RT FFT Convolver

rt-fft-convolver is a professional-grade, real-time digital signal processing (DSP) library for performing fast convolution. It is specifically engineered for guitar cabinet simulation (IR) and reverb effects where ultra-low latency is critical.

✨ Features

Zero-Latency Hybrid Engine: Combines Time-Domain Direct Form convolution for the initial impulse response (IR) head with Partitioned FFT convolution for the tail.
Optimized FFT: Powered by rustfft for high-performance frequency domain processing.
Real-Time Safe: Zero heap allocations in the processing loop, ensuring glitch-free audio.
Anti-Denormal Protection: Built-in Flush-To-Zero (FTZ) logic to prevent CPU spikes when processing near-silent signals.
Multi-Channel Support: Native support for Mono, Stereo, and True Stereo (4-channel) configurations.
Resampling Ready: Intelligent handling of IR sample rate mismatches.

Why rt-fft-convolver?

Standard FFT convolution libraries often introduce latency equal to the block size. rt-fft-convolver solves this by partitioning the impulse response into smaller segments and using a hybrid approach, allowing guitarists to play through IRs with a perceived latency of under 2ms.

Installation

Add this to your Cargo.toml:

[dependencies]
rt-fft-convolver = "0.1"

🚀 Quick start

The crate exposes composable primitives — assemble the ones you need for your signal path. A minimal mono chain using the partitioned FFT engine (one block of algorithmic latency) plus IR normalisation:

use rt_fft_convolver::{
    DenormalGuard, Mixer, UniformPartitionEngine, load_ir,
};

const BLOCK: usize = 64;

fn main() -> Result<(), Box<dyn std::error::Error>> {
    // ── Load and resample the IR to the host sample rate ──────────────────
    let ir = load_ir("cab_ir.wav", 48_000)?;
    let samples = &ir.channels[0]; // mono

    // ── FFT-partitioned convolution across the whole IR ───────────────────
    let mut engine = UniformPartitionEngine::new(samples, BLOCK);

    // ── Mixer normalises IR loudness and blends dry / wet ─────────────────
    let mixer = Mixer::new(samples, 1.0); // fully wet

    // Scratch buffers used by the render callback (allocate up front).
    let mut wet = [0.0f32; BLOCK];

    // ── Real-time render callback ─────────────────────────────────────────
    let mut process = move |input: &[f32; BLOCK], output: &mut [f32; BLOCK]| {
        let _guard = DenormalGuard::new(); // FTZ for the whole callback

        wet.fill(0.0);
        engine.process(input, &mut wet);   // convolve x ⊛ ir into wet

        mixer.process(input, &wet, output); // dry × cos + wet × sin·gain
    };

    // Plug `process` into your audio host of choice.
    let (input, mut output) = ([0.0f32; BLOCK], [0.0f32; BLOCK]);
    process(&input, &mut output);
    Ok(())
}

Zero-latency mode

For applications that need sub-block latency (live guitar rigs), the DirectConvolver primitive handles the first head_len IR taps in the time domain with zero algorithmic latency. Assembling it into a true hybrid engine requires delaying the tail input by head_len samples so the two paths don't double-count — a dedicated HybridConvolver type will land in a future release. Until then, use DirectConvolver alone for short IRs (≤ a few hundred taps) or UniformPartitionEngine for long IRs.

Stereo and true-stereo

StereoConvolver runs an independent engine per channel; TrueStereoConvolver runs the full 4-path routing matrix (L→L, L→R, R→L, R→R) needed for stereo-microphone IR captures. Both types expose the same process(in_l, in_r, out_l, out_r) signature.

🤝 Contributing

rt-fft-convolver is developed on Codeberg and mirrored to GitHub.

Primary Repo: Codeberg main repo
Mirror: GitHub repo

rt-fft-convolver 0.2.0