{HEAD ~ The low frequency and d.c. components} of the photo detector output are compared with {TAIL ~ a reference voltage} to derive an error signal 
are compared with
{ HEAD ~ The low frequency and d.c . components } of the photo detector output are compared with { TAIL ~ a reference voltage } to derive an error signal
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH OTH OTH REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH