An automatic gain controlling circuit controls a gain of a video signal
from an imager having an electronic shutter function according to a gain
control signal. An average detecting circuit detects an average level of a
luminance signal of the video signal. An automatic exposure controlling
circuit generates the gain control signal according to the luminance
average level in response to a mode signal indicative of the cycles per
second of the ac line and generates a timing signal corresponding to the
ac line and a shutter speed control signal together with electronic
shutter control signal generation circuit at a unit of the voltage
variation cycle of the ac line according to the luminance average level
such that a shutter interval of the imager is changed stepwise and each of
the shutter intervals is an integer times the voltage variation cycle of
the ac line. The automatic exposure controlling range is expanded by
changing the shutter speed stepwise from 1/100 sec, 2/100 sec 3/100 sec at
50-Hz-ac area, for example, to provide coarse exposure controlling and
fine automatic gain controlling is provided every shutter interval to
suppress flicker.
