1. In a signal processing apparatus for video image data compression, a system comprising: dithered threshold means for applying dithered threshold values to a multiple-level grey-scale image to convert said multiple grey-scale image into a reduced-level grey-scale image, the dithered threshold values varying spatially across an image and temporally across plural images; storage means for storing a reference reduced grey-scale image derived from image pixel values; comparison means for comparing a reduced grey-scale image of a current image frame with a reference image from said storage means, and for generating a luminance difference signal representative of the changes in reduced grey-scale pixel values between image frames; encoding means for encoding said difference signal for transmission through a transmission system; decoding means; and reconstruction means for generating an expanded grey-scale image from the luminance difference signal having expanded levels from the reduced grey-scale image pixel values with substantially the same number of pixel values as in the reduced grey-scale image by averaging pixel values of respective blocks of pixel values, a different block of pixel values for each expanded pixel value defined by a shifting window over pixel values of the reduced grey-scale image.