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