When {HEAD ~ the value} of the dc signal equals or exceeds {TAIL ~ a predetermined value} thereof, an indication of tone detection is provided 
exceeds
When { HEAD ~ the value } of the dc signal equals or exceeds { TAIL ~ a predetermined value } thereof , an indication of tone detection is provided
OTH HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH