Add the actual crossovers to the Crossover plugin

That feeling when you write a whole bunch of garbage in one without
testing it go and it actually works.

plugins/crossover/src/crossover/iir.rs

@@ -14,11 +14,163 @@
 // 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 nih_plug::buffer::ChannelSamples;
 use nih_plug::debug::*;
 use std::f32::consts;
 use std::ops::{Add, Mul, Sub};
 use std::simd::f32x2;
 
+use crate::NUM_BANDS;
+
+#[derive(Debug)]
+pub struct IirCrossover {
+    /// The kind of crossover to use. `.update_filters()` must be called after changing this.
+    mode: IirCrossoverType,
+
+    /// The crossovers. Depending on the number of bands argument passed to `.process()` one to four
+    /// of these may be used.
+    crossovers: [Crossover; NUM_BANDS - 1],
+}
+
+/// The type of IIR crossover to use.
+#[derive(Debug, Clone, Copy)]
+pub enum IirCrossoverType {
+    /// Clean crossover with 24 dB/octave slopes and one period of delay in the power band. Stacks
+    /// two Butterworth-style (i.e. $q = \frac{\sqrt{2}}{2}$) filters per crossover.
+    LinkwitzRiley24,
+}
+
+/// A single crossover using multiple biquads in series to get steeper slopes. This can do both the
+/// low-pass and the high-pass parts of the crossover.
+#[derive(Debug, Clone, Default)]
+struct Crossover {
+    /// Filters for the low-pass section of the crossover. Not all filters may be used dependign on
+    /// the crossover type.
+    lp_filters: [Biquad<f32x2>; 2],
+    /// Filters for the high-pass section of the crossover. Not all filters may be used dependign on
+    /// the crossover type.
+    hp_filters: [Biquad<f32x2>; 2],
+}
+
+impl IirCrossover {
+    /// Create a new multiband crossover processor. All filters will be configured to pass audio
+    /// through as it. `.update()` needs to be called first to set up the filters, and `.reset()`
+    /// can be called whenever the filter state must be cleared.
+    pub fn new(mode: IirCrossoverType) -> Self {
+        Self {
+            mode,
+            crossovers: Default::default(),
+        }
+    }
+
+    /// Split the signal into bands using the crossovers previously configured through `.update()`.
+    /// The split bands will be written to `band_outputs`. `main_io` is not written to, and should
+    /// be cleared separately.
+    pub fn process(
+        &mut self,
+        num_bands: usize,
+        main_io: &ChannelSamples,
+        mut band_outputs: [ChannelSamples; NUM_BANDS],
+    ) {
+        nih_debug_assert!(num_bands >= 2);
+        nih_debug_assert!(num_bands <= NUM_BANDS);
+        // Required for the SIMD, so we'll just do a hard assert or the unchecked conversions will
+        // be unsound
+        assert!(main_io.len() == 2);
+
+        let mut samples: f32x2 = unsafe { main_io.to_simd_unchecked() };
+        match self.mode {
+            IirCrossoverType::LinkwitzRiley24 => {
+                for (crossover, band_channel_samples) in self
+                    .crossovers
+                    .iter_mut()
+                    .zip(band_outputs.iter_mut())
+                    .take(num_bands as usize - 1)
+                {
+                    let (lp_samples, hp_samples) = crossover.process_lr24(samples);
+
+                    unsafe { band_channel_samples.from_simd_unchecked(lp_samples) };
+                    samples = hp_samples;
+                }
+
+                // And the final high-passed result should be written to the last band
+                unsafe { band_outputs[num_bands - 1].from_simd_unchecked(samples) };
+            }
+        }
+    }
+
+    /// Update the crossover frequencies for all filters. If the frequencies are not monotonic then
+    /// this function will ensure that they are.
+    pub fn update(&mut self, sample_rate: f32, mut frequencies: [f32; NUM_BANDS - 1]) {
+        // Make sure the frequencies are monotonic
+        for frequency_idx in 1..NUM_BANDS - 1 {
+            if frequencies[frequency_idx] < frequencies[frequency_idx - 1] {
+                frequencies[frequency_idx] = frequencies[frequency_idx - 1];
+            }
+        }
+
+        match self.mode {
+            IirCrossoverType::LinkwitzRiley24 => {
+                const Q: f32 = std::f32::consts::FRAC_1_SQRT_2;
+                for (crossover, frequency) in self.crossovers.iter_mut().zip(frequencies) {
+                    let lp_coefs = BiquadCoefficients::lowpass(sample_rate, frequency, Q);
+                    let hp_coefs = BiquadCoefficients::highpass(sample_rate, frequency, Q);
+                    crossover.update_coefficients(lp_coefs, hp_coefs);
+                }
+            }
+        }
+    }
+
+    /// Reset the internal filter state for all crossovers.
+    pub fn reset(&mut self) {
+        for crossover in &mut self.crossovers {
+            crossover.reset();
+        }
+    }
+}
+
+impl Crossover {
+    /// Process left and right audio samples through two low-pass and two high-pass filter stages.
+    /// The resulting tuple contains the low-passed and the high-passed samples. Used for the
+    /// Linkwitz-Riley 24 dB/octave crossover.
+    pub fn process_lr24(&mut self, samples: f32x2) -> (f32x2, f32x2) {
+        let mut low_passed = samples;
+        for filter in &mut self.lp_filters[..2] {
+            low_passed = filter.process(low_passed)
+        }
+        let mut high_passed = samples;
+        for filter in &mut self.hp_filters[..2] {
+            high_passed = filter.process(high_passed)
+        }
+
+        (low_passed, high_passed)
+    }
+
+    /// Update the coefficients for all filters in the crossover.
+    pub fn update_coefficients(
+        &mut self,
+        lp_coefs: BiquadCoefficients<f32x2>,
+        hp_coefs: BiquadCoefficients<f32x2>,
+    ) {
+        for filter in &mut self.lp_filters {
+            filter.coefficients = lp_coefs;
+        }
+        for filter in &mut self.hp_filters {
+            filter.coefficients = hp_coefs;
+        }
+    }
+
+    /// Reset the internal filter state.
+    pub fn reset(&mut self) {
+        for filter in &mut self.lp_filters {
+            filter.reset();
+        }
+        for filter in &mut self.hp_filters {
+            filter.reset();
+        }
+    }
+}
+
 /// A simple biquad filter with functions for generating coefficients for second order low-pass and
 /// high-pass filters. Since these filters have 3 dB of attenuation at the center frequency, we'll
 /// two of them in series to get 6 dB of attenutation at the crossover point for the LR24

plugins/crossover/src/lib.rs

@@ -19,16 +19,26 @@
 #[cfg(not(feature = "simd"))]
 compile_error!("Compiling without SIMD support is currently not supported");
 
+use crossover::iir::{IirCrossover, IirCrossoverType};
 use nih_plug::prelude::*;
 use std::sync::Arc;
 
 mod crossover;
 
+/// The number of bands. Not used directly here, but this avoids hardcoding some constants in the
+/// crossover implementations.
+pub const NUM_BANDS: usize = 5;
+
 const MIN_CROSSOVER_FREQUENCY: f32 = 40.0;
 const MAX_CROSSOVER_FREQUENCY: f32 = 20_000.0;
 
 struct Crossover {
     params: Arc<CrossoverParams>,
+
+    buffer_config: BufferConfig,
+
+    /// Provides the LR24 crossover.
+    iir_crossover: IirCrossover,
 }
 
 // TODO: Add multiple crossover types. Haven't added the control for that yet because the current
@@ -65,7 +75,14 @@ impl Default for CrossoverParams {
         let crossover_string_to_value = formatters::s2v_f32_hz_then_khz();
 
         Self {
-            num_bands: IntParam::new("Band Count", 2, IntRange::Linear { min: 2, max: 5 }),
+            num_bands: IntParam::new(
+                "Band Count",
+                2,
+                IntRange::Linear {
+                    min: 2,
+                    max: NUM_BANDS as i32,
+                },
+            ),
             // TODO: More sensible default frequencies
             crossover_1_freq: FloatParam::new("Crossover 1", 200.0, crossover_range)
                 .with_smoother(crossover_smoothing_style)
@@ -91,6 +108,15 @@ impl Default for Crossover {
     fn default() -> Self {
         Crossover {
             params: Arc::new(CrossoverParams::default()),
+
+            buffer_config: BufferConfig {
+                sample_rate: 1.0,
+                min_buffer_size: None,
+                max_buffer_size: 0,
+                process_mode: ProcessMode::Realtime,
+            },
+
+            iir_crossover: IirCrossover::new(IirCrossoverType::LinkwitzRiley24),
         }
     }
 }
@@ -134,35 +160,109 @@ impl Plugin for Crossover {
     fn initialize(
         &mut self,
         _bus_config: &BusConfig,
-        _buffer_config: &BufferConfig,
+        buffer_config: &BufferConfig,
         _context: &mut impl InitContext,
     ) -> bool {
-        // TODO: Setup filters
+        self.buffer_config = *buffer_config;
+
+        // Make sure the filter states match the current parameters
+        self.update_filters();
+
         true
     }
 
     fn reset(&mut self) {
-        // TODO: Reset filters
+        self.iir_crossover.reset();
     }
 
     fn process(
         &mut self,
         buffer: &mut Buffer,
-        _aux: &mut AuxiliaryBuffers,
+        aux: &mut AuxiliaryBuffers,
         _context: &mut impl ProcessContext,
     ) -> ProcessStatus {
-        // TODO: Do the splitty thing
+        let aux_outputs = &mut aux.outputs;
+        let (band_1_buffer, aux_outputs) = aux_outputs.split_first_mut().unwrap();
+        let (band_2_buffer, aux_outputs) = aux_outputs.split_first_mut().unwrap();
+        let (band_3_buffer, aux_outputs) = aux_outputs.split_first_mut().unwrap();
+        let (band_4_buffer, aux_outputs) = aux_outputs.split_first_mut().unwrap();
+        let (band_5_buffer, _) = aux_outputs.split_first_mut().unwrap();
 
-        // The main output should be silent as the signal is already evenly split over the other
-        // bands
-        for channel_slice in buffer.as_slice() {
-            channel_slice.fill(0.0);
+        // Snoclists for days
+        for (
+            (
+                (
+                    ((main_channel_samples, band_1_channel_samples), band_2_channel_samples),
+                    band_3_channel_samples,
+                ),
+                band_4_channel_samples,
+            ),
+            band_5_channel_samples,
+        ) in buffer
+            .iter_samples()
+            .zip(band_1_buffer.iter_samples())
+            .zip(band_2_buffer.iter_samples())
+            .zip(band_3_buffer.iter_samples())
+            .zip(band_4_buffer.iter_samples())
+            .zip(band_5_buffer.iter_samples())
+        {
+            // We can avoid a lot of hardcoding and conditionals by restoring the original array structure
+            let bands = [
+                band_1_channel_samples,
+                band_2_channel_samples,
+                band_3_channel_samples,
+                band_4_channel_samples,
+                band_5_channel_samples,
+            ];
+
+            // Only update the filters when needed
+            self.maybe_update_filters();
+
+            self.iir_crossover.process(
+                self.params.num_bands.value as usize,
+                &main_channel_samples,
+                bands,
+            );
+
+            // The main output should be silent as the signal is already evenly split over the other
+            // bands
+            for sample in main_channel_samples {
+                *sample = 0.0;
+            }
         }
 
         ProcessStatus::Normal
     }
 }
 
+impl Crossover {
+    /// Update the filter coefficients for the crossovers, but only if it's needed.
+    fn maybe_update_filters(&mut self) {
+        if self.params.crossover_1_freq.smoothed.is_smoothing()
+            || self.params.crossover_2_freq.smoothed.is_smoothing()
+            || self.params.crossover_3_freq.smoothed.is_smoothing()
+            || self.params.crossover_4_freq.smoothed.is_smoothing()
+        {
+            self.update_filters();
+        }
+    }
+
+    /// Update the filter coefficients for the crossovers.
+    fn update_filters(&mut self) {
+        // This function will take care of non-monotonic crossover frequencies for us, e.g.
+        // crossover 2 being lower than crossover 1
+        self.iir_crossover.update(
+            self.buffer_config.sample_rate,
+            [
+                self.params.crossover_1_freq.smoothed.next(),
+                self.params.crossover_2_freq.smoothed.next(),
+                self.params.crossover_3_freq.smoothed.next(),
+                self.params.crossover_4_freq.smoothed.next(),
+            ],
+        )
+    }
+}
+
 impl ClapPlugin for Crossover {
     const CLAP_ID: &'static str = "nl.robbertvanderhelm.crossover";
     const CLAP_DESCRIPTION: &'static str = "Cleanly split a signal into multiple bands";