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rtl_sdr_fm: fuse interleaved complex half-band decimation to avoid deinterleave/reinterleave in full_demod, reducing memory traffic and copies. Maintain per‑stage histories and equivalent output while improving throughput.

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This commit is contained in:
arancormonk
2025-09-03 22:35:34 -04:00
parent 391048a45a
commit 106aae1814
1 changed files with 69 additions and 27 deletions
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96
src/rtl_sdr_fm.cpp
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@@ -323,6 +323,72 @@ static inline int hb_decim2_real(const int16_t *in, int in_len, int16_t *out, in
/* One 2:1 decimation stage on interleaved I/Q, using per-stage histories. */
/* Removed hb_decim2_complex_stage in favor of inlined staged processing in full_demod() */
/* Fused interleaved complex half-band decimator: decimate I and Q in one pass without deinterleave/reinterleave.
in_len is number of interleaved int16_t (I,Q,I,Q,...). Returns interleaved output length (in_len/2). */
static inline int hb_decim2_complex_interleaved(const int16_t *in, int in_len, int16_t *out,
int16_t *hist_i, int16_t *hist_q)
{
const int hist_len = HB_TAPS - 1;
int ch_len = in_len >> 1; /* per-channel samples */
int out_ch_len = ch_len >> 1; /* decimated per-channel */
if (out_ch_len <= 0) {
return 0;
}
int16_t lastI = (ch_len > 0) ? in[in_len - 2] : 0;
int16_t lastQ = (ch_len > 0) ? in[in_len - 1] : 0;
for (int n = 0; n < out_ch_len; n++) {
int center_idx = hist_len + (n << 1); /* per-channel index */
int64_t accI = 0;
int64_t accQ = 0;
for (int t = 0; t < HB_TAPS; t++) {
int src_idx = center_idx - HB_HALF + t;
int16_t xi, xq;
if (src_idx < hist_len) {
xi = hist_i[src_idx];
xq = hist_q[src_idx];
} else {
int rel = src_idx - hist_len;
if (rel < ch_len) {
xi = in[(size_t)(rel << 1)];
xq = in[(size_t)(rel << 1) + 1];
} else {
xi = lastI;
xq = lastQ;
}
}
int16_t c = hb_q15_taps[t];
accI += (int32_t)c * (int32_t)xi;
accQ += (int32_t)c * (int32_t)xq;
}
accI += (1 << 14);
accQ += (1 << 14);
int32_t yI = (int32_t)(accI >> 15);
int32_t yQ = (int32_t)(accQ >> 15);
out[(size_t)(n << 1)] = sat16(yI);
out[(size_t)(n << 1) + 1] = sat16(yQ);
}
/* Update histories with last HB_TAPS-1 per-channel input samples */
if (ch_len >= hist_len) {
int start = ch_len - hist_len;
for (int k = 0; k < hist_len; k++) {
int rel = start + k;
hist_i[k] = in[(size_t)(rel << 1)];
hist_q[k] = in[(size_t)(rel << 1) + 1];
}
} else {
int need = hist_len - ch_len;
if (need > 0) {
memmove(hist_i, hist_i + ch_len, (size_t)need * sizeof(int16_t));
memmove(hist_q, hist_q + ch_len, (size_t)need * sizeof(int16_t));
}
for (int k = 0; k < ch_len; k++) {
hist_i[need + k] = in[(size_t)(k << 1)];
hist_q[need + k] = in[(size_t)(k << 1) + 1];
}
}
return out_ch_len << 1; /* interleaved length */
}
static void atan_lut_once_init(void)
{
@@ -1535,35 +1601,11 @@ void full_demod(struct demod_state *d)
int16_t *src = d->lowpassed;
int16_t *dst = d->hb_workbuf;
for (i = 0; i < ds_p; i++) {
/* Deinterleave src -> hb_i_buf/hb_q_buf */
int ch_len = in_len >> 1;
for (int k = 0, j = 0; j < in_len; j += 2, k++) {
d->hb_i_buf[(size_t)k] = src[(size_t)j];
d->hb_q_buf[(size_t)k] = src[(size_t)j + 1];
}
/* Define per-channel decimation task */
struct HBArg { const int16_t *in; int in_len; int16_t *out; int16_t *hist; int out_len; } aI, aQ;
aI.in = d->hb_i_buf; aI.in_len = ch_len; aI.out = d->hb_i_out; aI.hist = d->hb_hist_i[i]; aI.out_len = 0;
aQ.in = d->hb_q_buf; aQ.in_len = ch_len; aQ.out = d->hb_q_out; aQ.hist = d->hb_hist_q[i]; aQ.out_len = 0;
auto hb_task = [](void *arg){
struct HBArg *a = (struct HBArg*)arg;
a->out_len = hb_decim2_real(a->in, a->in_len, a->out, a->hist);
};
if (d->mt_enabled) {
demod_mt_run_two(d, hb_task, (void*)&aI, hb_task, (void*)&aQ);
} else {
hb_task((void*)&aI);
hb_task((void*)&aQ);
}
int out_ch_len = (aI.out_len < aQ.out_len) ? aI.out_len : aQ.out_len;
/* Interleave back to dst */
for (int n = 0; n < out_ch_len; n++) {
dst[(size_t)(2*n)] = d->hb_i_out[(size_t)n];
dst[(size_t)(2*n + 1)] = d->hb_q_out[(size_t)n];
}
/* Fused complex HB decimation on interleaved I/Q */
int out_len_interleaved = hb_decim2_complex_interleaved(src, in_len, dst, d->hb_hist_i[i], d->hb_hist_q[i]);
/* Next stage */
src = dst;
in_len = out_ch_len << 1;
in_len = out_len_interleaved;
dst = (src == d->hb_workbuf) ? d->lowpassed : d->hb_workbuf;
}
/* Final output resides in 'src' with length in_len */