David Dilks wrote [210117 MMDigest]:

> I have a homebuilt fairground organ having 320 pipes and playing
> from the Carl Frei 72-key scale. I am seeking information on the
> availability of a MIDI synthesizer that I can program to replicate
> the sound of my organ to enable me to test run MIDI files in my
> studio before playing them on the organ.

Answering this request in a useful way is a challenge because of what
has not been specified.

It is amazing how well we are able to control mechanical instruments
from MIDI files, but we depend on a lot of hardware and software which
does not conform to the MIDI standards. In fact, our mechanical systems
do not even conform to any single standard, so it is impossible to give
a definitive answer.

If David means a readily purchased piece of hardware -- a synthesizer
meeting the General MIDI Standard -- the answer is there isn't one.
Although both band organs (fairground organs) and MIDI synthesizers
are digital devices, they are based on very different design concepts.

First, they differ in digital data structure. The keyframe on his
organ reads a parallel 72 digit byte. A MIDI device sends, or responds
to, a serial signal of single bytes. Generally the speed at which MIDI
signals are sent allows us to disregard this difference.

Even when note playback is relatively simple, we can not easily
match organ tones. A Standard MIDI "trumpet" doesn't sound exactly
like a brass trumpet, nor like the "trumpet" pipes in a band organ.
The Standard MIDI organ "patch" collection includes several organ
sounds, listed as Hammond, Church, Percussive, and Reed, but it does
not include Pipe Stops. An arranger can use these to listen to where
tonal changes occur, but not to the tonal changes themselves which
will come from the actual organ.

There are also significant differences between synthesizers and band
organs with respect to percussion. Generally, band organs provide
a means to provide a "lead" for the bass drum, and sometimes other
sounds. This may be done in hardware, as on the Wurlitzer tracker
bar, or perhaps in marking the book templates for keyframe organs.

A synthesizer does not provide a lead. A MIDI file may need to include
a lead in the version for organ play, but not in the audition version.

Typically a band organ book or roll puts the bass drum and a cymbal
on a single hole, while in a synthesizer these are separate notes.
A band organ often has provision for a reiterating snare drum, but MIDI
synthesizers require a note on/off for each drum beat. Mapping software
can handle the timing issues easily, but not the reiteration or note
duplication.

The ability to roughly imitate the sound of a band organ through
a MIDI synthesizer is significantly related to the structure of the
MIDI file. For smaller instruments like crank organs with a single
register a pretty good simulation is easily achieved with a single
track containing only note on/off data in one Channel. This process is
also pretty straight forward for an instrument such as a Wurlitzer 125
scale organ, providing each voice is in a separate Channel.

David has not specified the source and structure of the MIDI files
he wishes to test. In order to play on a synthesizer, they must have
each voice in a separate Channel in order to even give a reasonable
audition. Many MIDI band organ files are in a compressed format, the
various AMES scales being an example, in which all data is in one or
two Channels. Mapping software (I use MIDI File Mapper) can be used to
move back and forth between various file structures accurately, so the
test file and instrument driving files do not have to have identical
structures, but each must match the hardware.

I have attached two MIDI files derived from a scan of Wurlitzer 135
Roll 6641 Tune D which illustrate this difference. The first is the
version as I received it, the second is one I created through mapping.
[ Links are at the end of the article. -- Robbie ]

The tune can be heard on the Seabreeze Wurlitzer 135 on YouTube at
https://www.youtube.com/watch?v=4iL0gxknOGE&list=OLAK5uy_lBOwFSxYmQYD-iHgFylVdZWuT5WlQXdIw&index=6 

My "audio" version of Silver Moon does not sound like the organ, but it
gives an arranger a lot of information about the notes and timing. The
Control signals are in Channel 8, with the Control 7 (VolCtl) set to 0.

The MIDI specification does not include any provision for directly
turning "stops" on and off. I think this can be simulated relatively
easily by inserting Velocity (Control 7) changes into Channels at
appropriate points, and that these can probably be mapped back and
forth with stop changes in the appropriate Channels if each stop for
a voice (bass, melody, etc.) is in a separate channel.

David has also not said whether he is an arranger, or simply attempting
to adapt other's work to his organ. Adapting is arranging, and requires
understanding things like key transpositions, and substituting for notes
missing from the organ scale. You don't need a degree in music to do
that, or even to read printed musical scores, but you need at least
basic musical skills.

MIDI files intended for use on synthesizers, including personal
computers, often include a lot of data which can not be duplicated
on a mechanical instrument. These include Note Velocity, and Bend.
In order to approximate how the file will sound on a band organ this
extraneous data must be stripped from the file. Again, MIDI File Mapper
can be used to accomplish that.

As Robbie noted, it was relatively easy to custom build a computer
based synthesizer back in the days when we ran Windows 95, but
Microsoft's "improvements" since then have messed up a lot of our
musical stuff through hardware and software changes related to sound
generation. I note with personal grief that they have also messed up
roll and book template generation by eliminating banner printing.

Construction of a custom synthesizer today might best be attempted with
a bank of Arduinos or other simple cards.

Wallace Venable

 [   W-165 6641-D_Silver_Moon MIDI sample - Ames format
 [ https://www.mmdigest.com/Attachments/21/01/21/210121_173710_6641-D_Silver_Moon-Ames.MID 
 [   W-165 6641-D_Silver_Moon MIDI sample - Audio format
 [ https://www.mmdigest.com/Attachments/21/01/21/210121_173710_6641-D_Silver_Moon-Audio.MID