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rtl_sdr_fm: correct deemphasis coefficient derivation and make it configurable

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This commit is contained in:
arancormonk
2025-09-03 19:08:22 -04:00
parent 402cef34a9
commit e2402d0328
1 changed files with 33 additions and 7 deletions
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40
src/rtl_sdr_fm.cpp
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@@ -24,6 +24,7 @@
#include <stdlib.h>
#include <math.h>
#include <string.h>
#include <strings.h>
#include <pthread.h>
#include <unistd.h>
#include <sched.h>
@@ -874,16 +875,16 @@ void deemph_filter(struct demod_state *fm)
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 */
/* Q15 alpha = (1 - a), where a = exp(-1/(Fs*tau)) */
const int kShiftDeemph = 15; /* Q15 */
int a = fm->deemph_a;
if (a <= 0) a = 1;
int recip_q = (1 << kShiftDeemph) / a;
/* Single-pole IIR: avg += (x - avg) * (1 - a) with fixed-point scaling */
int alpha_q15 = fm->deemph_a;
if (alpha_q15 < 0) alpha_q15 = 0;
if (alpha_q15 > (1 << kShiftDeemph)) alpha_q15 = (1 << kShiftDeemph);
/* Single-pole IIR: avg += (x - avg) * alpha */
DSD_FME_IVDEP
for (i = 0; i < fm->result_len; i++) {
d = res[i] - avg;
int64_t delta = (int64_t)d * recip_q;
int64_t delta = (int64_t)d * (int64_t)alpha_q15;
/* symmetric rounding */
if (d > 0) {
delta += (1LL << (kShiftDeemph - 1));
@@ -1714,7 +1715,32 @@ void open_rtlsdr_stream(dsd_opts *opts)
}
if (demod.deemph) {
demod.deemph_a = (int)round(1.0/((1.0-exp(-1.0/(demod.rate_out * 75e-6)))));
/* Configure deemphasis time constant via env DSD_FME_DEEMPH: 75 (default), 50, nfm, off */
double tau_s = 75e-6; /* default 75 microseconds */
const char *deemph_env = getenv("DSD_FME_DEEMPH");
if (deemph_env && deemph_env[0] != '\0') {
if (strcasecmp(deemph_env, "off") == 0) {
demod.deemph = 0;
} else if (strcmp(deemph_env, "50") == 0) {
tau_s = 50e-6;
} else if (strcasecmp(deemph_env, "nfm") == 0) {
/* Common NFM value */
tau_s = 750e-6;
} else if (strcmp(deemph_env, "75") == 0) {
tau_s = 75e-6;
}
}
if (demod.deemph) {
/* a = exp(-1/(Fs*tau)); store alpha=(1-a) in Q15 */
double Fs = (double)demod.rate_out;
if (Fs < 1.0) Fs = 1.0;
double a = exp(-1.0 / (Fs * tau_s));
double alpha = 1.0 - a;
int coef_q15 = (int)lrint(alpha * (double)(1 << 15));
if (coef_q15 < 1) coef_q15 = 1; /* ensure non-zero to move toward steady-state */
if (coef_q15 > (1 << 15)) coef_q15 = (1 << 15);
demod.deemph_a = coef_q15;
}
}
/* Set the tuner gain */