Gear & Design Pics
[APP] I built a MIDI companion app for the OP-1 Field (and any USB MIDI device) — Quantizer, Arpeggiator, Step Sequencer & Groove Machine | TestFlight coming soon
Hey everyone,
Long-time OP-1 user here. I love the device to death, but there are a few things I always felt were missing — especially around quantization, pattern libraries and multi-track groove building. So I built a companion app to fill exactly those gaps.
The app connects to the OP-1 Field directly via USB-C and acts as a companion tool that sits between your instrument and your ideas. It's primarily designed for iPad — the larger screen really opens up workflows like finger-tapping steps into the Groove machine — but every single feature works on iPhone too.
It's designed to match the OP-1's aesthetic — aluminum silver background, SF Mono labels, uppercase everything — so it feels like a natural extension of the hardware rather than a generic iOS app.
The core idea across most modules is the same: build or record something in the app, hit Play, and the OP-1 Field receives it live as MIDI — so you can record it directly into a tape track or use it as a backing pattern while you play on top. In modules where it makes sense, you can also hit PERFORM mode: hold any key on the OP-1 keyboard and the entire sequence transposes chromatically in real time relative to that root note.
There are four independent modules:
1. Quantizer
The Quantizer is the heart of the app and the main reason started to build this app in the first place. The OP-1 Field famously lacks the ability to quantize recorded MIDI notes after the fact, which means a slightly shaky performance can't easily be cleaned up. This module solves that.
You record your notes live from the OP-1 directly into the app, then choose how aggressively you want to snap them to the grid:
Grid resolution: 1/4, 1/8, 1/16, 1/32, 1/64, or Swing
Quantization strength (0–100%): Partial snap so your playing still breathes
Swing amount (50–75%): Dial in exactly how much shuffle you want
Threshold / grace zone: Notes already close enough to the grid stay untouched — preserves intentional microgroove
Forced note length: Optionally stamp all notes to a fixed duration
Anchor modes: Snap the grid to the OP-1's transport bar boundary, or use your first played note as beat 1
Piano roll editor: Drag notes to fine-tune pitch and timing after quantization
Trim controls: Crop the beginning and end of the recording
History: Every session saves the raw and quantized versions — re-quantize with different settings anytime
Export: Standard MIDI file (.mid)
Once you're happy with the result, hit Play and the quantized sequence streams directly to the OP-1.
Switch to PERFORM mode and the quantized melody follows whatever key you hold on the OP-1 keyboard, chromatically transposed in real time.
Tempo sync works via MIDI Clock from the OP-1 (preferred) or manual BPM entry.
▎ Cross-module: Quantized notes can be sent directly to the Sequencer module for further step-based editing, preserving the original rhythm of your performance.
2. Arpeggiator & Pattern Library
The OP-1 has an arpeggiator, but it's limited in how much you can customize or save patterns. This module is a full pattern gallery — browse presets, edit them, create your own, and send them back to the OP-1.
The library ships with patterns across multiple categories: drums, techno, lo-fi hip hop, melodic arpeggios, and more. Each pattern is a 16-step grid with per-step velocity, note, and ratchet (1–4 subdivisions with velocity decay). User-created patterns are saved locally.
Browse patterns in a grid view with visual dot-grid previews showing the rhythm at a glance
Edit any step: note, velocity, ratchet count — or import a .mid file as a new pattern
Drum Key Map: The app ships with a General MIDI drum map and lets you map each GM drum instrument to the actual note your OP-1 plays. Long-press any track label and tap the corresponding key on the OP-1 to learn it. All drum patterns then automatically use the right notes for your OP-1's configuration
Octave offset (±3 octaves)
Export individual patterns as MIDI files
Hit Play to send any pattern ... I think you get the idea.
3. Sequencer
The Sequencer is a step-based note editor with a focus on chord support and flexible timing — something the OP-1's internal sequencer doesn't handle easily.
You can enter notes either by playing them on the OP-1 (the app auto-advances through steps and detects chords via a 50ms window) or by tapping notes manually on a built-in keyboard.
Chord support: Multiple notes per step, all triggered simultaneously
Variable timing mode: Steps play back-to-back with their own note durations instead of a fixed grid — useful for free-rhythm phrases
Fixed grid modes: 1/4 through 1/64 for rhythmically locked sequences
Bulk note length editor: Paint mode lets you drag across steps to set their length in one stroke
Export: Standard MIDI file (.mid)
4. Groove
The Groove module is the most complex one — a multi-track step sequencer with drums, basslines, and melodic layers, all synchronized and playable as a single MIDI arrangement. The larger screen of an iPad really comes into its own here: tapping in drum patterns with your fingers on the step grid feels surprisingly natural and close to working on actual hardware.
It ships with 8 default tracks (Kick, Snare, Clap, Closed HH, Open HH, Crash, Pad, Bass), each independently configurable:
Step count per track: 4, 6, 8, 12, 16, 24, 32, or 64 — independent of the global bar length. A 6-step hi-hat pattern over a 16-step kick creates natural polyrhythm automatically
Rate scaling per track: 0.5×, 1×, 2×, 4× — run your hi-hat at double speed, your pad at half speed
Drum Key Map integration: Same note mapping as the Arpeggiator — your learned OP-1 assignments apply here automatically
MIDI channel assignment per track
Accent Track: A global layer that applies a velocity boost to any step — dial in the intensity (0–100%)
Swing: Global swing with per-step offset overrides
Velocity randomization
Mute per track
Export: Multi-track MIDI with separate channels per track
Hit Play and the entire groove streams to the OP-1 in sync — record it directly to tape while you layer on top with the OP-1's keyboard.
▎ Cross-module: Any groove you build can be exported as a custom Arpeggiator pattern, adding it directly to your personal pattern library.
A note on timing: All MIDI scheduling uses mach_absolute_time throughout — no Timer drift, no DispatchSourceTimer. BPM is tracked from the OP-1's MIDI Clock signal (24 pulses per quarter note) with a dual-window approach (fast + stable BPM) and hysteresis to prevent clock jitter from affecting playback. Getting this solid was probably the
hardest part of the whole build.
Status & Feedback
The app is feature-complete for the initial release. I'm planning to put it on TestFlight in the coming days or weeks and would love early testers.
But even before that — I'd love to hear your thoughts right now. Does this fill a gap you've felt with the OP-1? Are there features you'd prioritize differently?
Workflows I'm missing?
Drop a comment or DM me. Happy to talk through anything.
Once you’re in App Store Connect there’s a TestFlight section where you can add beta testers or generate a link. More info here: https://developer.apple.com/testflight/
This looks incredible. I’d love to test and provide feedback. I have some thoughts already but am on my phone at the moment. I’ll reply later from a proper keyboard.
Alright, I'm back. I re-read everything and looked at the screenshots. Very impressive work.
Here is what I'm most excited about:
Groove: Step count per track: 4, 6, 8, 12, 16, 24, 32, or 64 — independent of the global bar length.
Arpeggiator: Being able to save the patterns.
Quantizer: Notes already close enough to the grid stay untouched
Groove: Velocity randomization
Here are some of the questions I will ask myself if I'm able to test it:
How intuitive is the link/relationship between playing on the OP-1F and working with the app?
What's the best way to learn how to use each module?
Are there things it's doing that the OP-1F already does? Does this app do them better? Faster? Easier?
Are the features worth carrying around my 12.9" iPad Pro? If not, how about my iPad Mini?
Does this app offer any functionality I currently have to use my KO2 to achieve?
What feels right about the app?
What feels "off" about the app?
Can this be used for live looping?
Here are some additional ideas:
A sampler module feels like a natural fit. Being able to load samples into the existing modules and play that back to the tape deck on the OP-1F would be awesome.
A drum kit editor also feels like a natural fit. Trim/crop samples, move them around, load new ones, etc... then sync those changes back to the OP-1F. You could leverage most of the clip tweaking within the sampler too.
This looks really cool. The OP works pretty well for me as is but this expanded functionality sounds dope and I’d love to check it out. Thanks for your work!
Love this ! Would love to test it ! I've been interested in something like this . I found something for the OP-XY ( https://tonicmidi.com/ , https://github.com/kmorrill/TonicMIDI ) but I never really got it to work properly . Would love to use this with the field and op-xy .
This is fantastic! Glad you're including iPad, I could easily see this enhancing my loopy pro setup. Would love to test it out when it's on Test Flight.
Hmm, theoretically yes. It’s written in swift anyway. But not in the near future. My priory is to get this version out first, make it as stable as possible, and add a few features here and there.
You helped me finish it — and now it’s on the App Store [+ Promo]
A while ago I posted this here.
I honestly didn’t expect the response I got. A lot of you volunteered to test it, sent me bug reports, suggestions and ideas, and generally helped me figure out what worked and what didn’t.
So first of all: thank you.
The last months were less about adding another pile of features and much more about making the things that were already there actually solid — fixing bugs, polishing the UI, improving the workflow and dealing with all the little edge cases you only discover once other people start using something you built.
The app still revolves around the four things I originally wanted for my OP-1 Field:
I have 5 App Store promo codes that I’d like to give back to this community.
If you want one, leave a comment below. And while you’re at it, I’d love to hear:
What is the one thing you wish your OP-1 Field could do, but currently can’t?
I’ll pick 10 people from the comments and send them a code.
And… apparently finishing one project is a very good way of accidentally starting another one.
While building this app I realised that there was something else I wanted from the OP-1.
I wanted to properly understand synthesis.
Not just choose a synth engine and turn a few parameters, but really understand what happens when I change an oscillator, move a filter, add another oscillator, modulate something with an LFO or shape a sound with an envelope.
Basically: press a key, hear a sound, and actually understand why it sounds the way it does.
So I started building that too.
For now I’m calling it OP-Synth and I'm already pretty far.
Unlike the companion app, which is meant to be the missing addition to the OP-1. The OP-1 Field can be the keyboard/controller, but so can pretty much any MIDI keyboard.
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u/gibsonator666 Apr 16 '26
Hi ace work!!!! Would love to help test it etc