Version 0.9

Added: Filter type "IIR", which allows to specify a user-defined IIR filter of arbitrary order with custom coefficients.
Improved: For playback devices, the APO is now installed both as GFX (post-mix) and LFX (pre-mix) APO. Normally the filtering happens in GFX, but the command "Stage" allows to select LFX to apply effects based on the number of input channels.
Added: Expression language. The commands "If", "ElseIf", "Else", "EndIf", "Eval" and inline expressions allow to change the filter behaviour based on runtime conditions.
Added: Command "Copy", which copies audio data between channels.
Added: Command "Delay", which delays the audio on the currently selected channels.
Improved: Configuration files are now locked exclusively for writing while reading them.
Improved: There is now a transition (10 ms) from old to new configuration after loading.
Improved: Substantial internal restructuring to improve extensibility and maintainability. Filtering commands now work exactly in the order they were specified.
Fixed: When the microphone input was in stereo but the application requested a mono stream, the audio was heavily distorted.
Fixed: The sine sweep generated by the Benchmark application had floating-point precision issues leading to severe artifacts when certain filters were applied.

filters/BiQuad.h

@@ -0,0 +1,73 @@
+/*
+	This file is part of EqualizerAPO, a system-wide equalizer.
+	Copyright (C) 2013  Jonas Thedering
+
+	This program is free software; you can redistribute it and/or modify
+	it under the terms of the GNU General Public License as published by
+	the Free Software Foundation; either version 2 of the License, or
+	(at your option) any later version.
+
+	This program is distributed in the hope that it will be useful,
+	but WITHOUT ANY WARRANTY; without even the implied warranty of
+	MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+	GNU General Public License for more details.
+
+	You should have received a copy of the GNU General Public License along
+	with this program; if not, write to the Free Software Foundation, Inc.,
+	51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
+*/
+
+#pragma once
+
+#include <string>
+
+#define IS_DENORMAL(f) (((*(unsigned int *)&(f))&0x7f800000) == 0)
+
+class BiQuad
+{
+public:
+	enum Type
+	{
+		LOW_PASS, HIGH_PASS, BAND_PASS, NOTCH, ALL_PASS, PEAKING, LOW_SHELF, HIGH_SHELF
+	};
+
+	BiQuad() {}
+	BiQuad(Type type, double dbGain, double freq, double srate, double bandwidthOrQOrS, bool isBandwidth);
+
+	__forceinline
+	void removeDenormals()
+	{
+		if(IS_DENORMAL(x1))
+			x1 = 0.0f;
+		if(IS_DENORMAL(x2))
+			x2 = 0.0f;
+		if(IS_DENORMAL(y1))
+			y1 = 0.0f;
+		if(IS_DENORMAL(y2))
+			y2 = 0.0f;
+	}
+
+	__forceinline
+	float process(float sample)
+	{
+		// changed order of additions leads to better pipelining
+		float result = a0 * sample + a[1] * x2 + a[0] * x1 - a[3] * y2 - a[2] * y1;
+
+		x2 = x1;
+		x1 = sample;
+
+		y2 = y1;
+		y1 = result;
+
+		return result;
+	}
+
+	float gainAt(float freq, float srate);
+
+private:
+	__declspec(align(16)) float a[4];
+	float a0;
+
+	float x1, x2;
+	float y1, y2;
+};