AquaSynth/docs/renders/dx7-algorithm-32-native-diff-cause.md

dx7-algorithm-32-additive-organ: why native vs wasm reads -79.5 dB

Nothing here was heard. Offline buffers, 48 kHz, mono, 3 s, no params, same Faust text on both sides. Same toolchain as aqua-feel-native-diff-2026-11-10.md (wasm: faustwasm 0.20.0 / libfaust 2.90.19; native: FaustNativeRender, libfaust 2.90.20, -single -ftz 2 -vec -lv 1, faustlibraries master 3d8c6ef via AQUASYNTH_FAUST_HOME).

Verdict: cause found, two causes, neither a float32 floor and neither a bug in AquaSynth

The -79.5 dB is a real, deterministic phase drift of the sine voices, not noise, not a quiet signal and not a sample shift.

1. Increment rounding in os.phasor, selected by whether 1/ma.SR is shared (-79.3 to -93.0 dB when removed). Lowered patches define time = ba.time / ma.SR;. With that division in the graph, the native compiler computes the phasor increment as freq * fround(1/SR); the wasm compiler keeps freq / SR. For 392 Hz at 48 kHz: fround(392/48000) = 0.008166667074, fround(392 * fround(1/48000)) = 0.008166666143, one 2^-30 step (9.31e-10) apart. Measured effective increments: wasm 0.008166667074 on both, native 0.008166667074 without the division in the graph and 0.008166666143 with it. The phase then drifts linearly (about 1.3e-4 cycles over 3 s between the sides), so the waveform error grows linearly with time (0 to 1.65e-4 over 0.45 s, then follows the envelope down).
2. si.smoo library text on /macro/drawbar and /macro/shimmer (-93.0 to -141.2 dB when also pinned). This is the known old-form vs imp(3) difference. The 2026-11-10 imp3 pin did not show it because cause 1 sat 14 dB above it.

With both removed the patch reads -141.2 dB (max 1.8e-7), the same float floor as the Ding instruments (-130 to -142 dB).

Evidence (isolating experiments; each changes one thing)

Baseline reproduced through native-diff.mjs --items smoother: 1.65e-4 max, -79.27 dB. The signal is not quiet: RMS 0.193, peak 0.686. The last 2 s are digital silence and contribute zero error (73 % exactly-equal samples is that silence, not agreement). The error is zero at nothing but silence and rises linearly while the voices sound; lag test: lag 0 error 6.3e-5 vs 18.4 at lag +-1, so no shift.

experiment	rel RMS err dB	max err
whole patch, as lowered	-79.3	1.65e-4
filters stripped (fi.lowpass/fi.highpass)	-79.3	1.66e-4
per voice, no filters: v0 / v1 / v2 / v3 / v4 / v5	-85.8 / -79.3 / -143.3 / -73.8 / -143.1 / -109.0
sin(2pi*os.phasor(1,392))*0.5 alone	-163.7	3e-8
same sine times age+1, a ramp, or any slow signal	-64 to -74	1e-4 to 3e-3
os.phasor(1,392) alone	exact (0)	0
os.phasor(1,392) + (ba.time/ma.SR > 100) (never true)	-33.2	1.0 (one wrap sample)
same with ba.time > 1e9 or ma.SR > 1e9 instead	exact (0)	0
same with ba.time / 48000.0 > 100	exact (0)	0
effective phasor increment, native, with / without the division	0.008166666143 / 0.008166667074
whole patch, time = ba.time / 48000.0; only	-93.0	1.78e-5
whole patch, every ma.SR a literal	-93.0	1.78e-5
whole patch, phasor replaced by a float-accumulating one on both sides (library phasor text is fixed point in master, see below)	-84.0	9.4e-5
age literal + si.smoo pinned to old form / imp3	-114.0 / -141.2	1.3e-6 / 1.8e-7
si.smoo pinned to imp3 only (SR still shared)	-79.5	1.65e-4

Voices 2 and 4 (588 and 980 Hz) are clean because their increments round the same either way; 196, 392, 784 and 1176 are not. After cause 1 is removed only voice 1 (the one scaled by patch_param_0, the smoothed drawbar) stays at -91.5 dB, and voice 5 (shimmer) at -109 dB: cause 2.

Arithmetic check: Math.fround(392 * Math.fround(1/48000)) = 0.008166666142642498, matching the measured native increment to 10 digits.

Scope

• Other patches with the same time = ba.time / ma.SR; line are not affected the same way: with imp3 pinned and time made literal, advanced-bird-syrinx-graph stays -75.4, advanced-human-larynx-graph -94.2, dx7-algorithm-8-bright-pair -94.7, advanced-alien-multivalve-graph moves -97.9 to -110.9. Whether a patch is affected depends on whether its phasor increments round differently under division vs reciprocal multiply.
• native-diff.mjs --smoother imp3 therefore cannot reach the float floor for patches whose phasor increment is affected; its -60 dB limit still holds with the new reading (-79.5 is explained, not a worse mismatch).

Not verified

• I did not read the generated C++/wasm. "Native uses freq * fround(1/SR), wasm freq/SR" is inferred from the measured increments and the arithmetic above (exact match), not from emitted code (the wasm build has no C++ backend; no faust CLI here). It is also not separated whether the difference is libfaust 2.90.19 vs 2.90.20, or the -vec -lv 1 / LLVM path, or the library's phasor text (fixed point in master, which also looks different from what the wasm bundle holds). Settling it: emit the same DSP with one libfaust version's C++ backend and compare the instanceConstants lines, or render the wasm side with a 2.90.20 wasm build.
• Which side is nearer the ideal is not important: both are within one 2^-30 increment step of 392/48000 and in opposite directions (+4.1e-10, -5.2e-10).

Reproduce

. /var/lib/mind-aqua-self/env-review.sh
export AQUASYNTH_FAUST_HOME=/var/lib/mind-aqua-self/faust-home-overlay-20261031
cd tools/aqua-feel && node native-diff.mjs /var/lib/mind-aqua-self/native-diff-bin --items smoother --out "$TMPDIR/out"

then edit the lowered DSP text on both sides (as the table says) and render with renderOnce and FaustNativeRender. The experiment scripts are scratch in $TMPDIR and not committed.