Module mixer_fx

Audio filters and bus routing: the DSP behind the mixer panel.

BusGraph resolves Project.buses (Main first) into an evaluation order and owns one FilterState per (bus, filter) so IIR/delay state survives across blocks. Clips sum into their bus (Project::bus_of), each bus runs its filter chain, then gain/pan/mono, then sums into its output bus. Interleaved stereo f32 @ 48 kHz, processed in place, block-continuous.

Filters (model::FilterKind): Eq (5-band RBJ: low shelf, three peaks, high shelf — EQ_BANDS maps bands to parameter indices), HighPass/LowPass (RBJ), Reverb (Freeverb comb+allpass), Echo (delay line + feedback, optional ping-pong), Distortion (soft clip

• tone), Compressor (peak detector, attack/release, makeup), NoiseGate, Noise (white/pink/tone), Gain.

Parameters: every AudioFilter param is an Animated read once per block at the block-start timeline time t (AudioFilter::at(i, t)). Coefficients / delay lengths / thresholds are recomputed from those values and held constant for the block, so automation steps at block boundaries (≤ ~100 ms). The two params where a step is audible as a click — the Gain filter’s gain and the Noise filter’s level — are ramped linearly from the previous block’s value across the block. All buffers (delay lines, comb/allpass memories, bus scratch) are sized at construction / first block, so steady state allocates nothing.

Structs

Allpass 🔒

Biquad 🔒

Transposed direct-form II biquad, one per channel per band.

BusGraph

Bus routing + per-bus filter state, rebuilt when the bus set changes.

Comb 🔒

Delay 🔒

Interleaved-stereo delay line used by Echo.

FilterState

Per-filter DSP state (biquad histories, delay lines, envelopes).

Freeverb 🔒

Slot 🔒

Enums

Band

RBJ band shapes used by the EQ and the pass filters.

Dsp 🔒

The DSP memory a filter kind needs. Sized once in FilterState::new.

Constants

ALLP_LEN 🔒

COMB_LEN 🔒

EQ_BANDS

The EQ’s five bands, low to high, as (shape, gain param, freq param, Q param). The first seven F_EQ slots keep the meaning the 3-band EQ gave them, so projects saved before the extra peaks still load — hence the scattered indices.

MAX_ECHO_S 🔒

Longest Echo delay the parameter allows, in seconds (F_ECHO’s “Delay ms” max).

MAX_HOPS 🔒

Deepest routing chain we follow before declaring a cycle.

REV_IN_GAIN 🔒

Freeverb’s fixedgain: how much of the dry signal is pushed into the comb bank.

SPREAD 🔒

Right-channel stereo spread, in samples at 44.1 kHz (Freeverb’s stereospread).

Functions

add_into 🔒

coef 🔒

One-pole smoothing coefficient for a time constant in ms.

coeffs

Normalised RBJ coefficients [b0, b1, b2, a1, a2] (a0 divided out). Shelves take Q like the peaks (Q = 0.707 is the classic slope-1 shelf).

db_to_lin

Linear amplitude of a dB value.

filter_bands

The bands an EQ / pass filter is made of at time t, as (band, freq, q, gain_db). Empty for every other kind — the UI draws a response curve exactly when this is non-empty.

filter_response_db

Total response of an EQ / pass filter at freq, in dB (0 for other kinds).

hops 🔒

Hops from id to Main along resolved outputs (Main = 0, a broken cycle = 1).

lin_to_db

dB of a linear amplitude (floored at -120 dB).

pink_tick 🔒

Paul Kellet’s economy pink filter (~ -3 dB/oct over the audio band).

resolve 🔒

Where b really sends: its own id when terminal (Main, dangling or cyclic output).

response_db

|H(e^jw)| in dB at freq for one set of normalised coefficients.

white 🔒

xorshift32 white noise in [-1, 1).