The periods or, equivalently, frequencies of the divided CO signal and {TAIL ~ the divided reference signal} are compared, and the result is used to select an appropriate tuning curve for {HEAD ~ the CO} 
The periods or , equivalently , frequencies of the divided CO signal and { TAIL ~ the divided reference signal } are compared , and the result is used to select an appropriate tuning curve for { HEAD ~ the CO }
OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD