273 lines
7.7 KiB
Rust
273 lines
7.7 KiB
Rust
// Copyright (c) 2021 Weird Constructor <weirdconstructor@gmail.com>
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// This file is a part of HexoDSP. Released under GPL-3.0-or-later.
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// See README.md and COPYING for details.
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//
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// The implementation of this Biquad Filter has been adapted from
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// SamiPerttu, Copyright (c) 2020, under the MIT License.
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// See also: https://github.com/SamiPerttu/fundsp/blob/master/src/filter.rs
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//
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// You will find a float type agnostic version in SamiPerttu's code.
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// I converted this to pure f32 for no good reason, other than making
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// the code more readable (for me).
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use std::f32::consts::*;
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#[derive(Copy, Clone, Debug, Default)]
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pub struct BiquadCoefs {
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pub a1: f32,
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pub a2: f32,
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pub b0: f32,
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pub b1: f32,
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pub b2: f32,
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}
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// TODO:
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// https://github.com/VCVRack/Befaco/blob/v1/src/ChowDSP.hpp#L339
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// more coeffs from there ^^^^^^^^^^^^^ ?
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impl BiquadCoefs {
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#[inline]
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pub fn new(b0: f32, b1: f32, b2: f32, a1: f32, a2: f32) -> Self {
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Self { b0, b1, b2, a1, a2 }
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}
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/// Returns settings for a Butterworth lowpass filter.
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/// Cutoff is the -3 dB point of the filter in Hz.
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#[inline]
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pub fn butter_lowpass(sample_rate: f32, cutoff: f32) -> BiquadCoefs {
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let f = (cutoff * PI / sample_rate).tan();
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let a0r = 1.0 / (1.0 + SQRT_2 * f + f * f);
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let a1 = (2.0 * f * f - 2.0) * a0r;
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let a2 = (1.0 - SQRT_2 * f + f * f) * a0r;
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let b0 = f * f * a0r;
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let b1 = 2.0 * b0;
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let b2 = b0;
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BiquadCoefs { a1, a2, b0, b1, b2 }
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}
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/// Returns the Q for cascading a butterworth filter:
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fn calc_cascaded_butter_q(order: usize, casc_idx: usize) -> f32 {
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let order = order as f32;
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let casc_idx = casc_idx as f32;
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let b = -2.0 * ((2.0 * casc_idx + order - 1.0) * PI / (2.0 * order)).cos();
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1.0 / b
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}
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/// Returns settings for a lowpass filter with a specific q
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#[inline]
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pub fn lowpass(sample_rate: f32, q: f32, cutoff: f32) -> BiquadCoefs {
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let f = (cutoff * PI / sample_rate).tan();
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let a0r = 1.0 / (1.0 + f / q + f * f);
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/*
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float norm = 1.f / (1.f + K / Q + K * K);
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this->b[0] = K * K * norm;
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this->b[1] = 2.f * this->b[0];
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this->b[2] = this->b[0];
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this->a[1] = 2.f * (K * K - 1.f) * norm;
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this->a[2] = (1.f - K / Q + K * K) * norm;
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*/
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let b0 = f * f * a0r;
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let b1 = 2.0 * b0;
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let b2 = b0;
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let a1 = 2.0 * (f * f - 1.0) * a0r;
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let a2 = (1.0 - f / q + f * f) * a0r;
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BiquadCoefs { a1, a2, b0, b1, b2 }
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}
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/// Returns settings for a constant-gain bandpass resonator.
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/// The center frequency is given in Hz.
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/// Bandwidth is the difference in Hz between -3 dB points of the filter response.
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/// The overall gain of the filter is independent of bandwidth.
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pub fn resonator(sample_rate: f32, center: f32, bandwidth: f32) -> BiquadCoefs {
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let r = (-PI * bandwidth / sample_rate).exp();
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let a1 = -2.0 * r * (TAU * center / sample_rate).cos();
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let a2 = r * r;
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let b0 = (1.0 - r * r).sqrt() * 0.5;
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let b1 = 0.0;
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let b2 = -b0;
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BiquadCoefs { a1, a2, b0, b1, b2 }
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}
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// /// Frequency response at frequency `omega` expressed as fraction of sampling rate.
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// pub fn response(&self, omega: f64) -> Complex64 {
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// let z1 = Complex64::from_polar(1.0, -TAU * omega);
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// let z2 = Complex64::from_polar(1.0, -2.0 * TAU * omega);
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// (re(self.b0) + re(self.b1) * z1 + re(self.b2) * z2)
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// / (re(1.0) + re(self.a1) * z1 + re(self.a2) * z2)
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// }
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}
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/// 2nd order IIR filter implemented in normalized Direct Form I.
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#[derive(Debug, Copy, Clone, Default)]
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pub struct Biquad {
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coefs: BiquadCoefs,
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x1: f32,
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x2: f32,
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y1: f32,
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y2: f32,
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}
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impl Biquad {
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pub fn new() -> Self {
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Default::default()
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}
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#[inline]
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pub fn new_with(b0: f32, b1: f32, b2: f32, a1: f32, a2: f32) -> Self {
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let mut s = Self::new();
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s.set_coefs(BiquadCoefs::new(b0, b1, b2, a1, a2));
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s
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}
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#[inline]
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pub fn coefs(&self) -> &BiquadCoefs {
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&self.coefs
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}
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#[inline]
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pub fn set_coefs(&mut self, coefs: BiquadCoefs) {
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self.coefs = coefs;
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}
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pub fn reset(&mut self) {
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self.x1 = 0.0;
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self.x2 = 0.0;
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self.y1 = 0.0;
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self.y2 = 0.0;
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}
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#[inline]
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pub fn tick(&mut self, input: f32) -> f32 {
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let x0 = input;
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let y0 = self.coefs.b0 * x0 + self.coefs.b1 * self.x1 + self.coefs.b2 * self.x2
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- self.coefs.a1 * self.y1
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- self.coefs.a2 * self.y2;
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self.x2 = self.x1;
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self.x1 = x0;
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self.y2 = self.y1;
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self.y1 = y0;
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y0
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// Transposed Direct Form II would be:
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// y0 = b0 * x0 + s1
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// s1 = s2 + b1 * x0 - a1 * y0
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// s2 = b2 * x0 - a2 * y0
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}
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}
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#[derive(Copy, Clone)]
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pub struct ButterLowpass {
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biquad: Biquad,
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sample_rate: f32,
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cutoff: f32,
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}
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#[allow(dead_code)]
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impl ButterLowpass {
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pub fn new(sample_rate: f32, cutoff: f32) -> Self {
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let mut this = ButterLowpass { biquad: Biquad::new(), sample_rate, cutoff: 0.0 };
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this.set_cutoff(cutoff);
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this
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}
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pub fn set_cutoff(&mut self, cutoff: f32) {
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self.biquad.set_coefs(BiquadCoefs::butter_lowpass(self.sample_rate, cutoff));
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self.cutoff = cutoff;
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}
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fn set_sample_rate(&mut self, srate: f32) {
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self.sample_rate = srate;
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self.reset();
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self.biquad.reset();
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self.set_cutoff(self.cutoff);
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}
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fn reset(&mut self) {
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self.biquad.reset();
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self.set_cutoff(self.cutoff);
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}
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#[inline]
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fn tick(&mut self, input: f32) -> f32 {
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self.biquad.tick(input)
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}
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}
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// Loosely adapted from https://github.com/VCVRack/Befaco/blob/v1/src/ChowDSP.hpp
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// Copyright (c) 2019-2020 Andrew Belt and Befaco contributors
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// Under GPLv-3.0-or-later
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//
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// Which was originally taken from https://github.com/jatinchowdhury18/ChowDSP-VCV/blob/master/src/shared/AAFilter.hpp
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// Copyright (c) 2020 jatinchowdhury18
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/// Implements oversampling with a ratio of N and a 4 times cascade
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/// of Butterworth lowpass filters (~48dB?).
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#[derive(Debug, Copy, Clone)]
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pub struct Oversampling<const N: usize> {
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filters: [Biquad; 4],
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buffer: [f32; N],
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}
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impl<const N: usize> Oversampling<N> {
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pub fn new() -> Self {
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let mut this = Self { filters: [Biquad::new(); 4], buffer: [0.0; N] };
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this.set_sample_rate(44100.0);
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this
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}
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pub fn reset(&mut self) {
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self.buffer = [0.0; N];
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for filt in &mut self.filters {
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filt.reset();
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}
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}
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pub fn set_sample_rate(&mut self, srate: f32) {
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let cutoff = 0.98 * (0.5 * srate);
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let ovr_srate = (N as f32) * srate;
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let filters_len = self.filters.len();
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for (i, filt) in self.filters.iter_mut().enumerate() {
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let q = BiquadCoefs::calc_cascaded_butter_q(2 * 4, filters_len - i);
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filt.set_coefs(BiquadCoefs::lowpass(ovr_srate, q, cutoff));
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}
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}
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#[inline]
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pub fn upsample(&mut self, v: f32) {
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self.buffer.fill(0.0);
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self.buffer[0] = (N as f32) * v;
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for s in &mut self.buffer {
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for filt in &mut self.filters {
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*s = filt.tick(*s);
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}
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}
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}
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#[inline]
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pub fn resample_buffer(&mut self) -> &mut [f32; N] {
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&mut self.buffer
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}
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#[inline]
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pub fn downsample(&mut self) -> f32 {
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let mut ret = 0.0;
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for s in &mut self.buffer {
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ret = *s;
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for filt in &mut self.filters {
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ret = filt.tick(ret);
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}
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}
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ret
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}
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}
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