Here is another technical question for you player piano experts and
designers of roll readers:

As the paper hole moves over the tracker bar hole, the aperture increases
from zero to its full size, and then decreases back to zero.  At what
size aperture does the valve open, and then at what size aperture does it
close?  In other words, the note is not as long as the hole!

The above question came up as I was thinking about optical roll readers:
do roll readers correct for this, either in hardware or in software?
In an infrared photo-transistor system, the accuracy of the hole edges
depends on the whole light path, from the type, shape and position of the
light source to the aperture of the photo-transistors...

Laurent Coray