103 lines
3.3 KiB
Rust
103 lines
3.3 KiB
Rust
use core::f32::consts::PI;
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use serde::{Deserialize, Serialize};
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/// Generic vector for integer IIR filter.
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/// This struct is used to hold the x/y input/output data vector or the b/a coefficient
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/// vector.
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#[derive(Copy, Clone, Default, Deserialize, Serialize)]
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pub struct IIRState(pub [i32; 5]);
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impl IIRState {
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/// Lowpass biquad filter using cutoff and sampling frequencies. Taken from:
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/// https://webaudio.github.io/Audio-EQ-Cookbook/audio-eq-cookbook.html
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///
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/// # Args
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/// * `f` - Corner frequency, or 3dB cutoff frequency (in units of sample rate).
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/// This is only accurate for low corner frequencies less than ~0.01.
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/// * `q` - Quality factor (1/sqrt(2) for critical).
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/// * `k` - DC gain.
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///
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/// # Returns
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/// 2nd-order IIR filter coefficients in the form [b0,b1,b2,a1,a2]. a0 is set to -1.
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pub fn lowpass(f: f32, q: f32, k: f32) -> IIRState {
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// 3rd order Taylor approximation of sin and cos.
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let f = f * 2. * PI;
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let fsin = f - f * f * f / 6.;
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let fcos = 1. - f * f / 2.;
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let alpha = fsin / (2. * q);
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// IIR uses Q2.30 fixed point
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let a0 = (1. + alpha) / (1 << IIR::SHIFT) as f32;
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let b0 = (k / 2. * (1. - fcos) / a0) as _;
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let a1 = (2. * fcos / a0) as _;
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let a2 = ((alpha - 1.) / a0) as _;
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IIRState([b0, 2 * b0, b0, a1, a2])
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}
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}
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fn macc(y0: i32, x: &[i32], a: &[i32], shift: u32) -> i32 {
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// Rounding bias, half up
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let y0 = ((y0 as i64) << shift) + (1 << (shift - 1));
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let y = x
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.iter()
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.zip(a)
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.map(|(x, a)| *x as i64 * *a as i64)
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.fold(y0, |y, xa| y + xa);
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(y >> shift) as i32
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}
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/// Integer biquad IIR
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///
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/// See `dsp::iir::IIR` for general implementation details.
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/// Offset and limiting disabled to suit lowpass applications.
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/// Coefficient scaling fixed and optimized.
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#[derive(Copy, Clone, Default, Deserialize, Serialize)]
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pub struct IIR {
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pub ba: IIRState,
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// pub y_offset: i32,
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// pub y_min: i32,
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// pub y_max: i32,
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}
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impl IIR {
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/// Coefficient fixed point format: signed Q2.30.
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/// Tailored to low-passes, PI, II etc.
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pub const SHIFT: u32 = 30;
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/// Feed a new input value into the filter, update the filter state, and
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/// return the new output. Only the state `xy` is modified.
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///
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/// # Arguments
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/// * `xy` - Current filter state.
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/// * `x0` - New input.
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pub fn update(&self, xy: &mut IIRState, x0: i32) -> i32 {
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let n = self.ba.0.len();
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debug_assert!(xy.0.len() == n);
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// `xy` contains x0 x1 y0 y1 y2
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// Increment time x1 x2 y1 y2 y3
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// Shift x1 x1 x2 y1 y2
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// This unrolls better than xy.rotate_right(1)
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xy.0.copy_within(0..n - 1, 1);
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// Store x0 x0 x1 x2 y1 y2
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xy.0[0] = x0;
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// Compute y0 by multiply-accumulate
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let y0 = macc(0, &xy.0, &self.ba.0, IIR::SHIFT);
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// Limit y0
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// let y0 = y0.max(self.y_min).min(self.y_max);
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// Store y0 x0 x1 y0 y1 y2
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xy.0[n / 2] = y0;
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y0
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}
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}
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#[cfg(test)]
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mod test {
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use super::IIRState;
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#[test]
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fn lowpass_gen() {
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let ba = IIRState::lowpass(1e-3, 1. / 2f32.sqrt(), 2.);
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println!("{:?}", ba.0);
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}
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}
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