add lua DSP transfer function calculation

3 files changed, 36 insertions, 3 deletions

diff --git a/libs/ardour/ardour/dsp_filter.h b/libs/ardour/ardour/dsp_filter.h
index ddf61075cd..1ffccd3319 100644
--- a/libs/ardour/ardour/dsp_filter.h
+++ b/libs/ardour/ardour/dsp_filter.h
@@ -228,6 +228,13 @@ namespace ARDOUR { namespace DSP {
 			 * @param gain filter gain
 			 */
 			void compute (Type t, double freq, double Q, double gain);
+
+			/** filter transfer function (for spectrum visualization)
+			 * @param freq frequency
+			 * @return gain at given frequency in dB
+			 */
+			float dB_at_freq (float freq) const;
+
 			/** reset filter state */
 			void reset () { _z1 = _z2 = 0.0; }
 		private:
diff --git a/libs/ardour/dsp_filter.cc b/libs/ardour/dsp_filter.cc
index 2678d0008c..f82b5e46cf 100644
--- a/libs/ardour/dsp_filter.cc
+++ b/libs/ardour/dsp_filter.cc
@@ -145,14 +145,18 @@ BiQuad::run (float *data, const uint32_t n_samples)
 void
 BiQuad::compute (Type type, double freq, double Q, double gain)
 {
+	if (Q <= .001)     { Q = 0.001; }
+	if (freq <= 1.)    { freq = 1.; }
+	if (freq >= _rate) { freq = _rate; }
+
 	/* Compute biquad filter settings.
 	 * Based on 'Cookbook formulae for audio EQ biquad filter coefficents'
 	 * by Robert Bristow-Johnson
 	 */
-	const double     A = pow (10.0, (gain / 40.0));
+	const double A = pow (10.0, (gain / 40.0));
 	const double W0 = (2.0 * M_PI * freq) / _rate;
-	const double sinW0  = sin (W0);
-	const double cosW0  = cos (W0);
+	const double sinW0 = sin (W0);
+	const double cosW0 = cos (W0);
 	const double alpha = sinW0 / (2.0 * Q);
 	const double beta  = sqrt (A) / Q;
 
@@ -250,3 +254,24 @@ BiQuad::compute (Type type, double freq, double Q, double gain)
 	_a1 /= _a0;
 	_a2 /= _a0;
 }
+
+float
+BiQuad::dB_at_freq (float freq) const
+{
+	const double W0 = (2.0 * M_PI * freq) / _rate;
+	const float c1 = cosf (W0);
+	const float s1 = sinf (W0);
+
+	const float A = _b0 + _b2;
+	const float B = _b0 - _b2;
+	const float C = 1.0 + _a2;
+	const float D = 1.0 - _a2;
+
+	const float a = A * c1 + _b1;
+	const float b = B * s1;
+	const float c = C * c1 + _a1;
+	const float d = D * s1;
+
+#define SQUARE(x) ( (x) * (x) )
+	return 20.f * log10f (sqrtf ((SQUARE(a) + SQUARE(b)) * (SQUARE(c) + SQUARE(d))) / (SQUARE(c) + SQUARE(d)));
+}
diff --git a/libs/ardour/luabindings.cc b/libs/ardour/luabindings.cc
index cf1699021f..6531846ce2 100644
--- a/libs/ardour/luabindings.cc
+++ b/libs/ardour/luabindings.cc
@@ -1166,6 +1166,7 @@ LuaBindings::dsp (lua_State* L)
 		.addFunction ("run", &DSP::BiQuad::run)
 		.addFunction ("compute", &DSP::BiQuad::compute)
 		.addFunction ("reset", &DSP::BiQuad::reset)
+		.addFunction ("dB_at_freq", &DSP::BiQuad::dB_at_freq)
 		.endClass ()
 
 		/* DSP enums */