dsp

dsp/ — the Rust noise-suppression engine, behind a C ABI

A cdylib loaded by the APO from the driver store. It wraps DeepFilterNet3 (crate deep_filter, dual MIT/Apache-2.0) with the model embedded in the DLL.

cargo test --release
cargo clippy --release --all-targets -- -D warnings
cargo build --release        # -> target/release/kwiet_dsp.dll

Non-negotiable, with the reasoning in ../docs/architecture.md §9:

• Static CRT via .cargo/config.toml. Without it the DLL imports VCRUNTIME140.dll and may fail to load inside audiodg.
• No panic crosses the C boundary: catch_unwind at every entry point, and emphatically not panic = "abort" — an abort would kill audiodg, and with it every sound on the machine.
• kwiet_dsp_process is called from the worker thread, never the real-time one.

ABI

Declared in include/kwiet_dsp.h:

uint32_t  kwiet_dsp_abi_version(void);                               // guards against a stale DLL
KwietDsp* kwiet_dsp_create(uint32_t sample_rate, uint32_t channels); // not real-time
void      kwiet_dsp_destroy(KwietDsp*);
uint32_t  kwiet_dsp_block_frames(void);                              // fixed frame the host must feed
int32_t   kwiet_dsp_process(KwietDsp*, const float* in, float* out, uint32_t frames);
void      kwiet_dsp_set_attenuation_db(KwietDsp*, float db);         // thread-safe, atomic

Dependency pins, and why they are mandatory

The crate published on crates.io (deep_filter 0.2.5, 2022) is still DFN2, so we depend on the Git repository. Two pins are needed to compile at all, both commented in Cargo.toml:

• the tract family at =0.21.4 — deep_filter asks for ^0.21.4, but tract renamed a public field (symbol_table → symbols) in a release cargo considers compatible. The pin must live in the manifest: cargo update --precise downgrades crate by crate and breaks the family's internal coherence (tract-pulse-opl requires = its own version);
• kstring at 2.0.2 — 2.0.4 requires rustc 1.96.

The model ships inside the DLL: DfParams::default() embeds DeepFilterNet3_onnx.tar.gz (7.6 MB) through the default-model feature. There is no file to deploy next to it.

DeepFilterNet3 constraints

• 48 kHz only. kwiet_dsp_create returns NULL for any other rate and the host stays in passthrough. Resampling is still to be done.
• Mono. The host's channels are summed on the way in, and the mono result is written back across every channel.
• Fixed frame of kwiet_dsp_block_frames() (480 = 10 ms). The host uses that as its worker's block size; the rings decouple it from the APO's quantum.
• process allocates — tract allocates per inference. That is acceptable precisely because it runs on the worker and never on the real-time thread.