Metering and spectral processing

Analysis extracts information from sound; spectral processing changes its frequency or pitch content. Keep those roles clear: a pitch detector reports a period/frequency, while a pitch shifter needs a processing path and suitable timing. A meter reports levels; a loudness matcher also changes gain.

Choose the measurement

Question	Start with	Interpret the result
Is there a pitched monophonic note?	analysis/pitch-detector, analysis/voice-pitch-detector	Frequency/period, confidence and voiced state, updated at a hop.
What note should an input trigger?	analysis/pitch-tracking	Adds gating and continuous-pitch decisions.
How loud is this short window?	analysis/loudness-meter	Rolling stereo K-weighted/RMS power; not integrated LUFS.
What is programme loudness?	analysis/programme-meter	Gated integrated LUFS, momentary/short-term values, LRA and history.
What inter-sample peak is estimated?	analysis/true-peak	Four-phase BS.1770-5 detector; distinct from sample peak.
How do two paths compare?	analysis/loudness-match	Learn/hold/continuous compensation from paired inputs.

import { initTruePeak, truePeakTick, resetTruePeak } from "maruaudio/dsp/analysis/true-peak"
const meter = initTruePeak() // one state per measured channel
let peak = 0
function observe(sample: number): void {
  peak = Math.max(peak, truePeakTick(meter, sample))
}
// At an explicit measurement restart:
function restart(): void { resetTruePeak(meter); peak = 0 }

Tick the true-peak detector for every contiguous sample. At Stop, flush twelve zeros to observe its FIR tail, without counting those zeros as programme audio.

Meters need enough history before their reading is ready. Retain their readiness and reset/segment semantics in the module; publish telemetry through the host rather than updating React or the DOM from DSP. A loudness matcher belongs before the manual master stage and needs the actual pre-compensation candidate. It is separate from a limiter.

Choose the transformation

Frequency shifting adds an offset in Hz, so harmonic spacing changes. Pitch shifting multiplies frequencies, preserving musical interval ratios. The single-sideband frequency shifter, delay-based pitch shifter and TD-PSOLA shifters therefore solve different problems.

pitch-shifter requires an explicit input period. voice-shifter adds the control/voicing path for a monophonic voice and has declared fixed latency; align its dry path. The dual-delay shifter uses caller-owned rings and offers stereo detuning, without formant preservation. A vocoder needs an analysis signal and the selected carrier; it is a band-envelope system.

real-fft supplies transforms over prepared twiddle/bit-reversal tables with split real/imaginary output. Design the window, overlap, hop, reconstruction and latency around it. A transform alone is not a complete spectral effect.

Toolkit reference

Import paths below are suffixes of maruaudio/dsp/. Use the exported types and setters in your installed version for complete signatures and defaults.

Analysis and meters

Import suffix	Purpose	Selected API
analysis/pitch-detector	Streaming bounded YIN-lite monophonic pitch detector exposing frequency, period, confidence and voiced state per fixed hop.	initPitchDetectorNode, pitchDetectorNodeTick
analysis/pitch-tracking	Causal monophonic note gating, continuous pitch and experimental circular tuning compensation.	initPitchTracking, configurePitchTracking
analysis/voice-pitch-detector	Voice YIN pitch detector with decimated analysis, full-rate refinement, voicing hysteresis and configurable 50–1600 Hz range and hop.	initVoicePitchDetector, voicePitchDetectorTick
analysis/k-weighting	Shared BS.1770 K filter coefficient and sample operations for offline/streaming consumers.	initKWeighting, kWeightingTick
analysis/loudness-match	Bounded paired stereo window matching, fresh learn/hold and continuous correction.	initLoudnessMatch, loudnessMatchTick
analysis/loudness-meter	Bounded live summed-stereo K/RMS window meter independent of matching, signal gating, or gain control.	initLoudnessMeter, loudnessMeterTick
analysis/true-peak	BS.1770-5 four-phase true-peak detector with a twelve-tap phase kernel.	initTruePeak, truePeakTick
analysis/programme-meter	Continuous stereo gated integrated LUFS, momentary/short-term loudness, LRA, true peak and sample-timed dynamics/stereo history.	initProgrammeMeter, programmeMeterTick

Spectral processing

Import suffix	Purpose	Selected API
spectral/frequency-shifter	Single-sideband frequency shifter: two fixed four-section Niemitalo allpass banks and phase-locked quadrature carriers.	initFrequencyShifterNode, frequencyShifterAllpassTick
spectral/pitch-shifter	Bounded monophonic TD-PSOLA pitch shifter over a fixed ring, with the input period as an explicit control input.	initPitchShifterNode, pitchShifterNodeTick
spectral/vocoder	Channel vocoder with logarithmic filter bands, envelope followers, carrier noise and dry/wet.	initVocoderNode, vocoderNodeTick
spectral/voice-shifter	Monophonic TD-PSOLA voice pitch shifter for pitch correction: smoothed continuous ratio, independent formant factor, mark-spacing pitch, fixed declared latency and a voiced/unvoiced crossfade to an aligned dry path.	initVoiceShifter, voiceShifterTick
spectral/real-fft	Prepared real forward/inverse FFT with split real/imaginary bins, power-of-two sizes up to 2^19 and caller-owned tables.	initRealFft, realFftLog2
spectral/delay-pitch-shifter	Dual sweeping-delay pitch shifter over caller-owned prepared stereo rings.	initDelayPitchShifter, delayPitchShifterTick

Return to DSP fundamentals for I/O, lifecycle, units and the category map.