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rtl_sdr_fm: replace DC block with leaky integrator high-pass (IIR)

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arancormonk
2025-09-03 18:18:00 -04:00
parent 412a4dc3b1
commit f3ec024000
1 changed files with 12 additions and 10 deletions
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22
src/rtl_sdr_fm.cpp
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@@ -790,17 +790,19 @@ void deemph_filter(struct demod_state *fm)
void dc_block_filter(struct demod_state *fm)
{
int i, avg;
int64_t sum = 0;
for (i=0; i < fm->result_len; i++) {
sum += fm->result[i];
int i;
/* Leaky integrator high-pass: dc += (x - dc) >> k; y = x - dc */
int dc = fm->dc_avg;
const int k = 11; /* cutoff ~ Fs / 2^k (k in 10..12) */
int16_t *res = assume_aligned_ptr(fm->result, DSD_FME_ALIGN);
DSD_FME_IVDEP
for (i = 0; i < fm->result_len; i++) {
int x = (int)res[i];
dc += (x - dc) >> k;
int y = x - dc;
res[i] = sat16(y);
}
avg = sum / fm->result_len;
avg = (avg + fm->dc_avg * 9) / 10;
for (i=0; i < fm->result_len; i++) {
fm->result[i] -= avg;
}
fm->dc_avg = avg;
fm->dc_avg = dc;
}
long int rms(int16_t *samples, int len, int step)