Add Stereo Correlation Meter DSP

libs/ardour/ardour/dsp_filter.h

@@ -418,6 +418,24 @@ namespace ARDOUR { namespace DSP {
 			float            _bwcorr[513];
 	};
 
+	class LIBARDOUR_API StereoCorrelation {
+		public:
+			StereoCorrelation (float samplerate, float lp_freq = 2e3f, float tc_corr = .3f);
+
+			void process (float const*, float const*, uint32_t n_samples);
+			void reset ();
+			float read () const;
+
+		private:
+			float _zl;
+			float _zr;
+			float _zlr;
+			float _zll;
+			float _zrr;
+			float _w1;  // lowpass filter coefficient
+			float _w2;  // correlation filter coeffient
+	};
+
 	class LIBARDOUR_API Generator {
 		public:
 			Generator ();

libs/ardour/dsp_filter.cc

@@ -874,6 +874,71 @@ PerceptualAnalyzer::power_at_bin (const uint32_t b, float gain, bool pink) const
 	}
 }
 
+/* ****************************************************************************/
+StereoCorrelation::StereoCorrelation (float samplerate, float lp_freq, float tc_corr)
+{
+	_w1 = 6.28f * lp_freq / samplerate;
+	_w2 = 1.f / (tc_corr * samplerate);
+	reset ();
+}
+
+void
+StereoCorrelation::reset ()
+{
+	_zl  = 0;
+	_zr  = 0;
+	_zlr = 0;
+	_zll = 0;
+	_zrr = 0;
+}
+
+float
+StereoCorrelation::read () const
+{
+	return _zlr / sqrtf (_zll * _zrr + 1e-10f);
+}
+
+void
+StereoCorrelation::process (float const* pl, float const* pr, uint32_t n)
+{
+	float zl, zr, zlr, zll, zrr;
+
+	zl  = _zl;
+	zr  = _zr;
+	zlr = _zlr;
+	zll = _zll;
+	zrr = _zrr;
+	while (n--) {
+		zl += _w1 * (*pl++ - zl) + 1e-20f;
+		zr += _w1 * (*pr++ - zr) + 1e-20f;
+		zlr += _w2 * (zl * zr - zlr);
+		zll += _w2 * (zl * zl - zll);
+		zrr += _w2 * (zr * zr - zrr);
+	}
+
+	if (!isfinite (zl)) {
+		zl = 0;
+	}
+	if (!isfinite (zr)) {
+		zr = 0;
+	}
+	if (!isfinite (zlr)) {
+		zlr = 0;
+	}
+	if (!isfinite (zll)) {
+		zll = 0;
+	}
+	if (!isfinite (zrr)) {
+		zrr = 0;
+	}
+
+	_zl  = zl;
+	_zr  = zr;
+	_zlr = zlr + 1e-10f;
+	_zll = zll + 1e-10f;
+	_zrr = zrr + 1e-10f;
+}
+
 /* ****************************************************************************/
 
 Generator::Generator ()