spectral-halo

spectral-halo surrounds every note you play with the upper partials of its harmonic series, each one exactly in tune.

This effect needs an MPE-style setup on the receiving instrument; see Microtonality before wiring it in.

This is the instrumental synthesis of Grisey’s Partiels, where an orchestra plays the analyzed spectrum of a low trombone E: one note fanned out into the overtones that make up its timbre. Here the played note passes through dry as the fundamental, and partials 2 through partials are added above it, each on its own member channel. Partial k sits 12 * stretch * log2(k) semitones above the fundamental; the nearest key takes the note-on and the remainder rides as a pitch bend in cents, so the partials land on the harmonic series rather than on the equal-tempered keys nearest to it. At stretch=1.0 the offsets are:

Partial	Key offset	Residual detune
2	+12	0.00 cents
3	+19	+1.96 cents
4	+24	0.00 cents
5	+28	-13.69 cents
6	+31	+1.96 cents
7	+34	-31.17 cents
8	+36	0.00 cents

stretch is an inharmonicity control: 1.0 is the natural series, values above 1 stretch the partials apart the way stiff piano strings do, and values below 1 compress them toward the fundamental. Velocity rolls off geometrically, partial k playing at round(vel * rolloff^(k-1)), floored at 1, and a partial whose key would leave the keyboard is skipped. The mapping is deterministic, no seed.

Your note-off releases the fundamental and every partial it spawned, and a retrigger or scene switch cuts them first and resets the bends; if very dense playing forces the voice pool to steal, the stolen note’s later note-off is silently ignored rather than cutting the newcomer.

Parameters

Parameter	Type	Default	Range
partials	integer	4	2..=8
rolloff	number	0.7	0..=1
stretch	number	1.0	0.5..=2.0
channels	channel span	"2-16"	"1".."16", low end first
bend-range	semitones	48	1..=96

channels and bend-range are the MPE tail shared by all four microtonal effects; the Microtonality guide explains how to set them.

Example

The spectral scene from examples/microtonal.kdl, every note blooming into its first five overtones under a Feldman hush:

miditool.kdl

input "Roland"
output virtual="miditool Microtonal"

scene "spectral" {
    feldman-field seed=5 floor=8 ceiling=26 jitter=3
    spectral-halo partials=5 rolloff=0.6 stretch=1.0 channels="2-16" bend-range=48
}

Play a low note and hold it: the octave, the pure twelfth, the just major seventeenth all stack above it, each a little quieter than the last, and single notes start sounding like chords of nature.

Try this

Detune the series itself:

spectral-halo partials=6 rolloff=0.5 stretch=1.08

A slight over-stretch pushes every partial sharp of the true series, the sound of an upright piano’s inharmonic strings. Sweep stretch down toward 0.85 while playing and the halo squeezes into a dense, dark cluster around the octave.