FM brass timbres use c/m = 1/1 with index tracking amplitude, capturing the loudness-to-brightness coupling of brass instruments

Risset’s analysis of trumpet tones identified that brass instruments distribute energy over a wider spectral band as they grow louder — a loudness-to-brightness coupling. Chowning simulates this by coupling the FM modulation index envelope to the amplitude envelope: when amplitude is highest, index is highest (most sidebands, widest bandwidth); when amplitude decays, index decays in tandem (simpler spectrum). The carrier-to-modulator ratio c/m = 1/1 ensures all harmonics are present (odd and even), matching the full harmonic series of brass. The parameter set is minimal: carrier frequency equals modulating frequency, I sweeps from 0 to about 5 over the note duration. A rapid attack with slight overshoot is also characteristic.

Examples

Chowning’s brass patch: P5=P6=440Hz (c/m=1/1), I1=0, I2=5, envelope function rises fast and decays. The result is a recognizable trumpet-like timbre from just two oscillators and an envelope generator.

Assessment

Modify the FM brass patch to produce a softer horn-like timbre by reducing I2. What perceptual change do you expect and why?