Self-playing mechanical instruments were controlled by holes in paper or pins on a cylinder (like your wind-up music box), so each vertical column represented one note or action that the instrument would take. The layout of these vertical columns is called a "scale."
To save space and reduce size, most of these instruments would omit some notes, especially in the lower registers, and use those columns for something else. Some carousel and band organs only had four bass pipes! The arranger would compensate by transposing into a key that used those notes. In the early days of converting from paper to MIDI, these columns were simply translated to MIDI notes, which resulted in MIDI mappings like the one my machine uses.
Going forward, I plan to use MIDI channels to make it a lot easier to arrange music. Here's what I'm currently using:
My machine reads MIDI type 0, so everything is on one track.
Everything is also on one channel.
Its range is MIDI 36 to MIDI 103, divided into four sections plus percussion and functions.
I have 8 bass pipes from MIDI 36 - 47. That's the range staring two octaves below middle C, however the following notes are missing and used for other things;
C#, MIDI 37
D#, MIDI 39
G#, MIDI 44
A#, MIDI 46
The octave below middle C is used as "accompaniment." Think left hand stuff when playing the piano.
Accompaniment pipes are 48 - 61. That's actually C to D# with a couple of notes missing:
C#, MIDI 49
D#, MIDI 51
The melody pipes start at D right above middle C up to the C two octaves above middle C. That's MIDI 62 - 84, with A#, MIDI 82, missing.
There's one more rank of pipes -- the Counter-melody pipes go from MIDI 90 to 103, F# to G an octave above, but they actually sound two octaves lower so they're about in the same range as the melody pipes. There's two rows of melody pipes, "slightly" off tune to create a "chorus" effect.
There's also an "octave" rank, which plays with the melody pipes when it's enabled. The octave rank makes the notes way louder.
Countermelody pipes are 90 - 103 but they sound 2 octaves lower. These countermelody pipes are also very different from the melody pipes, so they add another "voice."
The percussion uses some of the stolen notes:
C#, MIDI 37 is the bass drum
D# MIDI 51 is the cowbell
The snare uses 82 and 85. That's how it can play so fast.
The functions steal four other notes and they only affect notes in the melody range. Playing the function notes toggles them on, so I usually just use an 1/8 note to trigger a function:
39 turns on the glockenspiel so the glock plays the same notes as the melody pipes.
44 mutes one row of melody pipes, disabling the "chorus" effect.
49 enables the octave pipes, which play the same notes as the melody but an octave higher.
46 resets all the functions.
Each function resets all the others, so if you want two functions active at the same time you have to play their notes together. Make sense?
I forgot to mention -- the whole thing is tuned a minor third down, so MIDI note 60, which is middle C, sounds at the A just below middle C.
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