A modulator is a file

FaustWave has no "LFO feature". A modulator is an ordinary .dsp document with a bargraph tagged [modout]; while it runs, that output is a modulation source in the Patchbay, and every live parameter of every running instrument, Builder and FX slot is a sink. Wiring is a cable with an amount, like audio is a cable with a gain.

This has three consequences you will feel immediately:

• You can read it. Open Mod LFO Breath.dsp in the demo and the whole modulator is twenty lines of Faust. Change the shape, add a second output, tempo-sync it — it is your code.
• It persists like a file. The modulator is in assets/, its edges are in the project's routing table, the values it swings around are in the project. Reopen the project and the breathing is back.
• It is a source like any other. An envelope follower on the drum bus, a random walk, a macro knob — anything with [modout] can drive anything with a slider.

#The corpus templates

You rarely write one from scratch. modulation.source.add { type } creates a modulator document from a template and runs it:

actions:
  - action_id: modulation.source.add
    input: { type: lfo }
    save_as: lfo
    label: "▶ Create an LFO document and run it"
  - action_id: modulation.source.list
    input: {}
    label: "▶ Every source that is running, with its ports and edge count"

Tempo sync works through two metadata tags a modulator (or any DSP) can carry: a slider tagged [bpm] receives the live transport tempo, a button tagged [transportreset] is pulsed on stop → play so the phase re-aligns with the downbeat. The demo's Mod SH Bell steps on every eighth because of exactly those two lines.

#Writing your own

declare name "Breath";
import("stdfaust.lib");
rate  = hslider("rate[unit:Hz][scale:log]", 0.135, 0.01, 20, 0.001);
depth = hslider("depth", 1, 0, 1, 0.001);
process = attach(0, os.osc(rate) * depth : hbargraph("out[modout]", -1, 1));

Three rules: the output range is what the bargraph declares (-1..1 or 0..1 — the amount maths below uses it); attach(0, …) keeps the DSP audio-silent while forcing the bargraph to compute; the modulator must run (dsp.run) — a source registers only while its patch runs, which is also why routing.list shows modulation sinks only for running instruments.

#Where you see it

The Modulators pane (shipped in Perform) lists every source with its live value and edges. The Inspector colours every modulatable widget: arm a source with modulation.assign.set (or the panel's assign toggle) and a drag on any widget creates the edge and sets its depth — the gesture; the next chapter is the arithmetic behind it.

• FaustWave — Faust DSP § Live params, modulation and observability — the [modout] tag from the instrument's side.
• FaustWave — Patchbay & Routing § Cable kinds — modulation is the third cable kind beside audio and MIDI.