dynth

A real-time subtractive synthesizer that turns a Pioneer DDJ-FLX4 into a proper synth controller. No DAW, no plugins — just raw audio computed sample-by-sample in Rust.

The jog wheels don't control tempo. They scrub a wavetable and drive FM self-modulation. The crossfader blends two independent synth voices. The EQ knobs sweep a Moog-style ladder filter.

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What it sounds like

Two independent synthesizer voices (Deck A and Deck B), each with:

• Wavetable oscillator — sine, saw, square, triangle
• 4-pole ladder filter with resonance
• ADSR envelope
• Sine LFO that can target cutoff, pitch, or amplitude

Blend them with the crossfader. Scratch the jog wheel to scrub the wavetable — the faster you scratch, the more FM distortion gets added to the tone.

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Requirements

• Rust 1.75+
• Pioneer DDJ-FLX4 connected via USB
• macOS (CoreAudio / CoreMIDI) — Linux should work with ALSA, untested

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Run

git clone <this repo>
cd dynth
cargo run --release

--release is not optional — the debug build can't keep up with the audio callback at 44100 Hz.

Custom config:

cargo run --release -- /path/to/my-config.toml

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DDJ-FLX4 controls

Hardware	Deck	What it does
Left jog wheel	A	Wavetable scrub + FM depth
Right jog wheel	B	Wavetable scrub + FM depth
EQ HIGH	A / B	Filter cutoff (80–8000 Hz, exponential)
EQ MID	A / B	Filter resonance
Channel fader	A / B	Deck volume
Crossfader	—	Blend Deck A ↔ Deck B

MIDI channel 1 → Deck A, channel 2 → Deck B for note input. Connect a keyboard to play notes; jog wheel touch pads also trigger voices.

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Keyboard controls (in the TUI)

Switch focused deck with a, b, or Tab. All controls below apply to the focused deck.

Key	Action
1 2 3 4	Waveform: sine / saw / square / triangle
e / E	Attack +/−
d / D	Decay +/−
s / S	Sustain +/−
r / R	Release +/−
] / [	LFO rate up/down
= / -	LFO depth up/down
t	Cycle LFO target: None → Cutoff → Pitch → Amp
q	Quit

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Config

CC mappings live in config.toml. Each entry looks like:

[[mapping]]
device   = "DDJ-FLX4"   # substring match on port name
type     = "cc"
channel  = 1             # 1 = Deck A, 2 = Deck B, 0 = any
cc       = 7
param    = "filter_cutoff"
deck     = 0
min      = 80.0
max      = 8000.0
exp      = true          # exponential scaling (recommended for frequency)

To find your device's CC numbers, temporarily add eprintln! in the MIDI callback and wiggle controls — see DYNTH.md for details.

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Tests

cargo test

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Technical overview

See DYNTH.md for the full architecture writeup — signal chain, threading model, jog wheel FM mechanics, filter stability details.