In addition, a subtractor produces an error signal by subtracting {HEAD ~ the output signal} from {TAIL ~ the reference input signal} 
In addition , a subtractor produces an error signal by subtracting { HEAD ~ the output signal } from { TAIL ~ the reference input signal }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL