Bitcrust.

Bitcrust’s SID engine is written from scratch — Oscillator, Envelope, Filter — against public datasheets and general DSP literature. Every quirk of the original silicon — the bit-crushing, the leaky integrators, the infamous filter resonance — is modelled deliberately, so what you hear is the real character of the chip, not an approximation.

Each of the twelve voices runs three independent chip instances — thirty-six in total — so per-oscillator volume is a real control, not a software overlay on top of a fixed chip architecture. The SID chip came in two revisions — the gritty, distorted 6581 and the cleaner, tighter 8580 — and their filters sound nothing alike. Bitcrust models both curves in the same engine and lets you blend between them continuously, so you can dial in tones that never existed in real hardware.