Art Reblitz points out a problem with MIDI files of piano rolls
derived from a roll scanner in which the paper speed remains constant
during scanning.  The musical tempo of the resulting MIDI file will
therefore slow towards the end.  Compensation is possible, but it can
be difficult to do this with high accuracy.

This is a fundamental reason I use a [take-up spool] roll reader
to convert piano rolls to MIDI files.  The transport system in the
reader matches that in the note recording systems used by the various
companies, ensuring not only correct tempo, but correct tempo over
the complete roll.  My MIDI files are therefore Ampico ready.

Peter Phillips
Sydney
https://petersmidi.com/