The present invention relates to an electric shutter control apparatus for a video camera using a liquid crystal shutter.
An image pick-up device such as a charge coupled device (CCD) in the video camera receives light transmitted from an object through a lens. The video camera generates a television signal by adding synchronizing signals which are synchronized with the vertical and horizontal synchronizing signals of a television signal to an electric signal supplied from the image pick-up device. To provide one video image in interlaced scanning for the television signal, time for two fields (one frame) is needed because the two fields are equivalent to one frame. When the object to be videoed is moving at high speed, the reproduced video image is distorted by a change of light from the image on the image pick-up device during one frame. Particularly in the interlaced scanning, the reproduced video image becomes indistinct due to a time lag of about one field between the two neighboring interlacing lines.
In order to provide a more distinct image of an object moving at high speed, the video camera is equipped with a shutter on its lens opening which opens at a predetermined time on the frame basis, and exposes the image pick-up device instantaneously to light from the object. However, the timing of the shutter opening is unstable and the synchronization with another system is difficult when using the shutter a mechanical structure as in the prior art. In addition, there is a problem with durability. If the video camera has no frequently moving mechanical structure other than such shutter, the timing of maintenance, inspection and repair will be determined by the shutter as the other parts all have long lives. Mechanism such as the shutter often cause trouble because of deterioration over time.
To solve this problem, it is disclosed in Japanese Patent Application Un-examined Publication No. 296866/1989 that a liquid crystal shutter can be used in place of a mechanical shutter. However, a means for synchronizing the operation of the video camera with an external system such as a strobe light on the object to be videoed is not disclosed in this invention, and therefore it is impractical to combine the liquid crystal shutter with the external system.