For those of you who are working with old 5.25 floppies and trying to build your own data recovery circuits, I loaned Al two old MPI Application notes. He has posted them: http://www.bitsavers.org/pdf/cdc/discs/77653447_5inchPrecomp_Aug83.pdf and http://www.bitsavers.org/pdf/cdc/discs/75897469_5inchFDD_Fmt_Jan80.pdf There's a lot of information here that might be useful in understanding some of the early formats and data recovery techniques. The first one also includes circuits and wave shapes using the WD1691, WD 179X and NEC 765.) Billy