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[nageru] / correlation_measurer.h

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+#ifndef _CORRELATION_MEASURER_H
+#define _CORRELATION_MEASURER_H 1
+
+// Measurement of left/right stereo correlation. +1 is pure mono
+// (okay but not ideal), 0 is no correlation (usually bad, unless
+// it is due to silence), strongly negative values means inverted
+// phase (bad). Typical values for e.g. music would be somewhere
+// around +0.7, although you can expect it to vary a bit.
+//
+// This is, of course, based on the regular Pearson correlation,
+// where µ_L and µ_R is taken to be 0 (ie., no DC offset). It is
+// filtered through a simple IIR filter so that older values are
+// weighed less than newer, depending on <falloff_seconds>.
+//
+//
+// Adapted from Adriaensen's project Zita-mu1 (as of January 2016).
+// Original copyright follows:
+//
+//  Copyright (C) 2008-2015 Fons Adriaensen <fons@linuxaudio.org>
+//    
+//  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 3 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, see <http://www.gnu.org/licenses/>.
+
+#include <vector>
+
+class CorrelationMeasurer {
+public:
+       CorrelationMeasurer(unsigned sample_rate, float lowpass_cutoff_hz = 1000.0f,
+                           float falloff_seconds = 0.150f);
+       void process_samples(const std::vector<float> &samples);  // Taken to be stereo, interleaved.
+       float get_correlation() const;
+
+private:
+       float w1, w2;
+
+       // Filtered values of left and right channel, respectively.
+       float zl = 0.0f, zr = 0.0f;
+
+       // Filtered values of l², r² and lr (where l and r are the filtered
+       // versions, given by zl and zr). Together, they make up what we need
+       // to calculate the correlation.
+       float zll = 0.0f, zlr = 0.0f, zrr = 0.0f;
+};
+
+#endif  // !defined(_CORRELATION_MEASURER_H)