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