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