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