Albert,

The term "phase locked loop" is a common term in the branch of
engineering called "control systems".  Electrical Engineers use this
term with the assumption that _EVERYONE_ knows what it means.
Oops... (Robbie and I are both EE's).

A "phase locked loop" is a control system which controls the rotation
of an object my comparing its rotational position (phase) with another
rotating object.  In Electrical Engineering this can be abstracted to
the "rotation" of a sine wave or other repeating signal but it has
meaning in mechanical systems as well.  This type of control system
can synchronize not only the speed, but the angular position of two
shafts which do not derive their power from the same source.

In Wayne Stahnke's scanner, the locations of hole "centers" are
measured by the scanner and the compared to a mathematical model of
the rotational position of the shaft in the original perforator.
Using a "phase locked loop" between the hole centers and the
mathematical model of the perforator shaft position, its actually
possible to determine where the hole centers "should have been" when
the roll being scanned has small but important copying imperfections.

This technique only works on rolls who's masters were read and
perforated synchronously in the first place, but AMPICO rolls fall
into this category and Wayne's program works really well at detecting
and correcting defects.

Jody