{HEAD ~ Each} of {TAIL ~ these bits} in a given line is compared with a dither coded second stream of bits generated in response to the background illumination along a line immediately adjacent to the line being scanned 
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{ HEAD ~ Each } of { TAIL ~ these bits } in a given line is compared with a dither coded second stream of bits generated in response to the background illumination along a line immediately adjacent to the line being scanned
HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH