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dsp.c: Added descriptive comments to Goertzel calculations.

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* Added doxygen to describe some struct members and what is going on in
the code.

Change-Id: I2ec706a33b52aee42b16dcc356c2bd916a45190d
This commit is contained in:
Richard Mudgett
2016-07-21 11:56:53 -05:00
parent 6dfb34cf13
commit a8cd5d255a
1 changed files with 22 additions and 3 deletions
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25
main/dsp.c
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@@ -238,9 +238,13 @@ static const int DEFAULT_SILENCE_THRESHOLD = 256;
#define CONFIG_FILE_NAME "dsp.conf" #define CONFIG_FILE_NAME "dsp.conf"
typedef struct { typedef struct {
/*! The previous previous sample calculation (No binary point just plain int) */
int v2; int v2;
/*! The previous sample calculation (No binary point just plain int) */
int v3; int v3;
/*! v2 and v3 power of two exponent to keep value in int range */
int chunky; int chunky;
/*! 15 bit fixed point goertzel coefficient = 2 * cos(2 * pi * freq / sample_rate) */
int fac; int fac;
} goertzel_state_t; } goertzel_state_t;
@@ -326,12 +330,22 @@ static inline void goertzel_sample(goertzel_state_t *s, short sample)
{ {
int v1; int v1;
/*
* Shift previous values so
* v1 is previous previous value
* v2 is previous value
* until the new v3 is calculated.
*/
v1 = s->v2; v1 = s->v2;
s->v2 = s->v3; s->v2 = s->v3;
/* Discard the binary fraction introduced by s->fac */
s->v3 = (s->fac * s->v2) >> 15; s->v3 = (s->fac * s->v2) >> 15;
/* Scale sample to match previous values */
s->v3 = s->v3 - v1 + (sample >> s->chunky); s->v3 = s->v3 - v1 + (sample >> s->chunky);
if (abs(s->v3) > 32768) {
if (abs(s->v3) > (1 << 15)) {
/* The result is now too large so increase the chunky power. */
s->chunky++; s->chunky++;
s->v3 = s->v3 >> 1; s->v3 = s->v3 >> 1;
s->v2 = s->v2 >> 1; s->v2 = s->v2 >> 1;
@@ -341,21 +355,26 @@ static inline void goertzel_sample(goertzel_state_t *s, short sample)
static inline float goertzel_result(goertzel_state_t *s) static inline float goertzel_result(goertzel_state_t *s)
{ {
goertzel_result_t r; goertzel_result_t r;
r.value = (s->v3 * s->v3) + (s->v2 * s->v2); r.value = (s->v3 * s->v3) + (s->v2 * s->v2);
r.value -= ((s->v2 * s->v3) >> 15) * s->fac; r.value -= ((s->v2 * s->v3) >> 15) * s->fac;
/*
* We have to double the exponent because we multiplied the
* previous sample calculation values together.
*/
r.power = s->chunky * 2; r.power = s->chunky * 2;
return (float)r.value * (float)(1 << r.power); return (float)r.value * (float)(1 << r.power);
} }
static inline void goertzel_init(goertzel_state_t *s, float freq, unsigned int sample_rate) static inline void goertzel_init(goertzel_state_t *s, float freq, unsigned int sample_rate)
{ {
s->v2 = s->v3 = s->chunky = 0.0; s->v2 = s->v3 = s->chunky = 0;
s->fac = (int)(32768.0 * 2.0 * cos(2.0 * M_PI * freq / sample_rate)); s->fac = (int)(32768.0 * 2.0 * cos(2.0 * M_PI * freq / sample_rate));
} }
static inline void goertzel_reset(goertzel_state_t *s) static inline void goertzel_reset(goertzel_state_t *s)
{ {
s->v2 = s->v3 = s->chunky = 0.0; s->v2 = s->v3 = s->chunky = 0;
} }
typedef struct { typedef struct {