The probability filter

The probability filter is a filter response drawn over the keyboard that decides how often a note plays depending on where it sits in the register. Think of the keys running left to right and a filter curve laid over them: where the curve is high, notes in that register play; where it dips, they thin out or drop. It shapes the notes the arpeggiator, chord lane, and clip produce, so a busy pattern can breathe, sparkle only up top, or dissolve and re-form by register over time.

It is a second dimension to the probability engine. The step sequencer’s probability gates a note in time (this step has a 70% chance of sounding). The probability filter gates a note in pitch (a note in this register has a 70% chance of sounding). Use them together and the pattern thins along two axes at once.

The probability filter starts off and at zero depth, so an existing patch sounds exactly as before. It does nothing until you turn it up.

What the curve does

You choose one of four filter shapes, and the curve’s height at each key is how likely a note in that register is to play:

Low-pass (LP) — keeps the low end and thins the top. The bass and mids come through; the higher you go, the sparser the notes.
High-pass (HP) — the reverse: keeps the top and thins the bottom, so the low mud clears and the sparkle survives.
Band-pass (BP) — favours a band around the cutoff and thins everything above and below it, so the pattern collapses toward one register.
Notch — scoops a band out and keeps the rest, so a chosen register goes quiet while the extremes keep playing.

Where the curve is at full height, every note in that register plays; where it falls away, notes drop out according to the Target you pick.

The two behaviours: Target

Target decides how the curve thins notes: stochastically, or as a hard gate.

Probability — the curve’s height becomes each note’s chance of playing, so the filter thins notes randomly. A note at 60% height plays roughly six times in ten. The pattern breathes and evolves: no two passes are quite the same, and density fades smoothly across the register instead of cutting off at a hard line. Use it for organic, ever-shifting textures.
Mask — the curve becomes a hard gate: above the line a register is cleanly present, below it, cleanly silent. No randomness, so whole registers switch in and out as a block. Use it for deliberate register edits, for example “mute the top octave of the arp for four bars” or “drop the bass out of the clip on the bridge.”

Probability is the performance/generative behaviour; Mask is the surgical one.

The three axes

Axis decides what the filter acts on. Two axes are spatial — the same curve read against a different ruler: fixed keyboard pitch, or the pattern’s own span. The third works by chord-tone role instead, and swaps the curve for a set of per-degree faders. This is the difference between a filter that stays put, one that follows your playing, and one that thinks in harmony.

Register — the curve sits over the actual keyboard, at absolute pitch. Cutoff is a real note. The filter stays where you drew it no matter what you play, so it behaves like a fixed zone on the keys: everything below C4 plays, everything above thins. Use it for a fixed “sparkle only up top” or “keep the bass solid” zone.
Range — the curve is measured relative to the pattern’s own span, from its lowest sounding note to its highest. Cutoff is a percentage of that range, so the filter tracks your playing: transpose up an octave, or play a wider voicing, and the curve stretches to follow. The same “thin the top third” setting works in any key. Use it for a filter that moves with the music.
Function — a different kind of axis, and the most musical. Harmonic role is a category, not a place on the keyboard, so Function drops the filter curve altogether and gives you a mixing board for the harmony: one fader per chord degree — Root, 3rd, 5th, 7th, 9th, 11th, and 13th — each set from 0 to 100%. A degree at 100% always plays; pull it down and notes in that harmonic role start to drop out. Switching the axis to Function swaps the curve view for these seven weight bars (the shape, Cutoff, and Slope controls disappear; only Depth and the Target stay), and it works hand in hand with the chord lane. Use it for harmonic control: keep the chord solid while its upper tensions flicker in and out — thin the extensions, then let Depth and Probability bloom them back in.

Cutoff, Slope, and Depth

Three knobs shape the curve:

Cutoff — where the corner sits (for LP/HP) or where the band is centred (for BP/Notch). In Register axis this is a pitch on the keyboard; in Range it is a position within the pattern’s span. Sweeping Cutoff slides the whole response up or down the register. (Cutoff, along with the shape and Slope, does not apply on the Function axis, which uses the per-degree faders instead.)
Slope — how sharply the curve falls off at the corner. A gentle slope fades density gradually across several octaves; a steep slope makes a near-hard edge between “plays” and “drops,” so under Mask it behaves like a clean register split.
Depth — how strongly the filter shapes the notes, and the master amount. At 0 the filter does nothing (every note plays, the shipping default). Turn it up and the dips in the curve bite harder, until at full depth the thinned registers drop out entirely. Depth is the one knob to reach for to dial the whole effect in or back it off, and it is a modulation destination too: an LFO or envelope on Depth fades the whole filter in and out for evolving generative intensity.

The headline: sweep the cutoff (and depth, and slope)

The filter gives you three modulation matrix destinations, so the whole effect can breathe hands-free:

Cutoff — assign an LFO or envelope and the whole probability response slides up and down the register over time. The pattern dissolves and re-forms by register: with an LP sweeping upward, the low notes fill in first, then the mids, then the top, so a held generative pattern grows from a sparse bass into a full spread and back down again, all on its own. This is the probability filter sweep, and it is the reason the whole feature exists: an evolving, breathing density that you set up once and let run.
Depth — the generative-intensity control. Because Depth is the master amount (0 does nothing, up thins harder), an LFO or envelope on it fades the whole filter in and out: the note stream dissolves and re-forms as the depth swells and recedes, a one-knob “how generative right now.” An envelope tied to note-on lets each new phrase build up from a gate into a full spread; a slow LFO breathes the density of the entire pattern.
Slope — modulate it to move the edge itself, sweeping between a gentle cross-register fade and a near-hard register split under a source.

Any matrix source works. A slow LFO gives you a continuous ebb and flow; an envelope tied to note-on makes each new phrase build up; the envelope follower can drive any of them from your own playing dynamics.

Where to find it and how to use it

The probability filter lives with the performance controls, next to the arpeggiator and the keys:

A Prob Filter toggle in the performance (Keys / arp) strip turns the whole filter on and off. This is the master switch: with it off the pattern plays untouched.
A small button beside the toggle opens the editor window — the curve-over-keyboard display with the shape, Axis, and Target selectors and the Cutoff, Slope, and Depth knobs. Draw or dial the response there and watch it laid over the keys.
Closing the editor window leaves the filter running. The window is just the editor; the toggle in the strip is the on/off. Set the curve, close the window, and keep playing.

Everything the filter does is saved with the patch, so a sculpted density travels with the preset.

See the filter on the keyboard: the heatmap

The on-screen performance keyboard can shade its keys by the filter’s response, so you can see which registers are thinning. Under the header Options gear → “Keyboard follows engine”, turn on Heatmap: because the keyboard is the Register axis, every key is shaded by how likely a note in that register is to play. A cutoff sweep walks the marked band across the keys, and a disabled or depth-0 filter marks nothing (every note passes).

The gear’s “Heatmap: Neutral” item picks the style, and both mark the same keys — the ones the filter is thinning:

Neutral (default) — darkens the filtered (low-probability) keys toward black, exactly matching the filter editor’s own keybed, so the two read identically.
Themed — tints those same filtered keys with the current theme’s accent instead of black.

Either way the wash marks what the filter removes, never what it passes. Heatmap composes with Live Notes (which lights the notes the generative engine is currently playing), so you can watch the live pattern play against the probability landscape it is drawn from. Both are display preferences for the current session, not part of the patch.

Try this

Sparkle only up top (fixed). Set Axis to Register, shape to Low-pass, Target to Probability, and put the Cutoff around the middle of the keyboard. Turn Depth up part way. Now the bass and mids of your arp stay solid while the top octaves thin to an occasional shimmer: a busy pattern with an airy, uncrowded top.

Evolving density (the sweep). Keep the same setup, then assign a slow LFO to the probability-filter Cutoff in the matrix. Hold a generative pattern (arp plus chord lane is ideal) and let it run: the response slides up and down the register, so the pattern grows and thins by octave on its own, hands-free, for minutes without repeating.

Generative intensity (Depth). For a different flavour, route a slow LFO or a note-on envelope to the probability-filter Depth instead of Cutoff. Now the whole filter fades in and out: the note stream dissolves toward the shaped curve and re-forms toward a full spread, so the pattern’s density breathes as one, a one-knob “how generative right now.”

Clean register edit (Mask). For an arrangement move rather than a texture, switch Target to Mask and Slope to steep. Now the curve is a hard split: drop the Cutoff to mute the bottom octave cleanly, or sweep it to bring registers in and out as blocks, with no stochastic flicker.