This invention relates to a binarizing system of picture image signals. In a facsimile device, for example, analog picture image signals produced by scanning a manuscript with a line sensor are binarized according to a predetermined threshold value. Generally, the level of the picture image signal varies depending upon such factors as the tone or concentration of the picture image, the thickness of lines, whether the lines are vertical or horizontal, states of surrounding picture images, or the like so that when the picture image signals are binarized as they are, it is impossible to obtain good image quality. For example, a fine black line in a white background, a continuous pattern of fine white and black lines, and a fine white line in a black background have different signal levels so that informations would be lost irrespective of the settings of threshold values as will be described later in more detail.
For this reason, in the prior art a correction circuit has been provided before the binarizing circuit for correcting contrast of the picture image signals. Although various types of correction circuits have been proposed, a satisfactory correction circuit is not yet available. For example, according to one type of correction circuit, where the difference in the signal levels of the picture image signals of adjacent picture elements and the picture image signal of a picture element to be corrected is small, substantially no correction is made, while for a picture image in which the level of the picture image signal varying slowly, that is, for a picture image whose contrast is vague, satisfactory correction could not be made.