An LFO controlling FM modulator amplitude creates tremolo-like timbral modulation (TM) without a filter

In FM synthesis, connecting the LFO to the amplitude of the modulating operator produces cyclic brightness changes analogous to filter modulation on subtractive synthesizers — called timbral modulation (TM). The LFO cycles between low and high modulation depth, alternately darkening and brightening the sound. LFO waveform choice shapes the character: triangle produces smooth sweeping, square creates hard stuttering. LFO rate controls the speed of the cycle. This technique enables the classic ‘filter wobble’ or ‘wah’ effect with no filter hardware. The DX7 implements this through the Amplitude Modulation Sensitivity (AMS) parameter on each operator.

Examples

TM Demo: LFO controls Op. 2’s amplitude at max AMS sensitivity (setting ‘3’). The sound cycles through dark and bright timbres at the LFO rate, mimicking a slowly swept filter.

Assessment

Describe how to set up LFO-based timbral modulation on a two-operator FM voice and distinguish this from LFO control of pitch (vibrato). Which operator gets the LFO assignment and why?