Improvement of Goertzel filter. Using Hamming window now. Add test routine.

This commit is contained in:
Andreas Eversberg 2021-11-28 13:11:19 +01:00
parent 252b9c08bf
commit 63f275d5db
2 changed files with 22 additions and 6 deletions

View File

@ -35,7 +35,7 @@
* For a perfect rectangualr wave, the result would equal the peak level.
* For a sine wave the result would be factor (2 / PI) below peak level.
*/
double audio_level(sample_t *samples, int length)
double audio_mean_level(sample_t *samples, int length)
{
double level, sk;
int n;
@ -54,8 +54,23 @@ double audio_level(sample_t *samples, int length)
return level;
}
/* use hamming window */
double window[256];
int window_generated = 0;
static void gen_window(void)
{
int i;
for (i = 0; i < 256; i++)
window[i] = 0.54 - 0.46 * cos(2.0 * M_PI * (double)i / 256.0);
window_generated = 1;
}
void audio_goertzel_init(goertzel_t *goertzel, double freq, int samplerate)
{
if (!window_generated)
gen_window();
memset(goertzel, 0, sizeof(*goertzel));
goertzel->coeff = 2.0 * cos(2.0 * M_PI * freq / (double)samplerate);
}
@ -67,10 +82,11 @@ void audio_goertzel_init(goertzel_t *goertzel, double freq, int samplerate)
/* filter frequencies and return their levels
*
* samples: pointer to sample buffer
* length: length of buffer
* length: length of buffer: -7.4 dB @ +-(1 / duration) Hz and -INF @ +-(1 / duration * 2) Hz
* -> if duration is 10 ms, we got -7.4 dB @ +-100 Hz and -INF at +-200 Hz
* offset: for ring buffer, start here and wrap around to 0 when length has been hit
* coeff: array of coefficients (coeff << 15)
* result: array of result levels (average value of the sine, that is 1 / (PI/2) of the sine's peak)
* result: array of result levels (peak value of the target frequency)
* k: number of frequencies to check
*/
void audio_goertzel(goertzel_t *goertzel, sample_t *samples, int length, int offset, double *result, int k)
@ -87,7 +103,7 @@ void audio_goertzel(goertzel_t *goertzel, sample_t *samples, int length, int off
cos2pik = goertzel[i].coeff;
/* note: after 'length' cycles, offset is restored to its initial value */
for (n = 0; n < length; n++) {
sk = (cos2pik * sk1) - sk2 + samples[offset++];
sk = (cos2pik * sk1) - sk2 + samples[offset++] * window[n * 256 / length];
sk2 = sk1;
sk1 = sk;
if (offset == length)
@ -98,7 +114,7 @@ void audio_goertzel(goertzel_t *goertzel, sample_t *samples, int length, int off
(sk * sk) -
(cos2pik * sk * sk2) +
(sk2 * sk2)
) / (double)length * 2.0 * 0.63662; /* 1 / (PI/2) */
) / (double)length * 4 / 1.08;
}
}

View File

@ -1,5 +1,5 @@
double audio_level(sample_t *samples, int length);
double audio_mean_level(sample_t *samples, int length);
typedef struct goertzel {
double coeff;