The apparatus 3, including a video monitor biased to the center of {HEAD ~ gray scale} so that said monitor registers zero difference between the original and copy and registers a difference signal the level of which is proportionate to the degree of degradation of {TAIL ~ the copy} over the original 
The apparatus 3 , including a video monitor biased to the center of { HEAD ~ gray scale } so that said monitor registers zero difference between the original and copy and registers a difference signal the level of which is proportionate to the degree of degradation of { TAIL ~ the copy } over the original
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD 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 TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH