The TG43 signal has NOTHING to do with pre-compensation. The original
IBM specification for single density (128 byte per sector, 26 sector
track, 77 tracks) floppies has an option to change the write current on
the drive head for the more densely packed sectors on the interior of
the disk. because of the constant angular velocity (360 rpm to be
exact) the interior tracks were more dense than the outer ones. To
prevent the bits from blasting into each other they (IBM) reduced the
write current on the inside (greater than 43, thus TG43) of the disk.
This was purely a drive option. Some later drives had more smarts and
could count track position on their own, and didn't need the signal.
Precompensation was NOT used on single density 8 inch drives. When
things changed to double density drives (MFM encoding) precompensation
was necessary when too many transitions were next to each other. The
floppy formatter chip usually did this internally, and since it had the
track register, it could do it "automagically". Some did it
differently, but it basically shifted the bits a bit to one side or the
other to "compensate" for bit crowding.
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