adding-synthdef

---

name: adding-synthdef description: Use when the user asks to "add a SynthDef", "create a new instrument", "ajouter un instrument", or names a synthesis technique to package (FM, granular, Karplus-Strong, formant, etc.). Generates a SynthDef under synth/<category>/ (drums, bass, lead, pad, world, master) for instruments, or fx/<type>/ (bus, insert, trick) for effects. The auto-loaders synth/_index.scd and fx/_index.scd discover new files recursively. Follows the rigid sound_algo convention: standard arg signature (freq/gate/ADSR/amp/pan/scBus/scAmt), EnvGen.kr(..., doneAction: 2), Pan2.ar final, LeakDC.ar after feedback, clip on freq, tanh saturation pattern. Forgetting doneAction: 2 creates Synth zombies that accumulate in RAM until the server crashes.

Adding a SynthDef

Where to put it

Pick a directory based on the SynthDef's role. The auto-loader (_index.scd) recursively loads every .scd in the tree, so a new file is picked up automatically without editing any index.

Directory	Theme
synth/drums/	Kicks, snares, claps, hats, percs, taiko, tabla
synth/bass/	Sub bass, FM bass, reese, hooverbass
synth/lead/	Lead synths, plucks, scratches
synth/pad/	Warm pads, choirs, strings, drones
synth/world/	Exotic/world instruments (koto, erhu, didgeridoo, ...)
synth/master/	Master-bus instruments / utility synths
fx/bus/	Send-bus FX (reverb, delay, chorus, ...)
fx/insert/	Insert FX on a single channel
fx/trick/	Live tricks (freeze, glitch, stutter, ...)
engine.scd	DO NOT add here -- it's the core, kept stable

Standard arg signature

Every SynthDef MUST accept these args (with defaults) :

arg freq = 220, gate = 1, cutoff = 6000, rq = 0.5,
    attack = 0.001, decay = 0.04, sustain = 0.0, release = 0.6,
    drive = 1.2, amp = 0.22, pan = 0,
    scBus = 0, scAmt = 0.4;

This way the SynthDef is a drop-in replacement in ~melodyInst, ~kickInst, etc.

Template

SynthDef(\<instName>, {
    arg freq = 220, gate = 1, cutoff = 6000, rq = 0.5,
        attack = 0.001, decay = 0.04, sustain = 0.0, release = 0.6,
        drive = 1.2, amp = 0.22, pan = 0,
        scBus = 0, scAmt = 0.4;
    var sig, env, duck;

    // --- 1. Synthesis core (oscillator / noise / sample / ...) -------
    sig = ...;

    // --- 2. Filter (LPF/RLPF/HPF/...) --------------------------------
    sig = LPF.ar(sig, freq.clip(40, 18000));

    // --- 3. Drive / saturation (preserve gain) -----------------------
    sig = (sig * drive).tanh / drive.sqrt;

    // --- 4. Leak DC if any feedback / waveshaping --------------------
    sig = LeakDC.ar(sig);

    // --- 5. Envelope with doneAction:2 (free node when env ends) ----
    env = EnvGen.kr(Env.adsr(attack, decay, sustain, release),
                    gate, doneAction: 2);

    // --- 6. Sidechain duck (kick -> instrument) ---------------------
    duck = 1 - (In.kr(scBus, 1) * scAmt);

    // --- 7. Output : Pan2 stereo ------------------------------------
    Out.ar(0, Pan2.ar(sig * env * amp * duck, pan));
}).add;

Mandatory invariants

Rule	Reason
doneAction: 2 on EnvGen.kr	Without it, Synth zombies pile up forever
Pan2.ar(sig, pan)	Mono [sig, sig] breaks the mixer's stereo bus
freq.clip(min, max)	Unclamped freq -> NaN/Inf at filter
(sig * drive).tanh / drive.sqrt	Preserves perceived loudness across drive
LeakDC.ar after feedback	Comb/Karplus need it or DC offset accumulates
In.kr(scBus, 1) * scAmt	Standard sidechain bus pattern
Defaults on EVERY arg	Pdef breaks if any arg is missing

Synthesis recipes

// Karplus-Strong (plucked string)
exciter = WhiteNoise.ar(0.6) * EnvGen.kr(Env.perc(0.0005, 0.003));
sig = LeakDC.ar(CombC.ar(exciter, 0.05, freq.reciprocal, 3.0));

// FM bell
mod = SinOsc.ar(freq * 1.4) * 200 * EnvGen.kr(Env.perc(0.001, 0.5));
sig = SinOsc.ar(freq + mod);

// Granular pad
sig = GrainSin.ar(2, Impulse.kr(20), 0.1, freq, pan);

// Formant vowel
sig = Formant.ar(freq, formantFreq, bw);

After creation

1. Run validating-scd-files on the synthdefs file (P:0 B:0, TLB:1).
2. Test SynthDef compilation : tests/e2e_04_synthdef_compile.scd should still pass.
3. Add the new symbol to the README's instrument list if user-facing.

Anti-patterns

• No doneAction: 2 -> Synth zombies, RAM leak, eventual crash.
• Out.ar(0, sig) -> mono, breaks panning + stereo mixer.
• Hardcoded 0.5 for amp -> not user-controllable.
• Missing default value for an arg -> Pdef crashes when scheduling.
• Duplicating an existing SynthDef name -> silently overwrites.
• SinOsc.ar(freq) without clip -> NaN if freq <= 0 from a modulator.