Small developers often add MPE as a checkbox: a line in the changelog, per-note pitch bend routed to the same place channel pitch bend already went. Unusable Engineering's September update does something more specific. In Curves & Membranes 1.4.0, MPE arrives as a mode called Expression Mode — and one of the three destinations it exposes is the shape of the oscillator itself.
That distinction is worth unpacking, because it says something about where per-note control is actually useful.
From Monosynth to Four Voices
The headline of version 1.4.0 is Four Voice Mode, which converts what was a monosynth into a four-voice paraphonic instrument. Voices are allocated round-robin, with voice stealing when you run out, and all four share a single membrane filter.
Paraphonic is the operative word: the voices have independent oscillators but a common filter path. That is a design decision with real consequences for expression — a shared filter means per-note timbral movement has to happen upstream of the filter rather than through it. Which is exactly what Expression Mode addresses.
What Expression Mode Actually Routes
Expression Mode adds per-note MPE control over three dimensions:
- Pressure — the vertical axis on a Seaboard or ERAE Touch, the Z-axis on a LinnStrument.
- Pitch bend — per-note, so a bend on one finger leaves the others alone.
- CC74-controlled oscillator geometry — the slide/timbre axis, mapped not to a filter cutoff but to the wave shape.
That third one is the interesting part. Most plugins route CC74 to filter cutoff by default, because that is the easiest destination to make sound musical. But on a paraphonic instrument with one shared filter, that route is unavailable per-note by definition. Mapping the timbre axis to oscillator geometry sidesteps the problem entirely: each voice's harmonic content changes at the source, before the shared filter, so four fingers on a controller can genuinely produce four different timbres.
Alongside it, Velocity Slot Morphing lets note velocity determine each voice's position between oscillator slots — a second, gesture-driven route into the same wave-shaping territory. Voice overlays now display each playing voice's waveform live in the editor, which makes the whole thing legible while you play rather than something you infer from listening.
Wave editing also improves: axis-locked movement, full-wave scaling and inversion, smoother morphing, and multi-step undo/redo.
The Rest of the Lineup
The September update is not limited to one plugin. Partials & Discrepancies moves to 1.3.2 with Live View, which follows an active voice and shows its slewed geometry, partial settings and sequencer position — again, a visibility feature aimed at making per-voice behaviour observable. X-Voice FM lets each voice modulate the next in a circular chain, sourced either from the preceding oscillator or its filtered signal, and Vel->Position hands velocity control over oscillator slot placement.
The effects lineup gets new modes across the board in 1.3.2. Frequency Guillotine and Harmonic Dissector both gain a Smooth Mode with wider filter slopes and softer, drive-dependent behaviour. Tension Reactor adds Bright Spring, a separate spring path with more upper-frequency content and lower CPU use. Cavitation Fractures now names its original algorithm Emission and adds two siblings: Erosion, which removes narrow frequency bands, and Vacuum, which subtracts from signal amplitude. Ensemble Diffuser sees revised filtering, modulation smoothing, BBD character and feedback behaviour.
Effect slot sequencers across the range now share the same editing model, including the ability to hold a parameter while the sequencer advances to record new values into incoming slots, plus Cmd/Ctrl-click slot preview and locking. Copy/paste and 30-step undo/redo are available across most effects.
Under the hood: CPU, memory, WebView and real-time state-management improvements, with visual processing now stopping more consistently when an editor is closed and audio buffers pre-allocated in more places. Basic user guides and changelogs are now published on every plugin page — a small thing that makes evaluating this kind of idiosyncratic software considerably easier.
Why This Matters for MPE Players
There is a recurring frustration among MPE players that per-note timbre control tends to sound the same across plugins, because it tends to be routed to the same place. A filter sweep per note is expressive, but it is a fairly narrow slice of what "timbre" could mean.
Routing the slide axis to oscillator geometry produces a different feel: rather than opening and closing a window onto a fixed harmonic spectrum, you are changing the spectrum itself. On a controller with a reliable Y-axis — a LinnStrument, a Seaboard, an ERAE Touch — the difference is immediately audible.
Whether four paraphonic voices with a shared filter is enough headroom for full MPE playing is a fair question. For chordal work with independent bends and timbre movement, it will be. For dense polyphonic passages, it will not. That seems to be a deliberate scope choice rather than an oversight.
Availability
Everything is available for Windows and macOS in VST3 and AU. The individual effect plugins are €29 each, Curves & Membranes is €59, and Partials & Discrepancies is €79. Binaural Modulator and Cavitation Fractures remain free — a reasonable way to audition whether this developer's approach suits you before committing.
Full details and downloads are on the Unusable Engineering press page.
Sources: Rekkerd