add lua DSP transfer function calculation

[ardour.git] / libs / ardour / dsp_filter.cc

diff --git a/libs/ardour/dsp_filter.cc b/libs/ardour/dsp_filter.cc
index 2678d0008c4671b64493acddea23e293b04fa4c5..f82b5e46cfd7e5e208b1d708c9deebbe2f55a76a 100644 (file)
--- 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)));
+}