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Diffstat (limited to 'src/include/lp4pole_filter.h')
-rw-r--r-- | src/include/lp4pole_filter.h | 137 |
1 files changed, 137 insertions, 0 deletions
diff --git a/src/include/lp4pole_filter.h b/src/include/lp4pole_filter.h new file mode 100644 index 0000000..adc5cba --- /dev/null +++ b/src/include/lp4pole_filter.h @@ -0,0 +1,137 @@ +/* + 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 */ |