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