A real-time audio playground running in your browser via WebAssembly, compiled from the audio-dsp Rust crate — a hand-rolled FFT, a seven-type biquad filter bank, and YIN pitch detection. The browser only does I/O (mic, file decoding, playback); every sample of math runs in Rust, and the same crate compiles to native targets unchanged.
Press an input button to start (browser needs a gesture for audio).
Gain only applies to peaking and shelf filters. Try: low-pass at 200 Hz on the tone, then sweep the frequency up and watch the spectrum's right side come alive.
The waveform is amplitude over time — the raw input in gray, the filtered output in teal, drawn on top. Bypass the filter and they sit exactly on top of each other; engage a low-pass and you watch the teal trace go smooth while the gray keeps buzzing. The spectrum is the filtered signal through a Fourier transform: energy per frequency, via a hand-rolled radix-2 FFT with a Hann window (which tames spectral leakage at the cost of slightly fattening each peak). Two domains, one cause.
Each filter is a biquad — a second-order IIR section with five coefficients computed from the RBJ Audio EQ Cookbook. Seven types cover every distinct thing a filter can do to a spectrum: remove above (low-pass), remove below (high-pass), keep a slice (band-pass), kill a slice (notch), boost/cut a slice (peaking), tilt the bass (low-shelf) or treble (high-shelf). Q controls how narrow the slice is.
The tuner doesn't pick the tallest FFT bin — on a guitar string that's usually a harmonic, and you'd be told you're an octave sharp. Instead it uses the YIN algorithm, which measures periodicity in the time domain: the lag at which the signal best predicts itself. Frequency then maps to the nearest note in 12-tone equal temperament (A4 = 440 Hz), with cents telling you which way to turn the peg.
fft.rs, filter.rs, pitch.rs,
note.rs — no dependencies, sample rate always a parameter,
unit-tested sample by sample. The page is a thin shell; the crate builds
for native targets unchanged.