selectively disabling {HEAD ~ the output} of {TAIL ~ said first processing means} if said periodic pulse is not detected 
selectively disabling { HEAD ~ the output } of { TAIL ~ said first processing means } if said periodic pulse is not detected
OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH