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Fix notch filter and add a test for it

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This commit is contained in:
Andreas Eversberg
2023-06-10 11:33:58 +02:00
parent 963be6e2c5
commit 20b5132ef7
3 changed files with 22 additions and 4 deletions
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5
src/libfilter/iir_filter.c
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@@ -88,13 +88,12 @@ void iir_bandpass_init(iir_filter_t *filter, double frequency, int samplerate, i
filter->b2 = (1 - K / Q + K * K) * norm; filter->b2 = (1 - K / Q + K * K) * norm;
} }
void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations) void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations, double Q)
{ {
double Fc, Q, K, norm; double Fc, K, norm;
memset(filter, 0, sizeof(*filter)); memset(filter, 0, sizeof(*filter));
filter->iter = iterations; filter->iter = iterations;
Q = pow(sqrt(0.5), 1.0 / (double)iterations); /* 0.7071 @ 1 iteration */
Fc = frequency / (double)samplerate; Fc = frequency / (double)samplerate;
K = tan(PI * Fc); K = tan(PI * Fc);
norm = 1 / (1 + K / Q + K * K); norm = 1 / (1 + K / Q + K * K);

2
src/libfilter/iir_filter.h
View File

@@ -10,7 +10,7 @@ typedef struct iir_filter {
void iir_lowpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations); void iir_lowpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations);
void iir_highpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations); void iir_highpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations);
void iir_bandpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations); void iir_bandpass_init(iir_filter_t *filter, double frequency, int samplerate, int iterations);
void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations); void iir_notch_init(iir_filter_t *filter, double frequency, int samplerate, int iterations, double Q);
void iir_process(iir_filter_t *filter, sample_t *samples, int length); void iir_process(iir_filter_t *filter, sample_t *samples, int length);
void iir_process_baseband(iir_filter_t *filter, float *baseband, int length); void iir_process_baseband(iir_filter_t *filter, float *baseband, int length);

19
src/test/test_filter.c
View File

@@ -85,6 +85,25 @@ int main(void)
printf("\n"); printf("\n");
} }
printf("testing notch filter with %d iterations, Q = 4\n", 1);
iir_notch_init(&filter_high, 2605.0, SAMPLERATE, 1, 4);
for (i = 0; i < 4001; i += 100) {
gen_samples(samples, (double)i);
iir_process(&filter_high, samples, SAMPLERATE);
level = get_level(samples);
printf("%s%4d Hz: %.1f dB", debug_db(level), i, level2db(level));
if (i == 2600)
printf(" filter frequency (2605 Hz)\n");
else if (i == 2800)
printf(" about 200 Hz above\n");
else if (i == 2400)
printf(" about 200 Hz below\n");
else
printf("\n");
}
printf("testing band-pass filter with %d iterations\n", iter); printf("testing band-pass filter with %d iterations\n", iter);
iir_lowpass_init(&filter_low, 2000.0, SAMPLERATE, iter); iir_lowpass_init(&filter_low, 2000.0, SAMPLERATE, iter);