From e991a077f9d2abcf0b766119e2a82f3bf0354fab Mon Sep 17 00:00:00 2001
From: "Steinar H. Gunderson" <sgunderson@bigfoot.com>
Date: Mon, 29 Aug 2016 00:29:25 +0200
Subject: [PATCH] Add support for EQ filters to the Filter class.

---
 filter.cpp | 31 ++++++++++++++++++++++++++++++-
 filter.h   | 14 +++++++++++++-
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/filter.cpp b/filter.cpp
index 3974ebc..c6f9997 100644
--- a/filter.cpp
+++ b/filter.cpp
@@ -36,6 +36,7 @@ Filter::Filter()
 {
 	omega = M_PI;
 	resonance = 0.01f;
+	A = 1.0f;
 
 	init(FILTER_NONE, 1);
 	update();
@@ -131,6 +132,33 @@ void Filter::update()
 		// a2 = 1 - alpha;
 		break;
 
+	case FILTER_PEAKING_EQ:
+		b0 = 1 + alpha * A;
+		b1 = -2*cs;
+		b2 = 1 - alpha * A;
+		a0 = 1 + alpha / A;
+		// a1 = -2*cs;
+		a2 = 1 - alpha / A;
+		break;
+
+	case FILTER_LOW_SHELF:
+		b0 =      A * ((A + 1) - (A - 1)*cs + 2 * sqrt(A) * alpha);
+		b1 =  2 * A * ((A - 1) - (A + 1)*cs                      );
+		b2 =      A * ((A + 1) - (A - 1)*cs - 2 * sqrt(A) * alpha);
+		a0 =           (A + 1) + (A - 1)*cs + 2 * sqrt(A) * alpha ;
+		a1 =     -2 * ((A - 1) + (A + 1)*cs                      );
+		a2 =           (A + 1) + (A - 1)*cs - 2 * sqrt(A) * alpha ;
+		break;
+
+	case FILTER_HIGH_SHELF:
+		b0 =      A * ((A + 1) + (A - 1)*cs + 2 * sqrt(A) * alpha);
+		b1 = -2 * A * ((A - 1) + (A + 1)*cs                      );
+		b2 =      A * ((A + 1) + (A - 1)*cs - 2 * sqrt(A) * alpha);
+		a0 =           (A + 1) - (A - 1)*cs + 2 * sqrt(A) * alpha ;
+		a1 =      2 * ((A - 1) - (A + 1)*cs                      );
+		a2 =           (A + 1) - (A - 1)*cs - 2 * sqrt(A) * alpha ;
+		break;
+
 	default:
 		//unknown filter type
 		assert(false);
@@ -266,7 +294,7 @@ void StereoFilter::init(FilterType type, int new_order)
 #endif
 }
 
-void StereoFilter::render(float *inout_left_ptr, unsigned n_samples, float cutoff, float resonance)
+void StereoFilter::render(float *inout_left_ptr, unsigned n_samples, float cutoff, float resonance, float dbgain_normalized)
 {
 #ifdef __SSE__
 	if (parm_filter.filtertype == FILTER_NONE || parm_filter.filter_order == 0)
@@ -277,6 +305,7 @@ void StereoFilter::render(float *inout_left_ptr, unsigned n_samples, float cutof
 
 	parm_filter.set_linear_cutoff(cutoff);
 	parm_filter.set_resonance(resonance);
+	parm_filter.set_dbgain_normalized(dbgain_normalized);
 	parm_filter.update();
 
 	__m128 b0 = _mm_set1_ps(parm_filter.b0);
diff --git a/filter.h b/filter.h
index 5e455d2..a4a3a13 100644
--- a/filter.h
+++ b/filter.h
@@ -32,6 +32,11 @@ enum FilterType
 	FILTER_BPF,
 	FILTER_NOTCH,
 	FILTER_APF,
+
+	// EQ filters.
+	FILTER_PEAKING_EQ,
+	FILTER_LOW_SHELF,
+	FILTER_HIGH_SHELF,
 };
 
 #define FILTER_MAX_ORDER 4
@@ -69,6 +74,12 @@ public:
 		resonance = new_resonance;
 	}
 
+	// For EQ filters only.
+	void set_dbgain_normalized(float db_gain_div_40)
+	{
+		A = pow(10.0f, db_gain_div_40);
+	}
+
 #ifdef __SSE__
 	// We don't need the stride argument for SSE, as StereoFilter
 	// has its own SSE implementations.
@@ -81,6 +92,7 @@ public:
 private:
 	float omega; //which is 2*Pi*frequency /SAMPLE_RATE
 	float resonance;
+	float A;  // which is 10^(db_gain / 40)
 
 public:
 	unsigned filter_order;
@@ -104,7 +116,7 @@ class StereoFilter
 public:
 	void init(FilterType type, int new_order);
 	
-	void render(float *inout_left_ptr, unsigned n_samples, float cutoff, float resonance);
+	void render(float *inout_left_ptr, unsigned n_samples, float cutoff, float resonance, float dbgain_normalized = 0.0f);
 #ifndef NDEBUG
 #ifdef __SSE__
 	void debug() { parm_filter.debug(); }
-- 
2.39.2