A tone switching logic circuit normally maintains {HEAD ~ the reference frequency} at {TAIL ~ a value} substantially equal to the frequency f1 
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A tone switching logic circuit normally maintains { HEAD ~ the reference frequency } at { TAIL ~ a value } substantially equal to the frequency f1
OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH