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