Glossary in plain language

Every term you met in this book, defined the way a friend would explain it.

Aliasing — The ghostly, gritty wrong-pitch tones you get when audio is converted to a lower sample rate without first removing the pitches that are too high for the new rate to hold. The audio version of wagon wheels spinning backwards in films.

Bit depth — How finely each single sample is written down (16-bit for CDs, 24-bit for studios). More bits = a quieter background fuzz floor.

Clipping — Distortion caused by a signal trying to go louder than the maximum the system can store, so its peaks get chopped flat. Sounds harsh and “broken.”

DAW (Digital Audio Workstation) — A full audio-editing program like Pro Tools, Logic, Ableton, Reaper, or Audition. The “Photoshop of sound.”

de- (prefix) — Just means “remove”: de-noise, de-hum, de-click.

Dequantization — Reducing the audible grain left when audio was stored with too few discrete levels (low bit depth). Cathar’s dequantize uses neighbour prediction on the quantisation lattice.

De-crackle — Suppressing dense, continuous micro-clicks (vinyl surface fizz) rather than isolated pops. Cathar: decrackle.

Pre-emphasis / de-emphasis — Boosting highs on record and cutting them on playback (or the reverse when correcting a capture). RIAA is the vinyl standard; FM and early CD use other curves. Cathar: riaa, deemphasis.

DSP (Digital Signal Processing) — The umbrella term for doing maths on digital audio (or any signal) to change it: filtering, denoising, all of it.

EBU R128 — The European broadcast loudness standard built on BS.1770; the reason broadcast audio targets −23 LUFS.

Elliptical mono — On stereo vinyl captures, summing only the low frequencies to mono while keeping highs in stereo — tames out-of-phase rumble without collapsing the mix. Cathar: riaa --elliptical <Hz>.

FFT (Fast Fourier Transform) — The fast machine that takes a chunk of sound and reads off its “recipe” of pitches. The workhorse behind the frequency view.

Filter — A tool that turns some pitches up or down. A high-pass filter keeps the highs and blocks the lows; a low-pass does the reverse; a notch removes one narrow band.

Frequency — How fast the wave wiggles; what you hear as pitch. Measured in cycles per second, or hertz (Hz). 1,000 Hz = 1 kHz.

Harmonics — Faint copies of a tone at exact whole-number multiples of its pitch. Why hum “buzzes” instead of being a pure tone.

Hum — Low, steady tone leaking in from the electrical mains (50 or 60 cycles per second, plus harmonics).

LUFS — “Loudness Units relative to Full Scale.” The modern unit for perceived loudness — two files at the same LUFS sound equally loud. Targets: −23 broadcast, −16 podcast, −14 streaming.

Mono — A single channel of audio; plays equally from both speakers.

Noise / hiss — Steady, random background energy spread across the high frequencies — the shhhh behind a recording.

Noiseprint / noise profile — A measurement of the recipe of a recording’s background noise, learned from a quiet patch, so a denoiser knows exactly what to subtract.

Normalization — Setting a recording to a target level. Peak normalization aims at the tallest sample (crude); loudness (LUFS) normalization aims at how loud it actually sounds (correct for delivery).

Nyquist frequency — The highest pitch a given sample rate can hold: exactly half the sample rate. Go above it and you get aliasing.

Overlap-add — The careful blending technique that glues the processed short slices of audio back into one seamless waveform.

Phase correlation — A single number from −1 to +1 that says how much the left and right channels agree at the same instant. Near +1 means mono-safe; near −1 means they cancel when summed. Cathar: stats (Phase corr line).

Mid-side (M/S) — A way to rewrite stereo as a shared centre (mid) and a difference (side). Scaling the side changes width without moving the centre. Cathar: stereo --width, --ms / --from-ms.

Phase coherence — Keeping a stereo file’s two channels “agreeing” when you process them, so the stereo image stays stable instead of wandering.

Plosive — The low thump on “p” and “b” sounds when a puff of breath hits the mic.

Quantization — Rounding each sample to the nearest allowed level on a fixed grid (determined by bit depth). Too coarse a grid leaves audible grain.

Resampling — Converting audio from one sample rate to another (e.g. 48,000 → 44,100). Done well, it’s a smart filter, not a copy.

Reverb — The trail of fading echoes a room adds as sound bounces off its surfaces. Makes recordings sound “roomy” or “boxy.”

RIAA curve — The standard EQ applied when cutting and playing back vinyl: bass cut and treble boost on playback (de-emphasis) so the groove stays narrow but the listener hears flat audio. Cathar: riaa.

Wow & flutter — Pitch wobble from unstable analog playback speed (warped disc, stretched tape). Cathar: dewow.

Azimuth — Stereo playback head tilt that puts the left and right channels slightly out of step. Cathar: azimuth.

WPE (Weighted Prediction Error) — A blind de-reverb method that predicts late spectral frames from earlier ones per frequency bin and subtracts the prediction. Cathar: dereverb --wpe.

HPSS — Harmonic / percussive source separation via median filtering on a spectrogram. Cathar: hpss.

Inpainting — Reconstructing a short gap of missing audio by predicting from samples on both sides. Cathar: inpaint.

Rustle — Scratchy mid-range noise from clothing brushing a clip-on (lavalier) microphone.

Sample — One single measurement of the wave’s height. Audio is a long list of these.

Sample rate — How many samples are taken per second (44,100 for CD, 48,000 for video/pro). Higher = can capture higher pitches.

Sibilance — Over-loud, piercing “s,” “sh,” and “ch” sounds; removed by de-essing.

Spectral subtraction — The core denoising method: measure the background haze at each pitch and subtract that amount.

Spectrogram — A heat-map picture of sound: time left-to-right, pitch bottom-to-top, brightness = how much of each pitch is present. Where most restoration tools “see.”

Stereo — Two channels (left and right) whose difference creates a sense of width and placement.

STFT (Short-Time Fourier Transform) — Taking an FFT of many short, overlapping slices in a row, to track how a sound’s pitches change over time. The engine behind the spectrogram.

Threshold — A “how much counts” cutoff: how loud a spike must be to count as a click, or how loud sibilance must get before a de-esser reacts.

True peak (inter-sample peak) — A hidden overshoot in the smooth curve drawn between samples on playback, which can distort even when no stored sample looked too loud. Why loudness tools keep a true-peak safety ceiling (e.g. −1 dBTP).

Waveform — The wiggly line of the wave’s height over time. Great for seeing how loud, poor for seeing what’s in it.

Wiener filter — A gentler denoising method: instead of subtracting the haze, scale each pitch by how likely it is to be real sound versus noise.

Window (Hann window) — The gentle taper applied to each short slice of audio before its FFT, so the slices blend together without clicks at the seams.