mode-lock

mode-lock forces every note onto one of Messiaen’s modes of limited transposition.

These are the seven scales Messiaen catalogued as modes of limited transposition: pitch-class sets so symmetrical that shifting them up a few semitones reproduces the same set, which is why each mode has only a handful of distinct transpositions rather than twelve. Mode 1 is the whole-tone scale of Voiles; mode 2 is the octatonic scale jazz players call half-whole diminished. Notes already in the (transposed) mode pass untouched; everything else snaps per snap.

The mapping is deterministic, no seed. Note-offs map the same way as their note-ons, so held notes always release correctly; a dropped note-on takes its note-off with it.

The seven modes

Pitch classes are given at transposition=0, with 0 as C.

Mode	Pitch classes	Shape
1	0 2 4 6 8 10	whole tone
2	0 1 3 4 6 7 9 10	octatonic (half-whole diminished)
3	0 2 3 4 6 7 8 10 11	tone plus two semitones, three times
4	0 1 2 5 6 7 8 11	two semitones, a minor third, a semitone, twice
5	0 1 5 6 7 11	semitone, major third, semitone, twice
6	0 2 4 5 6 8 10 11	two whole tones and two semitones, twice
7	0 1 2 3 5 6 7 8 9 11	three semitones, a whole tone, a semitone, twice

transposition rotates the whole set upward by that many semitones. Because the modes are symmetrical, only a few values sound distinct (2 for mode 1, 3 for mode 2, 4 for mode 3, 6 for the rest); higher values simply land on a set the mode already produced.

Parameters

Parameter	Type	Default	Range
mode	integer	required	1..=7
transposition	integer	0	0..=11 semitones
snap	string	"nearest"	"nearest", "up", "down", "drop"

The snap semantics are the same as sieve’s: "nearest" breaks ties downward, and since every mode repeats within the octave it always finds a member. "up" and "down" drop the note only past the last member at the very top or bottom of the keyboard. "drop" passes members and silences the rest.

Example

miditool.kdl

input "Roland"
output virtual="miditool Out"

mode-lock mode=2 transposition=0 snap="nearest"

Everything you play lands on the octatonic scale on C; runs come out with that even, rootless Messiaen glitter no matter what your fingers meant.

Try this

Thin the keyboard to mode 5, the sparsest of the seven, and keep only what is already there:

mode-lock mode=5 snap="drop"

Only six pitch classes survive per octave, so most keys go silent and chords arrive with holes in them. Then switch to snap="up" and the dead keys climb to the nearest member above instead, turning clusters into tight mode-5 voicings.