Restoration and Post-Production

Lorenz Schwarz
Karlsruhe University of Arts and Design (HfG)
Winter Semester 2025/26
Course info

Slides marked include a live REAPER demonstration.

← Overview

Restoration and Post-Production

Post-production

  • project interchange: transfers media, edits, and metadata between applications or collaborators
  • editing: prepares and structures the recorded material
  • re-recording: replaces or adds material such as dialogue, Foley, or other performances
  • restoration: reduces noise, defects, and unwanted artefacts
  • sound design: creates and transforms material for the production
  • mixing: integrates and balances the prepared material
  • delivery: creates the required output formats and documentation
Restoration and Post-Production

Common restoration problems

  • hiss and broadband noise
  • hum, buzz, and tonal whistles
  • clicks, crackle, and dropouts
  • wind, plosives, and handling noise
  • clipping, distortion, and excessive reverberation

A recording may contain several problems at once.

Related: Fundamentals of Sound · Spectral Analysis · Filters

Restoration and Post-Production

Local repair by editing

Local defects can often be treated directly on the timeline.

  • isolate the affected region
  • reduce or remove it with item gain, fades, or automation
  • rebuild boundaries or fill gaps with matching ambience
  • keep the original material available for comparison
Restoration and Post-Production

Replacement material

  • use another take, microphone, channel, or nearby clean passage
  • align the replacement with the surrounding material
  • match level, perspective, and ambience
  • audition the transition in context
Restoration and Post-Production

Sample editing in REAPER

At sample-level zoom, very short defects can be repaired non-destructively.

  • pencil editing: redraw individual sample values
  • interpolation or spectral repair: reconstruct a short selection from surrounding material
  • sample edits can be bypassed, deleted, or rendered to a new file

Sample-level editing in REAPER with redrawn sample values

Restoration and Post-Production

DC offset correction

DC offset shifts the average amplitude away from the zero line.

  • it reduces headroom and may produce clicks at edits or fades
  • remove it with a DC filter or correction tool

Waveform before and after DC offset correction

Restoration and Post-Production

Clicks and crackle

  • Click: short impulsive disturbance or waveform discontinuity
  • Crackle: a dense sequence of small clicks
  • declicking detects damaged samples and reconstructs them through interpolation

Waveform and spectrogram of a recording with isolated clicks as vertical lines across all frequencies

Restoration and Post-Production

Hum and buzz

  • mains hum is commonly based on 50 Hz in Europe or 60 Hz in some regions
  • harmonics appear at multiples of the fundamental frequency
  • notch filters or de-hum tools can attenuate the fundamental and its harmonics
  • excessive filtering may remove wanted tonal components

Waveform and spectrogram of 50 Hz mains hum with harmonics as steady horizontal lines

Restoration and Post-Production

De-hum in REAPER

  • ReaEQ: narrow Notch bands at 50 Hz and its harmonics; here, eight bands up to 400 Hz
  • keep the bandwidth narrow to preserve neighboring frequencies
  • higher harmonics remain; complete removal is not the goal

ReaEQ with eight narrow notch filters on a hum fundamental and its harmonics

Restoration and Post-Production

Wind and rumble

  • wind commonly produces strong low-frequency bursts, but turbulence can also add broadband noise
  • high-pass filtering can reduce rumble below the wanted range; when wind overlaps voice or music, repair also alters the source
  • severe gusts can overload the microphone or input and cause clipping

→ Use wind protection during recording.

Waveform and spectrogram of wind and rumble with gusty energy below 300 Hz

Restoration and Post-Production

High-pass filtering in REAPER

  • cutoff: place it below the wanted material; this wind-only demo uses an intentionally high setting
  • reduces low-frequency rumble, while gusts overlapping the wanted range remain
  • cannot reverse clipping caused by wind

ReaEQ high-pass filter attenuating the low-frequency region of a wind recording

Restoration and Post-Production

Profile-based denoising

A noise-only passage provides a spectral profile of the background noise.

  • the processor estimates and attenuates components resembling that profile
  • moderate reduction often sounds more natural than complete removal
  • excessive processing can produce metallic or watery artifacts

Waveform and spectrogram of steady room tone with broadband noise and low-frequency energy

Restoration and Post-Production

Denoising in REAPER

  • ReaFIR in Subtract mode works as a profile-based denoiser
  • enable Automatically build noise profile during a noise-only passage, then disable it
  • the captured profile is continuously subtracted from the signal
  • the subtraction depth decides between residual noise and processing artifacts

ReaFIR in Subtract mode with a captured noise profile of the room tone

Restoration and Post-Production

Spectral repair

Spectral editing selects material by time and frequency.

  • attenuate or remove isolated noises
  • process only the affected frequency region
  • reconstruct short damage from surrounding material

Spectral editing view in REAPER

Restoration and Post-Production

Spectral repair in REAPER

In spectrogram view, select a short defect and choose Spectral edits → Spectral repair samples in time selection.

  • Bias L/R: weights the material before or after the selection
  • Scale: balances frequency precision against time resolution
  • Balance: controls the relation between original and resynthesized material
  • DC / Fade: smooth the repair boundaries

Spectral Repair settings in REAPER

Restoration and Post-Production

Limits of restoration

  • Clipping: declipping estimates missing waveform peaks; severe distortion cannot be fully recovered
  • Recorded reverberation: gating can shorten exposed tails, while specialized de-reverb tools estimate reflections overlapping the wanted signal
  • Source separation: neural tools can isolate voice or other components, but may remove ambience, consonants, or spatial cues

Strong processing can introduce new defects.

Restoration and Post-Production

Project interchange and handover

  • native project: stores timeline, media references, routing, and settings for one application
  • AAF / OMF: exchange sequences, edit metadata, and linked or embedded media
  • EDL: transfers a comparatively simple list of edit decisions
  • FCPXML / XML: application-specific descriptions used by editing systems
  • consolidated audio or stems: robust fallback when plug-ins, routing, fades, or automation do not translate

RPP is REAPER's text-based native project format, not an interchange standard.

Restoration and Post-Production

Practical exercise

Restoration and Post-Production

Restoration and handover

Treat a short voice recording with one or two audible defects as a restoration and handover task. Preserve the original, diagnose the problems, and choose the least intrusive combination of editing, alternate material, or processing.

Render and verify a consolidated file. Compare it with the source and explain what improved, what changed, and which version sounds more natural.

Restoration and Post-Production

Selected references

  • Cockos Incorporated. Up and Running: A REAPER User Guide.
    Version 7.78, 2026.
  • Godsill, Simon J., and Peter J. W. Rayner.
    Digital Audio Restoration. Springer, 1998.
  • Vaseghi, Saeed V.
    Advanced Digital Signal Processing and Noise Reduction. 4th ed., Wiley, 2008.

Related: Fundamentals of Sound · Spectral Analysis · Filters · Digital Audio

© 2026 Lorenz Schwarz

Original text, diagrams, illustrations, photographs, audio, and video are licensed under CC BY 4.0, except where otherwise noted.

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