If desired, {HEAD ~ the scan logic} 20 may then compute from {TAIL ~ the error signal} the current position of the media 12 
compute from
If desired , { HEAD ~ the scan logic } 20 may then compute from { TAIL ~ the error signal } the current position of the media 12
OTH OTH OTH HEAD HEAD HEAD HEAD HEAD HEAD HEAD OTH OTH OTH REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH