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feat(analysis): add mel filterbank and log-mel/MFCC front end - #345
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Implement MelFilterbank<T,FftSize,NumMelBands> and Mfcc<T,FftSize,NumMelBands,NumCoefficients> for keyword-spotting feature extraction on resource-constrained devices. MelFilterbank constructs HTK triangular filters at runtime from per-band edge frequencies (no full weight matrix stored), supports optional Slaney area normalisation, and exposes static HzToMel / MelToHz helpers. Mfcc composes RealFastFourierTransform → power spectrum → MelFilterbank → log (with ε floor) → direct orthonormal DCT-II (identical to scipy dct(x, type=2, norm='ortho')). Three tests: - mel/Hz HTK round-trip for 200, 1000, 4000 Hz - Slaney per-band discrete area ≈ 1.0 (tolerance 0.15) - Reference frame: unit impulse (flat spectrum), values generated with numpy+scipy using float32 arithmetic Reference values produced by: python3 -c "import numpy as np; from scipy.fft import dct; ..." with FftSize=32, sampleRate=8000, numMelBands=8, numCoefficients=4, fMin=125, fMax=3800, HTK scale, epsilon=1e-12, norm='ortho'. Closes #340 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01N4pngRiBqeb1xiseKXNLG6
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| Descriptor | Linter | Files | Fixed | Errors | Max errors | Warnings | Elapsed time |
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| ✅ ACTION | actionlint | 7 | 0 | 0 | 0.53s | ||
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| ✅ CPP | cppcheck | 422 | 0 | 0 | 10.14s | ||
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| ✅ JSON | jsonlint | 8 | 0 | 0 | 0.08s | ||
| ✅ JSON | prettier | 8 | 8 | 0 | 0 | 0.45s | |
| markdownlint | 131 | 3 | 178 | 0 | 2.99s | ||
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| lychee | 240 | 24 | 0 | 20.9s | |||
| ✅ YAML | prettier | 12 | 3 | 0 | 0 | 0.53s | |
| ✅ YAML | yamllint | 12 | 0 | 0 | 0.6s |
Detailed Issues
⚠️ SPELL / lychee - 24 errors
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Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01N4pngRiBqeb1xiseKXNLG6
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Closes #340
analysis::HzToMel/MelToHzcover both the HTK and the Slaney (librosa default) scales.analysis::MelFilterbank<T, FftSize, NumMelBands>sampleRate,fMin,fMax,MelScale {Htk, Slaney}andMelNormalization {None, Slaney}.Applyis a single O(N) pass.Weight(band, bin)exposes the dense weight for inspection and tests.analysis::Mfcc<T, FftSize, NumMelBands, NumCoefficients>windowing::Window<T>) → real FFT → |X|² → filterbank →ln(max(S, ε))→ orthonormal DCT-II, keeping the firstNumCoefficients.Compute(frame)returns the cepstrum, andLogMelEnergies()exposes the log-mel vector for spectrogram/FBANK models.All of it is float-only, uses no heap, puts
OPTIMIZE_FOR_SPEEDonApply/Compute, and uses the scopedpush_options/pop_optionspragma (see #334).Tests (
TEST_Fonfloat):m(1000), and Slaneym(1000) = 15andm(4000).filters.melrow sums.filters.melplusscipy.fft.dct(norm='ortho'), checking both log-mel and MFCC to within 5e-3.The doc is
doc/analysis/MelFilterbankMfcc.md: theory, the sparse layout, complexity, a walkthrough and pitfalls.Validation:
numerical.analysis_testpasses 167 tests, plus 10 skips that already exist on main (host, GCC 13,-Werror). The coverage TUs compile, and clang-format is clean.🤖 Generated with Claude Code
https://claude.ai/code/session_01N4pngRiBqeb1xiseKXNLG6
Generated by Claude Code