Ingmar Krause summarized some queries and gave answers.  Here are my
few additional comments:

>  [ John Farmer wrote: ]
>
> > With some tunes I have found it impossible to convert them (to
> > a 20-note scale), because some vital notes were missing, and
> > the tune just wouldn't sound right if you left them out.
>
> I'd be very happy if you and everybody else in MMD would send me
> a list with their so called "impossible" tunes.  I'm always up for
> a challenge!

I'd love to do the same!  However, right now, my answer time is quite
long (weeks or months) because of the extreme workload in my job.

[...]
>  [ Mike Knudsen wrote:
>
> > You need to tell your program what notes your organ can play, and
> > find out how to make the program check your arrangement for illegal
> > (un-equipped) notes that your organ doesn't have.
[...]
> > But first, figure out what musical key(s) your organ plays best in
> > (depending on which "black" notes it has), and its range from high to
> > low.  Then transpose the given MIDI piece into those keys and see how
> > they fit the range, especially the melody.  Also beware of chords
> > that your bass section can't support well.
>
> As already was said: this can be done by marking the whole track.
>
> If you have problems with finding how many semitones you need to move
> the tune up or down, you can also just "grab" the whole marked tune and
> push it up or down (but beware: it might also move ahead or back, so
> keep that mouse movement straight!)
>
> > Finally, move out-of-range and un-equipped notes to notes your organ
> > has.
>

... and driven by a later remark by Ingmar Krause, namely:

> There is already enough "wrongly" arranged music out there

I find all of that a little too "mechanical"; arranging is _not_
a "check, look, and move" operation, but a musical operation.  When
and whether a chord sounds right/full/wrong/thin/meek, is a complex
thing and best decided by a (trained? able? no idea what ...) ear.

A very good test, in my opinion, is to have somebody play standard
(classically arranged) tunes on a piano with slight errors.  You should
be able to spot the errors without knowing the piece (intimately) and
of course without having a score.  Next, you should know that arranging
is mainly creating "melody lines", also in bass and intermediate
voices.  Then, you should know the standard chords and how they map to
your scale.  Equipped with that, you can, I think, quite easily write
(re-)arrangements.

However, I found that even with all that knowledge, I will create not
so good arrangements at first.  Then I will learn my arrangement by
heart -- each note in it, and under the shower, on the train to work,
in bed before dozing away, and everywhere else -- I'll play the piece
in my head and listen to it and find out which notes I do not like and
try re-arranging them.

I found out that replaying them with 'Ocarina' [voice] on a MIDI
player, or with my MIDI-controlled organ, does not improve them.  On
the contrary, I get too used to the "beautiful sound" and like
everything I wrote ...

[...]
> > Make sure any repeated notes have enough silence between them so your
> > organ valves can close and re-open.  When in doubt, shorten or delete
> > notes -- simple is better.
>
> Also you might have to check if notes are "double": underneath a bar
> there might just be sitting another bar!  Best way to check on this is,
> as has already been mentioned, the Event List.

Because of many such problems, I wrote a program for myself which does
all the checking and shortening/lengthening.  (The program is integrated
into a puncher-driving program, but I can also use it stand-alone to
check my arrangements.)  However, I still find that a "legato" as
created by my program does not sound completely similar in upper and
lower ranges of the scales, so when (if) I have time, I have to fiddle
with the "mathematical heuristics" in the program.

Regards

Harald M. Mueller
Grafing b. Muenchen
Germany