Probably things like the following have been done by the gurus on
this list too numerous times to report them, but when the (borrowed)
20er Drehorgel in my basement played my own arrangement directly from
my laptop on (almost) the first attempt, I felt somewhat proud.  After
all, this is the first time I have produced an "air-conducting
element"!

Can I tell you a little bit of what I did?  Actually, it is quite
simple:

1. The laptop feeds, from its serial port (COM1 on Windows NT), a
Midiman Biport 2x4, which converts its input to real MIDI on a MIDI
cable.

2. The Biport feeds its MIDI signal to an MTP-3 electronics board from
John Wale.

3. 20 of the MTP-3's 28 note outputs are connected via a long cable
to 20 organ valves (Peterson 90 ohm, 5/8" pallets; I found no cheaper
ones :-)  that open channels on a small wooden board.

4. Internally, the channels go to one end of the wooden board, where
they exit at a distance of 3.8 millimeters, the distance of the holes
on the 20er's tracker bar.

When a valve opens, it is exactly as if a hole appeared in the paper
strip: the air escapes, and the pipe sounds.

The wooden board with the valves sits on top of the tracker bar, no
change to the organ was needed.  The weight of the board plus valves
is enough to keep the connection air-tight (aided by a piece of a some
tight fabric I found in my workshop; leather would probably be better).

There were only two problems I encountered:

1. At first, I had the wires reversed in my home-made MIDI cable;
no reaction by the MTP-3.  It took me 2 hours to find that.  ;-)

2. Then, I had to open two of the bleed holes in the organ a little
bit.  The added channel length needed only so much more air to refill
that those two notes' repetition was no longer quick enough.

However, one other thing I had to create is a program that reads the
MIDI file output from the notation program (I use Noteworthy Composer)
and makes it "paper-strip like":

* The distance between two succeeding notes must be enlarged to at
least "one hole distance".  (Noteworthy inserts only two ticks between
two succeeding notes; this is about 1/100 of a second!  Neither the
valves nor the organ will react to such a short interruption, of
course.)

* Very short notes must be extended to "one hole's length".

I wrote the program in Java because, first of all, in my "other job"
I'm a professional software engineer working almost exclusively with
Java; and additionally, Java (in its version 1.3) has a complete MIDI
library which makes reading, handling, and writing MIDI messages very
simple.  And then I concatenated all the arrangements I had done last
year (without an organ ...) into one large file and listened to them.
I like them.  :-)

Oh yes: why did I do all that?

Well, my arranging is not yet world-class, so to speak, and wasting
a punched paper for every variation is out of the question, money-wise
and time-wise.  So I plan to hone the arrangements via the computer
set-up, but to have them punched in good old paper when I'm finally
satisfied!  And the same MIDIfication will therefore also go into the
26-scale organ that will come from Jaeger and Brommer sometime this
year...

The funny thing when playing directly from a computer is that you
cannot influence the tempo -- you can even stop turning for a short
moment.  You lose your ability to turn steadily very quickly,
therefore, I do not recommend this setup as the default.

The "professional" method is to measure the speed of the crank and play
the MIDI notes accordingly but, as I need the computer connection only
for my own "arrangement training", I'll not add this twist.  It would
mean (a) changing the organ; (b) building hardware and software that
sends the tempo information to the computer; (c) writing a sequencer of
my own that outputs the MIDI notes at a variable speed according to the
tempo information.  Others have done that, much more professionally than
I can do it.

Also, I found that I had to turn at a speed equivalent to a paper
speed of about 80 mm/s if I wanted to produce enough air to play my
arrangements (which frequently have 6- or 7-note chords).  Is that
normal or is the organ leaky or has a too small pump?

I thought that the "norm" for a 20er is 60 mm/s, or are my arrangements
too "demanding" (in terms of air)?

Regards

Harald M. Mueller
Grafing bei Muenchen
Germany