Architecture overview

TŒRN is a single Teensy sketch that behaves like a small OS for a sampler-sequencer: a mode machine drives the UI, a note grid is the musical source of truth, IntervalTimers advance playback, and the Teensy Audio library renders sound.

flowchart TB
  subgraph inputs [Inputs]
    ENC[Encoders + buttons]
    TOUCH[Touch switches]
    MIDI_IN[TRS MIDI]
    SD[Micro SD]
  end

  subgraph core [Core]
    MODE[currentMode]
    GLOB[GLOB / SMP]
    NOTE["note[x][y]"]
    LOOP[loop]
    ISR[playTimer ISR]
  end

  subgraph out [Outputs]
    LED[LED grid: 16×16 or 32×16]
    ENCLED[Encoder RGB]
    AUDIO[Audio graph]
    MIDI_OUT[MIDI out / clock]
  end

  ENC --> LOOP
  TOUCH --> LOOP
  MIDI_IN --> LOOP
  SD --> LOOP
  LOOP --> MODE
  LOOP --> GLOB
  LOOP --> NOTE
  LOOP --> LED
  LOOP --> ENCLED
  ISR --> NOTE
  ISR --> AUDIO
  ISR --> MIDI_OUT
  MODE --> LOOP
  GLOB --> LOOP
  NOTE --> ISR

Mental model in one paragraph

You edit a 2D grid of Note cells (step × pitch/row). Playback walks the X axis on a timer interrupt and triggers sample or synth voices through the Audio graph. The UI never owns music data directly — it mutates note[][], GLOB, and SMP, then redraws LEDs to mirror that state. Modes (DRAW, FILTERMODE, MENU, …) only change which encoder axes mean what and what loop() draws.

Runtime pillars

1. Mode machine

Mode *currentMode points at a struct with name, encoder min/max/pos, and knob colors. Switching modes (switchMode) rebinds encoder hardware to those ranges. See Modes.

2. Pattern + session state

• note[x][y] — packed note events for the loaded pattern (in EXTMEM)
• Device SMP — BPM, samplepack/paths, filter/synth params, mutes, song arrangement
• GlobalVars GLOB — cursor, channel, page, velocity, copy state, …

See Data model.

3. Timing

IntervalTimer playTimer calls playback logic in an ISR so the sequencer keeps running during SD/UI stalls. Heavy work (Serial, I2C, SD, FastLED) is deferred to loop() via flags. See Main loop.

4. Audio graph

Declared in audioinit.h: sample players, synth oscillators, envelopes, bitcrushers, filters, freeverb, mixers → SGTL5000 and USB audio output. Parameters are applied from filter/synth setting arrays. See Audio graph.

Channel map (quick)

Channels	Engine
1–8	Sample voices (AudioPlayArrayResmp)
11	3-voice poly synth
13–14	Mono synths (LFO / arp capable)

Details: Channels.

Where to change things

You want to…	Start in…
Change what a knob does in a screen	Mode defs + handlers in toern.ino, UI in toern_ui.ino / toern_filterUI.ino
Change how notes play	Playback / playNote path in toern.ino
Change sound routing or FX order	audioinit.h + toern_filter.ino / toern_parameters.ino
Change menu pages	toern_menu.ino
Change MIDI / clock	toern_midi.ino
Change sample load/browse	toern_sample.ino + toern_helpers.ino
Change LED drawing primitives	toern_ui.ino / toern_leds.ino