🎛️ 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.