Capicola for Schwung

A live time-stretch and pitch-shift audio FX for Schwung on Ableton Move.

Incoming stereo is continuously reduced to bandlimited local extrema ("keyframes") and replayed by a lagging read head under independent time and pitch control. A transient detector runs in parallel; every kept transient snaps the read head back to the present with a crossfade, so stretched tails ring out underneath while the output stays locked to the rhythm of the input.

Credit

This is a port of capicola by Heavylight Industries — the hardware embodiment of their independently authored, peer-reviewed DAFx26 paper Keyframe Time Stretching via Extrema Sampling. The DSP core in src/dsp/lib/ is their work, vendored unmodified. See VENDOR.md.

Their hardware module runs on the Alchemy Lab V2 (Daisy / STM32H750) and has a Eurorack panel with CV. This port keeps the engine and replaces the panel with Schwung param pages. Audio examples and the interactive manual for the original are linked from the upstream README.

Licence

AGPL-3.0, matching upstream. See LICENSE.

Implementation notes

Memory

Each instance holds two keyframe rings (one per channel) of 2^18 frames x 12 bytes = 3.1 MB per channel, 6.3 MB per instance — about 12 seconds of audio at the paper's typical keep ratio for musical material. Upstream uses 2^21 frames (25 MB/channel, 50 MB/instance), sized for a Eurorack module that freezes up to ~90 seconds on dedicated SDRAM. Schwung allows up to 12 concurrent instances (4 chain slots + 8 Master FX positions); at upstream's ring size that ceiling would be 600 MB. Move has ~1.28 GB available, so even upstream's size would technically fit, but there's no reason to spend it — the ring size is a single template parameter.

Known realtime violation

create_instance runs on the SPI audio callback thread — see src/dsp/audio_fx_api_v2.h / src/dsp/plugin_api_v1.h and Schwung's docs/REALTIME_SAFETY.md rule 4, which requires every plugin entry point (including create_instance) to be realtime-safe. The ~6.3 MB keyframe-ring allocation in create_instance is a genuine violation of that contract: it calls into the allocator from a SCHED_FIFO 90 thread. This is documented rather than hidden. Mitigating factors: it is one-shot, at module load time, on the same thread that already dlopen()s the plugin (a pre-existing violation of the same kind); it matches what the rest of the module fleet does. It is still real, and a device under load could in principle glitch on the frame a Capicola instance is created. set_param, get_param, on_midi and process_block perform no allocation, no blocking calls and no filesystem access.

Measured on hardware, create_instance originally took 14.3 ms against the SPI callback's ~900 µs budget — Engine::Init() allocated Impl value-initialized (new (std::nothrow) Impl()), which zero-fills the whole 6.3 MB block, immediately followed by KeyframeRecorder::Init overwriting every byte of it again (sparse.Init() → Clear() → buffer.fill(Keyframe{0.0f, 1.0f}), plus every scalar control field set explicitly). Switching to default-initialization (new (std::nothrow) Impl, no parens) removes that redundant zero-fill pass; KeyframeRecorder was already made trivially constructible upstream for exactly this reason (see its header comment). This does not eliminate the violation — the ring allocation itself still happens on the SPI thread — it removes a pass of work that was strictly wasted. On this dev machine, single-shot create_instance calls (which is what actually happens once per module load, unlike a warm-allocator loop) dropped from a ~1.2–1.9 ms range to a ~0.77–1.0 ms range; the ARM device's 14.3 ms baseline is expected to shrink by a similar proportion, not to disappear.

Differences from upstream

	Upstream	Here
Sample rate	48 kHz	44.1 kHz
Ring	2^21 keyframes/ch	2^18 keyframes/ch
Instances	2 SDRAM globals	per-instance
Panel	Eurorack, 4 pages + CV	Schwung param pages, 4 pages
Mod sources	input env / output env / CV IN	input env / output env
Presets	QSPI slot	Schwung slot autosave + user presets

Modulation depth and routing are ported in full. Upstream's CV IN source is dropped — Move has no such jack, and Schwung's chain LFOs and modulation routing already reach these parameters from outside, which is the same capability by another route.

Vendored DSP core

src/dsp/lib/ is copied byte-identical from upstream and must never be edited in place. Any fix goes in patches/, applied to a build-tree copy — see VENDOR.md for the mechanics.