Fundsp Reverb (Simple variant)

fundsp-reverb-simple plugin

Stereo plate reverb wired through a fundsp audio graph. Same topology / params / signal flow as fundsp-reverb-worker, but the graph rebuild happens inline on the audio thread instead of on a background worker.

Source: examples/truce-example-fundsp-reverb-simple/.

The fundsp integration guide walks through both variants in depth, explaining the rebuild trigger, why some params need a rebuild, and what the worker pattern buys you.

in (L,R) ──► high-pass (low cut)  ──► low-pass (high cut)  ──► reverb_stereo ──┐
                                                                                │
in (L,R) ─────────────────────────────────────────────────────────────────► dry ┤──► out
                                                                                │
                                                              mix ──────────────┘
Param Range
Low Cut 20 Hz → 2 kHz (log)
High Cut 500 Hz → 18 kHz (log)
Time 0.1 s → 20 s (log)
Mix 0 → 1

#Integration patterns

  • Graph rebuilt inline in process(). When Time crosses the hysteresis threshold, process() calls rebuild_graph directly. Box::new(...) and graph.allocate() run on the audio thread. The worker variant moves both off-thread; everything else about the graph wiring is identical between the two crates.
  • Hysteresis on Time changes. reverb_stereo bakes RT60 at construction, so each crossing means a full rebuild. The 5% threshold keeps tiny drifts (smoother ramps, automation jitter) from triggering rebuilds.
  • Read the raw target, not the smoothed value. A smoothed time.read() would crawl across the threshold for ~200 ms on each knob move and rebuild every block until it settled — audible as an unstable tail.
  • Params reach the graph through fundsp::Shared atomics. var(&shared) reads them per sample; the closure inside for_each_frame writes the smoothed truce-side value into the cell on the same tick (sample-accurate automation).
  • Box<dyn AudioUnit> for the field type. The concrete An<…> is hundreds of chars of nested generics; the vtable cost is one indirection per block.
  • AudioBuffer::for_each_frame::<2, _> transposes truce's per-channel layout into stack-allocated frames so fundsp's tick(in, out) callback can be called directly. No scratch field.

#Gotchas

  • Filter input order is positional and unchecked. highpass() / lowpass() take (signal, cutoff, Q). Every connection is f32, so (cutoff | Q | signal) >> highpass() compiles fine and silently feeds the filter cutoff in as audio — the resulting filter blows up the reverb FDN to peak ~7000 within a second. Test against constant input + assert_peak_below.
  • Type-level channels. dry * mix fails to compile when dry is stereo and mix is a 1-channel Shared read; broadcast the mix to stereo manually with var(&mix) | var(&mix). fundsp's payoff (graph composition with >> / | / &) costs this kind of explicit plumbing.

#Build

cargo build -p truce-example-fundsp-reverb-simple
cargo test  -p truce-example-fundsp-reverb-simple --release
cargo truce install -p truce-example-fundsp-reverb-simple
cargo truce run     -p truce-example-fundsp-reverb-simple