The squarer output is applied to a summing amplifier, which removes the multiplex frequency, and which provides {HEAD ~ a dc signal} which is applied to {TAIL ~ a comparator} 
The squarer output is applied to a summing amplifier , which removes the multiplex frequency , and which provides { HEAD ~ a dc signal } which is applied to { TAIL ~ a comparator }
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 OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL