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/*
* This file is part of AtracDEnc.
*
* AtracDEnc is free software; you can redistribute it and/or
* modify it under the terms of the GNU Lesser General Public
* License as published by the Free Software Foundation; either
* version 2.1 of the License, or (at your option) any later version.
*
* AtracDEnc is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
* Lesser General Public License for more details.
*
* You should have received a copy of the GNU Lesser General Public
* License along with AtracDEnc; if not, write to the Free Software
* Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
*/
#pragma once
#include <vector>
namespace NAtracDEnc {
// Result of TSpectralUpsampler::Process().
struct TProcessResult {
// kOutN upsampled samples; analysis region maps to [1024..3072).
std::vector<float> signal;
// Fraction of spectral energy in bins [LowCutBin, kInN/2] relative to
// total spectral energy, computed from the Planck-windowed input FFT
// *before* the HPF is applied. Range [0, 1].
// ≈ 0 : frame is dominated by sub-cutoff content (stopband leakage only)
// ≈ 1 : frame is dominated by supra-cutoff content (full passband)
float highFreqRatio;
};
// Preprocesses a 512-sample context window for improved spectral analysis.
//
// Input layout (512 samples total):
// [0 .. 127] : tail of the previous analysis frame
// [128.. 383] : current analysis region (will be passed to AnalyzeGain)
// [384.. 511] : look-ahead into the next frame
//
// Processing steps:
// 1. Apply a Planck-taper window with parameter epsilon across all 512
// samples. The flat top (w=1) covers n in [ε·N, N·(1-ε)], which
// fully contains the analysis region [128..384) for ε ≤ 0.25.
// 2. Forward real FFT (512-point).
// 3. Low-cut filter: zero all bins below lowCutHz (high-pass).
// 4. 8× upsample in the FFT domain by zero-padding the high-frequency
// region and scaling remaining bins to preserve amplitude.
// 5. Inverse real FFT (4096-point).
//
// The analysis region maps to [1024..3071] in the 4096-sample output.
// Passing each kInN/kUpsample-sample subframe of this region to AnalyzeGain
// gives 8× more stable RMS estimates than operating on the raw 8-sample
// subframes, which helps suppress false transient detections for low-
// frequency and chirp-like signals.
class TSpectralUpsampler {
public:
static constexpr int kInN = 512; // input window size
static constexpr int kUpsample = 8; // upsample factor
static constexpr int kOutN = kInN * kUpsample; // 4096
static constexpr float kDefaultEps = 0.15f; // Planck-taper ε
// Minimum TProcessResult::highFreqRatio to consider gain-curve analysis
// meaningful. Frames below this threshold are dominated by sub-cutoff
// leakage; the filtered output is at the noise floor and CalcCurve should
// be skipped (set ctx.LastLevel = 0 and continue).
// 5 % accounts for main-lobe spectral leakage from tones just below the
// cutoff into the 3-bin transition region (H² = 0.25 at LowCutBin).
static constexpr float kHighFreqThreshold = 0.05f;
// sampleRate : sample rate of the input (e.g. 11024 Hz for ATRAC3 sub-band)
// lowCutHz : frequencies below this are zeroed in the FFT domain
// epsilon : Planck-taper taper fraction in (0, 0.5); default 0.15
TSpectralUpsampler(float sampleRate, float lowCutHz, float epsilon = kDefaultEps);
~TSpectralUpsampler();
// Process a kInN-sample input window.
// Returns the upsampled signal and its high-frequency energy ratio.
TProcessResult Process(const float* in) const;
private:
const int LowCutBin; // first kept bin (inclusive); bins [0,LowCutBin) are zeroed
std::vector<float> Win;
void* FwdCfg; // kiss_fftr_cfg: kInN-point forward real FFT plan
void* InvCfg; // kiss_fftr_cfg: kOutN-point inverse real FFT plan
};
} // namespace NAtracDEnc
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