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perf(acars): halve decode CPU — hot-path trim + bit-exact contiguous convolution - #181
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Five changes to the two dominant costs (the demod's audio LPF and the channelizer's polyphase), each gated on an AWGN sensitivity A/B against the real demod, not just green tests. - demod: replace the per-sample atan2 frequency discriminator with the normalized cross-product form (d.im / |d|). Same phase info, no transcendental. NOTE the normalization is load-bearing: the raw cross product is amplitude-weighted, which lets loud-but-noisy samples dominate the bit accumulator and halved marginal-frame recovery (27/40 -> 13/40 CRC-OK at the cliff) while every existing test still passed. - dsp/nco: apply the per-sample rotation incrementally (one complex multiply) instead of calling f64 sin_cos per sample, re-deriving the rotator from the exact f64 phase every 256 samples so rounding cannot accumulate. Adds a drift test over 3M samples with uneven block boundaries (magnitude < 1e-4, phase < 1e-3 rad vs exact). - dsp/channelized_ddc: 8 polyphase taps/branch instead of 12 — measured 109 dB adjacent-bin rejection vs 113 dB, for 27% less CPU in one of the two dominant costs. (4 taps drops to 29 dB, too thin for a strong neighbour on the 25 kHz airband raster.) - dsp/channelized_ddc: when a channel needs no resampler, copy the bin straight into the caller's buffer and mix in place, saving a whole buffer pass at the channel rate. - demod: audio LPF 121 -> 101 taps. Sizing is set by sensitivity, not image rejection (51 taps already puts the -3000 Hz image 83 dB down). 101 is the measured floor that holds baseline yield; the sweep table is recorded at the constant so it is not lowered again on theory alone. Sensitivity unchanged vs master (60/56/42/21/5 vs 60/57/42/23/4 CRC-OK over 60 bursts at sigma 0.15/0.18/0.20/0.22/0.25 — within run-to-run noise). End-to-end 16-channel decode 6.47% -> 4.82% of one core.
…lyphase Convolution was ~77% of the decode pipeline. Both hot loops were written in the natural way and both defeated vectorisation; neither needed to. - fir: the circular delay line walked a wrapping index backwards, so the compiler could not prove the access affine and emitted a bounds-checked load plus a branch per tap. Store the window doubled and newest-first so it is always one contiguous run, and the inner loop becomes a plain zip of two equal-length slices. Measured: demod LPF 101t 99.5 -> 50.9 ms (1.95x), DDC stage1 59t/d10 9.1 -> 3.6 ms, stage2 95t/d10 13.0 -> 5.6 ms. - channelizer: the polyphase gathered with stride nch (48) per tap and memmoved the ENTIRE delay line every output block. Reindex the taps block-major (branch k tap l touches position nch-1-k of the block l steps back) so accumulation walks contiguous positions, and keep blocks in a ring that is overwritten in place instead of shifted. Measured: 57.0 -> 30.1 ms (1.9x) at M=48, taps/branch 8. Both are pure layout changes: the summation order is preserved, so the output is bit-identical. Each keeps the previous implementation as a test-only reference and asserts to_bits() equality over uneven streaming blocks — FIR across 6 tap/decimation combos, PFB across 5 (nch, taps) combos including the real (48, 8) and an odd (5, 6). That is why neither needs a sensitivity re-validation: the samples are the same samples. With convolution ~2x cheaper, the audio LPF goes back from 101 to its original 121 taps: full baseline sensitivity for ~0.2 points of one core, removing the only accuracy compromise made for speed. Sensitivity now matches master exactly (60/57/42/23/4 CRC-OK over 60 bursts at sigma 0.15/0.18/0.20/0.22/0.25). End-to-end 16-channel decode 4.82% -> 3.29% of one core.
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📝 WalkthroughWalkthroughThe PR updates PFB channelizer, FIR, NCO, channelized DDC, and ACARS demodulation code. It adds contiguous or block-based buffering, periodic NCO resynchronization, direct output handling, equivalent-output tests, and ACARS measurement examples. ChangesDSP processing updates
ACARS processing and measurement tools
Estimated code review effort: 4 (Complex) | ~45 minutes Merge Risk: 🔵 Low · up to The reported CPU-stage timings include setup work such as allocation, cloning, and NCO construction, which can overstate measured costs and make the performance comparison less reliable; the PR is otherwise mergeable with follow-up to move setup outside the timed regions. Sequence Diagram(s)sequenceDiagram
participant SensitivityExample
participant AcarsChannelDecoder
participant DSPPipeline
participant CRCValidator
SensitivityExample->>AcarsChannelDecoder: stream noisy ACARS burst
AcarsChannelDecoder->>DSPPipeline: process channelized and demodulated samples
DSPPipeline->>CRCValidator: validate decoded frame
CRCValidator-->>SensitivityExample: return payload match result
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✅ Passed checks (4 passed)
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In `@crates/xng-mode-acars/src/demod.rs`:
- Around line 27-45: Clarify the AWGN sweep description in the demodulator
documentation: either state that the table shows a three-sigma subset of the
five-sigma sweep, or change the documented sweep count from five to three if
those are the complete results. Keep the table values and surrounding
performance guidance unchanged.
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Pull request overview
This PR continues the ACARS CPU-reduction work by removing transcendentals from hot paths and restructuring the two dominant convolution kernels (FIR LPF and PFB channelizer) into contiguous, vectorizable memory access patterns while preserving bit-exact output for the layout-only changes.
Changes:
- Replaces the per-sample
atan2discriminator with a normalized cross-product discriminator in the ACARS demod hot loop. - Reworks
Nco::mixto use incremental complex rotation with periodic phase resync, plus adds a long-run drift/accuracy test. - Makes FIR and PFB convolution contiguous (doubled FIR history window; block-major taps + ring of blocks for PFB), with bit-exact reference tests; also trims PFB taps/branch to 8 and avoids an extra buffer pass when no resampler is needed.
Reviewed changes
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Show a summary per file
| File | Description |
|---|---|
| crates/xng-mode-acars/src/demod.rs | Updates discriminator to a normalized cross-product approach and documents LPF tap-count sensitivity tradeoffs. |
| crates/xng-dsp/src/nco.rs | Implements incremental rotation with periodic resync to amortize sin_cos; adds drift/accuracy test. |
| crates/xng-dsp/src/fir.rs | Replaces circular delay-line FIR with doubled contiguous history window; adds bit-exact reference test. |
| crates/xng-dsp/src/channelizer.rs | Rewrites PFB to use block-major tap layout and a ring of input blocks to enable contiguous accumulation; adds bit-exact reference test. |
| crates/xng-dsp/src/channelized_ddc.rs | Uses 8 taps/branch for PFB and avoids an extra per-channel buffer pass when resampling is not needed. |
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The comment claimed a five-sigma sweep and then printed three columns without explaining the gap. The sweep does run five; sigma 0.15 is saturated and 0.25 is floor noise, so only the middle three discriminate between tap counts. Say so.
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Review: no blockers — both bit-exactness claims verified far beyond the committed tests
I set out to break the two "bit-identical" claims and could not. I proved them over sweeps much wider than the committed tests, and found no indexing, wrap, or initial-state bug I could not disprove — zero unproven suspicions to hand back. The commit-2 restructures are sound and the split into "commit 1 changes results / commit 2 changes nothing observable" is exactly the right way to have organised this.
The issues that remain are concentrated in commit 1, and they're about evidence rather than correctness: the one change that is genuinely risky has no regression coverage, and several comment claims don't survive measurement.
Verified correct (so this doesn't get re-litigated)
Fir bit-exactness — CONFIRMED, structurally and empirically. Summation order is identical by construction (both walk taps 0..n-1 against a newest-first window), and the doubled buffer is provably right: the duplicate at head+n is read exactly once, at the step where head has wrapped past it. Proved bitwise against a faithful reimplementation of master's circular FIR over 810 combinations — ntaps ∈ {1,2,3,4,5,7,8,16,17,33} × decim ∈ {1,2,3,5,7,10,16,64,300} × block ∈ {1,2,3,5,16,17,100,1000,8000}, 8000 samples each. That covers what the committed test misses: ntaps = 1 and 2 (degenerate wrap), decim > ntaps, decim > block size (a decimation phase spanning several calls), and single-sample feeding. All bit-identical.
PfbChannelizer bit-exactness — CONFIRMED. The re-index is a clean transpose (scratch[p] = branch nch−1−p, undone by reverse()), the l loop order is unchanged so summation order is preserved, and the ring's initial zero-fill is equivalent to the old zero-filled delay line. Proved bitwise over 432 combinations — nch ∈ {2,3,4,5,6,7,9,12,16} × tpb ∈ {2,3,4,5,8,11} × block ∈ {1, 2, 3, 7, nch, nch·tpb, 1013, whole}. Covers tpb = 2 (the asserted minimum, where the ring wraps every step), nch = 2, prime nch, and blocks ≤ and exactly nch. All bit-identical.
NCO resync — sound, and the straddling-block concern doesn't apply. chunks_mut restarts per call, so resync happens at every call boundary and every 256 samples within a call — uneven blocks make it more accurate, never less. Phase advances by step * chunk.len(), so accumulated phase is identical regardless of chunking; there's no persistent counter to drift. Phase cannot grow unbounded, and the %= is required here (max excursion before reduction is 256·|step| ≤ 256·TAU, where the old single-subtract would not have sufficed). Swept 8 freq/rate ratios (including step ≈ 0 and |step| → π) × 8 block sizes {1, 255, 256, 257, 511, 512, 4096, 100000} over 400k samples each: worst magnitude error 1.0e-5, worst phase error 1.0e-5 rad (≈ −100 dBc), invariant to block size. The rotator needs no in-chunk renormalisation. Bonus: residual == 0 channels get step == 0 → rot == (1,0) → an exact identity multiply.
Discriminator NaN/zero/denormal safety — CONFIRMED. mag > 0.0 covers d == 0 (including y == 0 and the first sample where prev_sample == 0, matching master's atan2(0,0) == 0); a NaN makes the comparison false so disc becomes 0.0, never NaN. |d.im| ≤ |d| bounds the quotient to [−1,1]; denormal/denormal division is correctly rounded per IEEE-754, so no inf; extreme underflow flushes to 0 and takes the guard, same as master.
"Copy bin straight into caller's buffer" — no aliasing. self.bins[ch.bin] is never mutated (mixing lands in out_buf/binbuf), so two channels sharing a bin remain correct; out_buf is cleared first; the disjoint-field borrow is legal. And the path is exercised — offsets = [0.0] gives m = 100 → bin_rate 24000 = out_rate → no resampler, which is what rejects_neighbor_channel and output_rate_is_correct run.
PFB rejection numbers reproduced independently. 96 bins at 2.4 MS/s, tone dead-centre in the adjacent bin: tpb=4 → 29.6 dB, tpb=8 → 108.7 dB, tpb=12 → 112.9 dB — matching the comment's 29/109/113 exactly. Sweeping the neighbour's ±10 kHz occupied bandwidth, 8 taps holds ≥111 dB throughout. The 12→8 reduction is justified.
Speedups hold. FIR 2.24× (121 taps, decim 1); polyphase 1.91–2.28× (nch 16–96). "~2× on FIR and polyphase" is accurate.
Safety. No unsafe; every new index provably in range (head < n so head+n < 2n; next_block < tpb so write+nch ≤ tpb·nch; l < tpb for the tap slice); no debug integer-overflow risk; no new clippy warnings in any touched file (base and PR both emit 7 for xng-dsp, none in the four files here). cargo test -p xng-dsp -p xng-mode-acars fully green.
Findings
Two inline, on the discriminator and the tap table. Summarising the rest:
Minor — CPU percentages are unverifiable at this commit. "6.5% → 4.8% CPU", "27% less CPU" (channelized_ddc.rs:73-74), "~0.2 percentage points across 16 channels" (demod.rs:41-45): cpu_budget.rs lands only in the stacked PR, so none of these reproduce from #181 alone. What I could check lines up partially — the polyphase inner loop is 30% cheaper at 8 vs 12 taps (95.9 ms vs 137.3 ms, nch=96), consistent with "27%" once the FFT is included. But one channel's ACARS demod alone measures 0.40% of a core here, i.e. ~6.4% across 16 channels on the PR side, which doesn't obviously reproduce "4.8%". Different hardware is the likely explanation — please state the machine, or land the harness.
Nit — fir.rs:129: one permanently dead store. hist[2n-1] is written on every wrap but can never be read (the window at head = n-1 ends at 2n-2, and no later head reaches n). Harmless. Related: the doubled write costs one store per input sample while the win is on output samples, so the speedup narrows with decimation — 2.24× (121t, d1), 2.09× (121/10), 1.65× (101/25), 1.45× (31/8), 1.31× (9/1). Still a win everywhere; "~2×" should say "at decim=1".
Nit — channelized_ddc.rs:38,108: Channel::binbuf is now permanently dead for no-resampler channels. Cosmetic; consider moving it into the Some(rs) arm's state.
Nit — the description's "Audio LPF restored to 121 taps" is a no-op versus base. origin/master is already 121; 79bdc03 dropped it to 101 and a297538 put it back. Net constant diff is zero — worth saying so a reviewer doesn't go hunting for a change that isn't there.
Nit — one behavioural corner the swap changes: atan2(+0.0, −x) returned π (non-zero → counted as a zero-crossing), whereas d.im/|d| returns 0.0, which the disc != 0.0 gate at demod.rs:169 suppresses. Astronomically rare with real float data; noted for completeness only.
Recommendation
Commit 2 is ready as-is — the bit-exactness argument is the right one and it holds up under much harder testing than it shipped with. For commit 1, land the AWGN sweep harness in this PR (it's the difference between a justified change and an unfalsifiable comment), re-print the tap table at ≥400 trials, and soften the two overstated performance claims. The norm_sqr().sqrt() swap is free money while you're in there.
Generated by Claude Code
| // The raw cross product is amplitude-weighted where atan2 is not, | ||
| // and that difference is NOT harmless: under noise |y| fluctuates | ||
| // sample to sample, so loud-but-noisy samples dominate the bit | ||
| // accumulator and marginal frames are lost (measured: 27/40 → 13/40 | ||
| // CRC-OK at the sensitivity cliff). Dividing by |d| removes the | ||
| // weighting, leaving sin(Δφ) — pure phase, like atan2 — and one | ||
| // divide per sample is still far cheaper than the transcendental. | ||
| let d = y * self.prev_sample.conj(); | ||
| let mag = d.norm(); | ||
| let disc = if mag > 0.0 { d.im / mag } else { 0.0 }; |
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Major — this comment describes a safety gate that doesn't exist in the tree.
The comment is right that the normalisation is load-bearing, and I confirmed it hard. But nothing checks it. I built a variant with the divide removed (let disc = d.im;) and ran the full committed suite: 31/31 tests pass, all green. That variant is catastrophically worse — 400 paired trials × 3 seeds, identical noise draws:
| σ | 0.18 | 0.20 | 0.22 |
|---|---|---|---|
| raw cross | 315 | 153 | 44 |
| normalised | 372 | 271 | 159 |
So the PR ships a change whose entire safety argument rests on a divide that no test in the repo would notice being deleted. The AWGN sweep that justifies it exists only as an uncommitted harness (git ls-tree b12c12c crates/xng-mode-acars/examples/ → only gen_capture.rs). Please land the sweep alongside the change — this is exactly the "shipped comments pointing at tools that no longer exist" problem #182's description calls out about #178/#181.
Minor — the swap isn't free, and the comment implies it is. 4000 paired trials per σ (10 seeds × 400, identical noise draws), atan2 vs normalised cross:
| σ | 0.15 | 0.18 | 0.20 | 0.22 | 0.25 |
|---|---|---|---|---|---|
| atan2 | 3947 | 3623 | 2861 | 1693 | 274 |
| this PR | 3947 | 3620 | 2835 | 1674 | 249 |
| Δ | 0 | −3 | −0.9% | −1.1% | −9.1% |
Consistently negative — 9/10 seeds worse or tied at σ=0.20, none better. The cause is that "sin(Δφ) tracks Δφ to within 0.4%" (line 150) holds for the noiseless ±9°/sample case; under noise Δφ covers the whole circle, where sin saturates and beyond ±90° decreases. Sign is preserved so nothing breaks — it's a soft limiter, not an equivalent. Probably a fine trade for the CPU, but worth stating rather than implying exactness.
Minor — "hundreds of times cheaper than atan2" (line 151) is off by ~2 orders of magnitude. Complex::norm() is f32::hypot (num-complex 0.4.6 lib.rs:218) — another out-of-line libm call, not ALU ops. Measured over 20M samples: atan2 817 ms vs hypot + div 122 ms → 6.7×. End-to-end on one ACARS channel decoder over 600 s of channel audio: 0.422% → 0.398% of a core (~6%). Worth doing; the number isn't defensible.
Nit, actionable — line 162: d.norm_sqr().sqrt() beats d.norm(). 2.2× faster on the op (55 ms vs 122 ms per 20M) with bit-identical decode yield across 400 × 3 trials. |d| = |y||prev| is bounded far inside f32 range here, so hypot's overflow/underflow guarding buys nothing.
Generated by Claude Code
| /// Sizing here is set by **sensitivity**, not by image rejection: 51 taps | ||
| /// already puts the −3000 Hz mixing image 83 dB down, far below the capture's | ||
| /// noise floor, yet decode yield keeps improving well past that. An AWGN | ||
| /// sweep through the real demod runs 60 bursts at each of five σ values | ||
| /// (0.15/0.18/0.20/0.22/0.25); only the three middle columns discriminate — | ||
| /// σ = 0.15 is saturated (~60/60 for every tap count) and σ = 0.25 is down in | ||
| /// the floor (3–5/60) — so the table reports those: | ||
| /// | ||
| /// | taps | 0.18 | 0.20 | 0.22 | | ||
| /// |------|-------|-------|-------| | ||
| /// | 51 | 53 | 37 | 18 | | ||
| /// | 81 | 54 | 40 | 22 | | ||
| /// | 101 | 56 | 42 | 21 | | ||
| /// | 121 | 57 | 42 | 23 | | ||
| /// | ||
| /// This convolution used to be ~40% of pipeline CPU, which made trimming it | ||
| /// tempting; the contiguous-window `Fir` rewrite made it ~2× cheaper instead, | ||
| /// so the full 121 taps now cost ~0.2 percentage points of one core across 16 | ||
| /// channels. Buying back the top of that table for that price is the right |
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Minor — the table is quoted at 60 trials, where seed scatter is as large as the effect it reports.
I ran this identical 60-trial sweep at 5 seeds against the unmodified PR:
σ=0.18 → 52-56 σ=0.20 → 41-49 σ=0.22 → 20-29
The table's 51→121 deltas (53→57, 37→42, 18→23) sit entirely inside that run-to-run band, so the numbers as printed don't support the conclusion.
The conclusion is nevertheless correct — at 400 paired trials, 121 taps genuinely beats 51 (372/271/159/38 vs 351/250/129/14). So this is a problem with the evidence, not the decision. Re-run at ≥400 trials and reprint, or replace the table with a one-line statement.
Worth noting the same author reaches this exact conclusion in the follow-up work: the harness in the stacked squelch PR explicitly names 60 / 57 / 42 / 23 / 4 as numbers that should never have been quoted as a target — and that is precisely the 121-tap row here. Better to fix it in this PR than to leave a table that the next PR's own methodology disowns.
Generated by Claude Code
Independent verification of the test claims — exact matchBuilt and ran this branch in an isolated worktree off
Master baseline is 1230, so this branch is +3 — matching exactly the three No new clippy lints.
One thing worth fixing at the repo level
Methodology note for anyone reproducing: use a separate Generated by Claude Code |
…s with
Review found that the one genuinely risky change in this PR has no regression
coverage and that several comment claims do not survive measurement. Both are
addressed here; no decode behaviour changes except one bit-identical swap.
Land the harnesses
------------------
`examples/sensitivity.rs` and `examples/cpu_budget.rs` were written to justify
this PR, run, and deleted — leaving shipped comments citing tools that did not
exist and CPU figures nobody could reproduce. They are now committed.
This matters most for the discriminator. Deleting the `/ mag` normalisation
passes all 31 tests in the repository while collapsing marginal-frame recovery:
sigma 0.15 0.18 0.20 0.22 0.25
normalised 393 361 295 175 20
raw cross 382 295 159 46 0
Less than a third of the yield at sigma 0.22 and nothing at sigma 0.25, with a
green suite. The harness is the only thing standing between that and a silent
regression.
Correct the tap table
---------------------
The table was quoted at 60 trials, where reruns of the same build span
52-56 / 41-49 / 20-29 — scatter as large as the differences it attributed to
tap count. Re-run at 400 trials:
taps 0.18 0.20 0.22
51 336 262 133
81 353 269 150
101 356 282 172
121 361 295 175
Monotone across all three discriminating columns; 121 over 51 is +7/+13/+32%.
The conclusion was always right — the evidence for it was not.
Correct the discriminator comment
---------------------------------
- "hundreds of times cheaper than atan2" was wrong by orders of magnitude.
Measured here: 116.4 ms vs 28.6 ms per 20 M samples, i.e. 4.1x.
- The comment implied the cross product is equivalent to atan2. It is not: the
0.4% figure is the noiseless case, and under noise sin saturates, making this
a soft limiter. atan2 measures slightly better (394/362/296/179/23 against
393/361/295/175/20). Still the right trade at 4x the speed, but stated now
rather than implied away.
- The 27/40 -> 13/40 figure is replaced by the 400-trial numbers above.
norm() -> norm_sqr().sqrt()
---------------------------
`Complex::norm()` is `f32::hypot`, an out-of-line libm call whose overflow
guarding buys nothing here — |d| = |y||prev| sits far inside f32 range. 1.5x
faster on the op, and decode yield is bit-identical (393/361/295/175/20 both
ways), so no re-validation is owed.
Nits
----
`fir.rs`: document that the mirror write at hist[2n-1] is provably never read
and is kept deliberately (eliding it needs a per-input-sample branch to save
one store), and that the ~2x speedup is the undecimated figure — it narrows to
~1.3x by decim 9.
`cpu_budget.rs` labels the demod split as always-on vs gateable rather than
naming a squelch, which does not exist on this branch.
Tests: 1233 passing, 0 failing.
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In `@crates/xng-mode-acars/examples/cpu_budget.rs`:
- Around line 149-166: Update the cpu budget benchmark’s best timing flow and
the LPF/NCO closures to exclude setup work: preallocate and reuse FIR output and
mixer buffers, reuse the Nco state, and reset or initialize those buffers and
state before Instant::now() via untimed setup support in best. Ensure each timed
iteration measures only processing while preserving the existing LPF and NCO
results.
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The LPF closure allocated a fresh output `Vec` per call (a realloc chain up to 96 000 elements) and the NCO closure cloned the whole channel buffer and built a new `Nco` per call — ~12 MB of memcpy per repetition, timed as if it were filter cost. Both now reuse their buffers the way `MskDemod` does: clear and refill a preallocated output, mix in place with a long-lived `Nco`. These two lines are what the optimisation order is read off, so setup has no business inside them. Measured before/after, three runs each, machine idle: before LPF 48.3 / 48.4 / 47.7 ms NCO 2.5 / 2.5 / 2.5 ms after LPF 48.2 / 48.1 / 48.1 ms NCO 2.4 / 2.4 / 2.4 ms So the stage costs were **not** being overstated in practice, and no conclusion drawn from this table changes. `best` takes a min of five after a warm-up, so the allocator's free lists were already hot and the growth and the clone were amortised out. The fix is worth keeping regardless: it measures the stage rather than the allocator, the NCO line is consistently 0.1 ms tighter, and run-to-run spread narrows (48.1-48.2 against 47.7-48.4), which matters for a harness whose entire job is resolving small differences.
Round 2 of the ACARS CPU work, on top of the shared channelizer front end.
End-to-end 16-channel decode: 6.47% → 3.29% of one core (1.97×), with
sensitivity identical to master.
Where the time was
Re-audited from scratch rather than inheriting assumptions, and the picture
was not what the previous round's notes predicted:
sin_cos)~77% of the pipeline was scalar FIR convolution. Two prior assumptions were
wrong and are corrected here: the channelizer's bin count barely matters
(M=16 → 2.16%, M=128 → 2.59%), and
atan2was not the demod bottleneck(3% of the demod; its LPF was 81%).
Commit 1 — trim the hot path
Constant/identity changes, each gated on an AWGN sensitivity A/B against the
real demod:
atan2→ normalised cross product (d.im / |d|).The normalisation is load-bearing — the raw cross product is
amplitude-weighted, which lets loud-but-noisy samples dominate the bit
accumulator and halved marginal-frame recovery (27/40 → 13/40 CRC-OK at
the cliff) while every existing test still passed.
Nco: incremental complex rotation instead of a per-sample f64sin_cos,re-derived from the exact phase every 256 samples so rounding cannot
accumulate. New drift test: 3M samples, uneven blocks, magnitude < 1e-4 and
phase < 1e-3 rad vs exact.
113 dB, for 27% less CPU. (4 taps drops to 29 dB — too thin for a strong
neighbour on the 25 kHz raster.)
the caller's buffer and mix in place, saving a full buffer pass.
Commit 2 — make convolution contiguous (bit-exact)
Both hot loops were written the natural way and both defeated vectorisation:
Fir: the circular delay line walked a wrapping index backwards, so thecompiler could not prove the access affine — a bounds-checked load and a
branch per tap. Now the window is stored doubled and newest-first, always one
contiguous run, and the inner loop is a plain zip of two equal-length slices.
Demod LPF 101t 99.5 → 50.9 ms (1.95×); DDC stage1 59t/d10 9.1 → 3.6 ms;
stage2 95t/d10 13.0 → 5.6 ms.
PfbChannelizer: the polyphase gathered with stridench(48) per tapand memmoved the entire delay line every output block. Taps are now indexed
block-major (branch
ktapltouches positionnch−1−kof the blocklsteps back), so accumulation walks contiguous positions, and blocks live in a
ring that is overwritten in place rather than shifted. 57.0 → 30.1 ms
(1.9×) at M=48, taps/branch 8.
Both are pure layout changes — summation order is preserved, so output is
bit-identical. Each keeps the previous implementation as a test-only
reference and asserts
to_bits()equality over uneven streaming blocks (FIRacross 6 tap/decimation combos; PFB across 5
(nch, taps)combos including thereal
(48, 8)and an odd(5, 6)). That is why these need no sensitivityre-validation: the samples are the same samples.
Because convolution got ~2× cheaper, the audio LPF goes back to its original
121 taps. Commit 1 had trimmed it to 101 as a speed/sensitivity trade; the
full filter now costs ~0.2 points of one core, so the trade is bought back and
the PR ends with no accuracy compromise at all.
Verification
at σ = 0.15/0.18/0.20/0.22/0.25 (master: 60/57/42/23/4).
bit-exact, PFB bit-exact).
Notes for review
results and is justified statistically; commit 2 changes nothing
observable and is justified by bit-exactness. Worth reading in that order.
demod chain showed yield flat down to 51 taps; against the real demod,
51 taps costs 10–20% of marginal frames. The tap-count comment carries the
real table so it is not trimmed again on theory alone.
Still open (not this PR)
finish ~0.8%. Next candidates: the deframer's unconditional both-polarity
sync hunt on noise, and the per-channel finish.
generalising the decode loop (the two
TODO(perf)markers).SharedDdcremoval.capture (carried over from the previous PR).
Summary by CodeRabbit