Gordonjcp
/
nekobi-reworked
1
0
Fork 0
Code Issues Pull Requests Packages Projects Releases Wiki Activity

vcf, and expo converter calculation

This commit is contained in:
Gordon JC Pearce 2023-06-20 19:14:12 +01:00
parent bb5e064e1a
commit 3d0d07320a
2 changed files with 73 additions and 20 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

2
plugins/Nekobi/nekobee-src/nekobee_voice.h
View File

@ -90,7 +90,7 @@ struct _nekobee_voice_t {
#define _RELEASED(voice) ((voice)->status == XSYNTH_VOICE_RELEASED)
#define _AVAILABLE(voice) ((voice)->status == XSYNTH_VOICE_OFF)
extern float nekobee_pitch[128];
extern float nekobee_pitch[129];
/* nekobee_voice.c */
nekobee_voice_t *nekobee_voice_new();

91
plugins/Nekobi/nekobee-src/nekobee_voice_render.c
View File

@ -24,13 +24,14 @@
#include <stdint.h>
#include "nekobee_synth.h"
#include "nekobee_voice.h"
// centre oscillator around Middle C
// conveniently the middle of the 303's range
#define REF_NOTE 60
float nekobee_pitch[128];
float logpot[128];
float nekobee_pitch[129];
float logpot[129];
void nekobee_init_tables(void) {
// create tables used by Nekobee to save on expensive calculations
@ -51,9 +52,13 @@ void nekobee_init_tables(void) {
// log pot scale used for volume, decay, cutoff, and env mod
// for a range of "0 to 1" scaled to 0-127, gives a log response
// with 50% of "pot rotation" giving 15% output
x = i / 128.0f; // pot input from 0 to 1
logpot[i] = 0.0323 * powf(32, x) - 0.0323;
x = i / 127.0f; // pot input from 0 to 1
logpot[i] = 0.03225 * powf(32, x) - 0.03225;
}
// one extra value so we don't need to bounds check the linear interpolator
logpot[128] = logpot[127];
nekobee_pitch[128] = nekobee_pitch[127];
return;
}
@ -90,36 +95,84 @@ void vco(nekobee_synth_t *synth, uint32_t count) {
phase -= 1.0f;
}
delay += phase; // save value for next time
voice->osc_audio[i] = 0.5 - out; // save output in buffer, remove DC offset
voice->osc_audio[i] =
0.5 - out; // save output in buffer, remove DC offset
}
osc->phase = phase;
osc->delay = delay;
}
void vcf(nekobee_synth_t *synth, uint32_t count) {
void vcf(nekobee_synth_t *synth, float *out, uint32_t count) {
// run a 4-pole ladder filter over a block
// this is a crude implementation that only approximates the complex
// behaviour of the "real" ladder filter
nekobee_voice_t *voice = synth->voice;
printf("cutoff set to %f\n", synth->cutoff);
(void)voice;
(void)count;
// to calculate the cutoff frequency we need to solve the expo converter
// not as bad as it sounds!
// the equation is IcQ11 = IcQ10 * exp(-VbQ10 / 26)
// VbQ10 is the voltage on Q10's base, IcQ10 is the collector current
// this is supplied from the cutoff pot
nekobee_voice_t *voice = synth->voice;
float delay1 = voice->delay1, delay2 = voice->delay2,
delay3 = voice->delay3, delay4 = voice->delay4;
// to get the correct cutoff first we need Q10's collector current
// The top of VR3 Cutoff is fed from 12V, the bottom from the emitter
// of Q9 at around 3.2V through a 10k resistor. So, 8.8V between "rails"
// gives us (8.8*10k)/(10k+50k) = 1.47V at the bottom
// The wiper of VR3 goes through R73 100k and TM3 470k, which we'll assume
// is set to about half, call it 300k in total
// So IcQ10 is given by (Vcutoff - Vbias - Vbe) / 300
// For ease of working I just assume that Vbias is 0V and that the envelope
// can go negative, from about 7V to about -3V the range of Vcutoff is
// then 1.47 to 8.88-1.47 so 7.41V max
float Vcutoff = 1.47 + 7.41 * logpot[(int)floor(synth->cutoff)];
float Vbe1 = -5.077; // mV
// .3 is 300k expressed as MOhm
// if we expressed it in Ohms output would be in A
float IcQ10 = (Vcutoff - 0.65) / .3; // 100k + TM3, IcQ10 in uA
float IcQ11 = IcQ10 * exp(Vbe1 / 26.0); // in uA
// printf("Vbe1 = %04f, IcQ10 = %04f, IcQ11 = %04f\n", Vbe1, IcQ10, IcQ11);
float cutoff = IcQ11 * 96.67; // approximate Hz-per-uA
float ct = 6.2832 * cutoff * synth->deltat;
ct *= 0.25; // 4x oversampling
ct = ct / (1 + ct);
// printf("cutoff = %04fHz, ct=%f\n", cutoff, ct);
for (uint32_t i = 0; i < count; i++) {
for (uint32_t ovs = 0; ovs < 4; ovs++) {
float in = voice->osc_audio[i];
float fb = in - (delay4 * synth->resonance * 4);
if (fb > 3) fb = 3;
if (fb < -3) fb = -3;
delay1 = ((fb - delay1) * ct) + delay1;
delay2 = ((delay1 - delay2) * ct) + delay2;
delay3 = ((delay2 - delay3) * ct) + delay3;
delay4 = ((delay3 - delay4) * ct) + delay4;
}
out[i] = delay4;
}
voice->delay1 = delay1;
voice->delay2 = delay2;
voice->delay3 = delay3;
voice->delay4 = delay4;
}
void nekobee_voice_render(nekobee_synth_t *synth, float *out, uint32_t count) {
// generate "count" samples into the buffer at out
vco(synth, count);
vcf(synth, count);
for(uint32_t i=0; i<count; i++) {
out[i] = synth->voice->osc_audio[i];
}
vcf(synth, out, count);
return;
}