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rtl_sdr_fm: remove per-sample memmove from resampler, use ring buffer.

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This commit is contained in:
arancormonk
2025-09-04 09:34:09 -04:00
parent 283d000b59
commit dd8620bc39
1 changed files with 14 additions and 6 deletions
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20
src/rtl_sdr_fm.cpp
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@@ -709,7 +709,8 @@ struct demod_state
int resamp_taps_len; /* prototype taps length (padded to K*L) */
int resamp_taps_per_phase;/* K = ceil(taps_len/L) */
int16_t *resamp_taps; /* Q15 taps, length = K*L */
int16_t *resamp_hist; /* most-recent-first history, length = K */
int16_t *resamp_hist; /* circular history, length = K */
int resamp_hist_head; /* head index into circular history [0..K-1] */
/* Output buffer for resampler (worst-case 4x expansion) */
alignas(DSD_FME_ALIGN) int16_t resamp_outbuf[MAXIMUM_BUF_LENGTH * 4];
/* Residual CFO loop (FLL) state */
@@ -1807,6 +1808,7 @@ static void resamp_design(struct demod_state *s, int L, int M)
return;
}
memset(s->resamp_hist, 0, (size_t)taps_per_phase * sizeof(int16_t));
s->resamp_hist_head = 0;
/* Windowed-sinc (Hamming) */
double gain = 0.0;
@@ -1854,22 +1856,27 @@ static int resamp_process_block(struct demod_state *s, const int16_t *in, int in
const int K = s->resamp_taps_per_phase; /* taps per phase */
const int16_t *taps = s->resamp_taps; /* length K*L, phase-major stride L */
int phase = s->resamp_phase; /* 0..L-1 */
int head = s->resamp_hist_head; /* circular head index */
int out_len = 0;
for (int n = 0; n < in_len; n++) {
/* Push new sample into history (most-recent-first) */
memmove(s->resamp_hist + 1, s->resamp_hist, (size_t)(K - 1) * sizeof(int16_t));
s->resamp_hist[0] = in[n];
/* Push new sample into circular history at head */
s->resamp_hist[head] = in[n];
head++;
if (head == K) head = 0;
/* While we owe outputs with current input available */
int local_phase = phase;
while (local_phase < L) {
/* Dot: y = sum_{k=0..K-1} hist[k] * taps[k*L + local_phase] */
/* Dot: y = sum_{k=0..K-1} hist[idx] * taps[k*L + local_phase]
Access history most-recent-first starting from head-1. */
int64_t acc = 0;
const int16_t *tk = taps + local_phase;
int idx = head - 1; if (idx < 0) idx += K;
for (int k = 0; k < K; k++) {
acc += (int32_t)s->resamp_hist[k] * (int32_t)tk[0];
acc += (int32_t)s->resamp_hist[idx] * (int32_t)tk[0];
tk += L;
idx--; if (idx < 0) idx += K;
}
/* Q15 -> Q0 with rounding and saturation */
acc += (1 << 14);
@@ -1881,6 +1888,7 @@ static int resamp_process_block(struct demod_state *s, const int16_t *in, int in
}
s->resamp_phase = phase;
s->resamp_hist_head = head;
return out_len;
}