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rtl_sdr_fm: fix deemphasis state race

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This commit is contained in:
arancormonk
2025-09-03 18:01:06 -04:00
parent 9b399f31da
commit 2eedcc125b
1 changed files with 8 additions and 4 deletions
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12
src/rtl_sdr_fm.cpp
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@@ -149,6 +149,7 @@ struct demod_state
int comp_fir_size;
int custom_atan;
int deemph, deemph_a;
int deemph_avg;
int now_lpr;
int prev_lpr_index;
int dc_block, dc_avg;
@@ -750,7 +751,7 @@ void raw_demod(struct demod_state *fm)
void deemph_filter(struct demod_state *fm)
{
static int avg; // cheating...
int avg = fm->deemph_avg; /* per-instance state */
int i, d;
int16_t *res = assume_aligned_ptr(fm->result, DSD_FME_ALIGN);
/* Precompute fixed-point reciprocal of deemphasis constant to avoid per-sample division */
@@ -758,13 +759,12 @@ void deemph_filter(struct demod_state *fm)
int a = fm->deemph_a;
if (a <= 0) a = 1;
int recip_q = (1 << kShiftDeemph) / a;
// de-emph IIR
// avg = avg * (1 - alpha) + sample * alpha;
/* Single-pole IIR: avg += (x - avg) * (1 - a) with fixed-point scaling */
DSD_FME_IVDEP
for (i = 0; i < fm->result_len; i++) {
d = res[i] - avg;
/* Use multiply+shift with sign-aware rounding to mirror original behavior */
int64_t delta = (int64_t)d * recip_q;
/* symmetric rounding */
if (d > 0) {
delta += (1LL << (kShiftDeemph - 1));
} else if (d < 0) {
@@ -773,6 +773,7 @@ void deemph_filter(struct demod_state *fm)
avg += (int)(delta >> kShiftDeemph);
res[i] = (int16_t)avg;
}
fm->deemph_avg = avg; /* write back state */
}
void dc_block_filter(struct demod_state *fm)
@@ -1210,6 +1211,7 @@ void demod_init_analog(struct demod_state *s)
s->pre_j = s->pre_r = s->now_r = s->now_j = 0;
s->prev_lpr_index = 0;
s->deemph_a = 0; //
s->deemph_avg = 0;
s->now_lpr = 0;
s->dc_block = 1; //
s->dc_avg = 0;
@@ -1253,6 +1255,7 @@ void demod_init_ro2(struct demod_state *s)
s->pre_j = s->pre_r = s->now_r = s->now_j = 0;
s->prev_lpr_index = 0;
s->deemph_a = 0;
s->deemph_avg = 0;
s->now_lpr = 0;
s->dc_block = 1; //enabling by default, but offset tuning is also enabled, so center spike shouldn't be an issue
s->dc_avg = 0;
@@ -1296,6 +1299,7 @@ void demod_init(struct demod_state *s)
s->pre_j = s->pre_r = s->now_r = s->now_j = 0;
s->prev_lpr_index = 0;
s->deemph_a = 0;
s->deemph_avg = 0;
s->now_lpr = 0;
s->dc_block = 1; //enabling by default, but offset tuning is also enabled, so center spike shouldn't be an issue
s->dc_avg = 0;