selectively disabling the output of {HEAD ~ said second processing means} upon the detection of {TAIL ~ said periodic pulse}; and 
selectively disabling the output of { HEAD ~ said second processing means } upon the detection of { TAIL ~ said periodic pulse } ; and
OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH