fft-convolver

Fast, real-time safe FFT-based convolution for audio processing in Rust.

Port of HiFi-LoFi/FFTConvolver to pure Rust.

Features

• Real-time safe: No allocations, locks, or unpredictable operations during audio processing
• Highly efficient: Partitioned FFT convolution algorithm with uniform block sizes
• Zero latency: Output is sample-aligned with input (excluding processing time)
• Flexible: Handles arbitrary input/output buffer sizes through internal buffering
• Generic: Works with f32 and f64 floating-point types

Perfect for real-time audio applications like convolution reverbs, cabinet simulators, and other impulse response-based effects.

How it Works

The convolver uses a partitioned FFT convolution algorithm that divides the impulse response into uniform blocks. This approach provides:

• Consistent processing time regardless of buffer size
• Efficient computation through FFT
• Low latency suitable for real-time audio

All memory allocation happens during initialization (init()), making subsequent processing (process()) completely allocation-free and suitable for real-time audio threads.

Usage

Basic Example

use fft_convolver::FFTConvolver;

// Create an impulse response (e.g., a simple delay)
let mut impulse_response = vec![0.0_f32; 100];
impulse_response[0] = 0.8;  // Direct sound
impulse_response[50] = 0.3; // Echo

// Initialize the convolver
let mut convolver = FFTConvolver::default();
convolver.init(128, &impulse_response).unwrap();

// Process audio in any buffer size
let input = vec![1.0_f32; 256];
let mut output = vec![0.0_f32; 256];
convolver.process(&input, &mut output).unwrap();

Updating the Impulse Response

use fft_convolver::FFTConvolver;

let mut convolver = FFTConvolver::<f32>::default();
let ir1 = vec![0.5, 0.3, 0.2, 0.1];
convolver.init(128, &ir1).unwrap();

// Update to a different impulse response (must be ≤ original length)
let ir2 = vec![0.8, 0.6, 0.4];
convolver.set_response(&ir2).unwrap();

Handling Stream Discontinuities

use fft_convolver::FFTConvolver;

let mut convolver = FFTConvolver::<f32>::default();
let ir = vec![0.5, 0.3, 0.2];
convolver.init(128, &ir).unwrap();

// Process some audio...
let input = vec![1.0; 256];
let mut output = vec![0.0; 256];
convolver.process(&input, &mut output).unwrap();

// Clear state when seeking or handling playback discontinuities
convolver.reset();

// Continue processing with clean state
convolver.process(&input, &mut output).unwrap();

Performance Considerations

• Block size: Affects CPU efficiency. Larger blocks are more efficient (better FFT performance) but require more computation per block. Typical values: 64-512 samples.
• Impulse response length: Longer IRs require more computation. The algorithm scales well with IR length.
• Buffer size: Any input/output size is supported efficiently through internal buffering.

Real-Time Safety

The following operations are real-time safe (no allocations):

• process() - Audio processing
• set_response() - Updating impulse response
• reset() - Clearing internal state

The following operations are NOT real-time safe (perform allocations):

• init() - Initial setup

License

Licensed under the MIT license.