Address three issues introduced with the 3-oscillator synth:
- Crashes: ToolboxSlot.values was a plain HashMap read on the audio thread while
the UI wrote it (knob drags vs. the synth's ~40-param-per-note reads). Made it a
ConcurrentHashMap so get/put are atomic and reads are lock-free (RT-safe).
- Filter cutoff not affecting the sound: (a) the cheap ladder went unstable at high
cutoff (f > 1 -> NaN -> silent voice) — replaced with a stable Cytomic TPT
state-variable low-pass (with NaN recovery); (b) a voice captured its patch only
at note-on, so live knob turns were inaudible — the engine now refreshes active
voices' patches every block, so cutoff/level/mod changes affect held & playing
notes.
- Hiccups: every note allocated a SynthPatch (+arrays) causing GC dropouts — the
patch is now cached per bus and reparsed only on param/tempo change, so steady
playback allocates nothing on the audio thread.
Patch bump to 0.4.3 (versionCode 10).
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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