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