Door Manual

Overview

Door is a stereo low-pass gate with a strong sense of rhythm and pitch. Instead of a static gate that just opens and closes, Door listens and adapts its response depending on: how fast you trigger it, what pitch you feed it, and how high the level of your gates are. The result is a percussion instrument that sounds organic and physical, closer to striking a real object than opening and closing a VCF.

At its heart sits a pair of in-house designed filters (one per channel) that morph continuously from a gentle 1-pole low-pass gate into a punchy resonant 2-pole filter as you turn up Resonance. In front of the filters is a saturator / wavefolder that not only warms up the tone but also reshapes the attack and envelope to imitate different drum materials: from soft mallets to hard metallic strikes.

Door has two personalities, chosen automatically by whether the Trigger jack is patched:

  • Trigger patched: Low-Pass Gate (LPG). Each trigger fires an envelope that opens the gate. This is the percussion instrument. With no input signal, the audio path is normalized to an internal noise source, so Door can produce hi-hats and kicks (when self-oscillating) all by itself.
  • Trigger unpatched: Stereo Filter. No envelope runs; the gate stays open and Door becomes a growling, self-resonating stereo low-pass filter. In this mode the otherwise-idle Decay knob is repurposed to bias the filter's feedback path, sculpting the growl and generating different stereo images.

Signal Flow

  • In LPG mode: Input -> VCA(Envelope(pitch), Trigger Voltage) -> Saturator/Folder(Timbre) -> LPF(Envelope(pitch), Frequency, Resonance) -> Memory Unit(Rhythm) -> Soft Clip -> Out.
  • In Filter mode: Input -> Saturator/Folder(Timbre) -> LPF(Frequency, Resonance, Bias) -> Soft Clip -> Out.

Inputs

L In / R In (Audio)

Stereo audio input. Both channels run through independent filters, so a true stereo source stays stereo.

Noise normalization (LPG mode only): when the Trigger jack is patched and nothing is plugged into the audio inputs, the signal path is normalized to an internal noise generator. This lets Door make hi-hats, kicks and noise-based percussion with no external oscillator. In Filter mode there is no noise normalization, the filter needs real input (or its own self-oscillation).

Trigger

Gate input and the mode switch for the whole module:

  • Patched: LPG mode. A Schmitt-triggered gate detector fires the envelopes on each rising edge. It responds cleanly to both slow gates and fast CV triggers. The max voltage value of the gate controls the output volume, lower gates will return quieter sounds.
  • Unpatched: Filter mode. Envelope is always open; Door is a continuous stereo filter.

On each trigger in LPG mode:

  • The main envelope and the transient envelope both fire.
  • The envelope's floor is set to the current output value so retriggers don't click.
  • The time between the last two triggers is measured and used by the rhythm-sensitivity engine (Memory Unit).

Controls

Door has four knobs. Each knob has a corresponding CV input that adds to the knob value (bipolar, summed and clamped).

All CV inputs accept audio-rate modulation. Every CV is captured by a 16-bit ADC and read/summed per-sample at 48 kHz, so you can drive Frequency, Decay, Resonance and Timbre with full-bandwidth audio, not just slow control voltages. No slow control-rate bottleneck: the CVs are as fast as the audio path itself. This opens up filter FM, cross-modulated timbres, and (on the Decay input in Filter mode) a third audio input into the filter core (see Decay below).

Frequency (cutoff)

Sets the filter cutoff on a logarithmic scale.

  • LPG mode: Frequency sets the ceiling the envelope opens toward. So the envelope sweeps from nearly closed up to your Frequency setting on every hit and closes again as the envelope decays.
  • Filter mode: Frequency directly sets the cutoff over a 10 Hz to 18 kHz range.

Frequency CV: dedicated CV input (added to the knob). Use it for filter FM or envelope sweeps from an external source.

Decay

This knob does two completely different jobs depending on mode. (We apologize for repurposing an otherwise idle knob. We know this is terrible UX practice. But the possibilities and sound results of direct feedback bias control at audio rate were simply too tempting to resist.)

LPG mode: envelope decay time. Sets how long the gate stays open after each trigger. The knob curve is exponential:

Fine resolution for short, tight ticks at the low end; long tails at the top. The decay time is further scaled by the input pitch and the rhythm period, so high-pitched sources decay faster and fast rhythms tighten up automatically.

Filter mode: feedback bias. With no envelope running, Decay is repurposed as the bias offset of the filter's internal feedback path. Turning it moves a DC offset into the resonant feedback loop before the filter's internal feedback clipper. This asymmetric clipping control enable you to adjust the growl sound to taste:

  • Centered bias: cleaner, symmetric resonance.
  • Pushed either way: the feedback clips asymmetrically, adding grit and flavor as the filter rings.

Critically, the two channels get slightly different bias. Because the growl is generated only in the clipped, resonating portions of the signal, the two channels diverge only while resonating, producing a wide stereo image on the resonant peaks while the rest of the signal stays phase-coherent. This is the stereo trick unique to Filter mode.

Decay CV: a third audio input into the filter (Filter mode). Because the Decay CV is summed at audio rate and, in Filter mode, controls the feedback bias, modulating it with an audio signal does something special: a bias modulated at audio rate is itself an audio input to the filter. The signal you patch here does two things at once:

  1. It leaks into the outputs: its content passes through the filter alongside the main audio inputs.
  2. It modulates the resonant feedback path: warping the growl, pitch and stereo behavior of the resonance in response to the injected signal.

This makes the Decay CV a route for chaotic signal interaction: feed a second oscillator, another voice, or feedback from elsewhere in your patch and let it collide with Door's self-oscillation. Ideal for drones, cross-patched feedback systems, and unpredictable textures. (In LPG mode this input behaves as ordinary decay-time modulation and the internal bias is set to zero.)

Resonance

Controls both the character and the amount of resonance by doing something unusual: it morphs the filter's pole count.

First ~10% of knob travel: pole morph (1-pole to 2-pole). Resonance stays at zero, but the filter crossfades from a single-pole low-pass gate (the classic, gentle, buttery LPG response) into a two-pole filter (steeper, more focused). The blend uses a squared curve for a smooth hand-off. This lets you dial the slope/flavor of the gate before any resonance appears: subtle vactrol-style LPG at fully counter-clockwise, sharper modern LPG as you exit this zone.

Remaining ~90% of knob travel: resonance amount. The filter is now fully 2-pole, and this range raises the resonant feedback from none up to self-oscillation.

So this single knob takes you seamlessly from subtle classic 1-pole LPG, to 2-pole filtering and then to screaming resonant filter.

Resonance CV: dedicated CV input (added to the knob).

Timbre

Door's signature material / drive control. It combines a saturator + wavefolder with attack- and envelope-shaping so a single knob sweeps through different "drum materials," from soft and organic to hard and metallic.

First ~30% of knob travel: soft materials.

  • Attack time is lengthened at the far-left and shortens toward the center, giving a soft, rounded onset.
  • The transient layer's gain rises, so the strike gradually gains definition.
  • Folder drive stays low and clean, gentle tone.

Think soft mallets and muted membranes.

Remaining ~70% of knob travel: hard materials.

  • Transient punch scales up.
  • The transient tail is extended slightly for body.
  • Folder drive climbs steeply, pushing the signal into the wavefolder/saturator: added harmonics, then folding, then aggressive metallic overtones at the extreme.

Think wood, then metal, then a full wavefolder scream.

Timbre CV: dedicated CV input (added to the knob).

Push Button

The push button's function depends on mode:

Mode Action Result
LPG (Trigger patched) Press Manual trigger: fires the envelopes exactly as an external gate would. Play the module by hand.
Filter (Trigger unpatched) Press Toggles cross-modulation on/off: an extra layer of spice.

Cross-modulation (Filter mode): when enabled, each channel's previous filter output is fed back into that channel's cutoff and resonance. This makes the filter modulate itself with its own resonant voice: adding movement, chaos and extra growl. The amount of cutoff cross-mod is also scaled by the Decay (bias) knob, so Decay and cross-mod interact. LED 2 shows the cross-mod state (see below).

Envelopes (LPG mode)

Two AD envelopes run internally, both with a soft exponential curve:

  • Main envelope: the body/decay. Attack ≈ 1.5 ms + Timbre-derived attack; decay set by the Decay knob and scaled by input pitch and rhythm. Its peak level tracks trigger rhythm (Memory Unit) and gate's max voltage (Gate Level).
  • Transient envelope: a short, punchy layer for the initial strike. Its level is set by Timbre's transient gain and its short tail by Timbre + Decay.

Audio-Reactive Behavior

Three internal sensitivities (configuration options via web programmer) make Door respond to how you play. They all read the incoming trigger stream and audio:

  • Rhythm sensitivity (Memory Unit): measures the time between triggers. Fast playing tightens decays, shifts folder character and boosts output gain; slow playing loosens them. Makes rolls and fills feel natural.
  • Frequency sensitivity: detects the spectral peak of the input. Higher-pitched material decays faster (like a real resonator), lower pitchs ring longer.
  • Gate Level sensitivity (Gate Level): the trigger's peak voltage level sets the envelope's maximum, so higher gates hit louder. With it off, every hit is full level.

Outputs

L Out / R Out

Stereo output. Each channel is the filtered, folded signal, multiplied by the rhythm gain term and passed through a soft-clip stage for a musical ceiling instead of hard digital clipping.

Audio Quality

Door runs a high-fidelity signal path end to end. Audio input and output are converted at 48 kHz / 32-bit, and all internal DSP (filters, folder, envelopes, feedback) is computed in 32-bit floating point. The extra internal headroom means the saturator and resonant feedback push hard without quantization grit or fixed-point overflow: overdrive stays smooth, quiet tails keep their detail, and the soft-clip ceiling stays clean.

LED Indicators

Door has three LEDs. Their meaning changes with mode.

LED 1: Input Level

Green, brightness proportional to the RMS of the input.

LED 2: Output Level

Green, brightness proportional to the RMS of the output.

LED 3: Mode Activity

  • Filter mode: red, brightness follows the cross-modulation mixer. Off when cross-mod is disabled, glowing red as you engage it with the push button.
  • LPG mode: white, driven by the rhythm "Memory Unit". It brightens with the current rhythm period and fades out as time passes since the last trigger: a visual pulse of the internal Memory Unit, brightest right after a hit and when you're playing fast.