Audio compressor implementation.

firmware/baseband/audio_compressor.cpp

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+/*
+ * Copyright (C) 2016 Jared Boone, ShareBrained Technology, Inc.
+ *
+ * This file is part of PortaPack.
+ *
+ * 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, 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; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#include "audio_compressor.hpp"
+
+float GainComputer::operator()(const float x) const {
+	const auto abs_x = std::abs(x);
+	const auto db = (abs_x < lin_floor) ? db_floor : log2_db_k * fast_log2(abs_x);
+	const auto overshoot_db = db - threshold_db;
+	if( knee_width_db > 0.0f ) {
+		const auto w2 = knee_width_db / 2.0f;
+		const auto a = w2 / (knee_width_db * knee_width_db);
+		const auto in_transition = (overshoot_db > -w2) && (overshoot_db < w2);
+		const auto rectified_overshoot = in_transition ? (a * std::pow(overshoot_db + w2, 2.0f)) : std::max(overshoot_db, 0.0f);
+		return rectified_overshoot * slope;
+	} else {
+		const auto rectified_overshoot = std::max(overshoot_db, 0.0f);
+		return rectified_overshoot * slope;
+	}
+}
+
+void FeedForwardCompressor::execute_in_place(const buffer_f32_t& buffer) {
+	constexpr float makeup_gain = std::pow(10.0f, (threshold - (threshold / ratio)) / 20.0f);
+	for(size_t i=0; i<buffer.count; i++) {
+		// TODO: Terrible hack here due to not normalizing float samples to +/-1.0.
+		buffer.p[i] = execute_once(buffer.p[i] * (1.0f / 32768.0f)) * (makeup_gain * 32768.0f);
+	}
+}
+
+float FeedForwardCompressor::execute_once(const float x) {
+	const auto gain_db = gain_computer(x);
+	const auto peak_db = -peak_detector(-gain_db);
+	const auto gain = fast_pow2(peak_db * (3.321928094887362f / 20.0f));
+	return x * gain;
+}

firmware/baseband/audio_compressor.hpp

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+/*
+ * Copyright (C) 2016 Jared Boone, ShareBrained Technology, Inc.
+ *
+ * This file is part of PortaPack.
+ *
+ * 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, 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; see the file COPYING.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street,
+ * Boston, MA 02110-1301, USA.
+ */
+
+#ifndef __AUDIO_COMPRESSOR_H__
+#define __AUDIO_COMPRESSOR_H__
+
+#include "dsp_types.hpp"
+#include "utility.hpp"
+
+#include <cmath>
+
+/* Code based on article in Journal of the Audio Engineering Society
+ * Vol. 60, No. 6, 2012 June, by Dimitrios Giannoulis, Michael Massberg,
+ * Joshua D. Reiss "Digital Dynamic Range Compressor Design – A Tutorial
+ * and Analysis"
+ */
+
+class GainComputer {
+public:
+	constexpr GainComputer(
+		float ratio,
+		float threshold
+	) : ratio { ratio },
+		slope { 1.0f / ratio - 1.0f },
+		threshold_db { threshold }
+	{
+	}
+
+	float operator()(const float x) const;
+
+private:
+	const float ratio;
+	const float slope;
+	const float threshold_db;
+
+	static constexpr float knee_width_db = 0.0f;
+
+	static constexpr float db_floor = -120.0f;
+	static constexpr float lin_floor = std::pow(10.0f, db_floor / 20.0f);
+	static constexpr float log2_db_k = 20.0f * std::log10(2.0f);
+};
+
+class PeakDetectorBranchingSmooth {
+public:
+	constexpr PeakDetectorBranchingSmooth(
+		float att_a,
+		float rel_a
+	) : att_a { att_a },
+		rel_a { rel_a }
+	{
+	}
+
+	float operator()(const float db) {
+		const auto a = (db > state) ? att_a : rel_a;
+		state = db + a * (state - db);
+		return state;
+	}
+
+private:
+	float state { 0.0f };
+	const float att_a;
+	const float rel_a;
+};
+
+class FeedForwardCompressor {
+public:
+	void execute_in_place(const buffer_f32_t& buffer);
+
+private:
+	static constexpr float fs = 12000.0f;
+	static constexpr float ratio = 10.0f;
+	static constexpr float threshold = -30.0f;
+
+	GainComputer gain_computer { ratio, threshold };
+	PeakDetectorBranchingSmooth peak_detector { tau_alpha(0.010f, fs), tau_alpha(0.300f, fs) };
+
+	float execute_once(const float x);
+
+	static constexpr float tau_alpha(const float tau, const float fs) {
+		return std::exp(-1.0f / (tau * fs));
+	}
+};
+
+#endif/*__AUDIO_COMPRESSOR_H__*/