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+/*
+ Header for lp4pole_filter struct, and functions to run instance.
+ Copyright 2011 David Robillard
+ Copyright 2003 Mike Rawes
+
+ Originally originally appeared in CSound as Timo Tossavainen's (sp?)
+ implementation from the Stilson/Smith CCRMA paper.
+
+ See http://musicdsp.org/archive.php?classid=3#26
+
+ Originally appeared in the arts softsynth by Stefan Westerfeld:
+ http://www.arts-project.org/
+
+ First ported to LADSPA by Reiner Klenk (pdq808[at]t-online.de)
+
+ Tuning and stability issues (output NaN) and additional audio-rate
+ variant added by Mike Rawes (mike_rawes[at]yahoo.co.uk)
+
+ This 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 software 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 software. If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef blip_lp4pole_filter_h
+#define blip_lp4pole_filter_h
+
+#include "lv2/lv2plug.in/ns/lv2core/lv2.h"
+#include "common.h"
+
+typedef struct {
+ float f;
+ float coeff;
+ float fb;
+ float in1;
+ float in2;
+ float in3;
+ float in4;
+ float inv_nyquist;
+ float out1;
+ float out2;
+ float out3;
+ float out4;
+ float max_abs_in;
+} LP4PoleFilter;
+
+/**
+ Allocate a new LP4PoleFilter instance.
+ @param sample_rate Intended playback (DAC) rate
+ @return Allocated LP4PoleFilter instance
+*/
+LP4PoleFilter* lp4pole_new(double sample_rate);
+
+/**
+ Cleanup an existing LP4PoleFilter instance.
+ @param lpf Pointer to LP4PoleFilter instance allocated with initFilter
+*/
+void lp4pole_cleanup(LP4PoleFilter* lpf);
+
+/**
+ Initialise filter.
+ @param lpf Pointer to LP4PoleFilter instance allocated with initFilter
+*/
+void lp4pole_init(LP4PoleFilter* lpf);
+
+/**
+ Set up filter coefficients for given LP4Pole instance.
+ @param lpf Pointer to LP4PoleFilter instance
+ @param cutoff Cutoff frequency in Hz
+ @param resonance Resonance [Min=0.0, Max=4.0]
+*/
+static inline void
+lp4pole_set_params(LP4PoleFilter* lpf,
+ float cutoff,
+ float resonance)
+{
+ float fsqd;
+ float tuning;
+
+ /* Normalise cutoff and find tuning - Magic numbers found empirically :) */
+ lpf->f = cutoff * lpf->inv_nyquist;
+ tuning = f_clip(3.13f - (lpf->f * 4.24703592f), 1.56503274f, 3.13f);
+
+ /* Clip to bounds */
+ lpf->f = f_clip(lpf->f * tuning, lpf->inv_nyquist, 1.16f);
+
+ fsqd = lpf->f * lpf->f;
+ lpf->coeff = fsqd * fsqd * 0.35013f;
+
+ lpf->fb = f_clip(resonance, -1.3f, 4.0f) * (1.0f - 0.15f * fsqd);
+
+ lpf->f = 1.0f - lpf->f;
+}
+
+/**
+ Run given LP4PoleFilter instance for a single sample.
+ @param lpf Pointer to LP4PoleFilter instance
+ @param in Input sample
+ @return Filtered sample
+*/
+static inline float
+lp4pole_run(LP4PoleFilter* lpf,
+ float in)
+{
+ float abs_in = fabsf(16.0f * in); /* ~24dB unclipped headroom */
+
+ lpf->max_abs_in = f_max(lpf->max_abs_in, abs_in);
+
+ in -= lpf->out4 * lpf->fb;
+ in *= lpf->coeff;
+
+ lpf->out1 = in + 0.3f * lpf->in1 + lpf->f * lpf->out1; /* Pole 1 */
+ lpf->in1 = in;
+ lpf->out2 = lpf->out1 + 0.3f * lpf->in2 + lpf->f * lpf->out2; /* Pole 2 */
+ lpf->in2 = lpf->out1;
+ lpf->out3 = lpf->out2 + 0.3f * lpf->in3 + lpf->f * lpf->out3; /* Pole 3 */
+ lpf->in3 = lpf->out2;
+ lpf->out4 = lpf->out3 + 0.3f * lpf->in4 + lpf->f * lpf->out4; /* Pole 4 */
+ lpf->in4 = lpf->out3;
+
+ /* Simple hard clip to prevent NaN */
+ lpf->out4 = f_clip(lpf->out4, -lpf->max_abs_in, lpf->max_abs_in);
+
+ lpf->max_abs_in *= 0.999f;
+
+ return lpf->out4;
+}
+
+#endif /* blip_lp4pole_filter_h */