20. The apparatus of claim 19 , wherein a sequential collection of LD and LD/OCR-Key trellis step errors comprises an LD and an LD/OCR-Key critical path, wherein the critical path is a minimum error path.
21. The apparatus of claim 20 , wherein the LD and the LD/OCR-Key critical paths associated with the LD trellis and the LD/OCR-Key trellis are constructed concurrently.