The repair toolbox: what breaks, and the “de-” family
Before we open up each tool, here’s the lay of the land — the common ways a recording goes wrong, and the tool that addresses each. You’ll notice almost everything starts with “de-”: de-noise, de-hum, de-click. That little prefix just means “take away.”
| What you hear | What it is | The tool |
|---|---|---|
| A steady background shhhhh | Noise / hiss — even, random energy across the high end | denoise |
| Hiss that won’t yield to classical denoise | Learned noise — complex, speech-like background | ml-denoise (opt-in) |
| A low hummmm or buzz | Mains hum — electrical 50/60-cycle leakage and its echoes | de-hum |
| Hum that wanders as the grid drifts | Drifting hum — fundamental and harmonics not perfectly fixed | de-hum –adaptive |
| Sharp ticks and pops | Clicks — brief spikes (vinyl dust, bad digital edits) | de-click |
| Constant vinyl fizz under the music | Crackle — dense micro-spikes, not isolated pops | decrackle |
| A harsh, fuzzy, “broken speaker” tone on loud bits | Clipping — peaks chopped flat by overload | de-clip |
| It sounds like it’s in a bathroom | Reverb — the room’s echoes smearing the sound | de-reverb |
| Long, smeary room on speech | Heavy reverb tail — late reflections dominate | de-reverb –wpe |
| Piercing ssss and sshhh | Sibilance — over-loud consonants | de-ess |
| A low whoomph on outdoor recordings | Wind — turbulence rumbling the mic | de-wind |
| A thump on every “p” and “b” | Plosives — breath bursts hitting the mic | de-plosive |
| Scratchy noise when someone moves | Rustle — clothing against a clip-on mic | de-rustle |
| A held note that breathes flat/sharp | Wow & flutter — unstable playback speed | dewow |
| Stereo sounds thin; mono sums badly | Azimuth skew — L/R channels slightly out of step | azimuth |
| Two takes of the same moment don’t line up | Timing offset — multi-mic or reference misalignment | align |
| A silent gap or dropout | Missing samples — splice, skip, or mute | inpaint |
| Too quiet / too loud / inconsistent | Level — wrong loudness for delivery | normalize |
| Muffled, “telephone-y” | Lost highs — squashed by heavy compression | enhance |
| Boomy, dull digitized vinyl | RIAA curve — playback de-emphasis not applied | riaa |
| Dull FM or early-CD capture | Broadcast/CD pre-emphasis — wrong playback curve | deemphasis |
| Gritty “stair-step” quiet passages | Quantization grain — too few stored levels | dequantize |
| Stereo too wide / bass not mono-safe | Stereo image — mid/side width and mono-maker | stereo |
| Phase / mono check | Phase correlation — L/R agreement meter | stats |
| Clipped peaks, hissy gaps, hot true-peak | Restoration triage — noise floor, clip runs, suggested command | stats |
| Too fast / slow / chipmunk | Time & pitch — duration vs pitch edits | tempo / pitch / speed |
Two big families
Look closely and the tools split into two families, and the split matters because it tells you what’s possible.
Reducers turn down something unwanted that’s mixed alongside the good sound: hiss, hum, reverb, sibilance, wind. These work in the frequency view from the last chapter — find the unwanted pitches, turn them down, leave the rest. The good news: the wanted sound is still there underneath, so a careful reduction can be nearly invisible. The catch: if the unwanted thing overlaps the wanted thing too much, turning one down dents the other (this is where the “underwater” artefact comes from when people over-do noise reduction).
Repairers rebuild sound that’s been destroyed — clicks that punched a hole in the waveform, or clipping that chopped the tops off. Here the original is genuinely gone, and the tool has to invent a plausible replacement from the surrounding good audio, like an art restorer repainting a scratched corner of a canvas. The good news: done well, you can’t tell. The honest catch: it’s a guess, and the bigger the hole, the more it’s guessing.
flowchart TD
P["a problem in the recording"] --> Q{"is the good sound <b>destroyed</b>,<br/>or just <b>mixed</b> with junk?"}
Q -->|"mixed alongside it"| R["<b>REDUCER</b><br/>find the unwanted pitches,<br/>turn them down<br/><br/>denoise · de-hum · de-reverb<br/>de-ess · de-wind"]
Q -->|"destroyed"| S["<b>REPAIRER</b><br/>invent a plausible<br/>replacement<br/><br/>de-click · de-clip"]
R --> RR["good sound still there<br/>underneath → can be<br/>nearly invisible"]
S --> SS["original is gone → it's an<br/>educated guess; bigger hole,<br/>more guessing"]
Keeping these two families straight saves you a lot of disappointment. Asking a reducer to remove hiss that’s quieter than the voice? Easy. Asking a repairer to perfectly rebuild a badly clipped scream? It’ll help, but don’t expect a miracle — there was no original left to recover.
A golden rule of order
When you chain several fixes, order matters. A good default, and roughly the order cathar’s chapters follow:
- Repair destruction first — de-click, de-clip. (You don’t want later tools analysing damaged samples.)
- Remove steady offenders — de-hum, then de-noise.
- Tame the spectral stuff — de-reverb, de-ess.
- Shape and deliver last — enhance, then set the loudness.
With the map in hand, let’s open the tools one at a time — starting with the most common complaint of all: hiss.