At the same time, {HEAD ~ a reduced size block picture} is generated by {TAIL ~ second block generating unit} 
is generated by
At the same time , { HEAD ~ a reduced size block picture } is generated by { TAIL ~ second block generating unit }
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL