From 9c7d9abc18ea3d8a8f4f19dcc3a457f47ad5cf7d Mon Sep 17 00:00:00 2001
From: Chris <chris@ternar.tech>
Date: Thu, 31 Jul 2025 16:26:40 +0200
Subject: [PATCH] refined oneknob compressor

---
 dynamics/oneknob.h | 73 +++++++++++++++++++++++-----------------------
 1 file changed, 36 insertions(+), 37 deletions(-)

diff --git a/dynamics/oneknob.h b/dynamics/oneknob.h
index 343dcc6..b6058f7 100644
--- a/dynamics/oneknob.h
+++ b/dynamics/oneknob.h
@@ -24,26 +24,26 @@ inline sample rms_process(rms_detector& det, sample input)
 }
 
 struct hp_filter {
-    float a0, a1, b1;
-    float z1; // filter state
+	float a0, a1, b1;
+	float z1; // filter state
 };
 
 inline void hp_filter_init(hp_filter& f, float samplerate)
 {
-    float cutoff = 100.0f;
-    float w0 = 2.0f * 3.14159265359f * cutoff / samplerate;
-    float alpha = (1.0f - std::tan(w0 / 2.0f)) / (1.0f + std::tan(w0 / 2.0f));
-    f.a0 = 0.5f * (1.0f + alpha);
-    f.a1 = -0.5f * (1.0f + alpha);
-    f.b1 = alpha;
-    f.z1 = 0.0f;
+	float cutoff = 100.0f;
+	float w0 = 2.0f * 3.14159265359f * cutoff / samplerate;
+	float alpha = (1.0f - std::tan(w0 / 2.0f)) / (1.0f + std::tan(w0 / 2.0f));
+	f.a0 = 0.5f * (1.0f + alpha);
+	f.a1 = -0.5f * (1.0f + alpha);
+	f.b1 = alpha;
+	f.z1 = 0.0f;
 }
 
 inline float hp_filter_process(hp_filter& f, float x)
 {
-    float y = f.a0 * x + f.a1 * f.z1 - f.b1 * f.z1;
-    f.z1 = x;
-    return y;
+	float y = f.a0 * x + f.a1 * f.z1 - f.b1 * f.z1;
+	f.z1 = x;
+	return y;
 }
 
 struct oneknob_comp {
@@ -63,49 +63,48 @@ inline void oneknob_init(oneknob_comp& comp, float samplerate, float window_ms)
 {
 	rms_init(comp.detector, samplerate, window_ms);
 	hp_filter_init(comp.filter, samplerate);
-	comp.amount = 0.0f;
 
-	const float attack_ms = 10.f;
-	const float release_ms = 100.f;
+	const float attack_ms = 0.2f;
+	const float release_ms = 150.f;
 
-	comp.attack_coef = expf(-1.0f / (samplerate * (attack_ms * 0.001f)));
-	comp.release_coef = expf(-1.0f / (samplerate * (release_ms * 0.001f)));
-	comp.envelope_level = 0.f;
+	comp.attack_coef = expf(-1.0f / (attack_ms * 1e-6 * samplerate));
+	comp.release_coef = expf(-1.0f / (release_ms * 1e-3 * samplerate));
+	comp.envelope_level = -60.f;
 	comp.sidechain_in = 0.f;
 }
 
 template <typename sample>
 inline void oneknob_process_block(oneknob_comp& comp, sample** audio, int frames)
 {
-	const float threshold = -18.f;
-	const float min_ratio = 1.0f;
-	const float max_ratio = 10.0f;
-	float ratio = min_ratio + comp.amount * (max_ratio - min_ratio);
+	const float min_user_ratio = 1.0f;
+	const float max_user_ratio = 20.0f;
+	const float threshold_db = -9.f;
+
+	const float amount = fmaxf(0.0f, fminf(powf(comp.amount, 2.f), 1.0f)); // clamp to [0, 1]
+	float ratio = min_user_ratio + amount * (max_user_ratio - min_user_ratio);
 
 	for (int i = 0; i < frames; ++i) {
 		float rms_value = rms_process(comp.detector, comp.sidechain_in);
-		float absolute_rms_db = lin_2_db(fabs(rms_value));
+		float envelope_in = lin_2_db(fmaxf(fabs(rms_value), 1e-20f));
 
-		// cut envelope below threshold
-		float overshoot = absolute_rms_db - threshold;
-		if (overshoot < 0.f) overshoot = 0.f;
-
-		if (overshoot > comp.envelope_level) {
-			comp.envelope_level = overshoot + comp.attack_coef * (comp.envelope_level - overshoot);
-		} else {
-			comp.envelope_level = overshoot + comp.release_coef * (comp.envelope_level - overshoot);
+		// attack
+		if (envelope_in > comp.envelope_level) {
+			comp.envelope_level = envelope_in + comp.attack_coef * (comp.envelope_level - envelope_in);
+		}
+		// release
+		else {
+			comp.envelope_level = envelope_in + comp.release_coef * (comp.envelope_level - envelope_in);
 		}
 
-		if (comp.envelope_level < 0.f) comp.envelope_level = 0.f;
+		float x = comp.envelope_level;
+		float y;
 
-		float slope = 1.f / ratio;
+		if (x < threshold_db) y = x;
+		else y = threshold_db + (x - threshold_db) / ratio;
 
-		float gain_reduction_db = comp.envelope_level * (slope - 1.f);
+		float gain_reduction_db = y - x;
 		float gain_reduction_lin = db_2_lin(gain_reduction_db);
 
-		sample input_l = audio[0][i];
-		sample input_r = audio[1][i];
-
 		audio[0][i] *= gain_reduction_lin;
 		audio[1][i] *= gain_reduction_lin;