scaffold-control-source

name	scaffold-control-source
description	Scaffold a nanoTracker control-source plugin — a MIDI emitter triggered by tracker rows. Step sequencers, virtual keyboards, drum machines, arpeggiators, chord triggers, LFO-to-CC. Plugins with manifest.type "control-source" emit TrackerMidiEvents over cables instead of producing audio.

Scaffold a nanoTracker control-source plugin

When to use

Use this when the user wants to author a plugin whose primary output is MIDI events, not audio:

• Step sequencers (16-step × N-lane grids)
• Virtual keyboards (on-screen keys → MIDI notes)
• Drum machines (per-track step patterns → channel-routed notes)
• Arpeggiators / chord triggers / scale quantisers (transform incoming MIDI and emit downstream)
• Euclidean sequencers, LFO-to-CC emitters, random generators

Do NOT use this for audio instruments (use scaffold-instrument — those respond to noteOn and produce audio). Do NOT use this for audio effects (use scaffold-pedal).

If the plugin transforms incoming MIDI (arp, chord trigger) AND also plays audio, prefer scaffold-instrument with the midi-thru-custom capability — instruments can own their thru emission too.

Procedure

1. Confirm the shape with the user

Control-source plugins:

• Get an implicit midi-out port (no manifest work needed).
• Get NO implicit midi-in or midi-thru — declare one explicitly if the plugin needs to receive cabled MIDI.
• Do NOT need an audio output (engine.output falls back to a silent GainNode).
• Are triggered by tracker rows via the voice engine's noteOn / noteOff — the plugin converts those triggers into whatever MIDI it wants to emit.

Ask the user:

• "How is the plugin clocked? Off its own noteOn triggers, the transport clock (via consumes-song-position), or a cabled MIDI input?"
• "What MIDI events does it emit? Just notes, or also CC / pitch bend / clock / transport?"
• "Does it have a webview UI (step grid, XY pad) or native controls (knobs, sliders)?"

2. Start from the template

cp -r templates/control-source my-ctrl-src

The template is a minimal 8th-note MIDI pulse generator clocked by the host transport. Replace the worklet's scheduling logic with whatever pattern / arp / keyboard / chord behaviour the plugin needs; the songPosition intake and midiOut emission stay the same.

3. Manifest essentials

{
  "schemaVersion": 4,
  "manifest": {
    "name":    "STEP SEQ 16",
    "version": "1.0.0",
    "type":    "control-source",
    "description": "16-step × 4-lane clock-driven sequencer"
  },
  "requires": [
    "portsV4",
    "worklet-v3",               // if you have a worklet processor
    "consumes-song-position"    // if you want host songPosition messages
  ],
  "parameters": [
    { "key": "gate_length", "label": "GATE", "min": 0.05, "max": 1.0, "default": 0.5, "step": 0.01 },
    { "key": "step_count",  "label": "STEPS","min": 1,    "max": 16,  "default": 16,  "step": 1    }
  ],
  "ui": {
    "layout":   "flex",
    "controls": [
      { "type": "knob",  "parameter": "gate_length", "label": "GATE" },
      { "type": "knob",  "parameter": "step_count",  "label": "STEPS" }
    ]
  },
  "dsp": {
    "voices": 1,
    "voiceStealing": "none",
    "worklet": {
      "processorName": "com.example.step-seq",
      "voiceParams":   ["gate_length", "step_count"]
    }
  }
}

Key points:

• No ports block needed if the implicit midi-out is your only output. Declare one if you want a different id / label or additional ports (e.g. a midi-in for live-play override).
• voices: 1 and voiceStealing: "none" is the usual shape. Control-source plugins don't have per-voice state; the single "voice" is really the plugin's event emission engine.
• consumes-song-position opts into host songPosition messages on the worklet port — essential for clock-driven sequencers.

4. Worklet emission

Control-source worklets post midiOut messages. The host picks up the event and dispatches through the MIDI bus:

class StepSeqProcessor extends AudioWorkletProcessor {
  constructor() {
    super();
    this._steps   = [60, 64, 67, 72, 60, 64, 67, 72,
                     65, 69, 72, 77, 65, 69, 72, 77];
    this._stepIdx = 0;
    this._gate    = 0.5;
    this._stepN   = 16;
    this._nextId  = 1_000_000;

    this.port.onmessage = (e) => {
      const m = e.data;
      if (m.type === "param") {
        if (m.key === "gate_length") this._gate  = m.value;
        if (m.key === "step_count")  this._stepN = Math.max(1, Math.min(16, m.value | 0));
      }
      if (m.type === "songPosition") {
        // 24 PPQN → 16th note = every 6 ticks.
        if (m.position.ppq24Counter % 6 === 0) {
          this.fireStep(m.position.audioTime);
        }
      }
    };
  }

  fireStep(time) {
    const note = this._steps[this._stepIdx % this._stepN];
    const id = this._nextId++;
    this.port.postMessage({
      type:  "midiOut",
      event: { kind: "noteOn", note, velocity: 100, channel: 0, time, id },
    });
    // Schedule the release at the gate length (quarter note * gate).
    this.port.postMessage({
      type:  "midiOut",
      event: { kind: "noteOff", note, channel: 0, time: time + this._gate * 0.25, id },
    });
    this._stepIdx = (this._stepIdx + 1) % this._stepN;
  }

  process() { return true; }
}

registerProcessor("com.example.step-seq", StepSeqProcessor);

The id field is essential — noteOff matches noteOn by id, not by (note, channel). Maintain a monotonic counter in your processor.

5. Tracker-row triggering (optional)

If you want the plugin to ALSO react to tracker row notes (so the user can trigger the sequencer from the pattern), implement the voice engine's noteOn / noteOff as normal. The tracker row's noteOn(note, velocity, time) can be mapped to "start the sequence on this root", "restart from step 0", or whatever the plugin's semantic is.

6. Validate and pack

node tools/ntvalidate.mjs my-step-seq
node tools/ntpack.mjs    my-step-seq --out my-step-seq.ntins

Common gotchas

"Nothing plays when I cable midi-out to a synth."

• Check you're posting midiOut messages from the worklet.
• Check the destination's midi-in is actually connected (drag a cable in the workspace).
• Open the browser console; un-dispatched events often log warnings about missing endpoints.

"My step sequencer drifts out of time."

• Your clocking is likely in setTimeout-land. Use the worklet's songPosition messages (opt in with consumes-song-position) or cable __clock-source.clock-out into your midi-in.

"Tracker row triggers produce no audio."

Correct — control-source plugins don't emit audio by default. If you want preview audio (a metronome click or a keyboard feedback), declare an audio output in ports.outputs[]:

"ports": {
  "outputs": [
    { "id": "midi-out", "label": "MIDI OUT", "kind": "midi" },
    { "id": "preview",  "label": "PREVIEW",  "kind": "audio" }
  ]
}

Once declared, the host creates a per-audio-output gain and routes through the normal master mix.

"My noteOff doesn't release on the receiver."

You're probably allocating a fresh id for the noteOff. Reuse the same id you used on the noteOn. The bus matches by id; a mismatched id falls back to (note, channel) which breaks under transposition / chord expansion.

See also

• docs/23-control-source-plugins.md — full reference
• docs/22-midi-ports.md — MIDI cable layer
• docs/08-worklet-v3.md — worklet contract
• examples/step-sequencer/ — canonical reference plugin