{HEAD ~ A second coding device} is located in {TAIL ~ the write path} and removes runs of zeros in interleaved subsequences in the scrambled user data 
is located in
{ HEAD ~ A second coding device } is located in { TAIL ~ the write path } and removes runs of zeros in interleaved subsequences in the scrambled user data
HEAD HEAD HEAD HEAD HEAD HEAD HEAD HEAD REL REL REL TAIL TAIL TAIL TAIL TAIL TAIL TAIL OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH OTH