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CalcCurve (transient_detector.cpp/h):
- Recursive divide-and-conquer FindTransients scans the gain vector for
monotonic 3-subframe windows (rising or falling); kMinScore=2.0 filters
out oscillations smaller than a factor of 2 (no-op at Level 4).
- RelationToIdx maps an amplitude ratio to an ATRAC3 gain Level index.
- TCurveBuilderCtx carries LastLevel across frames; CalcCurve prepends it
as a virtual boundary element to detect Location=0 attacks.
- budget=8 matches ATRAC3 SubbandInfo::MaxGainPointsNum.
TSpectralUpsampler (transient_spectral_upsampler.cpp/h):
- Applies a Planck-taper window (ε=0.15) to a 512-sample context window,
forward-FFTs, applies a 3-bin raised-cosine HPF, zero-pads to 4096 bins,
and inverse-FFTs to give an 8× upsampled output.
- Returns highFreqRatio = Σ|X[k]·H[k]|²/Σ|X[k]|²; callers skip CalcCurve
when this is below kHighFreqThreshold=0.05, preventing false transients
from Planck noise-floor variation in sub-cutoff frames.
Tests:
- gain_processor_ut: upsampled-path blocks added to all FreqDomain tests;
CalcCurve negative tests (NegativeTests suite).
- transient_spectral_upsampler_ut: OutputSize, DCIsRemovedByLowCutFilter,
HighFreqSinePreservesRMS (parametrised), ChirpNoTransient (0→5510 Hz
sweep at 689 Hz low-cut, Len1024/16384/262144).
Co-Authored-By: Claude Sonnet 4.6 <[email protected]>
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