In FM synthesis, increasing the modulator's amplitude makes the carrier sound brighter

In linear FM synthesis, a carrier oscillator is frequency-modulated by a modulator oscillator. The amplitude of the modulator’s signal determines the depth of modulation — called the modulation index. As the modulator’s amplitude increases, sidebands proliferate further from the carrier frequency, adding upper harmonics and making the sound brighter. As it decreases, sidebands collapse and the sound darkens. This is directly analogous to opening and closing a low-pass filter on an analog synthesizer: increasing modulator amplitude ≈ opening the filter cutoff. The key insight is that FM has no filter; timbral brightness is sculpted entirely through modulation depth.

Examples

On the DX7, raising Op. 2 Output Level from 60 to 99 brightens the timbre; controlling Op. 2’s EG shapes how brightness evolves over time — a slow attack gives the ‘filter opening’ effect.

Assessment

Describe what happens to a two-operator FM sound when the modulator’s output level is swept from 0 to 99. At which output level is the sound purest (sine-like) and at which is it brightest?