal intervals therebetween in the column direction. Therefore, the false colors easily occur. Influence of the false colors becomes stronger when the number of pixels not read out in the thin-out readout operation is increased.
A unique problem regarding false colors in a digital still camera is a problem of mismatching of an optical low-pass filter. An optical low-pass filter is disposed to reduce occurrence of the false color. In digital still camera, a pitch of the birefringence of the optical low-pass filter is determined based on the still picture imaging operation.
Therefore, when the non-uniform thin-out readout operation is conducted for a mobile picture, the readout pitch is not fixed because of the unequal intervals described above, and hence does not match the pitch of the optical low-pass filter for still pictures. There exists a tendency that as the number of pixels of the solid-state image pickup device increases, the mismatching between the readout pixel pitch for the operation to shoot a mobile picture and the pitch of the optical low-pass filter increases.