src/c/ks.c — sigil-dsp

1/*

2 * ks.c - Karplus-Strong plucked string

3 *

4 * Buffer sized from a ceiling on the lowest intended fundamental

5 * (~20 Hz) so retuning up to any audible pitch is a pointer trick,

6 * not a reallocation.

7 */

9#include "ks.h"

10#include <stdlib.h>

11#include <string.h>

12#include <math.h>

14#define KS_MIN_FREQ 20.0f

16static uint32_t xorshift32(uint32_t *s)

17{

18 uint32_t x = *s;

19 x ^= x << 13;

20 x ^= x >> 17;

21 x ^= x << 5;

22 *s = x ? x : 0x1u;

23 return *s;

24}

26static SPFLOAT ks_noise(sp_ks *p)

27{

28 uint32_t r = xorshift32(&p->rng);

29 /* map to [-1, 1] */

30 return ((SPFLOAT)(int32_t)r) * (1.0f / 2147483648.0f);

31}

33int sp_ks_create(sp_ks **p)

34{

35 *p = (sp_ks *)calloc(1, sizeof(sp_ks));

36 return SP_OK;

37}

39int sp_ks_destroy(sp_ks **p)

40{

41 if (!*p) return SP_OK;

42 free((*p)->buf);

43 free(*p);

44 *p = NULL;

45 return SP_OK;

46}

48int sp_ks_init(sp_data *sp, sp_ks *p, SPFLOAT freq, SPFLOAT decay,

49 SPFLOAT excite_gain)

50{

51 if (freq < KS_MIN_FREQ) freq = KS_MIN_FREQ;

52 if (decay <= 0.0f) decay = 0.5f;

53 p->freq = freq;

54 p->decay = decay;

55 p->excite_gain = excite_gain;

56 p->bufcap = (int)ceilf((SPFLOAT)sp->sr / KS_MIN_FREQ) + 2;

57 p->buf = (SPFLOAT *)calloc(p->bufcap, sizeof(SPFLOAT));

58 p->bufpos = 0;

59 p->last = 0.0f;

60 p->gate_prev = 0;

61 p->rng = 0xC0FFEE01u;

62 sp_ks_retune(sp, p);

63 return SP_OK;

64}

66int sp_ks_retune(sp_data *sp, sp_ks *p)

67{

68 SPFLOAT f = p->freq < KS_MIN_FREQ ? KS_MIN_FREQ : p->freq;

69 int n = (int)lroundf((SPFLOAT)sp->sr / f);

70 if (n < 2) n = 2;

71 if (n > p->bufcap) n = p->bufcap;

72 p->bufsize = n;

73 /* Per-trip gain so signal decays to -60 dB after `decay` seconds.

74 * After T seconds the loop runs T*freq times, want g^(T*freq)=1e-3. */

75 SPFLOAT d = p->decay > 0.0f ? p->decay : 0.001f;

76 SPFLOAT g = powf(10.0f, -3.0f / (d * f));

77 if (g > 0.99995f) g = 0.99995f;

78 if (g < 0.0f) g = 0.0f;

79 p->feedback = g;

80 if (p->bufpos >= p->bufsize) p->bufpos = 0;

81 return SP_OK;

82}

84int sp_ks_trigger(sp_data *sp, sp_ks *p)

85{

86 (void)sp;

87 SPFLOAT amp = p->excite_gain;

88 for (int i = 0; i < p->bufsize; i++)

89 p->buf[i] = ks_noise(p) * amp;

90 p->bufpos = 0;

91 p->last = 0.0f;

92 return SP_OK;

93}

95int sp_ks_compute(sp_data *sp, sp_ks *p, SPFLOAT *gate, SPFLOAT *out)

96{

97 /* Rising-edge retrigger on gate */

98 int g_now = (*gate > 0.5f) ? 1 : 0;

99 if (g_now && !p->gate_prev) sp_ks_trigger(sp, p);

100 p->gate_prev = g_now;

101

102 SPFLOAT y = p->buf[p->bufpos];

103 /* 2-tap averaging lowpass in feedback */

104 SPFLOAT avg = 0.5f * (y + p->last);

105 SPFLOAT newval = avg * p->feedback;

106 p->buf[p->bufpos] = newval;

107 p->last = y;

108 p->bufpos++;

109 if (p->bufpos >= p->bufsize) p->bufpos = 0;

110 *out = y;

111 return SP_OK;

112}