A sound barrier wall suitable for new installations and for retrofitting existing sound barrier walls includes a plurality of vertically extending posts and a plurality of wall sections disposed between pairs of the posts. Each pixel value represents an optical density of a pixel of the image and is generated mainly on the basis of a signal of only one of the sensor elements. Any of the factors at the other signal levels is obtained by performing an interpolating operation using the basic factors in the basic factor table. The very late collision occurs after a predetermined amount of data has been transmitted in the one of the first frames.