126 lines
3.7 KiB
C
126 lines
3.7 KiB
C
/* nekobee DSSI software synthesizer plugin
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*
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* Copyright (C) 2023 Gordonjcp, with attributions inline
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*
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* This program is free software; you can redistribute it and/or
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* modify it under the terms of the GNU General Public License as
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* published by the Free Software Foundation; either version 2 of
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* the License, or (at your option) any later version.
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*
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* This program is distributed in the hope that it will be
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* useful, but WITHOUT ANY WARRANTY; without even the implied
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* warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR
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* PURPOSE. See the GNU General Public License for more details.
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*
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* You should have received a copy of the GNU General Public
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* License along with this program; if not, write to the Free
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* Software Foundation, Inc., 59 Temple Place, Suite 330, Boston,
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* MA 02111-1307, USA.
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*/
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// complete rewrite of the voice engine
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#include <math.h>
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#include <stdint.h>
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#include "nekobee_synth.h"
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// centre oscillator around Middle C
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// conveniently the middle of the 303's range
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#define REF_NOTE 60
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float nekobee_pitch[128];
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float logpot[128];
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void nekobee_init_tables(void) {
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// create tables used by Nekobee to save on expensive calculations
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// mostly involving exponentiation!
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// tables are scaled to 128 values for ease of calculation with MIDI
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// it's worth noting that a real 303 only responds over four octaves
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// although in theory its DAC could do five
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// it's a bit of a waste defining 128 MIDI notes in the expo scale
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uint8_t i;
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float x;
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for (i = 0; i < 128; i++) {
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// expo pitch scale (MIDI note number to VCO control current)
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nekobee_pitch[i] = powf(2, (i - REF_NOTE) / 12.0f);
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// log pot scale used for volume, decay, cutoff, and env mod
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// for a range of "0 to 1" scaled to 0-127, gives a log response
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// with 50% of "pot rotation" giving 15% output
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x = i / 128.0f; // pot input from 0 to 1
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logpot[i] = 0.0323 * powf(32, x) - 0.0323;
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}
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return;
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}
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void vco(nekobee_synth_t *synth, uint32_t count) {
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// generate a bandlimited oscillator
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// uses polyblep for bandlimiting
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// massive and endless thanks to Mystran
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// https://www.kvraudio.com/forum/viewtopic.php?t=398553
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nekobee_voice_t *voice = synth->voice;
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blosc_t *osc = &voice->osc;
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uint32_t i;
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float phase = osc->phase; // current running phase 0..1
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float delay = osc->delay; // delay sample for polyblep
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float out, t; // output sample, temporary value for blep
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// calculate omega for phase shift
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float w = nekobee_pitch[voice->key] * 261.63 * synth->deltat;
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// FIXME this only does saws
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for (i = 0; i < count; i++) {
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phase += w;
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out = delay;
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delay = 0;
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if (phase > 1.0f) {
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t = (phase - 1) / w;
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out -= 0.5 * t * t; // other polynomials are available
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t = 1 - t;
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delay += 0.5 * t * t;
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phase -= 1.0f;
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}
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delay += phase; // save value for next time
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voice->osc_audio[i] = 0.5 - out; // save output in buffer, remove DC offset
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}
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osc->phase = phase;
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osc->delay = delay;
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}
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void vcf(nekobee_synth_t *synth, uint32_t count) {
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// run a 4-pole ladder filter over a block
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// this is a crude implementation that only approximates the complex
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// behaviour of the "real" ladder filter
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nekobee_voice_t *voice = synth->voice;
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printf("cutoff set to %f\n", synth->cutoff);
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(void)voice;
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(void)count;
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}
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void nekobee_voice_render(nekobee_synth_t *synth, float *out, uint32_t count) {
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// generate "count" samples into the buffer at out
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vco(synth, count);
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vcf(synth, count);
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for(uint32_t i=0; i<count; i++) {
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out[i] = synth->voice->osc_audio[i];
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}
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return;
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}
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