Rubin Manual

Overview

Rubin is a stereo ADSR envelope generator + VCA & VCF. It takes a stereo audio input and shapes both channels independently, either as an amplitude envelope (VCA mode) or as a filter sweep (Filter mode). Three operating modes cover all the needs on envelope shaping: Normal for conventional ADSR and gating, Inverted for ducking and sidechain shapes, and Looped for continuous, self-running motion.

What sets Rubin apart is that it listens. The module continuously reads the pitch, spectral content, and rhythmic density of the incoming signal and uses what it hears to shape the envelope on its own. The idea is to mimic how acoustic instruments behave, where the force a player applies, the note they play, and the tempo they're playing at all feed into the shape and intensity of every sound. The goal is an envelope that feels natural and organic, one that breathes with the music instead of stamping a fixed, mechanical contour on top of it. All of this happens with no extra patching and no external modulation.

Signal Flow

Two envelope generators run in parallel, one per channel, both fed by the same real-time analysis of the input. Each drives either a VCA or a low-pass filter depending on the mode. The second generator carries a small randomized offset that opens the stereo image without ever fighting the first.

Inputs

In L / In R (Audio)

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

If the signal at either input exceeds the module's full-scale level, an internal clipping flag is set. Keep input levels within the standard Eurorack audio range to avoid saturation before the envelope stage.

Trigger

Trigger input. Fires the envelope on each rising edge. A trigger runs a full envelope cycle on its own (see Operating Modes for how each mode handles retriggering). This mode is probably prefered for eurorack sequencing style.

Gate

Gate input. Holds the envelope open while the signal is high and releases it when the signal goes low. Gate and Trigger can be patched together for retrigger behavior, and the way they interact changes per mode (see the patching tables under each operating mode). This mode is probably prefered for users with keyboards.

The peak level of the gate signal sets the peak of the envelope in Normal mode: a stronger gate opens the envelope further, a quieter gate keeps it low. More on this below.

Controls

Rubin has four knobs and one button. The knobs keep their familiar ADSR names. Each knob has a corresponding CV input that adds to the knob value (bipolar, summed and clamped), except Sustain.

All CV inputs accept audio-rate modulation. Every CV is captured by a 16-bit ADC and read/summed per-sample at 48 kHz. No slow control-rate bottleneck.

Attack

Sets the envelope attack time. The exact range and curve depend on the operating mode (see below).

Decay

Sets the envelope decay time. If Rhythm Sensitivity (Memory Unit) is active, the detected tempo adds a little to this time. If Frequency Sensitivity is active, higher input pitches shorten it.

Sustain

Sets the level the envelope rests at after decay.

Release

Sets the release time, with the same pitch and rhythm modifiers as Decay. In Looped mode it becomes the Autopan control (see Looped Mode).

Mode (button)

Short press toggles VCA / Filter. Long press cycles Normal, Inverted, Looped. Covered under Push Button below.

Stereo Processing

Rubin runs two envelope generators in parallel, one for the left channel and one for the right. They are deliberately not identical: the module adds a small, continuously randomized difference in timing and level between them, recalculated on every envelope cycle. This builds a natural stereo image out of a mono source with no comb filtering and no phase inversion. The difference is subtle enough to keep transient punch and spectral coherence intact while still giving the output real width.

In VCA mode, each channel's envelope multiplies the audio directly, so output level tracks the envelope curve exactly.

In Filter mode, each channel's envelope drives the cutoff of an independent low-pass filter, sweeping from 1 Hz up to 19 kHz across the full envelope range. The audio is filtered instead of gated.

Audio-Reactive Behavior

Rubin continuously analyzes the audio at its inputs. Three pieces of information are extracted and optionally folded into the envelope:

  • Frequency Sensitivity (Pitch detection): The module estimates the dominant frequency of the input. When Frequency Sensitivity is on, higher-pitched material makes the decay and release stages finish faster, while lower-pitched material earns longer tails. This mirrors acoustic instruments, where higher harmonics decay faster. The pivot sits around 300 Hz: well above it the envelope shortens noticeably, below it the shortening is minimal. The effect applies equally to Decay and Release.
  • Rhythm sensitivity (Memory Unit): The module measures the interval between consecutive gate triggers and derives a tempo. When Rhythm Sensitivity is on, that tempo shapes the intensity of the envelope: at slower tempos the sustain and release gently stretch, at faster tempos the effect fades away. The intent is to emulate how a human player leans in harder or backs off depending on the pace and feel of a passage. The envelope ends up breathing in time with the music without a single extra CV.
  • 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.

Operating Modes

Long-press the Mode button to cycle through the three modes.

Normal Mode (green LED)

Standard ADSR. The envelope fires on a gate or trigger and follows Attack, Decay, Sustain, Release in the usual order.

Attack: roughly 0.08 ms to 20 s on an exponential curve.

Decay: 10 ms to 20 s.

Sustain: the level the envelope rests at after decay, from zero to full.

Release: 10 ms to 20 s.

Gate signal as an expressive input

The peak of the envelope is directly proportional to the amplitude of the incoming gate. A stronger gate produces a higher peak, a quieter gate a lower one. This is intentional and expressive: by sending gates of varying voltage instead of a fixed +5 V, you get real-time control over the intensity of every note. Harder hits open louder, softer hits stay quiet.

This matters. Combined with the rhythm and pitch sensitivity above, a gate sequence with dynamics baked into its voltage levels produces an envelope that responds like a player rather than a sequencer. The module becomes a way to write musical intention straight into the gate signal, something a conventional envelope generator can't do, where gate voltage only sets on and off. The result is envelopes that feel fluid and alive.

Gate and Trigger patching in Normal mode

Patched inputs Behavior
Gate only Envelope opens and holds while the gate is high, releases when it goes low
Trigger only Each trigger fires a full ADSR cycle. The envelope sustains until the internal gate extensor expires (scaled to the Decay knob), then releases. A new trigger soft-retriggers from the current level
Both (Retrigger) Gate holds the envelope open. A trigger arriving while the gate is high restarts the attack from the current level
Neither TODO

Inverted Mode (red LED)

The output is flipped: full level when idle, dipping down when a gate or trigger arrives, then returning to full. This is the classic ducking or sidechain shape, loudest when nothing is happening and pulling back when a trigger/gate lands.

The character and shape of the envelope are adapted in this mode to better fit the sidechain effect and make it work as a LFO shaping tool.

Attack: how quickly the dip happens, roughly 3 ms to 20 s.

Decay: how long it takes from the dip to the floor of the envelope, up to 10 s. The attack time is internally subtracted from the decay calculation, so the total timing stays accurate no matter how attack is set (after long hours of testing this felt more intuitive for this parameter).

Sustain: the floor of the dip, that is, how far down the signal settles.

Release: how long the return to full level takes, up to 20 s, with a steep curve for natural fade-backs.

Gate and Trigger patching in Inverted mode

Patched inputs Behavior
Gate only Output ducks while the gate is high, returns to full when it goes low
Trigger only Each trigger hard-retriggers the envelope. The gate extensor keeps the envelope open until the envelope reaches the sustain stage, then closes. This ties the floor duration tightly to the programmed decay
Both (Retrigger) Gate holds the duck open. A trigger while the gate is high hard-retriggers the envelope, restarting the dip from the top
Neither Output stays at full level; the inverted envelope has nothing to trigger it

Looped Mode (cyan LED)

The envelope loops continuously, building a periodic amplitude or filter-sweep shape. Both generators loop in sync, with an automatic re-sync every five cycles to stop drift over time. The loop shape is AD, attack then decay, returning to zero before the next cycle starts. There is no sustain or release phase in the loop itself.

Attack: the rise time of each loop cycle, roughly 5 ms to 10 s.

Decay: the fall time of each loop cycle, same range.

Sustain: "repurposed" here, but tuned so it still behaves like a sustain control. Turning it up feels like the sound is sustaining longer or sitting at a higher resting level, so the musical result stays familiar even though the mechanism underneath is different. Specifically, in Filter mode, below the midpoint it scales the sweep depth from a minimal flutter up to the full frequency range, and above the midpoint it also lifts the filter floor so the filter never fully closes between cycles. In VCA mode, it sets the minimum level the envelope dips to, blending the looping motion toward a constant level as the knob rises.

φ/⊙: Autopan control, described in detail below.

Autopan in Looped Mode

Since Release has no conventional job in a looping AD envelope, the knob steers the stereo movement of the output by adjusting the phase relationship between the two envelope generators.

φ, Phase mode (knob left half). Both generators run at the same speed, but the right channel is delayed relative to the left by a selectable phase offset. At the far left the offset is 360°, so both channels are effectively in phase and the image is centered, giving a clean, click-free entry when you sweep in from center. At the midpoint of this half the offset is 180°, a classic ping-pong where the image alternates fully left and right in time with the loop. Turning from left to center sweeps continuously between centered and ping-pong.

⊙, Spin mode (knob right half). Instead of a fixed phase offset, the right channel's attack and decay times are stretched relative to the left. This gives a continuous rotation of the stereo image that never locks into a fixed pattern. The further right you turn, the greater the timing divergence and the stronger the spin.

Gate and Trigger patching in Looped mode

When Trigger or Gate is patched, a third fade envelope (the "tail") layers over the looping signal to give the loop a start and stop shape.

Patched inputs Behavior
Gate only Loop runs continuously. The tail opens on the rising edge of the gate and fades out when the gate goes low. A new gate resets both loopers and the tail to their start point
Trigger only Loop runs continuously at full tail level, no fade-out. Each trigger resets both loopers to their start point for rhythmic re-sync
Both (Retrigger) Gate controls the tail fade. A gate resets both loopers and the tail. A trigger independently resets the loopers mid-cycle without touching the tail, allowing rhythmic re-sync while the gate stays open
Neither Loop runs continuously at full level with no tail shaping

VCA / VCF

Short-press the Mode button to toggle between VCA and VCF. The right-hand LED is lit in VCF mode and dark in VCA mode.

VCA mode: the envelope controls output amplitude directly.

VCF mode: the envelope drives the cutoff of an independent low-pass filter on each channel. A fully open envelope sweeps the filter to 19 kHz, a fully closed envelope closes it to 1 Hz. Because the two generators differ slightly in timing, the two filters differ slightly too, adding a subtle spectral dimension to the stereo image.

Push Button

The Mode button has two gestures:

Action Result
Short press Toggles VCA / Filter processing
Long press Cycles the operating mode: Normal → Inverted → Looped

A 100 ms crossfade smooths every mode change.

Outputs

Out L / Out R

Stereo output. Each channel carries its own envelope, VCA or filter, including the subtle randomized offset that widens the stereo image. A true stereo input stays stereo all the way through.

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 operations are computed in 32-bit floating point.

LED Indicators

Rubin has three LEDs.

LED 1

A three-color LED showing the current operating mode:

  • Normal: green (top)
  • Inverted: red (middle)
  • Looped: cyan (bottom)

LED 2

Lit in VCF mode, dark in VCA mode.

LED 3

Envelope follower.