Doppler Manual
Overview
Doppler turns any audio source into percussion. Feed it an oscillator, a sample, a synth voice, anything, and it shapes it into kicks, snares, hi-hats, and textures in between.
The core mechanism is a delay-line pitch processor. Audio written into a delay line is read back at a moving position, and that movement creates pitch shifts, exactly like the Doppler effect when a sound source passes you. A trigger resets the read head, which then sweeps toward a target depth, creating a falling pitch envelope. That's the kick.
Beyond Doppler, the module offers FM synthesis using the input audio itself as a reference for frequency tracking, and harmonic chording using multiple pitch-shifted delay taps. The Mod knob sweeps continuously through all of these: from pitch-shifted chords at one end, through Doppler kick territory in the middle, to aggressive FM at the other end.
Inputs
Body
The primary audio input. This signal is written into the delay line every sample. Before that write, it passes through a gentle one-pole low-pass filter to remove DC and high-frequency content that would cause artifacts in the delay processing.
All pitch processing (Chord, Doppler, FM) reads back from this delay line.
Trans (Transient)
A secondary audio input intended for a transient or noise source, a snare, a hi-hat, white noise, anything with edge content. It is filtered by an auto-tuned band-pass. This is needed to morph noise depending on pitch input so we can better create different types of drums.
The filtered Trans signal is mixed into Out via the noise envelope. It creates the transient attack layer, the "click" before the body of a kick, or the "snap" of a snare.
Trigger
Gate/CV trigger input. Uses a Schmitt trigger internally so it responds cleanly to both gate signals and fast CV triggers. On each rising edge:
- The main envelope fires
- The noise envelope fires only if Attack is turned left of 12 o'clock (see Attack section)
Controls
Attack
The Attack knob has three distinct behavioral regions:
Left of 12 o'clock (CCW half) Attack time is near-instant (1 ms, essentially a click). In this range, the knob controls a noise transient decay instead. The further left you go, the longer the noise envelope lasts, from a brief crack to a long hiss that decays at the same rate as the main Decay setting. This is the snare/hi-hat territory for the Trans input.
At exactly 12 o'clock, both the noise decay and the attack time are at their minimum.
Just right of 12 o'clock (roughly 12–1 o'clock zone) Attack sweeps from 0.1 ms to 4 ms. Noise envelope is off. Short, snappy transient into the body.
Right of that (1 o'clock to fully clockwise) Attack sweeps from 4 ms to 100 ms. Creates a fade-in or swell on the body of the sound. Noise envelope stays off.
Attack CV jack: The CV input adds to the Attack knob value. Modulate it to animate transient behavior from an external source.
At high Mod settings (FM territory) and long Attack times, the output is automatically attenuated to compensate for the increased energy the FM accumulation produces over a long attack.
Decay
Controls the decay time of the main AD envelope. The knob curve is exponential.
This gives you fine resolution at short times (tight clicks and snaps) and a wide range up to long sustained tails. The Decay setting also influences how fast the Doppler pitch sweep resolves, slower Decay = slower pitch movement = slower pitch drop.
Decay CV jack: CV-controllable for external sequencing or modulation.
Mod
This is the main mode selector. Turning it clockwise sweeps through the module's processing engine continuously. There are six regions:
| Range | Mode |
|---|---|
| 0–10% | Chord / Harmonic |
| 10–40% | Transition (chord → Doppler) |
| 40–50% | Pure Doppler pitch envelope |
| 50–80% | Doppler → FM blend |
| 80–90% | Full FM |
| 90–100% | FM + cross modulation |
The color of the LED tracks this knob: warm amber/green in the chord and Doppler range, transitioning to blue as you enter FM territory.
Mod CV jack: Modulating this CV in the audio range sweeps the processing mode continuously, which can produce aggressive, complex textures.
Amount
Controls the character within whichever mode Mod has selected. Its behavior changes completely depending on where Mod is set:
In Chord mode: Selects one of 13 chord voicings applied to the 4 pitch-shifted delay taps:
| Position | Voicing |
|---|---|
| 0 | Unison (no pitch shift) |
| 1 | Major |
| 2 | Sus2 |
| 3 | Maj7 |
| 4 | Maj9 |
| 5 | Sus4 |
| 6 | Major |
| 7 | Maj6 |
| 8 | Minor |
| 9 | m6 |
| 10 | m7 |
| 11 | m9 |
| 12 | m11 |
When Mod is below ~5%, a low-register voicing set is used, which extends the chord with octave doublings for a fuller, darker sound.
In Doppler mode: Sets the target pitch-drop depth in five steps:
| Position | Pitch Drop |
|---|---|
| 1 | –1 octave |
| 2 | –1.5 octaves |
| 3 | –2 octaves |
| 4 | –2.5 octaves |
| 5 | –3 octaves |
In FM mode: Sets the FM oscillator frequency as a ratio of the detected spectral peak of the Body signal, from 0.1× to 1.15× (exponential curve). This is harmonic FM tracking, the modulator frequency follows the input's pitch, so the character of the FM stays consistent as you play different notes into Body.
Amount CV jack: Modulate chord voicing, Doppler depth, or FM ratio from CV.
Processing Modes - Detailed
Chord / Harmonic Mode (Mod 0–10%)
Four delay read heads advance through the delay line at rates derived from the semitone offsets of the selected chord. Each heads moves forward at a constant speed proportional to a pitch ratio, which causes the delay line to be read faster or slower than it was written, producing a pitch shift.
The output is a mix of the dry signal and the four pitch-shifted taps.
Transition Zone (Mod 10–40%)
A crossfade parameter blends between chord-rate tap increments and the Doppler head sweep. As you turn Mod clockwise through this zone:
- The four taps gradually shift from fixed chord intervals toward the Doppler pitch sweep rate
- The dry Body signal fades out.
- By Mod = 40%, only the Doppler-swept delay read is audible
Doppler Mode (Mod 40–50%)
This is the kick drum engine.
On each trigger, all ramp heads reset to zero and the current sweep speed resets to zero.
A slow Decay → slow slew → gradual pitch drop. A fast Decay → fast slew → the pitch snaps down quickly.
Each sample, the ramp head advances. This means the delay read head accelerates, creating an increasing frequency offset, the pitch appears to rise briefly then level off. From the listener's perspective: a sharp transient, a descending pitch sweep, settling at normal pitch. Classic kick drum.
The Amount knob's five depth values correspond to final head speed positions of 0.5, 0.625, 0.75, 0.8125, and 0.875. These translate to approximately –1, –1.5, –2, –2.5, and –3 octaves of pitch drop at the bottom of the sweep.
FM Blend Zone (Mod 50–80%)
A second crossfade blends between the Doppler ramp and an FM synthesis ramp.
The spectral peak is tracked with a one-pole filter to avoid sudden jumps.
This section already creates a delay read position that oscillates according to the FM oscillator, shaped by the main envelope, a percussive FM pitch modulation applied to the delay line.
Full FM Mode (Mod 80–90%)
The Doppler sweep is gone. The FM depth grows over the course of the note, the deeper into FM range, the more aggressive the modulation at its peak.
The main envelope shapes the FM modulation depth, so attack and decay directly control how aggressively the FM punches in and tails off.
Amount continues to control the FM frequency ratio, keeping the modulator harmonically related to Body's input pitch.
FM + Cross Modulation (Mod 90–100%)
The Body audio signal itself is added to the FM oscillator's frequency input.
At low amounts, the audio contribution is subtle. Past a certain point, the cross modulation dominates and the result becomes chaotic and self-referential, the character of the incoming audio directly warps the FM spectrum.
This range is best for industrial percussive sounds, clangs, metallic impacts, and unpitched noise textures.
Outputs
Out
The main output. The mix formula depends on which mode is active (see VCA vs LPG below). Both modes share the following structure:
- Main body signal (pitch-processed Body through delay line) shaped by the main envelope
- Transient layer (band-pass filtered Trans) shaped by the noise envelope
Anti-click: On every new trigger, the output sample from the previous cycle is captured. That value is immediately subtracted from the new output, then decays at 0.999x per sample (~0.1 s to inaudibility). This tries to prevent audible clicks when retriggering at high rates or while the previous hit is still decaying.
Noise
An independent dust/crackle oscillator output, separate from Out. The density of the dust is automatically modulated by the Body signal's spectral content:
- Body pitch peaks in the low range (kick territory) → lower density, sparse pops
- Body pitch peaks in the high range (hi-hat territory) → higher density, denser crackle
- If Body appears to be broadband noise → fixed medium density
This output can be used freely on its own, or mixed externally with Out to add texture.
VCA Mode vs LPG Mode
Long-pressing the push button (≥ 500 ms) switches between two envelope architectures. The setting is saved to non-volatile memory and persists across power cycles. Transitions between modes are crossfaded smoothly.
LED 2 is green in LPG mode and off in VCA mode.
VCA Mode (LED 2 off)
The main envelope is a standard AD amplitude envelope, it controls how loud the signal is over time. Attack and Decay directly shape the volume contour. Predictable, punchy, good for tight percussion.
LPG Mode (LED 2 green)
Instead of a VCA, the main envelope drives a low-pass gate: an SVF filter whose cutoff frequency is swept by the envelope on a logarithmic curve from 1 Hz to 10 kHz. As the envelope rises, the filter opens and the body brightens. As it decays, the filter closes and the sound darkens and quiets simultaneously.
This produces an organic, Buchla-style pluck character, the timbre and amplitude breathe together. Well suited for pitched percussion and melodic transients. The Trans layer is slightly quieter in this mode to keep the character consistent.
Push Button
| Action | Result |
|---|---|
| Short press | Manual trigger, fires both envelopes as if a gate arrived at Trigger |
| Long press (≥ 500 ms) | Toggles VCA / LPG mode (saved to memory) |
LED Indicators
LED 1 - Mode Color
Tracks the Mod knob position. Changes color continuously:
- Mod 0–50% (chord and Doppler): Warm range, amber through yellow-green
- Mod 50–90% (FM blend and full FM): Shifts toward blue
- Mod 90–100% (cross mod): Red/green tint returns as blue recedes
Use this as a visual reference for which processing region Mod is currently in.
LED 2 - Envelope Mode
- Green: LPG mode active
- Off: VCA mode active
LED 3 - Envelope Activity
- Red: Noise envelope level (the Attack transient)
- Green: Main envelope level (the body decay)
Patch Ideas
Basic kick drum
- Patch a sine oscillator or low-frequency oscillator into Body
- Trigger from sequencer into Trigger
- Mod at ~45% (Doppler mode), Amount at 12 o'clock (–2 octave drop)
- Decay to taste, Attack at 12 o'clock
Snare
- Patch sine into Body
- Attack fully CCW (maximum noise decay), Decay moderate
- Mod at 45–50%, Amount to adjust pitch drop
Hi-hat
- Patch something into Body
- Attack fully CCW for a crisp transient
- Short Decay
- Mod and Amount to the right for complex and metal sound generation
FM tom
- Mod at 80–85%, Amount to dial in the FM harmonic
- Medium Decay
- Attack just right of center for a slight swell
Melodic pluck
- LPG mode on
- Patch a tuned oscillator into Body
- Chord mode (Mod < 10%), Amount selects voicing
- Short Decay, Attack at center
Chaos texture
- Mod past 90%
- Patch a complex signal into Body (multiple oscillators, FM output, etc.)
- Modulate Amount CV with an LFO