A tape drive system which includes the ability to reliably reproduce LPOS
information even if both of the servo channels of the tape drive system
could not detect the LPOS information due to defects. The system
recognizes that an LPOS sequence is sequentially incremented by one if a
tape is moving in a forward direction and decremented by one if the tape
is moving in a backward direction. This sequential property of the LPOS
information lends itself to LPOS prediction, where the LPOS prediction is
the previous LPOS value plus one if the tape is moving forward and the
previous LPOS value minus one if the tape is moving backward. Even if the
LPOS words from the two servo channels are both bad, but not all of the
bits in the LPOS word are bad. The system uses the remaining good bits
from the two servo channels to verify all of the bits of the LPOS
prediction word.