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rtl_sdr_fm: replace sample duplication with simple resampling (linear interpolation)

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arancormonk
2025-09-03 18:30:38 -04:00
parent f3ec024000
commit 29be104345
1 changed files with 28 additions and 5 deletions
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33
src/rtl_sdr_fm.cpp
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@@ -28,6 +28,7 @@
#include <unistd.h>
#include <sched.h>
#include <atomic>
#include <vector>
#include <rtl-sdr.h>
#include "dsd.h"
@@ -975,15 +976,37 @@ static void *demod_thread_fn(void *arg)
safe_cond_signal(&controller.hop, &controller.hop_m);
continue;
}
/* Write demod block to SPSC ring, duplicating per bandwidth_multiplier as before.
Wake consumer once per produced block. */
/* Write demod block to SPSC ring. If upsampling (bandwidth_multiplier > 1),
linearly interpolate between adjacent samples instead of duplicating. */
if (bandwidth_multiplier <= 1) {
ring_write_no_signal(o, d->result, (size_t)d->result_len);
} else {
for (int i = 0; i < d->result_len; i++) {
for (int j = 0; j < bandwidth_multiplier; j++) {
ring_write_no_signal(o, &d->result[i], 1);
const int M = bandwidth_multiplier;
const int N = d->result_len;
if (N <= 0) {
/* nothing to write */
} else if (N == 1) {
/* Degenerate case: only one sample, replicate M times */
std::vector<int16_t> tmp(M);
for (int m = 0; m < M; m++) tmp[m] = d->result[0];
ring_write_no_signal(o, tmp.data(), (size_t)M);
} else {
/* N >= 2: perform linear interpolation between successive samples */
const size_t up_len = (size_t)N * (size_t)M;
std::vector<int16_t> upsampled;
upsampled.resize(up_len);
for (int n = 0; n < N - 1; n++) {
int16_t x0 = d->result[n];
int16_t x1 = d->result[n + 1];
int32_t dx = (int32_t)x1 - (int32_t)x0;
for (int m = 0; m < M; m++) {
int32_t interp = (int32_t)x0 + (dx * m) / M;
upsampled[(size_t)n * (size_t)M + (size_t)m] = (int16_t)interp;
}
}
/* Last original sample maps to the last position */
upsampled[(size_t)(N - 1) * (size_t)M] = d->result[N - 1];
ring_write_no_signal(o, upsampled.data(), up_len);
}
}
safe_cond_signal(&o->ready, &o->ready_m);