The Filter

Phaseburn One’s four sound sources — the Phase-Locked Loop (PLL) voice, the VECTOR phase-shaping oscillator, MORPH and the sub — are cabled into FILTER 1, and a second, identical filter, FILTER 2, waits behind it. Because the instrument runs a single voice there are no per-voice filter copies to manage: two filters with the whole CPU budget behind them, sitting after the oscillators and ahead of every effect. By default all four sources go through FILTER 1, sub included, and FILTER 1 runs on into FILTER 2; the patchbay lets you re-cable which sources reach each one and in what order. Both filters live on the FX tab; the filter envelope they share is edited in the envelope panel at the bottom of the Oscillators tab. The coloration stage — the analog-mess controls that keep the voice from sitting perfectly still — belongs to the PLL and is covered in the oscillators chapter (Chapter 2 — Oscillators).

The filter

Pick the machine before you sculpt. The LADDER / SVF switch at the top right of the filter, beside its power button, chooses between two complete filter designs that share one set of controls; it shows the machine that is running, and a host lists it as Filter Topology (Filter 2 Topology for the second filter). The transistor-ladder is four saturating filter stages in series — the thick, vintage-flavored lowpass lineage — built after D’Angelo and Välimäki’s published research on nonlinear ladder filtering. It colors everything it touches: push level into it and the stages bend, sweeps sound expensive, resonance turns vocal. The state-variable filter (SVF) is the surgical alternative, built on Zavalishin’s zero-delay-feedback (ZDF) designs: its resonance peak stays exactly where you put it and the response is clean until you deliberately dirty it. When a patch needs weight and grime, start with the ladder; when it needs a precise, repeatable sweep, start with the SVF. Switching it on a sounding note crossfades from one machine to the other, so the switch never clicks, even when switches come in quick succession, as from fast automation.

The response shape is one knob: the pole morph. It blends the filter smoothly from one landmark shape to the next, and the in-between positions are real filter responses, not crossfades between two signals; the knob’s tick marks name the landmarks. On the ladder the journey runs from a steep lowpass through a gentler lowpass and a bandpass, past a notch, to a steep highpass. On the SVF it is longer — lowpass, bandpass, highpass, notch, peak and allpass — and its last leg, from peak to allpass, passes through one null at the cutoff on the way. Steep slopes cut decisively — bass patches keep their focus; the gentler lowpass leaves more of the upper harmonics breathing. Park the knob on a tick when you want a textbook response; sweep it slowly from an LFO and you get response morphing that no fixed shape selector can play.

On a patch started from INIT, FILTER 1 is already engaged — a 24 dB ladder lowpass wide open at 20 kHz with no resonance — so it passes everything audible while its cutoff, resonance and envelope answer the moment you reach for them. A filter that is off is bypassed until you engage it, and it opens fully wide when you do — so switching it on never dulls a patch by surprise. By default all four sources feed it together, each through its own cable into FILTER 1 — sub included, so engaging the filter shapes the low end along with everything else. Each of those cables has its own level, so the balance into the filter is set right there. The patchbay on the Mixer tab re-cables this. Pull a source’s cable and it leaves the filtered path — and the output — until you patch it somewhere else: into FILTER 2, straight into an effect, or around the filters with a direct-to-master cable that arrives unfiltered. To keep a sub solid under a closing filter, route the raw sub around the whole chain with the sub’s NO FX switch (see the oscillators chapter (Chapter 2 — Oscillators)).

MIX, the last knob in the filter row, sets a dry/wet blend between the signal that entered the filter and the filtered output — fully wet by default. Back it off to fold parallel dry signal back in: a way to keep the low end and transient attack intact while the ladder does its coloring above, or to dial the whole effect in gently. The dry side waits for the filter’s oversampling to catch up before the two meet, so a half blend adds body instead of hollowing out the upper mids. It is the filter’s own internal blend, and there is no output-gain knob to manage beside it: at normal program level an open filter with DRIVE at zero is level-matched to bypass, so switching it in does not jump the level — DRIVE is the knob that pushes. The blend is click-free, switching the filter on included, and only acts while the filter is engaged; with the filter off, its input passes through untouched.

STEREO SEP splits the left and right cutoff frequencies apart — left shifts down, right up, by up to an octave each — which widens a mono patch without touching a single effect. The first half of the dial covers the subtle range; past it the two sides start to read as two different filters. Each side’s resonance is level-matched at its own cutoff, so opening SEP widens the image without leaning it to one side, however high the resonance.

Two filters

FILTER 2 is a complete second filter, identical to the first: both machines, the same pole morph, drive, TYPE, CHARACTER, bass lock, resonance, key tracking, STEREO SEP and MIX, each with its own settings. It ships switched off, and a filter that is off passes its signal through untouched, so a new patch — which already runs FILTER 1 into FILTER 2 — sounds exactly like one filter until you switch the second one on. Presets and projects from before FILTER 2 existed keep their own routing, with FILTER 2 arriving unconnected.

There is still one filter panel on the FX tab. The FILTER 1 | FILTER 2 tabs to the right of its ON switch choose which filter the panel shows: the knobs, the ON switch and the LADDER/SVF toggle all belong to the selected filter, and the editor remembers which one you were looking at. Because both filters live on the FX tab, the FX switch in the app header takes them out along with the effects, and switching it back on returns them exactly as they were.

Series is the obvious move — a ladder lowpass into an SVF notch, or a driven ladder into a clean one that tames what the drive added. The patchbay also takes the pair in parallel: give each filter its own sources, or the same sources at different levels, and send both onward — a bandpass on the PLL beside a lowpass on the sub. Either filter can sit anywhere in the chain, after an effect as easily as before one; the patchbay chapter (Chapter 5 — Mixer & Patchbay) covers the wiring. Either filter’s output can also feed one of the four feedback returns, so a filter can sit inside a delay’s, chorus’s, reverb’s or freezer’s loop.

When what you want is not character but a clean cut — trimming the low end off a reverb return, or narrowing a source to a band — the patchbay carries two more filters for exactly that: Filter 3 and Filter 4, the bay’s HP/LP filters, with six slopes, two notches and one frequency knob each and no envelope, drive or colour of any kind. They live only in the bay; see the patchbay chapter (Chapter 5 — Mixer & Patchbay).

Resonance and self-oscillation

Turn RESONANCE up until the filter starts to sing. It emphasizes the region around the cutoff while the level barely moves: as the peak grows, the level below it eases down a few decibels and then holds, the same curve on both machines, and on band-pass, high-pass and notch shapes RESONANCE never acts as a volume knob. In the last stretch of the knob both machines cross into self-oscillation, at the same point and at the same level. On the ladder the feedback is deliberately allowed past the point where the filter becomes its own oscillator, with the per-stage saturators bounding the level instead of a damping fudge that would thin the sound. The SVF whistles cleaner, held in bounds by damping that only engages where the signal grows loud, so everything short of the whistle stays clean. On the SVF the resonance climbs smoothly into the whistle, with no jump in character at the threshold. Just under the threshold the SVF’s resonance rings on for seconds after each hit, longer than the ladder’s.

A self-oscillating filter is a playable voice in its own right. The volume envelope comes after both filters, so the whistle belongs to the note: it follows velocity, the envelope and note-off, and the gaps in a sequenced line go quiet. Add full KEY TRACK (see The filter envelope below) and it follows the keyboard. The SVF’s whistle sits on the cutoff to within a few cents; the ladder’s runs a shade flat, a little more so at the very top of RESONANCE, so nudge the cutoff up to bring it into tune. Tune it, sequence it, treat it as one more oscillator. The exception is a filter with live audio cabled into it from outside the voice — AUDIO IN, or an effect’s output: that filter runs as an insert with the envelope ahead of it, so a whistling insert keeps droning through the gaps in its input and fades out once the input stays away.

Resonance costs bass: as the peak grows, the passband under it dips — the easing described above. BASS LOCK buys it back. It is a low shelf on the filter’s own output that restores what resonance took and nothing more: at full amount the low end sits exactly where it was with RESONANCE at zero, on both machines and every shape, with no dry signal leaking through a closed filter. Shapes that pass no bass, such as band-pass and high-pass, have nothing to restore, so it stays out of their way, and at low resonance there is little to buy back, so it never bloats a non-resonant patch. BL FREQ sets the shelf’s corner — low values restore only the sub range, higher values reach into the low mids. The corner always stays well below the running cutoff, so the shelf lifts the body under a resonant peak and the peak itself only slightly, and BL FREQ glides smoothly when automated.

Drive, TYPE and CHARACTER

Push the filter until it growls, then pick the grain of the growl. DRIVE pushes the input into the filter’s saturating stages, from clean through thick to overdriven — one knob spanning the full 0 to +24 dB, with no separate boost step. At zero the filter is near-linear at program level; DRIVE is where the push comes from. The TYPE knob beside it selects the curve every stage saturates with: TRAN, the transistor curve, is the smooth, rounded one; DIODE bends earlier and more gradually and never quite flattens; TUBE starts out identical to TRAN and grows a firmer shoulder as CHARACTER rises. All three are symmetric, so the grit they add is odd-harmonic — they differ in how the knee feels, not in which harmonics appear. Because TYPE feeds both the drive input and the filter’s own per-stage saturation, it colours the sound whenever the filter is working, not only when DRIVE is up. It is level-matched at program level and crossfades when switched on a sounding note, even in quick succession, so sweep it and keep the curve that flatters the patch. CHARACTER sharpens the saturation curve itself across all stages while the cutoff frequency stays put, from smooth to snarling, and it is level-matched at program level too, so at ordinary levels the knob sweeps as a pure texture control. Hard settings still read as more squash — the peaks come down and the harmonic content thickens — and with a hot input, several sources at full volume, the top of the knob compresses audibly harder, because a harder curve squashes a loud signal harder by its nature. It is audible at any resonance setting, not only extreme ones.

The filter envelope

This is where the filter starts playing notes with you. The filter has its own envelope — attack, hold, decay, sustain, release, fully independent from the volume envelope — so the brightness of a note can move on a completely different schedule from its loudness: a slow volume swell with an instant filter bite, or a percussive amplitude hit whose harmonics bloom afterwards. The stages are edited in the envelope panel at the bottom of the OSC tab — its tab strip reads VOLUME ENV / FILTER ENV / CUSTOM ENV / TRANSIENT / VOICING — switched to FILTER ENV. Both filters follow this one envelope, each by its own amount: the last two sliders in the envelope row, Filter 1 and Filter 2, are those amounts, and each filter’s panel on the FX tab carries the same control as its ENVELOPE knob. An amount dims while its filter is off, but stays editable. A patch started from INIT gives the envelope a 400 ms decay to zero sustain and a 300 ms release, with both amounts at zero, so turning an amount up plays a sweep on the very next note rather than a fixed offset. The envelope also reaches past the filters: the FILTER ENV knob on VECTOR’s FM tab lets it shape the FM depth, and it runs for that even with both filters off — see VECTOR FM (Chapter 2 — Oscillators).

The amount sets how far the envelope reaches. It is bipolar: positive values sweep the cutoff up from where you parked it, negative values flip the whole movement downward — the same envelope becomes a closing gesture instead of an opening one — and at full amount the sweep spans eight octaves. KEY TRACK completes the picture by making the cutoff follow the played note, semitone for semitone at the full setting: without it, high notes duck under a fixed cutoff and low notes sail through; with it, the filter keeps the same voicing across the keyboard (each filter has its own KEY TRACK) — mandatory for basslines that walk, and for playing a self-oscillating filter in tune.

A second envelope can join this one on the cutoff. The CUSTOM ENV tab holds an envelope built the same way — the same stages, shapes, sync, DIP, RESET, LOOP, key track and velocity routes — with its own Filter 1 and Filter 2 amounts at the end of its row. Where both envelopes reach a filter they move its cutoff together, each by its own amount, and the sum stays inside the same eight-octave reach: a slow opening sweep from one and a quick pluck from the other, on the same note. COPY FILTER, in that tab’s header, starts it from this envelope’s settings. The custom envelope is a modulation source as well; see The custom envelope (Chapter 4 — Modulation).

Shaping the filter envelope

Bend the sweep’s curves, lock its stages to the tempo and let it answer your touch. The filter envelope’s stages are shapeable. Attack, decay and release each have a shape control that bends the curve from logarithmic (LOG, at the bottom of the slider) through linear (LIN) to exponential (EXP, at the top), plus an S-curve switch that splits the curve into an ease-in/ease-out pair — the shape slider then sets how pronounced the S is. HOLD pins the envelope at its peak between attack and decay, useful when a sweep should sit fully open for a beat before falling. DIP pulls the envelope down briefly when the sequencer retriggers a note that is still sounding, giving re-articulated steps a percussive filter pluck; like the volume envelope’s Dip it acts on sequenced lines only — live MIDI takes the smooth path — and it runs the same law, where depth buys length as well as depth — see The volume envelope (Chapter 4 — Modulation). RESET, the switch under the DIP slider, changes what a retrigger does: with it lit, every note that retriggers the envelope restarts the attack from zero instead of from wherever the envelope stood. The envelope first falls to zero over 4 ms, smoothly enough not to click, so a run of overlapping notes plucks the filter open from the bottom on every one instead of riding a half-open sweep. Legato and tied notes still do not retrigger, and with DIP raised, a sequenced re-attack under RESET falls and pauses the way the dip does. LOOP switches the envelope from one-shot to a repeating attack–hold–decay cycle for as long as the note is held — a rhythmic wobble with no LFO involved. The four time stages — attack, hold, decay and release — can also leave milliseconds behind: each has its own sync toggle and division selector, locking that stage’s time to the host tempo.

The filter envelope also answers your playing the way the volume envelope does. A key-track slider shortens its stages as you play higher and lengthens them lower down — this is envelope timing, distinct from the KEY TRACK knob above, which moves the cutoff itself — and four bipolar velocity routes shift the attack, decay and release times — up to a second either way at full depth — and offset the sustain level with how hard you hit, using the same formulas as the volume envelope’s routes (see Velocity and note strength (Chapter 4 — Modulation)). All five sit at zero by default and only act when raised, so a patch that never touches them keeps its filter movement identical on every note. Under each velocity slider sits a VEL/STR switch, and the slider’s caption follows it — Vel›A becomes Str›A. VEL reads how hard the note was played; STR reads the note’s strength instead: the value painted for that step on the Groove page’s Strength view, or, on a patch saved with Velocity As Strength on — a setting with no panel control — the struck velocity. A note with no strength counts as the neutral middle, so a STR route leaves its stage where the stage’s own slider puts it. Each note takes the switch’s setting when it starts, so a flip lands on the next note. A copy button (COPY VOL) pulls the volume envelope’s settings across — DIP, RESET, key track and the velocity routes with their VEL/STR choices included — when you want both movements to start in lockstep. One housekeeping note: RANGE, in the table below, rescales the envelope panel’s editing time axis so long stages stay visible — it does not itself change the sound.

Cutoff, resonance, drive, envelope amount, pole morph, bass lock, CHARACTER, STEREO SEP and MIX are live modulation targets on both filters, beyond this envelope. The destination list files them under FILTER 1 and FILTER 2, and every entry carries its filter’s number — Filter 1 Cutoff, Filter 2 Cutoff and so on — so a route always says which filter it moves; see the modulation chapter (Chapter 4 — Modulation) for LFOs and step modulators pointed at the filter.

Formant

A formant is a resonant peak that stays put while the pitch moves underneath it — it is why a singer holding two different notes still sounds like the same vowel. Phaseburn One has no single formant knob; it has three honest ways to build one, and they stack.

The first is the filter you just met. A bandpass response with moderate resonance and KEY TRACK left at zero is a formant by definition: the emphasized band stays fixed while the sequencer walks notes under it, and the patch takes on a vowel-like identity. Automate the cutoff gently between two positions and the vowel mouths open and closed; on the SVF, the peak region of the pole-morph path is made for exactly this. Keep the resonance short of self-oscillation — a formant is an emphasis, not a whistle.

The second lives in the VECTOR oscillator. Its FORMANT switch retunes the Y control into a formant-position control, planting resonant peaks directly on chosen harmonics of the played note — formant synthesis as described in the original Vector Phase Shaping research (Kleimola et al. 2011). This produces vocal, resonant character at the source, before any filtering happens. The third is MORPH’s formant wavetable mode, which sweeps through vowel shapes directly with the morph slider. Both are covered in the oscillators chapter (Chapter 2 — Oscillators).

The combinations are the point. Generate a vowel at the source with the VECTOR FORMANT switch, then focus it with a fixed bandpass from this chapter — the oscillator supplies the character, the filter supplies the mouth. Or run MORPH’s vowel sweep into a slowly morphing SVF and let two formant systems drift against each other. Because everything upstream is one voice by design, the result stays a single voice with a single throat — which is precisely what makes it read as vocal.