A hybrid optical-digital encoder incorporating a new optical preprocessing method for reducing visible error instability noise in the reconstructed video display of adaptive delta modulation (ADM) encoded signals is presented. The method employs the edge contrast dependent phase gradients, produced by the localized transition surface reliefs of the input image absorption transparency. Phase gradients add a local angular carrier to the input illumination angle, causing the position of the Fourier transformed spatial spectra of the light from high contrast, instability producing transitions to be displaced from the axially centered and close lying transformed spectra. The close lying spectra of slowly varying and low contrast transitions do not cause highly visible disturbing error instabilities.;In the transform domain, directional and differential weighting of the composite transformed spatial spectra is implemented using an asymmetric linearly transmitting absorption spatial filter. The filter is produced photographically on Kodak 649F photographic emulsion by an analog exposure technique. The effect of the filtering on the retransformed output image distribution is essentially to superimpose a visualized image of the high contrast transition phase gradients on the retransformed real amplitude input image.;The resulting superposition modifies the slope of the original edge detail. Thus error instability usually seen at high contrast transitions in the video reconstruction of ADM encoded scenes sampled at low rates is reduced.;The method of optical preprocessing used modifies the input spatial spectrum distribution in a way not possible by either simple binary symmetric low pass filtering or image defocusing--optical methods used to reduce image resolution-bandwidth requirements.;The method is explored for three illumination image class cases: Temporally and spatially coherent illumination of monochrome still and moving scenes, temporally incoherent partially spatially coherent polychromatic illumination of monochrome still and moving scenes, and temporally incoherent partially spatially coherent polychromatic illumination of still and moving color scenes.;In each case optical preprocessing reduces the subjective image noise when encoder rates fall below 10 Mbit/sec. The increase in image stability in each case results in the compression of the bandwidth needed to produce a given quality of the transmitted scene.
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