Enveloping the Modulator's amplitude brightens an FM tone over time, replacing the subtractive filter sweep

In subtractive synthesis a contour generator sweeps a filter’s cutoff to make a tone brighter or darker over time. In FM you get the equivalent by enveloping the Modulator’s amplitude: since (for a fixed modulator frequency) the modulator amplitude sets ß and thus the bandwidth of the output, driving the modulator level with an envelope makes the spectrum expand and the tone brighten with no filter at all. The article’s seven-module patch shows this — as one envelope fades the output volume, another raises the modulator amplitude so the tone brightens as it decays, the opposite of most natural sounds. Crucially, a low-pass VCF sweep cannot reproduce the complex spectral reshaping FM produces, only a simple cutoff move.

Examples

Patch: EG on the modulator’s VCA raises ß during the note so brightness evolves; a separate amplitude EG shapes loudness. A DX7 electric piano gets its brightness-decay from an envelope on the modulating operator’s level.

Assessment

Explain why enveloping the modulator amplitude gives a filter-sweep-like brightness change, and name one tonal change FM produces this way that a plain low-pass VCF cannot.