For each digital input signal, a pair of {HEAD ~ references} is selected according to the state that represents {TAIL ~ the input signal} 
For each digital input signal , a pair of { HEAD ~ references } is selected according to the state that represents { TAIL ~ the input signal }
OTH OTH OTH OTH OTH OTH OTH OTH OTH HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH OTH OTH OTH TAIL TAIL TAIL TAIL TAIL TAIL TAIL