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rtl_sdr_fm: add saturating arithmetic for decimation and volume scaling

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arancormonk
2025-09-03 18:05:28 -04:00
parent 2eedcc125b
commit 412a4dc3b1
1 changed files with 18 additions and 5 deletions
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23
src/rtl_sdr_fm.cpp
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@@ -72,6 +72,16 @@ static int atan_lut_coef = 8;
static pthread_once_t atan_lut_once = PTHREAD_ONCE_INIT;
static pthread_mutex_t atan_lut_mutex = PTHREAD_MUTEX_INITIALIZER;
/* =====================
Saturating helpers
===================== */
static inline int16_t sat16(int32_t x)
{
if (x > 32767) return 32767;
if (x < -32768) return -32768;
return (int16_t)x;
}
static void atan_lut_once_init(void)
{
int i;
@@ -447,8 +457,9 @@ void low_pass(struct demod_state *d)
if (d->prev_index < d->downsample) {
continue;
}
d->lowpassed[i2] = d->now_r; // * d->output_scale;
d->lowpassed[i2+1] = d->now_j; // * d->output_scale;
/* Saturate accumulated sums when writing back to int16 */
d->lowpassed[i2] = sat16(d->now_r);
d->lowpassed[i2+1] = sat16(d->now_j);
d->prev_index = 0;
d->now_r = 0;
d->now_j = 0;
@@ -467,7 +478,8 @@ int low_pass_simple(int16_t *signal2, int len, int step)
sum += (int)signal2[i + i2];
}
//signal2[i/step] = (int16_t)(sum / step);
signal2[i/step] = (int16_t)(sum);
/* Saturate accumulated sum on write */
signal2[i/step] = sat16(sum);
}
signal2[i/step + 1] = signal2[i/step];
return len / step;
@@ -1590,9 +1602,10 @@ int get_rtlsdr_samples(int16_t *out, size_t count, dsd_opts * opts, dsd_state *
if (got <= 0) {
return -1;
}
/* Apply volume scaling */
/* Apply volume scaling with saturation */
for (int i = 0; i < got; i++) {
out[i] = out[i] * volume_multiplier;
int32_t y = (int32_t)out[i] * (int32_t)volume_multiplier;
out[i] = sat16(y);
}
return got;
}