Buying an MPE controller means buying someone else's idea of how a playing surface should be laid out. That's usually fine — the LinnStrument grid and the Seaboard keywave are both strong designs — but it leaves out anyone whose instrument doesn't map cleanly onto either. Numerical Audio released MidiRack on 24 August 2026, and it takes the opposite approach: build the surface yourself.
A grid you fill with modules
MidiRack is a modular MIDI toolkit and performance environment for iPadOS. Developer Kai Aras describes it as bridging touch performance and deep MIDI architecture, and the app splits cleanly into two layers.
The base layer is a grid you drag modules onto. The available modules include standard keyboards, scale-locked keyboards, diatonic chord pads, drum pads, faders, knobs, sequencers — and, most relevant here, Seaboard-style MPE ribbons and MPE string grids. These come from Numerical Audio's existing KB-1 Keyboard Suite app, which offered eight fixed performance layouts; MidiRack includes those layouts and makes them arrangeable.
That's the practical hook for MPE players. A guitarist can build a string-grid layout with per-string transposition. A wind player can put a single ribbon next to a breath-mapped fader. Someone working in a microtonal scale can combine a scale-locked surface with the app's microtonality support. None of these require a specific piece of hardware — just an iPad and a USB or Bluetooth connection.
Sequencers that carry expression
The sequencer modules are more capable than the "drag a few pads on screen" framing suggests. MidiRack includes drum, melody, chord and CC sequencers, plus analog-style and matrix sequencers, all with a moving playhead, microtonality support, per-step probability and trigger conditions.
The CC sequencer deserves attention from anyone thinking about expression. Because MPE's expressive dimensions are ultimately CC74, channel pressure and per-channel pitch bend, a sequencer that can shape those over time gives you programmatic articulation to sit alongside whatever you're playing by hand. Sequencers can be clocked from the host transport, from node triggers or from external pulses.
The MIDIGraph: a modular synth for MIDI
Under the surface modules sits a node-based MIDI engine, unlocked with a separate in-app purchase. Numerical Audio describes it as a modular synth for MIDI functions, and the description is apt: over 60 processing nodes that you patch visually, including scale quantizers, generative models, Turing machines, Markov chains and a Conway's Game of Life node.
For MPE work this is the most interesting part of the app. Per-note expression data is just streams of numbers, and a node graph is a natural place to transform them — rescaling a pressure curve, deriving one dimension from another, quantizing pitch bend to a scale while leaving vibrato intact. The app also ships built-in subtractive synth and sampler engines, which exist mainly so you can audition what the graph is producing without wiring up a host first.
Connections and pricing
MidiRack connects to Ableton Live, Bitwig and Logic over USB (via IDAM) or Bluetooth MIDI with no drivers required, and runs as both a standalone app and an AUv3 plugin. It's iPadOS only — no iPhone version.
Pricing is unusually low for what's included: $7.99 introductory, rising to $14.99, with the MIDIGraph editor a further $7.99 introductory / $14.99 regular in-app purchase. Fully unlocked, that's under $30 for a configurable MPE-capable surface. It's on the App Store now.
Where a tablet surface actually helps
Nobody should pretend an iPad screen matches a Continuum's neoprene or an Osmose's keybed. Glass gives you no key edges, no resistance and no Z-axis travel — pressure on a capacitive screen is inferred from contact area, not measured directly, and that difference is audible on sustained notes.
What a tablet does offer is reconfigurability and price. If you're not sure whether you want a ribbon, a grid or an isomorphic layout, working through those questions on a $30 app before spending four figures on hardware is a sensible order of operations. It's also a reasonable second surface for players who already own a controller: put macros, XY pads and CC sequencers on the tablet, keep the hardware free for notes.
One early user on the Synth Anatomy report made a fair point about the category — tools like TouchOSC are powerful GUI builders but push you into engineering mode, whereas MidiRack stays playable at every stage of construction. For anyone who has abandoned a half-configured custom layout, that distinction matters more than the feature list.