One of our members asked me a question in a private email but I don't
really have a good answer so I am posting this in case anyone else
can help. I think this is a useful question too and by posting these
discussions they remain available for the next person who might
wonder about the subject. So here is part of my original post, the
question, and my stab at an answer.

Jim Henry

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I wrote:

The sound card is precisely the
MIDI device that doesn't really fit the way a pipe organ is
constructed. To be able to add and subtract stops at will you need
to put each stop on a seperate MIDI channel and use software to
translate the key and stop MIDI messages into appropriate MIDI note
messages on appropriate channels to send to the sound card. Let me
know if would like to discuss this in more detail. I am guessing
that much if not all of this is something you have already considered.

Jim Henry

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I was asked:

Hi Jim,

I hope you don't mind if I ask a question. For a long time
I have wanted to do this very thing, and have not figured out how to
do it. What kind of midi message will turn off a channel and what
will turn it on. I am sure there is a way.

======

A MIDI Channel is nothing more than an address embedded in the MIDI
message. 16 MIDI Channels are carried in the same wires.

At the receiving end I don't know of a MIDI command to say something
like ignore all MIDI messages on this MIDI Channel. You could set
the channel volume to zero but that may or may not do what you want.

Otherwise you need to stop the sender from sending MIDI message to
the channels that are "off" or put a filter in between the sender and
the receiver that only passes on the messages for channels that
are "on." If at least one of the sender or receiver is in a PC, I
think MIDI-OX could be used as a filter and I think you can send MIDI
messages that MIDI-OX will recognize to change the filtering.

If MIDI-OX doesn't do that, it would be pretty easy to write a MIDI
filter program that used the channel volume message, or some other
message, to turn channels on and off.

This doesn't really come up in trying to do a theatre organ because
you have many stops controlling a single rank (MIDI channel). You
can't simply turn off the channel. A real theatre organ uses a
rather elaborate relay to receive all the key inputs and stop inputs
and produce the pipe outputs. A MIDI theatre organ needs a
corresponding MIDI relay which is further complicated because MIDI
messages just mark the start and end of events while a real organ
provides signals that maintain a continuous state. The MIDI relay
has to maintain continuous state based on the messages it receives.

As an example, when Solo Channel Note On C4 is received, the relay
has to remember that the Solo C4 key is now on so that if Solo
Channel Stop On Diapason is received, the MIDI Relay will generate
the output MIDI message of Diapason Channel Note On C4 to add the
Diapason sound to the C4 notes that are already sounding. Likewise,
the state of all the stops has to be remembered so that the MIDI
Relay knows what channels to send note events to as it receives the
key inputs. On top of all this, the Relay needs to keep track of how
many thing are keeping the "pipe" on so that it doesn't send out a
Note Off message until everything that could have sent a Note On
message provides the corresponding Note Off.

Hope there is a useful answer somewhere in all of that ;-)

Jim Henry