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