Surge XT supports full microtonal retuning via Scala SCL/KBM files with two options for how pitch modulation tracks the tuning

Surge XT’s SCL/KBM mode reads Scala plain-text files to retune all MIDI notes to arbitrary equal temperaments, just intonation systems, or any custom scale. A KBM file specifies how scale degrees map to MIDI note numbers. An important fork in behavior: ‘Apply tuning at MIDI input’ (default) retunes only note pitch while modulation operates in 12-TET cents — so pitch bend of 2 always means 200 cents. ‘Apply tuning after modulation’ retunes the combination of note + modulation, so a pitch bend of 2 lands on the next scale step regardless of unequal interval sizes. This setting is stored per patch because it fundamentally changes how the patch plays in tuned mode. Remap A4 directly lets users set A=432 Hz or other reference frequencies without a KBM file.

Examples

Load a 31-TET SCL for meantone harmony. With ‘Apply tuning after modulation’, a pitch bend of 1 step arrives at the correct 31-TET interval — not a standard 100-cent semitone.

Assessment

A composer loads a just-intonation scale and notices that FM2 intervals sound out of tune relative to the scale. Which tuning application mode should they switch to and why?