From c164d0848926ab363470eb685e5150aa579b9017 Mon Sep 17 00:00:00 2001
From: Petr Horacek <hrck@protonmail.com>
Date: Sat, 21 Jan 2023 12:13:30 +0100
Subject: [PATCH] Smoothen wow depth control

Signed-off-by: Petr Horacek <hrck@protonmail.com>
---
 CHANGELOG.md               |  1 +
 dsp/src/dc_blocker.rs      |  1 +
 dsp/src/lib.rs             |  1 +
 dsp/src/one_pole_filter.rs | 31 +++++++++++++++++++++++++++++++
 dsp/src/wow_flutter/wow.rs | 31 ++++++++++++++++++++++---------
 5 files changed, 56 insertions(+), 9 deletions(-)
 create mode 100644 dsp/src/one_pole_filter.rs

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9d821f5..d22957b 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -20,6 +20,7 @@ backwards compatibility.
 * Delay start to prevent pops.
 * Increase pot buffer to prevent clicks.
 * Randomize intensity and spacing of flutter.
+* Smoothen wow depth control.
 
 ## 0.6.0
 
diff --git a/dsp/src/dc_blocker.rs b/dsp/src/dc_blocker.rs
index cd83380..813764f 100644
--- a/dsp/src/dc_blocker.rs
+++ b/dsp/src/dc_blocker.rs
@@ -2,6 +2,7 @@
 //!
 //! Based on <https://ccrma.stanford.edu/~jos/fp/DC_Blocker_Software_Implementations.html>.
 
+// XXX: Assuming 48 kHz sample rate.
 const POLE: f32 = 0.995;
 
 #[derive(Default, Debug)]
diff --git a/dsp/src/lib.rs b/dsp/src/lib.rs
index fee3c50..e814379 100644
--- a/dsp/src/lib.rs
+++ b/dsp/src/lib.rs
@@ -30,6 +30,7 @@ mod compressor;
 mod dc_blocker;
 mod linkwitz_riley_filter;
 mod math;
+mod one_pole_filter;
 mod pre_amp;
 mod ring_buffer;
 mod state_variable_filter;
diff --git a/dsp/src/one_pole_filter.rs b/dsp/src/one_pole_filter.rs
new file mode 100644
index 0000000..2792ab9
--- /dev/null
+++ b/dsp/src/one_pole_filter.rs
@@ -0,0 +1,31 @@
+//! Simple one-pole low-pass filter.
+//!
+//! Based on <https://www.earlevel.com/main/2012/12/15/a-one-pole-filter/>,
+//! this filter may be useful for attribute smoothening.
+
+use core::f32::consts::PI;
+
+use libm::expf;
+
+#[derive(Default, Debug)]
+#[cfg_attr(feature = "defmt", derive(defmt::Format))]
+pub struct OnePoleFilter {
+    y_m1: f32,
+    a0: f32,
+    b1: f32,
+}
+
+impl OnePoleFilter {
+    // Cutoff is normalized frequency. 10.0 / sample_rate would be 10 Hz.
+    pub fn new(sample_rate: f32, cutoff: f32) -> Self {
+        let normalized_frequency = cutoff / sample_rate;
+        let b1 = expf(-2.0 * PI * normalized_frequency);
+        let a0 = 1.0 - b1;
+        Self { y_m1: 0.0, a0, b1 }
+    }
+
+    pub fn tick(&mut self, x: f32) -> f32 {
+        self.y_m1 = x * self.a0 + self.y_m1 * self.b1;
+        self.y_m1
+    }
+}
diff --git a/dsp/src/wow_flutter/wow.rs b/dsp/src/wow_flutter/wow.rs
index 50e2aa4..7efa4d8 100644
--- a/dsp/src/wow_flutter/wow.rs
+++ b/dsp/src/wow_flutter/wow.rs
@@ -3,10 +3,16 @@ use micromath::F32Ext as _;
 
 use super::ornstein_uhlenbeck::OrnsteinUhlenbeck;
 use super::wavefolder;
+use crate::one_pole_filter::OnePoleFilter;
 use crate::random::Random;
 use crate::state_variable_filter::StateVariableFilter;
 use crate::trigonometry;
 
+// Smoothening of the depth attribute to make sure that wow does not
+// scroll to the present too abruptly, causing pops when hitting 0.
+const DEPTH_CUTOFF: f32 = 0.1;
+const CONTROL_SAMPLE_RATE: f32 = 1000.0;
+
 // These constants were obtained through design in hack/wow.py and
 // experimentation with sound.
 const BASE_FREQUENCY: f32 = 0.07;
@@ -25,9 +31,10 @@ pub struct Attributes {
 pub struct Wow {
     sample_rate: f32,
     depth: f32,
+    depth_filter: OnePoleFilter,
     phase: f32,
     ornstein_uhlenbeck: OrnsteinUhlenbeck,
-    filter: StateVariableFilter,
+    modulation_filter: StateVariableFilter,
 }
 
 impl Wow {
@@ -36,10 +43,11 @@ impl Wow {
             sample_rate > 500,
             "Wow may be unstable for low sample rates"
         );
-        let filter = {
-            let mut filter = StateVariableFilter::new(sample_rate);
-            filter.set_frequency(MODULATION_CUTOFF);
-            filter
+        let depth_filter = OnePoleFilter::new(CONTROL_SAMPLE_RATE, DEPTH_CUTOFF);
+        let modulation_filter = {
+            let mut modulation_filter = StateVariableFilter::new(sample_rate);
+            modulation_filter.set_frequency(MODULATION_CUTOFF);
+            modulation_filter
         };
         let ornstein_uhlenbeck = {
             let mut ornstein_uhlenbeck = OrnsteinUhlenbeck::new(sample_rate as f32);
@@ -50,9 +58,10 @@ impl Wow {
         Self {
             sample_rate: sample_rate as f32,
             depth: 0.0,
+            depth_filter,
             phase: 0.5, // Start the offset sine wave on 0.0
             ornstein_uhlenbeck,
-            filter,
+            modulation_filter,
         }
     }
 
@@ -68,11 +77,11 @@ impl Wow {
 
             x
         };
-        wavefolder::fold(self.filter.tick(target).low_pass, 0.0, 1000.0)
+        wavefolder::fold(self.modulation_filter.tick(target).low_pass, 0.0, 1000.0)
     }
 
     pub fn set_attributes(&mut self, attributes: &Attributes) {
-        self.depth = attributes.depth;
+        self.depth = self.depth_filter.tick(attributes.depth);
     }
 }
 
@@ -96,7 +105,11 @@ mod tests {
     #[test]
     fn it_spans_in_expected_range() {
         let mut wow = Wow::new(SAMPLE_RATE);
-        wow.set_attributes(&Attributes { depth: 1.0 });
+
+        // Depth is filtered, let it reach the destination.
+        for _ in 0..10000 {
+            wow.set_attributes(&Attributes { depth: 1.0 });
+        }
 
         let x = wow.pop(&mut TestRandom);
         let (mut min, mut max) = (x, x);