Crate bosai_dsp 

Beat-synced DJ effects, in pure Rust.

A library of DSP building blocks modelled on the Beat FX section of a professional DJ mixer: effects whose time base is musical rather than absolute. Every time-based effect here takes a beat_value (in beats) plus a live BPM, and derives its own delay time, loop length, or LFO rate from them — so a 1/2-beat echo stays a 1/2-beat echo when the deck’s tempo fader moves.

That is the distinction from general-purpose audio DSP crates, which overwhelmingly express time in seconds and leave tempo mapping to the caller.

What’s here

Beat-synced effects — all take set_bpm() + beat_value:

• echo — stereo echo with DJ-mixer pre-filter variants (HPF / LPF / BPF)
• ping_pong — cross-fed L/R bouncing delay
• roll_fx — Roll, Slip Roll, Rev Roll and Helix buffer loopers
• gater — rhythmic gate with noise fill
• filter_fx — resonant HP/LP filter with beat-rate LFO sweep
• flanger, phaser, peak_filter — beat-rate modulated

Free-running blocks:

• reverb — Freeverb, RT60-parameterised
• eq_chain — 3-band Linkwitz-Riley crossover EQ + sweep filter
• biquad, svf — filter primitives
• smoother — one-pole parameter smoothing
• resample — linear/cubic interpolation helpers

Conventions

Stereo only. CHANNELS is fixed at 2. Every DJ mixer signal path is stereo, and fixing it keeps frames on the stack as [f64; 2] with no allocation on the audio path.

f64 internally. Effects process in f64 and are intended to sit in an f64 mix bus; convert at the device boundary.

Click-free by construction. Parameter changes are smoothed with one-pole filters, and changes that would jump a read pointer (delay time, beat value) are handled with short equal-power crossfades rather than discontinuities. Filter state is deliberately not reset on parameter change.

No allocation on the audio path. Buffers are sized once at construction from a documented maximum; process() never allocates.

Modules

biquad

Second-order IIR biquad filter.

echo

BPM-synced stereo echo with optional pre-filter variants.

eq_chain

3-band parallel crossover EQ + SVF sweep filter.

filter_fx

Beat FX: resonant filter with LFO-modulated cutoff.

flanger

Beat FX: Flanger.

freeverb

Freeverb — a Rust implementation of the public-domain Freeverb reverb algorithm.

gater

Beat FX: Gater.

peak_filter

Beat FX: Peak Filter.

phaser

Beat FX: Phaser.

ping_pong

Beat FX: Ping Pong delay (Pioneer DJM-900NXS2 style, locked variant).

resample

Resamplers for variable-speed playback and vinyl scratching.

reverb

Reverb Beat FX: Freeverb algorithm wrapped with DJM-style depth + RT60 control.

roll_fx

Beat FX: Roll, Slip Roll, Rev Roll, Helix.

smoother

One-pole lowpass parameter smoother.

svf

Topology Preserving Transform (TPT) State Variable Filter.

Constants

CHANNELS

Channel count for every frame in this crate. Stereo, fixed — see the crate-level docs for why.