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