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