gate 52 the other input to each of these and gates is from a ram store 54 56 and 58 respectively representing binary image information derived respectively from the vertical slit filter the horizontal slit filter and the reference filter more particularly these ram storage units 54 56 and 58 hold the most recent forty two lines of binary image for each of the three mini field filters involved the output from the comparator circuit 46 enables the appropriate and gate to pass on to the microprocessor the binary value for that pixel pxy from either 1 the image developed through the vertical average filter or 2 the image developed through the horizontal filter or 3 in the default situation where v h the image developed through the reference filter the image ram storage units 54 56 and 58 hold the binary values for each of the three images for the most recent forty two lines of image so as to have available the appropriate binary value for transmission through the or gate 60 to the microprocessor the output from the comparators 32 34 36 are the three video images indicated each image in turn being constituted by a series of binary value pixels the processing applied by the exclusive nor gates 38 40.