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