op1-lfo-hero
Custom MIDI LFOs (low frequency oscillators) for the Teenage Engineering OP-1 Field synthesizer. Controls per-track volume, pan, mute, FX, LFO parameters, and master FX/compressor parameters with beat-synchronized automation curves. Handles these OP-1 tempo modes:
BEAT MATCH aka op1 (ext clock) -> op1 sends clock to app: used to sync the LFOsMIDI SYNC aka app (int clock) -> app sends clock to op1: generates tempo, transport, and tape navigation commands
Running the App
Requirements:
- Teenage Engineering OP-1 Field
- USB-C cable or Bluetooth
- Python 3.11 or newer
- macOS (tested) or Windows/Linux (untested but should work)
With the OP-1 connected and the virtual environment activated:
source venv/bin/activate
python -m src.app
The app auto-detects the OP-1 by port name (looks for "op-1" case-insensitively). If exactly one OP-1 port is found it is selected automatically. If multiple OP-1 ports are detected (e.g. USB and Bluetooth both connected) you will be prompted to choose from the full port list.
On startup the terminal also prints a remote control URL:
Remote control: http://192.168.x.x:8765/
Open that address in Safari on your iPhone (same WiFi network required) to get a mobile control surface — BPM, transport, per-track volume/pan/mute, and LFO controls. No app install needed.
Bluetooth MIDI
Bluetooth LE MIDI is supported alongside USB. When the connected port name contains "Bluetooth" the status bar appends (bt) with a tooltip noting the ~15ms additional latency. BPM smoothing doubles automatically to absorb BLE jitter. Note that Tempo LFO requires the app to be clock master and is unavailable over Bluetooth.
Command-line flags
| Flag | Description |
|---|
--debug | Enable verbose debug logging to the terminal |
--no-device | Run without a MIDI device — all controls are live but no MIDI is sent |
No-device mode also activates automatically if no MIDI input or output ports are found at startup, or if no device is selected during manual port selection. The status bar will show ● no device.
Using the App
Transport Controls (left column)
| Button | Action |
|---|
| Play | Sends MIDI Start (first press) or Continue (subsequent presses). OP-1 tape begins playing from the current position. |
| Stop | Sends MIDI Stop. OP-1 tape halts. |
| ← | Sends CC 82 (tape previous bar) and updates the song position pointer. |
| → | Sends CC 83 (tape next bar) and updates the song position pointer. |
Note: The OP-1 must be in Midi Sync mode for Play/Stop/←/→ to affect the tape. In other modes these buttons have no effect on tape transport.
BPM
- Displays the current tempo with one decimal place (e.g.
120.0)
- The app generates a continuous 24 PPQN MIDI clock signal — the OP-1 locks to this tempo in MIDI Sync mode
- Arrow keys — increment/decrement by 1.0 BPM
- Type a value and press Enter to set an exact tempo
- Range: 20.0 - 300.0 BPM
Changes take effect immediately; the OP-1's BPM display updates in real time.
Track Strips
Each of the four tracks corresponds to an OP-1 Field mixer channel (tracks 1-4 = MIDI channels 1-4).
Mute button (top)
- Colored header button showing the track number
- Click to toggle mute on/off; the button dims when muted
- Sends CC 9 (127 = muted, 0 = unmuted)
Pan knob
- Dark circular dial below the fader
- Center position (12 o'clock) = center pan (MIDI 64) — line is orange
- Off-center — line is white, pointing toward the pan direction
- Click and drag up/down to adjust; sends CC 10
- Labels: L (left) and R (right)
Volume fader
- Vertical slider, range 0-99 (maps to MIDI 0-127)
- Default: 99
- The filled portion below the handle turns red to show the current level
- The numeric value is shown below the fader
- Drag or click to adjust; sends CC 7
LFO Panel
The LFO panel (below the track strips) applies beat-synchronized automation curves to any track parameter.
Waveform selector
Choose the LFO shape:
| Wave | Description |
|---|
| Sine | Smooth S-curve oscillation |
| Triangle | Linear ramp up and down |
| Saw | Linear ramp up, instant reset |
| Square | Instant snap between high and low |
| Log | Fast start, slow arrival each half-cycle |
| Exp | Slow start, fast arrival each half-cycle (complement of Log) |
| Sweep Up | Chirp — oscillation frequency increases linearly across the cycle (4 sweeps total) |
| Sweep Down | Chirp — oscillation frequency decreases linearly across the cycle |
| Random | Sample-and-hold — 8 deterministic steps per cycle, each holding a pseudo-random value |
Waveform preview
A live preview of the selected waveform is drawn in the panel. The number of visible cycles scales with the Rate setting.
When LFOs are running, selecting rows in the active LFO list replaces the preview with each selected LFO's actual waveform drawn in its track color. Deselects when you click elsewhere in the app.
Track and Master buttons
Each LFO targets one or more tracks. The numbered buttons (1–4) select per-track targets; the green M button targets master parameters.
- Click once to activate (normal phase)
- Click again to invert (flipped phase, marked
[inv] in the active list)
- Click a third time to deactivate
For Tempo and other master-only parameters, the track 1–4 buttons are disabled and M activates automatically. For per-track parameters (volume, pan, mute, FX, LFO), M is disabled.
Parameter selector
| Parameter | Target | CC |
|---|
| volume | Per-track channel volume | CC 7 (ch 1-4) |
| pan | Per-track pan | CC 10 (ch 1-4) |
| mute | Per-track mute toggle | CC 9 (ch 1-4) |
| tempo | App clock BPM (modulates LFO engine tempo) | — |
| par 1–4 | Per-track patch par parameters | CC 46-49 (ch 1-4) |
| env attack/decay/sustain/release | Per-track synth envelope | CC 50-53 (ch 1-4) |
| fx 1–4 | Per-track patch FX parameters | CC 54-57 (ch 1-4) |
| lfo 1–4 | Per-track patch LFO parameters | CC 58-61 (ch 1-4) |
| rec level | Global record level — also the main level of the synth and drum sound, so it shapes what reaches the tape | CC 78 (master) |
| eq low / mid / high | Global master EQ, flat at the middle | CC 90-92 (master) |
| sustain | Global sustain pedal for live notes on the selected instrument — not the tape | CC 64 (master) |
| pitch bend | Global pitch bend of the selected instrument's live notes — not the tape. 14-bit | Pitch bend, ch 1 (master) |
Tempo LFO: modulates the app's BPM directly with float precision. The OP-1 display tracks in real time. When the Tempo LFO is stopped the BPM restores to its original value. Requires app-clock mode (MIDI Sync on the OP-1); unavailable when the OP-1 is the clock master or when connected via Bluetooth.
Controls (per row)
| Control | Description |
|---|
| Rate | How many LFO cycles per bar (beat-synced) |
| Depth | How much the parameter value swings from center |
| Center | The midpoint value the LFO oscillates around |
| Range | Readout showing the min-max value the LFO will reach, derived from Center ± Depth |
Inverted LFOs
When two or more tracks are selected, the Invert 2nd+ checkbox becomes available. With it checked, the first selected track gets the normal waveform and every additional track gets the same waveform flipped upside-down — its output is center - (wave x depth) instead of center + (wave x depth).
This means when the primary track's value rises, the secondary tracks' values fall by the same amount, and vice versa. Some uses:
- Stereo pan spread — assign the same LFO to tracks 1 and 2 with Pan selected and Invert enabled. Track 1 sweeps left while track 2 sweeps right, keeping an even stereo image.
- Pumping sidechain feel — automate Volume on two tracks so one ducks while the other swells.
- Call and response — run an inverted LFO on a second melody track so phrases naturally dovetail rather than stack.
The active LFO list marks inverted clips with [inv] so you can tell them apart at a glance. The waveform preview draws each track's curve in its own color.
Buttons
| Button | Action |
|---|
| Start | Begin LFO automation on the selected tracks and parameter |
| Stop Selected | Stop automation on the currently selected clip |
| Stop All | Stop all running automation clips immediately |
Installation
1. Clone the repository
git clone https://github.com/andrewralon/op1-lfo-hero.git
cd op1-lfo-hero
2. Create a virtual environment
python3 -m venv venv
3. Activate the virtual environment
macOS / Linux:
source venv/bin/activate
Windows:
venv\Scripts\activate
You should see (venv) in your terminal prompt.
4. Install dependencies
pip install -r requirements.txt
This installs:
mido — MIDI I/O librarypython-rtmidi — low-latency MIDI backend for midoPyQt6 — desktop UI frameworknumpy — used by the automation engine
Troubleshooting
App shows "MIDI Connection Failed"
- Make sure the OP-1 is powered on and connected via USB-C (or paired via Bluetooth) before launching
- Try a different USB-C cable (some are charge-only)
- On macOS, check System Settings → Privacy & Security if any MIDI access prompt was dismissed
Controls send but OP-1 doesn't respond (volume, pan, mute, transport all ignored)
- macOS CoreMIDI can get into a bad state where the port appears available but no data flows
- Fix:
sudo pkill -9 coremidi in the terminal, then unplug and replug the OP-1 — no restart needed
OP-1 tape doesn't respond to Play/Stop
- Confirm the OP-1 is in MIDI Sync mode (COM → MIDI → SYNC: MIDI)
- The OP-1 BPM display should show
-- when waiting for clock — if it shows an internal BPM, Sync mode is not active
BPM on the OP-1 doesn't match the app
- The OP-1 needs a few seconds of continuous clock signal to lock in; the display may lag slightly after a BPM change
Left/right buttons stop playback
- This is a known OP-1 behavior: receiving a Song Position Pointer message while playing causes the OP-1 to stop and wait for a Continue or Start. Press Play after navigating to resume.
Pan knob jumps unexpectedly
- QDial uses click-to-set by default. Click and drag vertically for fine control.
MIDI Reference
Per-track (channels 1-4)
| CC | Function | Range | Notes |
|---|
| 7 | Volume | 0-127 | Per channel (track 1-4 = channel 1-4) |
| 9 | Mute | 0-127 | ≥ 64 = muted |
| 10 | Pan | 0-127 | 64 = center |
| 46-49 | Patch par params 1-4 | 0-127 | Per track |
| 50-53 | Patch synth envelope (A/D/S/R) | 0-127 | Per track |
| 54-57 | Patch FX params 1-4 | 0-127 | Per track |
| 58-61 | Patch LFO params 1-4 | 0-127 | Per track |
Global (channel 1)
| CC | Function | Range | Notes |
|---|
| 70-73 | Master FX params 1-4 | 0-127 | |
| 74-77 | Master compressor params 1-4 | 0-127 | |
| 64 | Sustain pedal | 0-127 | ≥ 64 = down (holds live notes, not tape playback). Measured: 63 releases, 127 holds |
| 78 | Tape record level | 0-127 | Also the main level of the synth/drum sound. Answers on any channel. Starts at 95 on power-up |
| 79 | Octave | 0-127 | < 64 = down, ≥ 64 = up (keyboard/synth mode only) |
| 90-92 | Master EQ low / mid / high | 0-127 | 64 = flat. Answers on any channel |
| 82 | Tape prev bar | 127 | Tape navigation |
| 83 | Tape next bar | 127 | Tape navigation |
Pitch bend (status 0xE0, 14-bit, 0-16383, 8192 = no bend): channel 1 only. It bends the
selected instrument's live notes; recorded and played-back tape is not affected, and channels 2-4
are ignored. The OP-1 holds the level once sent, so a bend stays on until it is sent back to 8192
or the patch is reset.
Transport messages
| Message | MIDI Status | Description |
|---|
| Clock | 0xF8 | Sent at 24 PPQN continuously |
| Start | 0xFA | Begin playback from position 0 |
| Continue | 0xFB | Resume from current song position |
| Stop | 0xFC | Halt playback |
| Song Position Pointer | 0xF2 | Sets resume position in MIDI beats |
OP-1 Field Midi Reference: https://teenage.engineering/guides/op-1#midi-reference
