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rtl_sdr_fm: hoist divisions and use integer scalers in hot loops

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This commit is contained in:
arancormonk
2025-09-03 16:14:26 -04:00
parent 0decc5934c
commit 2ec3f9a263
1 changed files with 19 additions and 4 deletions
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23
src/rtl_sdr_fm.cpp
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@@ -458,6 +458,11 @@ void low_pass_real(struct demod_state *s)
int i=0, i2=0;
int fast = (int)s->rate_out;
int slow = s->rate_out2;
/* Precompute fixed-point reciprocal of decimation factor to avoid per-sample division */
int decim = (slow != 0) ? (fast / slow) : 1;
if (decim < 1) decim = 1;
const int kShiftLPR = 15; /* Q15 reciprocal */
int recip_decim_q = (1 << kShiftLPR) / decim;
while (i < s->result_len) {
s->now_lpr += s->result[i];
i++;
@@ -465,7 +470,9 @@ void low_pass_real(struct demod_state *s)
if (s->prev_lpr_index < fast) {
continue;
}
s->result[i2] = (int16_t)(s->now_lpr / (fast/slow));
/* Multiply by reciprocal and shift instead of dividing by (fast/slow) */
int64_t scaled = ((int64_t)s->now_lpr * recip_decim_q);
s->result[i2] = (int16_t)(scaled >> kShiftLPR);
s->prev_lpr_index -= fast;
s->now_lpr = 0;
i2 += 1;
@@ -720,15 +727,23 @@ void deemph_filter(struct demod_state *fm)
{
static int avg; // cheating...
int i, d;
/* Precompute fixed-point reciprocal of deemphasis constant to avoid per-sample division */
const int kShiftDeemph = 15; /* Q15 */
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;
for (i = 0; i < fm->result_len; i++) {
d = fm->result[i] - avg;
/* Use multiply+shift with sign-aware rounding to mirror original behavior */
int64_t delta = (int64_t)d * recip_q;
if (d > 0) {
avg += (d + fm->deemph_a/2) / fm->deemph_a;
} else {
avg += (d - fm->deemph_a/2) / fm->deemph_a;
delta += (1LL << (kShiftDeemph - 1));
} else if (d < 0) {
delta -= (1LL << (kShiftDeemph - 1));
}
avg += (int)(delta >> kShiftDeemph);
fm->result[i] = (int16_t)avg;
}
}