Per Robbie's inquiry, I'll repeat what I posted some time ago, based
on a little book entitled "Piano Playing Mechanisms" or something like
that, by Dr. William Braid White.  Maybe the White of Wilcox & White?

This book was reprinted by Vestal Press back in the early 1970s, which
is where I got it.  Originally written in the 20s, at the height of
player pianodom, it gives a thorough description of every system known
in the USA at least.

The Angelus system appears to be based on the Duo-Art, but potentially
superior.  Its rolls use the same snake-bite Theme holes and changeover
valves.  But instead of allocating four bits each to theme and
accompaniment, Angelus gave five bits to Theme and only three to
Accompaniment.

However, the accompaniment regulator was in series with the theme
regulator, i.e., it gave a percentage of the current Theme suction, so
in effect it had 8 bits (256 levels) available.  In practice, it could
just ride along a few degrees of loudness behind the Theme, swelling
and diminishing along with the theme.

The regulators were simple and very fast.  They worked by establishing
a reference version of the desired suction in a small chamber, where-
upon the large regulator pneumatic would then hold the main stack
suction to that same pressure.  The expression holes in the tracker bar
opened different sized bleeds into the reference chamber, to set its
suction.

Any leakage in the reference chamber would certainly upset the relation
of suction to roll perforations.  Porosity in the regulator pneumatics'
cloth would do that.  However, stack leakage should not have been a
problem, since the regulator would just open a little wider.

Note that I'm reading a lot into Mr. White's diagrams; he doesn't
actually say any of the above in his text.  But in trying to figure out
how the Angelus worked at all, by studying the drawings, I came to
these conclusions back in the early 70s.  I also once owned a dozen
Angelus rolls, which were great to play using my Duo-Art's theme
system, just like Themodist rolls.  I later traded them off.

I'll dig out the little book in anticipation of questions, but I'll
be away from my computer for a couple of weeks.

Mike Knudsen

 [ Thanks for the reply, Mike.  Dr. White's books are considered
 [ tantalizing but short on details.  The Themodist/Solodant switching
 [ scheme is not a patent issue, and the appearance of the music roll
 [ is incidental.  The key questions: (1) What is so unique about the
 [ system that Wilcox & White did not have to license the Welte patent
 [ like everyone else did?  (2) Why didn't the regulator pneumatic
 [ which you describe infringe upon the Welte patent?  (3) How did the
 [ control system in the production units actually work?  -- Robbie