148 lines
6.6 KiB
Rust
148 lines
6.6 KiB
Rust
// Diopser: a phase rotation plugin
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// Copyright (C) 2021-2023 Robbert van der Helm
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//
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// This program is free software: you can redistribute it and/or modify
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// it under the terms of the GNU General Public License as published by
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// the Free Software Foundation, either version 3 of the License, or
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// (at your option) any later version.
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//
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// This program is distributed in the hope that it will be useful,
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// but WITHOUT ANY WARRANTY; without even the implied warranty of
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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// GNU General Public License for more details.
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//
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// You should have received a copy of the GNU General Public License
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// along with this program. If not, see <https://www.gnu.org/licenses/>.
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use nih_plug::prelude::*;
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use nih_plug::util::window::multiply_with_window;
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use realfft::num_complex::Complex32;
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use realfft::{RealFftPlanner, RealToComplex};
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use std::f32;
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use std::sync::Arc;
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use triple_buffer::TripleBuffer;
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pub const SPECTRUM_WINDOW_SIZE: usize = 2048;
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// Don't need that much precision here
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const SPECTRUM_WINDOW_OVERLAP: usize = 2;
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/// The time it takes for the spectrum to go down 12 dB. The upwards step is immediate like in a
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/// peak meter.
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const SMOOTHING_DECAY_MS: f32 = 100.0;
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/// The amplitudes of all frequency bins in a windowed FFT of Diopser's output. Also includes the DC
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/// offset bin which we don't draw, just to make this a bit less confusing.
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pub type Spectrum = [f32; SPECTRUM_WINDOW_SIZE / 2 + 1];
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/// A receiver for a spectrum computed by [`SpectrumInput`].
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pub type SpectrumOutput = triple_buffer::Output<Spectrum>;
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/// Continuously compute spectrums and send them to the connected [`SpectrumOutput`].
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pub struct SpectrumInput {
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/// A helper to do most of the STFT process.
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stft: util::StftHelper,
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/// The number of channels we're working on.
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num_channels: usize,
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/// The spectrum behaves like a peak meter. If the new value is higher than the previous one, it
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/// jump up immediately. Otherwise the old value is multiplied by this weight and the new value
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/// by one minus this weight.
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smoothing_decay_weight: f32,
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/// A way to send data to the corresponding [`SpectrumOutput`]. `spectrum_result_buffer` gets
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/// copied into this buffer every time a new spectrum is available.
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triple_buffer_input: triple_buffer::Input<Spectrum>,
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/// A scratch buffer to compute the resulting power amplitude spectrum.
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spectrum_result_buffer: Spectrum,
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/// The algorithm for the FFT operation used for our spectrum analyzer.
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plan: Arc<dyn RealToComplex<f32>>,
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/// A Hann window window, passed to the STFT helper. The gain compensation is already part of
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/// this window to save a multiplication step.
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compensated_window_function: Vec<f32>,
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/// The output of our real->complex FFT.
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complex_fft_buffer: Vec<Complex32>,
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}
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impl SpectrumInput {
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/// Create a new spectrum input and output pair. The output should be moved to the editor.
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pub fn new(num_channels: usize) -> (SpectrumInput, SpectrumOutput) {
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let (triple_buffer_input, triple_buffer_output) =
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TripleBuffer::new(&[0.0; SPECTRUM_WINDOW_SIZE / 2 + 1]).split();
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let input = Self {
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stft: util::StftHelper::new(num_channels, SPECTRUM_WINDOW_SIZE, 0),
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num_channels,
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// This is set in `initialize()` based on the sample rate
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smoothing_decay_weight: 0.0,
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triple_buffer_input,
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spectrum_result_buffer: [0.0; SPECTRUM_WINDOW_SIZE / 2 + 1],
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plan: RealFftPlanner::new().plan_fft_forward(SPECTRUM_WINDOW_SIZE),
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compensated_window_function: util::window::hann(SPECTRUM_WINDOW_SIZE)
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.into_iter()
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// Include the gain compensation in the window function to save some multiplications
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.map(|x| x / SPECTRUM_WINDOW_SIZE as f32)
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.collect(),
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complex_fft_buffer: vec![Complex32::default(); SPECTRUM_WINDOW_SIZE / 2 + 1],
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};
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(input, triple_buffer_output)
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}
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/// Update the smoothing using the specified sample rate. Called in `initialize()`.
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pub fn update_sample_rate(&mut self, sample_rate: f32) {
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// We'll express the dacay rate in the time it takes for the moving average to drop by 12 dB
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// NOTE: The effective sample rate accounts for the STFT interval, **and** for the number of
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// channels. We'll average both channels to mono-ish.
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let effective_sample_rate = sample_rate / SPECTRUM_WINDOW_SIZE as f32
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* SPECTRUM_WINDOW_OVERLAP as f32
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* self.num_channels as f32;
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let decay_samples = (SMOOTHING_DECAY_MS / 1000.0 * effective_sample_rate) as f64;
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self.smoothing_decay_weight = 0.25f64.powf(decay_samples.recip()) as f32
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}
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/// Compute the spectrum for a buffer and send it to the corresponding output pair.
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pub fn compute(&mut self, buffer: &Buffer) {
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self.stft.process_analyze_only(
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buffer,
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SPECTRUM_WINDOW_OVERLAP,
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|_channel_idx, real_fft_scratch_buffer| {
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multiply_with_window(real_fft_scratch_buffer, &self.compensated_window_function);
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self.plan
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.process_with_scratch(
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real_fft_scratch_buffer,
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&mut self.complex_fft_buffer,
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// We don't actually need a scratch buffer
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&mut [],
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)
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.unwrap();
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// We'll use peak meter-like behavior for the spectrum analyzer to make things
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// easier to dial in. Values that are higher than the old value snap to the new
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// value immediately, lower values decay gradually. This also results in quasi-mono
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// summing since this same callback will be called for both channels. Gain
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// compensation has already been baked into the window function.
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for (bin, spectrum_result) in self
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.complex_fft_buffer
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.iter()
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.zip(&mut self.spectrum_result_buffer)
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{
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let magnetude = bin.norm();
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if magnetude > *spectrum_result {
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*spectrum_result = magnetude;
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} else {
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*spectrum_result = (*spectrum_result * self.smoothing_decay_weight)
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+ (magnetude * (1.0 - self.smoothing_decay_weight));
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}
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}
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self.triple_buffer_input.write(self.spectrum_result_buffer);
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},
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);
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}
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}
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