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