add one-knob compressor with feedback topology

dynamics/oneknob.h

@@ -0,0 +1,118 @@
+#pragma once
+#include "audio_math.h"
+#include <cmath>
+
+namespace trnr {
+
+struct rms_detector {
+	float alpha;
+	float rms_squared;
+};
+
+inline void rms_init(rms_detector& det, float samplerate, float window_ms)
+{
+	float window_seconds = 0.001f * window_ms;
+	det.alpha = 1.0f - expf(-1.0f / (samplerate * window_seconds));
+	det.rms_squared = 0.0f;
+}
+
+template <typename sample>
+inline sample rms_process(rms_detector& det, sample input)
+{
+	det.rms_squared = (1.0f - det.alpha) * det.rms_squared + det.alpha * (input * input);
+	return sqrtf(det.rms_squared);
+}
+
+struct hp_filter {
+    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;
+}
+
+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;
+}
+
+struct oneknob_comp {
+	// params
+	float amount;
+
+	// state
+	rms_detector detector;
+	hp_filter filter;
+	float attack_coef;
+	float release_coef;
+	float envelope_level;
+	float sidechain_in;
+};
+
+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;
+
+	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.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);
+
+	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));
+
+		// 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);
+		}
+
+		if (comp.envelope_level < 0.f) comp.envelope_level = 0.f;
+
+		float slope = 1.f / ratio;
+
+		float gain_reduction_db = comp.envelope_level * (slope - 1.f);
+		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;
+
+		// feedback compression
+		float sum = sqrtf(0.5f * (audio[0][i] * audio[0][i] + audio[1][i] * audio[1][i]));
+		comp.sidechain_in = hp_filter_process(comp.filter, sum);
+	}
+}
+
+} // namespace trnr