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