Assuming that no tone signals have been applied to {TAIL ~ the decoder} 14, {HEAD ~ the output signal} from comparator 56 is at a low logic level 
Assuming that no tone signals have been applied to { TAIL ~ the decoder } 14 , { HEAD ~ the output signal } from comparator 56 is at a low logic level
OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH OTH