Algorithm References

This page records algorithm, paper, standard, and compatibility references that are visible in current libsonare source, tests, or README. It is not a general DSP bibliography; every entry is tied to local evidence in the project.

What You Will Learn

By the end of this page you should be able to:

• distinguish source-backed references, reference-tested compatibility claims, and algorithm-family names;
• find the standard or paper basis for loudness, true peak, denoise, EQ, pitch, room acoustics, sequence helpers, and reverb families;
• avoid treating this page as a promise that every high-level estimate is identical to another library;
• know when to pair this page with Implementation Validation for test coverage.

Evidence Levels

Level	Meaning
Source-backed	Current source, header comments, tests, or README explicitly name the standard, algorithm, author, or paper
Reference-tested	The implementation is checked against generated reference values, mainly librosa 0.11.0
Algorithm-named	Current source names an algorithm family, but does not embed a full paper citation

Source-Backed References

Room-acoustic terms used below

RIR means room impulse response. Equivalent-room estimation fits a useful room model from audio rather than recovering exact geometry. Room morphing is a creative room effect, not dereverberation.

Area	Reference basis	Evidence
Loudness and true peak	ITU-R BS.1770-4/5, EBU R128, EBU Tech 3342 loudness range	README.md, src/rt/biquad_design.h, src/rt/true_peak_filter.h, src/metering/lufs.cpp, tests/rt/true_peak_filter_test.cpp
K-weighting	BS.1770 K-weighting biquads: reference coefficients at 48 kHz and analytic design for other rates	src/rt/biquad_design.h, src/rt/biquad_design.cpp
True-peak interpolation	Self-designed 2x/4x/8x/16x polyphase FIRs matching the BS.1770 target frequency response. Use 4x or higher for standards-compliant measurement; the 2x path is a deliberately non-compliant fast approximation, below BS.1770's 4x minimum.	src/rt/true_peak_filter.h, src/rt/true_peak_filter.cpp
Classical denoise	Ephraim-Malah MMSE-STSA (1984), Ephraim-Malah LogMMSE (1985), Berouti spectral subtraction with over-subtraction (1979), MCRA, IMCRA	src/mastering/repair/denoise_classical.h
Tape hysteresis	Jiles-Atherton hysteresis; Chowdhury, "Real-time Physical Modelling for Analog Tape Machines", DAFx-19 2019, equations 6-10	src/mastering/common/hysteresis_ja.cpp, src/mastering/saturation/tape.h
EQ filters	RBJ biquads, Vicanek matched-Z IIR, Butterworth Q, Linkwitz-Riley crossovers with all-pass phase compensation	README.md, src/rt/biquad_design.h, src/rt/biquad_design.cpp, EQ routing and cut-filter sources
Anti-aliased nonlinearities	ADAA / antiderivative anti-aliasing	README.md, src/rt/adaa.h, src/rt/aliasing_control.h
Sliding maximum	Lemire sliding max	README.md, limiter and true-peak helper usage
Tonnetz	Harte et al. 2006 via librosa.feature.tonnetz behavior	src/feature/tonnetz.h, src/feature/tonnetz.cpp, tests/librosa/tonnetz_test.cpp
Pitch tracking	YIN and pYIN; pYIN uses probabilistic pitch candidates and Viterbi decoding	src/feature/pitch.h, tests/librosa/pitch_test.cpp
Room acoustics	Energy-decay metrics, blind decay fitting, RIR synthesis, room estimation, and room morphing	src/analysis/acoustic_analyzer.*, src/acoustic/*, src/analysis/room_estimator.*, src/effects/acoustic/room_morph.*, tests/acoustic/*, tests/effects/room_morph_test.cpp
Sequence helpers	DTW, Viterbi, discriminative Viterbi, recurrence quantification analysis, mirroring librosa.sequence	src/util/sequence.h, src/feature/segment.*, librosa sequence references
Time stretch and pitch shift	Native spectral stretch, phase-vocoder fallback, pitch shift by time stretch followed by resampling	src/effects/time_stretch.h, src/effects/pitch_shift.h
Reverb families	Convolution, Dattorro plate, FDN, velvet-noise, and geometric room reverb engines	README.md, src/effects/reverb/*, src/effects/acoustic/*, tests/effects/*

What is Tonnetz?

Tonnetz ("tone network") is a feature that maps harmony onto a geometric space where musically related chords sit close together. It is derived from chroma and is useful for measuring harmonic change and chord relationships.

Plain-language gloss of the named algorithms

This page cites algorithms by name; here is what the less-obvious ones do.

• Viterbi decoding — picks the most likely sequence of states (e.g. pitch candidates over time) rather than the best guess at each isolated frame, so pYIN produces a smooth note line.
• DTW (dynamic time warping) — aligns two sequences that may run at different speeds by stretching/compressing time, used to compare performances or feature sequences.
• MMSE-STSA / LogMMSE — statistical denoisers that estimate, per frequency bin, how much is signal vs. noise before subtracting (see Mastering Processors for the plain version). Over-subtraction removes a bit extra to kill residue, at the cost of some signal.
• Spectral subtraction / MCRA / IMCRA — spectral subtraction estimates the noise spectrum and subtracts it from each frame; MCRA and IMCRA track that noise floor over time even while speech or music is present, so the subtraction adapts instead of using a fixed guess.
• Jiles-Atherton hysteresis — a physics model of how magnetic tape "remembers" its recent signal, giving tape saturation its nonlinear, history-dependent character.
• ADAA (antiderivative anti-aliasing) — a trick that runs a distortion curve through its mathematical integral to suppress the aliasing that nonlinear processing would otherwise create.
• Schroeder energy-decay curve — integrates a room impulse response backward in time to get a smooth decay curve, which is what RT60/EDT are measured from.
• RBJ biquads / Linkwitz-Riley crossovers — standard recipes for EQ filters and for splitting audio into frequency bands with correct phase behavior.

librosa Reference Compatibility

The reference fixtures under tests/librosa/reference/ are generated with librosa 0.11.0.

tests/librosa/reference/NOTICE.md states that the values are generated by tests/librosa/generate_librosa_reference.py to verify numerical compatibility between libsonare and librosa.

This is a compatibility target, not copied implementation code.

Feature family	Reference coverage
Spectral features	STFT, reassigned spectrogram, mel filterbank, mel spectrogram, MFCC, chroma, CQT/iCQT, VQT/wavelet filters, spectral centroid/bandwidth/rolloff/flatness/contrast, zero crossing, RMS
Rhythm and onset	onset strength, beat, tempo, tempogram, Fourier tempogram, PLP
Pitch and tonal features	YIN, pYIN, piptrack-like helpers, pitch utilities, tonnetz
Decomposition and transforms	HPSS, harmonic/percussive, decompose, remix, inverse mel/MFCC helpers, Griffin-Lim-related synthesis tests
Sequence and structure	segment helpers, cross-similarity, recurrence matrix, DTW/Viterbi-style sequence helpers
Utilities	frame/sample/time conversion, dB conversion, pre/de-emphasis, padding, fix length/frames, peak pick, vector normalize, trim/split silence, weighting
NNLS	NNLS tests compare against SciPy/librosa reference data and support NNLS chroma behavior

Scope Boundaries

The source explicitly frames repair as classical DSP.

That means these paths are not DNN restoration, source separation, or interactive spectral repair:

Repair path	How to read it
repair.denoiseClassical, repair.declick, repair.declip, repair.decrackle, repair.dehum, repair.dereverbClassical, repair.trimSilence	Classical or deterministic DSP in the current implementation.

Room acoustics has five different entry points:

Entry point	Input	What it does	How to treat the result
analyze_impulse_response / analyzeImpulseResponse	A clap, pop, sweep-derived IR, or other recording where the room response is isolated	Measures the decay curve directly after the impulse	Easier to treat as a measurement
detect_acoustic / detectAcoustic	Normal music, speech, or environmental audio	Searches for regions where room decay appears to fall naturally, then estimates from them	Treat as a hint and always inspect confidence
estimate_room / estimateRoom	A recording or impulse response	Estimates an equivalent room with volume, dimensions, DRR, absorption bands, RT60 bands, and confidence	Treat as a model fit, not exact geometry
synthesize_rir / synthesizeRir	Shoebox dimensions plus source/listener placement	Synthesizes a mono RIR	Check hasError / has_error for invalid geometry
room_morph / roomMorph	Source recording plus target room geometry	Adds a target-room character after gentle source-tail suppression	Creative effect, not dereverberation

A decay curve shows how quickly energy falls after the sound stops. In impulse-response analysis, that curve is measured directly and RT60/EDT are read from it.

Blind estimation is different because the input is ordinary audio. The algorithm has to decide whether an apparent decay is really room reverberation. Instrument tails, sustained speech, compression, and background noise can all look like decay.

synthesize_rir / synthesizeRir uses image-source early reflections and a deterministic late tail. That implementation detail matters when you compare generated RIRs, but users can usually think of it as "make a repeatable room response from dimensions."

For that reason, detect_acoustic / detectAcoustic and estimate_room / estimateRoom intentionally return confidence. If the input does not expose a clear free-decay region, the result may be unavailable or low confidence.

Read low confidence as "this recording does not reveal the room clearly enough," not as a firm room measurement.