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Move FIR filters to their own module

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This commit is contained in:
Robbert van der Helm 2022-06-07 15:19:18 +02:00
parent ac5796ee59
commit b32cd27e8c
3 changed files with 211 additions and 188 deletions
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192
plugins/crossover/src/crossover/fir.rs
View file

@ -17,11 +17,14 @@
use nih_plug::buffer::ChannelSamples; use nih_plug::buffer::ChannelSamples;
use nih_plug::debug::*; use nih_plug::debug::*;
use std::f32; use std::f32;
use std::simd::{f32x2, StdFloat}; use std::simd::f32x2;
use crate::crossover::iir::biquad::{Biquad, BiquadCoefficients, NEUTRAL_Q}; use self::filter::{FirCoefficients, FirFilter};
use crate::crossover::iir::biquad::{BiquadCoefficients, NEUTRAL_Q};
use crate::NUM_BANDS; use crate::NUM_BANDS;
pub mod filter;
// TODO: Move this to FFT convolution so we can increase the filter size and improve low latency performance // TODO: Move this to FFT convolution so we can increase the filter size and improve low latency performance
/// The size of the FIR filter window, or the number of taps. The low frequency performance is /// The size of the FIR filter window, or the number of taps. The low frequency performance is
@ -52,53 +55,6 @@ pub enum FirCrossoverType {
LinkwitzRiley24LinearPhase, LinkwitzRiley24LinearPhase,
} }
/// A single FIR filter that may be configured in any way. In this plugin this will be a
/// linear-phase low-pass, band-pass, or high-pass filter.
#[derive(Debug, Clone)]
struct FirFilter {
/// The coefficients for this filter. The filters for both channels should be equivalent, this
/// just avoids broadcasts in the filter process.
///
/// TODO: Profile to see if storing this as f32x2 rather than f32s plus splatting makes any
/// difference in performance at all
pub coefficients: FirCoefficients,
/// A ring buffer storing the last `FILTER_SIZE - 1` samples. The capacity is `FILTER_SIZE`
/// rounded up to the next power of two.
delay_buffer: [f32x2; RING_BUFFER_SIZE],
/// The index in `delay_buffer` to write the next sample to. Wrapping negative indices back to
/// the end, the previous sample can be found at `delay_buffer[delay_buffer_next_idx - 1]`, the
/// one before that at `delay_buffer[delay_buffer_next_idx - 2]`, and so on.
delay_buffer_next_idx: usize,
}
/// Coefficients for an FIR filter. This struct includes ways to design the filter. Parameterized
/// over `f32x2` only for the time being since that's what we need here.
#[repr(transparent)]
#[derive(Debug, Clone)]
struct FirCoefficients([f32x2; FILTER_SIZE]);
impl Default for FirFilter {
fn default() -> Self {
Self {
coefficients: FirCoefficients::default(),
delay_buffer: [f32x2::default(); RING_BUFFER_SIZE],
delay_buffer_next_idx: 0,
}
}
}
impl Default for FirCoefficients {
fn default() -> Self {
// Initialize this to a delay with the same amount of latency as we'd introduce with our
// linear-phase filters
let mut coefficients = [f32x2::default(); FILTER_SIZE];
coefficients[FILTER_SIZE / 2] = f32x2::splat(1.0);
Self(coefficients)
}
}
impl FirCrossover { impl FirCrossover {
/// Create a new multiband crossover processor. All filters will be configured to pass audio /// Create a new multiband crossover processor. All filters will be configured to pass audio
/// through as is, albeit with a delay. `.update()` needs to be called first to set up the /// through as is, albeit with a delay. `.update()` needs to be called first to set up the
@ -258,141 +214,3 @@ impl FirCrossover {
} }
} }
} }
impl FirFilter {
/// Process left and right audio samples through the filter.
pub fn process(&mut self, samples: f32x2) -> f32x2 {
// TODO: Replace direct convolution with FFT convolution, would make the implementation much
// more complex though because of the multi output part
let coefficients = &self.coefficients.0;
let mut result = coefficients[0] * samples;
// Now multiply `self.coefficients[1..]` with the delay buffer starting at
// `self.delay_buffer_next_idx - 1`, wrapping around to the end when that is reached
// The end index is exclusive, and we already did the multiply+add for the first coefficient.
let before_wraparound_start_idx = self
.delay_buffer_next_idx
.saturating_sub(coefficients.len() - 1);
let before_wraparound_end_idx = self.delay_buffer_next_idx;
let num_before_wraparound = before_wraparound_end_idx - before_wraparound_start_idx;
for (coefficient, delayed_sample) in coefficients[1..1 + num_before_wraparound].iter().zip(
self.delay_buffer[before_wraparound_start_idx..before_wraparound_end_idx]
.iter()
.rev(),
) {
// `result += coefficient * sample`, but with explicit FMA
result = coefficient.mul_add(*delayed_sample, result);
}
let after_wraparound_begin_idx =
self.delay_buffer.len() - (coefficients.len() - num_before_wraparound);
let after_wraparound_end_idx = self.delay_buffer.len();
for (coefficient, delayed_sample) in coefficients[1 + num_before_wraparound..].iter().zip(
self.delay_buffer[after_wraparound_begin_idx..after_wraparound_end_idx]
.iter()
.rev(),
) {
result = coefficient.mul_add(*delayed_sample, result);
}
// And finally write the samples to the delay buffer for the enxt sample
self.delay_buffer[self.delay_buffer_next_idx] = samples;
self.delay_buffer_next_idx = (self.delay_buffer_next_idx + 1) % self.delay_buffer.len();
result
}
/// Reset the internal filter state.
pub fn reset(&mut self) {
self.delay_buffer.fill(f32x2::default());
self.delay_buffer_next_idx = 0;
}
}
impl FirCoefficients {
/// A somewhat crude but very functional and relatively fast way create linear phase FIR
/// **low-pass** filter that matches the frequency response of a fourth order biquad low-pass
/// filter. As in, this matches the frequency response magnitudes of applying those biquads to a
/// signal twice. This only works for low-pass filters, as the function normalizes the result to
/// hae unity gain at the DC bin. The algorithm works as follows:
///
/// - An impulse function (so all zeroes except for the first element) of length `FILTER_LEN / 2
/// + 1` is filtered with the biquad.
/// - The biquad's state is reset, and the impulse response is filtered in the opposite
/// direction.
/// - At this point the bidirectionally filtered impulse response contains the **right** half of
/// a truncated linear phase FIR kernel.
///
/// Since the FIR filter will be a symmetrical version of this impulse response, we can optimize
/// the post-processing work slightly by windowing and normalizing this bidirectionally filtered
/// impulse response instead.
///
/// - A half Blackman window is applied to the impulse response. Since this is the right half,
/// this starts at unity gain for the first sample and then tapers off towards the right.
/// - The impulse response is then normalized such that the final linear-phase FIR kernel has a
/// sum of 1.0. Since it will be symmetrical around the IRs first sample, the would-be final
/// sum can be computed as `ir.sum() * 2 - ir[0]`.
///
/// Lastly the linear phase FIR filter simply needs to be constructed from this right half:
///
/// - This bidirectionally filtered impulse response is then reversed, and placed at the start
/// of the `FILTER_LEN` size FIR coefficient array.
/// - The non-reversed bidirectionally filtered impulse response is copied to the second half of
/// the coefficients. (one of the copies doesn't need to include the centermost coefficient)
///
/// The corresponding high-pass filter can be computed through spectral inversion.
pub fn design_fourth_order_linear_phase_low_pass_from_biquad(
biquad_coefs: BiquadCoefficients<f32x2>,
) -> Self {
const CENTER_IDX: usize = FILTER_SIZE / 2;
// We'll start with an impulse (at exactly half of this odd sized buffer)...
let mut impulse_response = [f32x2::default(); FILTER_SIZE];
impulse_response[CENTER_IDX] = f32x2::splat(1.0);
// ...and filter that in both directions
let mut biquad = Biquad::default();
biquad.coefficients = biquad_coefs;
for sample in impulse_response.iter_mut().skip(CENTER_IDX - 1) {
*sample = biquad.process(*sample);
}
biquad.reset();
for sample in impulse_response.iter_mut().skip(CENTER_IDX - 1).rev() {
*sample = biquad.process(*sample);
}
// Now the right half of `impulse_response` contains a truncated right half of the
// linear-phase FIR filter. We can apply the window function here, and then fianlly
// normalize it so that the the final FIR filter kernel sums to 1.
// Adopted from `nih_plug::util::window`. We only end up applying the right half of the
// window, starting at the top of the window.
let blackman_scale_1 = (2.0 * f32::consts::PI) / (impulse_response.len() - 1) as f32;
let blackman_scale_2 = blackman_scale_1 * 2.0;
for (sample_idx, sample) in impulse_response.iter_mut().enumerate().skip(CENTER_IDX - 1) {
let cos_1 = (blackman_scale_1 * sample_idx as f32).cos();
let cos_2 = (blackman_scale_2 * sample_idx as f32).cos();
*sample *= f32x2::splat(0.42 - (0.5 * cos_1) + (0.08 * cos_2));
}
// Since this final filter will be symmetrical around `impulse_response[CENTER_IDX]`, we
// can simply normalize based on that fact:
let would_be_impulse_response_sum =
(impulse_response.iter().skip(CENTER_IDX).sum::<f32x2>() * f32x2::splat(2.0))
- impulse_response[CENTER_IDX];
let would_be_impulse_response_recip = would_be_impulse_response_sum.recip();
for sample in &mut impulse_response {
*sample *= would_be_impulse_response_recip;
}
// And finally we can simply copy the right half of the filter kernel to the left half
// around the `CENTER_IDX`.
for source_idx in CENTER_IDX + 1..impulse_response.len() {
let target_idx = CENTER_IDX - (source_idx - CENTER_IDX);
impulse_response[target_idx] = impulse_response[source_idx];
}
Self(impulse_response)
}
}

206
plugins/crossover/src/crossover/fir/filter.rs Normal file
View file

@ -0,0 +1,206 @@
// Crossover: clean crossovers as a multi-out plugin
// Copyright (C) 2022 Robbert van der Helm
//
// This program is free software: you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation, either version 3 of the License, or
// (at your option) any later version.
//
// This program is distributed in the hope that it will be useful,
// but WITHOUT ANY WARRANTY; without even the implied warranty of
// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
// GNU General Public License for more details.
//
// You should have received a copy of the GNU General Public License
// along with this program. If not, see <https://www.gnu.org/licenses/>.
use std::f32;
use std::simd::{f32x2, StdFloat};
use super::{FILTER_SIZE, RING_BUFFER_SIZE};
use crate::crossover::iir::biquad::{Biquad, BiquadCoefficients};
/// A single FIR filter that may be configured in any way. In this plugin this will be a
/// linear-phase low-pass, band-pass, or high-pass filter.
#[derive(Debug, Clone)]
pub struct FirFilter {
/// The coefficients for this filter. The filters for both channels should be equivalent, this
/// just avoids broadcasts in the filter process.
///
/// TODO: Profile to see if storing this as f32x2 rather than f32s plus splatting makes any
/// difference in performance at all
pub coefficients: FirCoefficients,
/// A ring buffer storing the last `FILTER_SIZE - 1` samples. The capacity is `FILTER_SIZE`
/// rounded up to the next power of two.
delay_buffer: [f32x2; RING_BUFFER_SIZE],
/// The index in `delay_buffer` to write the next sample to. Wrapping negative indices back to
/// the end, the previous sample can be found at `delay_buffer[delay_buffer_next_idx - 1]`, the
/// one before that at `delay_buffer[delay_buffer_next_idx - 2]`, and so on.
delay_buffer_next_idx: usize,
}
/// Coefficients for an FIR filter. This struct includes ways to design the filter. Parameterized
/// over `f32x2` only for the time being since that's what we need here.
#[repr(transparent)]
#[derive(Debug, Clone)]
pub struct FirCoefficients(pub [f32x2; FILTER_SIZE]);
impl Default for FirFilter {
fn default() -> Self {
Self {
coefficients: FirCoefficients::default(),
delay_buffer: [f32x2::default(); RING_BUFFER_SIZE],
delay_buffer_next_idx: 0,
}
}
}
impl Default for FirCoefficients {
fn default() -> Self {
// Initialize this to a delay with the same amount of latency as we'd introduce with our
// linear-phase filters
let mut coefficients = [f32x2::default(); FILTER_SIZE];
coefficients[FILTER_SIZE / 2] = f32x2::splat(1.0);
Self(coefficients)
}
}
impl FirFilter {
/// Process left and right audio samples through the filter.
pub fn process(&mut self, samples: f32x2) -> f32x2 {
// TODO: Replace direct convolution with FFT convolution, would make the implementation much
// more complex though because of the multi output part
let coefficients = &self.coefficients.0;
let mut result = coefficients[0] * samples;
// Now multiply `self.coefficients[1..]` with the delay buffer starting at
// `self.delay_buffer_next_idx - 1`, wrapping around to the end when that is reached
// The end index is exclusive, and we already did the multiply+add for the first coefficient.
let before_wraparound_start_idx = self
.delay_buffer_next_idx
.saturating_sub(coefficients.len() - 1);
let before_wraparound_end_idx = self.delay_buffer_next_idx;
let num_before_wraparound = before_wraparound_end_idx - before_wraparound_start_idx;
for (coefficient, delayed_sample) in coefficients[1..1 + num_before_wraparound].iter().zip(
self.delay_buffer[before_wraparound_start_idx..before_wraparound_end_idx]
.iter()
.rev(),
) {
// `result += coefficient * sample`, but with explicit FMA
result = coefficient.mul_add(*delayed_sample, result);
}
let after_wraparound_begin_idx =
self.delay_buffer.len() - (coefficients.len() - num_before_wraparound);
let after_wraparound_end_idx = self.delay_buffer.len();
for (coefficient, delayed_sample) in coefficients[1 + num_before_wraparound..].iter().zip(
self.delay_buffer[after_wraparound_begin_idx..after_wraparound_end_idx]
.iter()
.rev(),
) {
result = coefficient.mul_add(*delayed_sample, result);
}
// And finally write the samples to the delay buffer for the enxt sample
self.delay_buffer[self.delay_buffer_next_idx] = samples;
self.delay_buffer_next_idx = (self.delay_buffer_next_idx + 1) % self.delay_buffer.len();
result
}
/// Reset the internal filter state.
pub fn reset(&mut self) {
self.delay_buffer.fill(f32x2::default());
self.delay_buffer_next_idx = 0;
}
}
impl FirCoefficients {
/// A somewhat crude but very functional and relatively fast way create linear phase FIR
/// **low-pass** filter that matches the frequency response of a fourth order biquad low-pass
/// filter. As in, this matches the frequency response magnitudes of applying those biquads to a
/// signal twice. This only works for low-pass filters, as the function normalizes the result to
/// hae unity gain at the DC bin. The algorithm works as follows:
///
/// - An impulse function (so all zeroes except for the first element) of length `FILTER_LEN / 2
/// + 1` is filtered with the biquad.
/// - The biquad's state is reset, and the impulse response is filtered in the opposite
/// direction.
/// - At this point the bidirectionally filtered impulse response contains the **right** half of
/// a truncated linear phase FIR kernel.
///
/// Since the FIR filter will be a symmetrical version of this impulse response, we can optimize
/// the post-processing work slightly by windowing and normalizing this bidirectionally filtered
/// impulse response instead.
///
/// - A half Blackman window is applied to the impulse response. Since this is the right half,
/// this starts at unity gain for the first sample and then tapers off towards the right.
/// - The impulse response is then normalized such that the final linear-phase FIR kernel has a
/// sum of 1.0. Since it will be symmetrical around the IRs first sample, the would-be final
/// sum can be computed as `ir.sum() * 2 - ir[0]`.
///
/// Lastly the linear phase FIR filter simply needs to be constructed from this right half:
///
/// - This bidirectionally filtered impulse response is then reversed, and placed at the start
/// of the `FILTER_LEN` size FIR coefficient array.
/// - The non-reversed bidirectionally filtered impulse response is copied to the second half of
/// the coefficients. (one of the copies doesn't need to include the centermost coefficient)
///
/// The corresponding high-pass filter can be computed through spectral inversion.
pub fn design_fourth_order_linear_phase_low_pass_from_biquad(
biquad_coefs: BiquadCoefficients<f32x2>,
) -> Self {
const CENTER_IDX: usize = FILTER_SIZE / 2;
// We'll start with an impulse (at exactly half of this odd sized buffer)...
let mut impulse_response = [f32x2::default(); FILTER_SIZE];
impulse_response[CENTER_IDX] = f32x2::splat(1.0);
// ...and filter that in both directions
let mut biquad = Biquad::default();
biquad.coefficients = biquad_coefs;
for sample in impulse_response.iter_mut().skip(CENTER_IDX - 1) {
*sample = biquad.process(*sample);
}
biquad.reset();
for sample in impulse_response.iter_mut().skip(CENTER_IDX - 1).rev() {
*sample = biquad.process(*sample);
}
// Now the right half of `impulse_response` contains a truncated right half of the
// linear-phase FIR filter. We can apply the window function here, and then fianlly
// normalize it so that the the final FIR filter kernel sums to 1.
// Adopted from `nih_plug::util::window`. We only end up applying the right half of the
// window, starting at the top of the window.
let blackman_scale_1 = (2.0 * f32::consts::PI) / (impulse_response.len() - 1) as f32;
let blackman_scale_2 = blackman_scale_1 * 2.0;
for (sample_idx, sample) in impulse_response.iter_mut().enumerate().skip(CENTER_IDX - 1) {
let cos_1 = (blackman_scale_1 * sample_idx as f32).cos();
let cos_2 = (blackman_scale_2 * sample_idx as f32).cos();
*sample *= f32x2::splat(0.42 - (0.5 * cos_1) + (0.08 * cos_2));
}
// Since this final filter will be symmetrical around `impulse_response[CENTER_IDX]`, we
// can simply normalize based on that fact:
let would_be_impulse_response_sum =
(impulse_response.iter().skip(CENTER_IDX).sum::<f32x2>() * f32x2::splat(2.0))
- impulse_response[CENTER_IDX];
let would_be_impulse_response_recip = would_be_impulse_response_sum.recip();
for sample in &mut impulse_response {
*sample *= would_be_impulse_response_recip;
}
// And finally we can simply copy the right half of the filter kernel to the left half
// around the `CENTER_IDX`.
for source_idx in CENTER_IDX + 1..impulse_response.len() {
let target_idx = CENTER_IDX - (source_idx - CENTER_IDX);
impulse_response[target_idx] = impulse_response[source_idx];
}
Self(impulse_response)
}
}

1
src/event_loop/linux.rs
View file

@ -7,7 +7,6 @@ use std::sync::Weak;
use std::thread::{self, JoinHandle, ThreadId}; use std::thread::{self, JoinHandle, ThreadId};
use super::{EventLoop, MainThreadExecutor}; use super::{EventLoop, MainThreadExecutor};
use crate::nih_log;
use crate::util::permit_alloc; use crate::util::permit_alloc;
/// See [`EventLoop`][super::EventLoop]. /// See [`EventLoop`][super::EventLoop].