lowpass-filter

High performance no_std lowpass filter for digital signal processing.

This crate implements a simple first-order digital lowpass filter for f32 and f64 samples. Use it, for example, to extract the bass from a song or to smooth noisy sensor data. It has no dependencies, no unsafe code, and performs no allocations, making it suitable for any target from desktop to embedded.

Samples must be in range -1.0..=1.0, which is the default in DSP.

Usage

To filter a buffer of samples in one go, use lowpass_filter_slice (or lowpass_filter_slice_f64). This is the fastest option: it processes samples in blocks that compilers auto-vectorize (SIMD), several times faster than per-sample processing.

use lowpass_filter::lowpass_filter_slice;

// Mono audio samples, recorded at 44.1 kHz sample rate.
let mut samples = [0.0, 0.3, -0.6, 0.8, 0.5, -0.2];
// Only keep frequencies below 120 Hz; mutates the buffer in-place.
lowpass_filter_slice(&mut samples, 44100.0, 120.0);

For streaming data, e.g. in an audio callback, keep a LowpassFilter around: its state carries over between calls, so chunked processing equals processing everything at once. It also filters single samples, e.g. inside iterator chains.

use lowpass_filter::LowpassFilter;

let mut filter = LowpassFilter::<f32>::new(44100.0, 120.0);
// Process data as it arrives (fast block processing) ...
for mut chunk in [[0.0, 0.3, -0.6, 0.8], [0.5, -0.2, 0.1, 0.4]] {
    filter.run_slice(&mut chunk);
}
// ... or one sample at a time.
let filtered = filter.run(0.25);

The iterator-based lowpass_filter and lowpass_filter_f64 functions are convenient when the samples do not live in a slice.

Comparison with `biquad`

For the equivalent first-order lowpass (Type::SinglePoleLowPass), this crate outperforms the biquad crate: roughly 1.5x throughput with the iterator-based API and 5-6x with the slice-based API (x86_64), as biquad processes samples strictly one at a time.

biquad is the better choice for sharp frequency separation or other filter types (highpass, bandpass, notch, EQ): its second-order filters roll off at 12 dB/octave instead of 6, with a configurable Q factor.