🎛️ Decap uses a fixed MIDI channel per rank and a fixed MIDI note range per rank.

This is the standard Decap mapping used in modern MIDI conversions:

Ahh — now we’re talking about the musical logic of a Decap organ, not the MIDI‑valve hardware.
This is the fun part, because Decap’s mapping is not like a church organ, not like a Wurlitzer, and not like a standard street organ.
It’s its own ecosystem.

Let me give you the clean, normalized, engineering‑accurate breakdown of how Decap distributes voices into ranks and how those ranks map to MIDI notes.

🎯 First principle:

Decap organs are orchestrions, not pipe organs.

Their “ranks” are not flute/violin/principal like a church organ.
They are instrument families:

• Melody pipes
• Counter‑melody pipes
• Accompaniment pipes
• Bass pipes
• Trombone / Trumpet pipes
• Saxophone pipes
• Clarinet pipes
• Violins (string pipes)
• Accordions
• Drums
• Percussion
• Effects
• Jazz pipes
• Celeste pipes
• Etc.

Each family is a separate rank, with its own compass and its own MIDI mapping.

🎼 How Decap distributes pipe voices into ranks

Decap ranks are built around musical function, not timbre class.

✔ Melody rank

• Usually 37–49 notes
• Bright, penetrating pipes
• Often saxophone or clarinet tone
• Plays the tune

✔ Counter‑melody rank

• 24–37 notes
• Often violin or clarinet tone
• Plays obligato lines

✔ Accompaniment rank

• 24–37 notes
• Softer pipes
• Plays chords and rhythmic patterns

✔ Bass rank

• 12–16 notes
• Big wooden pipes
• Plays bassline

✔ Trombone / Trumpet rank

• 12–24 notes
• Very loud reed pipes
• Used for accents and punch

✔ Celeste / Violin rank

• 24–37 notes
• Beating pairs or string‑like pipes
• Used for lush textures

Each rank has its own compass, wind pressure, voicing, and MIDI channel.

These ranges vary slightly between organs, but the pattern is consistent:

• Higher ranks = higher MIDI channels
• Each rank has its own compass
• Each rank responds only to its assigned channel
• No rank overlaps another unless intentionally voiced that way

🧠 How the mapping is actually implemented

Each rank has:

• A MIDI decoder board
• A set of electromagnets (pallet magnets)
• One magnet per pipe
• A fixed mapping table:

MIDI note 60 → Magnet #1 (Melody C4)
MIDI note 61 → Magnet #2 (Melody C#4)
MIDI note 62 → Magnet #3 (Melody D4)
...

Each rank has its own table.

So the organ doesn’t “decide” which pipe to play.
The rank’s decoder simply fires the magnet assigned to that note.

🎹 Example: Melody rank mapping

If the melody rank has 37 pipes starting at G3 (MIDI 55):

MIDI 55 → Pipe 1 (G3)
MIDI 56 → Pipe 2 (G#3)
MIDI 57 → Pipe 3 (A3)
...
MIDI 91 → Pipe 37 (G6)

If the counter‑melody starts at C4 (MIDI 60):

MIDI 60 → Pipe 1 (C4)
MIDI 61 → Pipe 2 (C#4)
...

Each rank is independent.

🎵 Why Decap does it this way

Because Decap organs are arranged like orchestras, not like organs.

Each rank is a section:

• Melody = lead instrument
• Counter‑melody = obligato
• Accompaniment = harmony
• Bass = bassline
• Trumpet = accents
• Violins = pads
• Drums = rhythm

MIDI simply gives each section its own staff.

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 at C2 (two octaves below middle C), however the following notes are missing and used for other things;

C#2, MIDI 37

D#2, MIDI 39

G#2, MIDI 44

A#2, MIDI 46

The octave below middle C (C3) is used as "accompaniment."  Think left hand stuff when playing the piano.  

Accompaniment pipes are 48 - 61.  That's actually C3 to C#4 with a couple of notes missing:

C#3, MIDI 49

D#3, MIDI 51

The melody pipes start at D4 (right above middle C) up to C6 (the C two octaves above middle C).  That's MIDI 62 - 84, with A#5, 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.