The plugin's real RFFT (th1a90/th2180) uses custom twiddle operations
with buf548 (cos/sin table) and mask598 (SIMD masks) that are NOT
standard FFT butterflies. Our implementation uses a simplified approach
(ln → negate → exp2 → IFFT → window → FFT) which is not bit-exact.
Current state:
- Default path (no FIRCONV): TOTAL 1.825 dB
- FIRCONV=2 (real RFFT): TOTAL 10.377 dB (much worse)
The default path provides better results, so we use it as the primary
approach. Bit-exact FIR construction would require reverse-engineering
the plugin's exact twiddle operations from disassembly.
Added real RFFT functions (execute_real_forward, execute_real_inverse)
to fft.hpp/cpp. These implement the standard algorithm for real-valued
FFT using complex FFT of half size.
Updated buildFirFromMask to use real RFFTs matching the plugin's pipeline:
1. log(mask) → negate
2. forward real RFFT (opB)
3. EXP in-place
4. inverse real RFFT (opC)
5. Window
6. forward real RFFT (opD)
However, the real RFFT implementation makes results worse (10.377 dB vs
1.825 dB default). The plugin's real RFFT likely has subtle differences
(normalization, twiddle factors) that are not captured by the standard
algorithm.
The default path (no FIRCONV) remains the best approach with 1.825 dB
TOTAL error.
Future work: Reverse-engineer the plugin's exact real RFFT implementation
from disassembly (th1a90/th2180) to achieve bit-exact FIR construction.
The plugin's FIR construction pipeline (52b550-52b8bb) uses real RFFTs
(real-valued FFT) with twiddle operations (opA/B/C/D). These twiddle
operations use buf548 (cos/sin table) and mask598 (SIMD masks) and are
specific to real RFFTs.
Our implementation uses complex FFTs, which cannot replicate the plugin's
real RFFT twiddle operations. The simplified approach (ln → negate → exp2
→ IFFT → window → FFT) provides reasonable results but is not bit-exact.
Key findings:
- Plugin uses real RFFTs (th1a90=forward, th2180=inverse)
- Twiddle operations are FMA-complex with precomputed cos/sin tables
- Complex FFTs cannot replicate real RFFT behavior
- FIRCONV=2 path makes results worse (10.377 dB vs 1.825 dB default)
Future work: Implement real RFFT to achieve bit-exact FIR construction.
- log2_ln.hpp/cpp: Plugin's exact ln(float) polynomial from 535a70
(0x1802a24c0). IEEE 754 bit extraction + Horner evaluation.
Coefficients extracted from binary at 0x181f81f80..0x181f821c0.
Max error ~3e-6 for typical inputs.
- spectral.cpp: Updated buildFirFromMask to use plugin's ln→negate→exp2
pipeline instead of naive 1/mask reciprocal.
- exp2_tables.hpp/cpp: Already contains plugin's exp2 tables (0x26b820).
Remaining: twiddle stages (ops B/C/D with cos/sin tables from buf548)
are the missing piece for bit-exact FIR construction. These are
FFT butterflies already implemented in fft.hpp but need integration
into the FIR pipeline.
Implement minimum-phase FIR design from BLOCKMAP:
- buildFirFromMask: mask → 1/mask (reciprocal via log→negate→exp) →
IFFT → causal window → FFT → normalize → complex multiply
- RT_FIRCONV=2 activates the new path
- RT_FIRCONV=1 preserved as simple mask × audio (legacy)
Results (tone1kq single band):
default (pointwise): 500Hz=-25.35 dB, 1kHz=-50.60 dB
FIRCONV=1 (mask mul): 500Hz=-1.31 dB, 1kHz=-25.92 dB
FIRCONV=2 (min-phase): same as FIRCONV=1
The twiddle stages (ops B/C/D with cos/sin tables) are the missing
piece for bit-exact FIR construction. They perform FMA operations
with twiddle factors that modify the mask shape.
Note: dual_b1q_0.5.wav reference is empty (0 bytes) — corpus can't run.
Needs regeneration.
- th_b3c0 (0x18000b3c0) = pure complex multiply FIR × audio in freq-domain
- scan3.py: pre-scan approach finds ctx in 1.5s, multi-instance detection
- RT_FIRCONV=1: FIR from mask + complex multiply (spectral.cpp)
- RT_FIRPOWER=1: power-law mask from raw spectrum (framed_model.cpp)
- Root cause: plugin uses FIR convolution (OLA), not per-bin multiply
- Live captures: FIR@43=0.524, mask@43=0.510, final gain=0.305
- Best result: RT_LUT_OFF gives cut@500=-8.18 dB (ref -10.32)
- NOTES_LEVEL 24e/24f/24g appended
- Fixed execute_inverse: removed conj bug, now uses positive twiddle only
- Added WOLA normalization factor (wola_sum/hop_ for Hann+hop=N/4)
- New detector model: floor(level) + bell_curve * boost, calibrated from
measured data (summary.md level sweep, 7 data points)
- Transcribed twiddle loader (FUN_18014ec20): Cody-Waite 4-level reduction
with minimax sin/cos polynomial, constants from Frida memory dump
- Added soothe_constants.hpp with extracted polynomial coefficients
- HARNESS parameters updated to match burst500_b1.rpp (depth=0.864)
Results: burst500.wav reduction now -5.8 dB vs Ref -6.8 dB (was -14.4 dB)
Matiq