{TAIL ~ The output} of sensor 80 is applied to summer 82 where it is compared to a signal representing {HEAD ~ a selected common reference frequency} from reference frequency source 81 
{ TAIL ~ The output } of sensor 80 is applied to summer 82 where it is compared to a signal representing { HEAD ~ a selected common reference frequency } from reference frequency source 81
TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH