Oops! In my previous posting [000426 MMD], I forgot to mention where
most of us would find the logic "gates" that are used to cut off
tracker bar signals from reaching the pouches. Such a cutoff block is
used in Duo-Arts to prevent the top and bottom four notes from
activating the expression pneumatics when playing a non-Duo-Art roll.
I still have my cutoff block in my piano bench (I didn't have thin
enough pouch leather at the time to rebuild it). Pre-1925 Duo-Arts
would have a second such block, to keep Duo-Art expression codes from
playing the bottom and top notes, but after 1925 the outermost eight
stack pneumatics were simply omitted.
These gates are very simple. The tracker tube to be controlled is cut
and the two ends are fed into a pair of holes in a flat surface of
wood, close together, with a pouch membrane glued around and over the
holes. The other side of the pouch is the control chamber. If that
chamber has stack vacuum, the pouch is pulled away from the two holes
and tracker signals can pass normally. But if atmosphere is admitted
to the chamber, the pouch is pressed against the holes and the passage
is cut off. At least one of the holes will contain vacuum, so the
pouch is plastered against it.
One chamber can control the eight gates of the Duo-Art, or dozens of
them in a tubular pneumatic organ or Orchestrelle (if the Orchestrelle
uses this). The scheme works just as well for pressure systems, except
that pressurizing the chamber cuts off the gates, while atmosphere
opens them. No moving parts except the little leather pouches. I'd
assume the shiny side of the leather should bear against the holes.