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